diff --git a/README.md b/README.md
index b16391f7..da863255 100644
--- a/README.md
+++ b/README.md
@@ -388,6 +388,19 @@ Run `codeburn` for the dashboard, or use a subcommand below. Most commands also
 | `codeburn export` | CSV covering today, 7 days, and 30 days |
 | `codeburn export -f json` | Export as JSON instead of CSV |
 
+**Sync (team telemetry)** _preview_
+
+| Command | What it does |
+|---------|--------------|
+| `codeburn sync setup <url>` | One-time setup: OIDC login via browser, stores token securely |
+| `codeburn sync push` | Push unsent usage to remote endpoint (default: last 7 days) |
+| `codeburn sync push --since 30d` | Push a larger window |
+| `codeburn sync status` | Show endpoint, auth state, last sync time |
+| `codeburn sync logout` | Revoke token and remove credentials |
+| `codeburn sync reset --confirm` | Clear sent-ledger (re-send all data on next push) |
+
+Sync sends token counts, costs, models, and projects — never prompts or code. This feature is in preview; the protocol may change between releases. See [docs/sync/](docs/sync/) for details.
+
 **Web & devices**
 
 | Command | What it does |
diff --git a/docs/sync/DEVELOPER.md b/docs/sync/DEVELOPER.md
new file mode 100644
index 00000000..130edf11
--- /dev/null
+++ b/docs/sync/DEVELOPER.md
@@ -0,0 +1,215 @@
+# Sync — Developer Documentation
+
+Architecture, protocol, server contract, and testing for `codeburn sync`.
+
+## Architecture
+
+```
+Developer machine                          Remote backend
+──────────────────                         ──────────────
+~/.config/codeburn/sync.json  (config)
+~/.config/codeburn/.sync-token (credential)
+~/.cache/codeburn/sync-ledger.json (sent-ledger)
+
+codeburn sync push
+  │
+  ├─ Read config → baseUrl, clientId, issuer, tracesPath
+  ├─ Read refresh token from OS store
+  ├─ POST {issuer}/oauth2/token (refresh_token grant) → access_token
+  ├─ Collect ParsedProviderCall[] for window
+  ├─ Filter against sent-ledger (only unsent calls)
+  ├─ Build OTLP/HTTP JSON payload
+  ├─ POST {baseUrl}{tracesPath} with Bearer token
+  ├─ On success → append deduplicationKeys to ledger
+  └─ Update lastSync in config
+```
+
+## Discovery Protocol
+
+### Server discovery document
+
+```
+GET {baseUrl}/.well-known/codeburn-export.json
+```
+
+```json
+{
+  "version": 1,
+  "issuer": "https://cognito-idp.us-west-2.amazonaws.com/us-west-2_XXXX",
+  "client_id": "70e6sgst2ju6ff9dnrmv4l1tcb",
+  "scopes": ["openid", "email"],
+  "traces_path": "/v1/traces",
+  "max_batch_size": 1000
+}
+```
+
+| Field | Required | Default | Description |
+|---|---|---|---|
+| `version` | No | `1` | Client rejects `version > 1` |
+| `issuer` | Yes | — | OIDC issuer URL. Client fetches `{issuer}/.well-known/openid-configuration` |
+| `client_id` | Yes | — | OAuth client ID for this deployment |
+| `scopes` | No | `["openid"]` | Scopes to request. `offline_access` added dynamically if IdP supports it |
+| `traces_path` | No | `/v1/traces` | Path for OTLP POST |
+| `max_batch_size` | No | `1000` | Max spans per HTTP request |
+
+### Why not proxy `.well-known/openid-configuration`?
+
+OIDC requires the `issuer` claim inside the discovery doc to match the URL it was fetched from. Serving Cognito's doc from a different domain violates this constraint. The `codeburn-export.json` doc decouples the metrics endpoint from the identity provider.
+
+## OIDC Authentication
+
+### Flow: Authorization Code + PKCE
+
+1. Client generates `code_verifier` (32 random bytes, base64url) and `code_challenge` (SHA-256 of verifier, base64url)
+2. Client starts callback server on `127.0.0.1:19876` (fallback: 19877, 19878)
+3. Browser opens `{authorization_endpoint}?response_type=code&client_id=...&redirect_uri=http://127.0.0.1:{port}/callback&code_challenge=...&code_challenge_method=S256&state=...&scope=...`
+4. User logs in at IdP → IdP redirects to `http://127.0.0.1:{port}/callback?code=...&state=...`
+5. Callback server validates `state`, extracts `code`
+6. Client POSTs to `{token_endpoint}` with `grant_type=authorization_code`, `code`, `code_verifier`, `redirect_uri`, `client_id`
+7. IdP returns `access_token` + `refresh_token`
+
+### Fixed ports
+
+Cognito (and Okta) do exact string comparison on callback URLs. Ephemeral ports fail. We register three fixed ports: `19876`, `19877`, `19878`. The client tries in order, falling back if a port is in use.
+
+RFC 8252 recommends `127.0.0.1` (IP literal) over `localhost` to avoid IPv6 `::1` resolution.
+
+### Token refresh
+
+On every `sync push`:
+1. Read refresh token from OS store
+2. POST `{token_endpoint}` with `grant_type=refresh_token`
+3. Store whatever refresh token the server returns (handles rotation transparently)
+4. On `invalid_grant` → stop, prompt user to re-run `sync setup`
+
+### Scope resolution
+
+- Request scopes from `codeburn-export.json`
+- Add `offline_access` only if `scopes_supported` in OIDC discovery includes it
+- Cognito rejects `offline_access` as `invalid_scope` — it issues refresh tokens without it
+
+## Credential Storage
+
+| Platform | Method | Implementation |
+|---|---|---|
+| macOS | Keychain | `security add-generic-password` / `find-generic-password` |
+| Linux | libsecret | `secret-tool store` / `secret-tool lookup` |
+| Windows | DPAPI | PowerShell `ConvertTo-SecureString` / `ConvertFrom-SecureString` |
+| Fallback | File | `~/.config/codeburn/.sync-token` with `0600` permissions |
+
+No native modules (`keytar` is archived). Shell out to OS CLIs. Fallback reported honestly in `sync status`.
+
+## OTLP Encoding
+
+Strict protobuf-JSON mapping of `ExportTraceServiceRequest`. lowerCamelCase fields, hex-encoded IDs, integer enums.
+
+### Span identity (deterministic)
+
+```
+span_id   = first 8 bytes of SHA-256(deduplicationKey) → hex (16 chars)
+trace_id  = first 16 bytes of SHA-256(sessionId)       → hex (32 chars)
+```
+
+Re-sends are byte-identical. Server-side dedup is defense-in-depth.
+
+### Resource attributes
+
+```json
+{
+  "resource": {
+    "attributes": [
+      { "key": "codeburn.device_id", "value": { "stringValue": "<SHA-256(hostname+username)[:16]>" } }
+    ]
+  }
+}
+```
+
+### Span attributes
+
+```json
+{
+  "attributes": [
+    { "key": "ai.provider", "value": { "stringValue": "kiro" } },
+    { "key": "ai.model", "value": { "stringValue": "claude-sonnet-4-6" } },
+    { "key": "ai.input_tokens", "value": { "intValue": "12500" } },
+    { "key": "ai.output_tokens", "value": { "intValue": "3200" } },
+    { "key": "ai.cost_usd", "value": { "doubleValue": 0.085 } },
+    { "key": "ai.project", "value": { "stringValue": "my-app" } },
+    { "key": "ai.tools", "value": { "arrayValue": { "values": [{ "stringValue": "Edit" }] } } },
+    { "key": "ai.speed", "value": { "stringValue": "standard" } },
+    { "key": "ai.cost_estimated", "value": { "boolValue": true } }
+  ]
+}
+```
+
+## Sent-Ledger
+
+Client-side deduplication source of truth at `~/.cache/codeburn/sync-ledger.json`.
+
+Format: JSON array of `{ key: string, ts: string }` objects.
+
+**Push logic**: collect all calls in window → subtract ledger entries → send remainder → append to ledger on success.
+
+**Pruning**: entries older than 6 months removed on every push.
+
+**Why not a watermark?** Timestamp watermarks silently skip late-arriving calls (long sessions, providers that update rows). The ledger is exact.
+
+### Partial success
+
+OTLP returns `partial_success.rejected_spans` in the response body. Because OTLP does not identify *which* spans were rejected, the client ledgers nothing for a partially-rejected batch — the entire batch retries on the next push. This is safe: span IDs are deterministic (derived from the deduplication key), so servers that store by span ID treat re-sent spans as idempotent upserts.
+
+### Rate limiting (429)
+
+A push runs to completion — there is no routine per-push cap (only a 50,000-call safety valve). Server rate limits are the intended brake:
+
+- On HTTP 429 the client honors `Retry-After` (delta-seconds or HTTP-date), capped at 120 seconds per wait, defaulting to 5 seconds when the header is absent
+- The same batch is retried up to 3 consecutive times; if the server is still rate-limiting after that, the push stops and the remaining (unledgered) calls are sent on the next push
+- On 401 or 5xx the push stops immediately with the same resume-on-next-push behavior
+
+### Server contract
+
+The backend must implement:
+
+1. `GET {baseUrl}/.well-known/codeburn-export.json` — returns the discovery doc (public, no auth)
+2. `POST {baseUrl}{traces_path}` — accepts OTLP/HTTP JSON with Bearer token
+   - Validate JWT (issued by the configured IdP)
+   - Derive developer identity from token's `sub` claim
+   - Accept `startTimeUnixNano` up to 6 months in the past
+   - Return standard OTLP response body
+
+No PII is included in the payload. The server derives identity solely from the authenticated token.
+
+## Testing
+
+### Unit tests (`tests/sync.test.ts`)
+
+26 tests covering pure functions: discovery parsing, PKCE generation, auth URL construction, scope resolution, callback server, config read/write. No network, no browser.
+
+### Mock IdP e2e (`tests/sync-e2e.test.ts`)
+
+6 tests with a localhost mock IdP server. Exercises the full auth round-trip, token refresh, rotation, revocation — fully offline, runs in CI.
+
+### Headless browser e2e (`tests/sync-headless-e2e.test.ts`)
+
+1 test with Playwright headless Chromium against real Cognito. Proves the actual browser PKCE flow works including Cognito Hosted UI form submission and localhost redirect.
+
+**Developer-only** — requires:
+- Deployed test backend (CDK stack at `../codeburn-sync-backend/`)
+- Cognito user with confirmed password
+- Environment variables: `CODEBURN_SYNC_URL`, `CODEBURN_SYNC_EMAIL`, `CODEBURN_SYNC_PASSWORD`
+- Playwright Chromium installed (`PLAYWRIGHT_BROWSERS_PATH`)
+
+Skipped by default when env vars are not set. Never runs in CI.
+
+### Test CDK stack (`codeburn-sync-backend/`)
+
+Minimal AWS backend for the headless e2e test:
+- Cognito User Pool (PKCE, fixed callback ports)
+- HTTP API with JWT authorizer
+- Discovery Lambda (serves `codeburn-export.json`)
+- Ingest Lambda (logs OTLP spans to CloudWatch)
+
+Deploy: `npx cdk deploy --profile andklee-dev`
+Cost: ~$0/mo idle (pay-per-request)
+
+This is a **test fixture**, not a production reference. Any OIDC provider + OTLP-accepting endpoint satisfies the server contract.
diff --git a/docs/sync/README.md b/docs/sync/README.md
new file mode 100644
index 00000000..5f1343ed
--- /dev/null
+++ b/docs/sync/README.md
@@ -0,0 +1,130 @@
+# codeburn sync
+
+Push your AI usage telemetry to a shared backend so teams can track adoption, budgets, and ROI across developers.
+
+Everything stays local-first: codeburn never sends data without your explicit action, and prompts/code are never included.
+
+## Quick Start
+
+```bash
+# One-time setup (opens browser for login)
+codeburn sync setup https://metrics.your-team.com
+
+# Push recent usage
+codeburn sync push
+
+# Check status
+codeburn sync status
+```
+
+## Commands
+
+### `codeburn sync setup <url>`
+
+Configures sync with a remote endpoint. Opens your browser for a one-time OIDC login.
+
+```bash
+codeburn sync setup https://metrics.your-team.com
+```
+
+What happens:
+1. Fetches server configuration from `<url>/.well-known/codeburn-export.json`
+2. Opens your browser to the identity provider's login page
+3. After login, stores a refresh token securely in your OS keychain
+4. Saves the endpoint configuration (no secrets) to `~/.config/codeburn/sync.json`
+
+You only need to do this once. The token refreshes silently on every push.
+
+### `codeburn sync push`
+
+Sends unsent AI usage data to the configured endpoint.
+
+```bash
+# Push unsent calls from the last 7 days (default)
+codeburn sync push
+
+# Push a larger window
+codeburn sync push --since 30d
+
+# Preview what would be sent
+codeburn sync push --dry-run
+```
+
+### `codeburn sync status`
+
+Shows the current sync configuration and authentication state.
+
+```
+Endpoint: https://metrics.your-team.com
+Traces path: /v1/traces
+Issuer: https://auth.your-team.com
+Auth: configured
+Token storage: keychain
+Last sync: 2h ago
+```
+
+### `codeburn sync logout`
+
+Removes stored credentials and revokes the token at the identity provider.
+
+```bash
+codeburn sync logout
+```
+
+### `codeburn sync reset --confirm`
+
+Clears the sent-ledger, causing the next push to re-send all data in the window. Use after a backend migration or if you suspect missing data.
+
+```bash
+codeburn sync reset --confirm
+```
+
+## What Gets Sent
+
+Each AI interaction becomes one OTLP span with these attributes:
+
+| Field | Example | Description |
+|---|---|---|
+| `ai.provider` | `kiro`, `cursor`, `claude` | Which AI tool |
+| `ai.model` | `claude-sonnet-4-6` | Model used |
+| `ai.input_tokens` | `12500` | Prompt tokens |
+| `ai.output_tokens` | `3200` | Response tokens |
+| `ai.cost_usd` | `0.085` | Estimated cost |
+| `ai.project` | `my-app` | Project name |
+| `ai.tools` | `["Edit", "Bash"]` | Tools invoked |
+
+A pseudonymous `device_id` distinguishes your machines without revealing hostnames.
+
+### What is NOT sent
+
+- **Prompts** — your actual messages to AI are never included
+- **Code** — file contents, diffs, and paths stay local
+- **Bash commands** — may contain secrets, never sent
+- **Your name/email** — identity is derived server-side from your login token
+
+There is no flag to override this. Privacy is structural, not configurable.
+
+## Authentication
+
+Sync uses standard OIDC (the same protocol as "Sign in with Google"). Your team's admin sets up the identity provider — you just click through the browser login once.
+
+- **Token storage**: macOS Keychain, Windows Credential Manager, or Linux libsecret. Falls back to a `0600` file if no keychain is available.
+- **Token lifetime**: typically 30–90 days (set by your admin). You'll be prompted to re-login when it expires.
+- **Re-login**: run `codeburn sync setup <url>` again.
+
+## FAQ
+
+**Q: Does sync run automatically?**
+A: No. You run `codeburn sync push` when you want. A future version may offer opportunistic push (after each `codeburn report`), but it's always explicit.
+
+**Q: What if I push the same data twice?**
+A: Safe. A local sent-ledger tracks what's been sent. Re-pushing the same window doesn't create duplicates.
+
+**Q: What if I'm offline for a week?**
+A: Next push catches up. The default window is 7 days; use `--since 30d` or `--since all` (up to 6 months) for longer gaps. A push runs to completion regardless of size — server rate limits (429) are waited out automatically.
+
+**Q: Can my admin see my prompts?**
+A: No. Prompts are never included in the payload. The server only sees token counts, costs, model names, and project names.
+
+**Q: How do I stop syncing?**
+A: `codeburn sync logout` removes everything. Or just stop running `push`.
diff --git a/package-lock.json b/package-lock.json
index e649491f..16b2e23f 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -27,6 +27,7 @@
         "@types/node": "^22.19.17",
         "@types/react": "^19.2.14",
         "@types/selfsigned": "^2.0.4",
+        "playwright": "^1.61.1",
         "tsup": "^8.4.0",
         "tsx": "^4.19.0",
         "typescript": "^5.8.0",
@@ -2834,6 +2835,53 @@
         "node": ">=16.0.0"
       }
     },
+    "node_modules/playwright": {
+      "version": "1.61.1",
+      "resolved": "https://registry.npmjs.org/playwright/-/playwright-1.61.1.tgz",
+      "integrity": "sha512-DWnY5o3YbLWK4GovuAVwpqL+1VwGNdUGrRr++8j8PtQQzvAVZUIMjKQ90fY689sEJZJBbZVw1rXaOKSTitkzPQ==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "dependencies": {
+        "playwright-core": "1.61.1"
+      },
+      "bin": {
+        "playwright": "cli.js"
+      },
+      "engines": {
+        "node": ">=18"
+      },
+      "optionalDependencies": {
+        "fsevents": "2.3.2"
+      }
+    },
+    "node_modules/playwright-core": {
+      "version": "1.61.1",
+      "resolved": "https://registry.npmjs.org/playwright-core/-/playwright-core-1.61.1.tgz",
+      "integrity": "sha512-h7Qlt6m4REp25qvIdvbDtVmD4LqVXfpRxhORv9L0jzETM05p4fuPJ3dKyuSXQxDSbXnmS79HAgi9589lGSpLkg==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "bin": {
+        "playwright-core": "cli.js"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    },
+    "node_modules/playwright/node_modules/fsevents": {
+      "version": "2.3.2",
+      "resolved": "https://registry.npmjs.org/fsevents/-/fsevents-2.3.2.tgz",
+      "integrity": "sha512-xiqMQR4xAeHTuB9uWm+fFRcIOgKBMiOBP+eXiyT7jsgVCq1bkVygt00oASowB7EdtpOHaaPgKt812P9ab+DDKA==",
+      "dev": true,
+      "hasInstallScript": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "darwin"
+      ],
+      "engines": {
+        "node": "^8.16.0 || ^10.6.0 || >=11.0.0"
+      }
+    },
     "node_modules/postcss": {
       "version": "8.5.15",
       "resolved": "https://registry.npmjs.org/postcss/-/postcss-8.5.15.tgz",
diff --git a/package.json b/package.json
index f3f59c03..9fda978c 100644
--- a/package.json
+++ b/package.json
@@ -63,6 +63,7 @@
     "@types/node": "^22.19.17",
     "@types/react": "^19.2.14",
     "@types/selfsigned": "^2.0.4",
+    "playwright": "^1.61.1",
     "tsup": "^8.4.0",
     "tsx": "^4.19.0",
     "typescript": "^5.8.0",
diff --git a/src/main.ts b/src/main.ts
index a4e435d4..6b4abbcd 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -26,6 +26,7 @@ import { formatDateRangeLabel, parseDateRangeFlags, parseDayFlag, parseDaysFlag,
 import { runOptimize } from './optimize.js'
 import { registerActCommands } from './act/cli.js'
 import { registerGuardCommands } from './guard/cli.js'
+import { registerSyncCommands } from './sync/cli.js'
 import { runContextCommand } from './context-tree.js'
 import { renderCompare } from './compare.js'
 import {
@@ -1597,5 +1598,6 @@ program
 
 registerActCommands(program)
 registerGuardCommands(program)
+registerSyncCommands(program)
 
 program.parse()
diff --git a/src/sync/auth.ts b/src/sync/auth.ts
new file mode 100644
index 00000000..fa912322
--- /dev/null
+++ b/src/sync/auth.ts
@@ -0,0 +1,346 @@
+/**
+ * codeburn sync — OIDC authentication.
+ *
+ * Authorization Code + PKCE flow with localhost callback server.
+ */
+
+import { createHash, randomBytes } from 'crypto'
+import { createServer, type Server } from 'http'
+import { URL } from 'url'
+import { assertHttps } from './discovery.js'
+
+export interface OidcConfig {
+  authorization_endpoint: string
+  token_endpoint: string
+  revocation_endpoint?: string
+  scopes_supported?: string[]
+}
+
+export interface TokenResponse {
+  access_token: string
+  refresh_token?: string
+  expires_in?: number
+  token_type: string
+}
+
+export class AuthError extends Error {
+  constructor(message: string) {
+    super(message)
+    this.name = 'AuthError'
+  }
+}
+
+// --- OIDC Discovery ---
+
+export async function fetchOidcConfig(issuer: string): Promise<OidcConfig> {
+  assertHttps(issuer, 'OIDC issuer')
+  const url = `${issuer.replace(/\/$/, '')}/.well-known/openid-configuration`
+
+  let response: Response
+  try {
+    response = await fetch(url)
+  } catch (err) {
+    throw new AuthError(`Cannot reach OIDC discovery at ${url}: ${(err as Error).message}`)
+  }
+
+  if (!response.ok) {
+    throw new AuthError(`OIDC discovery returned HTTP ${response.status}: ${url}`)
+  }
+
+  let body: Record<string, unknown>
+  try {
+    body = await response.json() as Record<string, unknown>
+  } catch {
+    throw new AuthError(`OIDC discovery returned invalid JSON: ${url}`)
+  }
+
+  // Issuer mix-up defense (OIDC Discovery §4.3): the issuer claim in the
+  // metadata must match the issuer we fetched it from.
+  const issuerClaim = typeof body.issuer === 'string' ? body.issuer.replace(/\/$/, '') : ''
+  if (issuerClaim !== issuer.replace(/\/$/, '')) {
+    throw new AuthError(
+      `OIDC issuer mismatch: metadata claims "${body.issuer}" but was fetched from "${issuer}"`
+    )
+  }
+
+  const authorization_endpoint = body.authorization_endpoint
+  const token_endpoint = body.token_endpoint
+  if (typeof authorization_endpoint !== 'string' || typeof token_endpoint !== 'string') {
+    throw new AuthError('OIDC discovery missing authorization_endpoint or token_endpoint')
+  }
+  // Tokens travel on these endpoints — enforce https (loopback exempt)
+  assertHttps(authorization_endpoint, 'authorization_endpoint')
+  assertHttps(token_endpoint, 'token_endpoint')
+  if (typeof body.revocation_endpoint === 'string') {
+    assertHttps(body.revocation_endpoint, 'revocation_endpoint')
+  }
+
+  return {
+    authorization_endpoint,
+    token_endpoint,
+    revocation_endpoint: typeof body.revocation_endpoint === 'string' ? body.revocation_endpoint : undefined,
+    scopes_supported: Array.isArray(body.scopes_supported)
+      ? (body.scopes_supported as unknown[]).filter((s): s is string => typeof s === 'string')
+      : undefined,
+  }
+}
+
+// --- PKCE ---
+
+export interface PkceChallenge {
+  code_verifier: string
+  code_challenge: string
+  code_challenge_method: 'S256'
+}
+
+export function generatePkce(): PkceChallenge {
+  // RFC 7636: 43-128 chars, unreserved characters
+  const code_verifier = randomBytes(32).toString('base64url')
+  const code_challenge = createHash('sha256').update(code_verifier).digest('base64url')
+  return { code_verifier, code_challenge, code_challenge_method: 'S256' }
+}
+
+// --- Auth URL ---
+
+export const CALLBACK_PORTS = [19876, 19877, 19878] as const
+
+export interface AuthUrlParams {
+  authorization_endpoint: string
+  client_id: string
+  redirect_uri: string
+  scopes: string[]
+  state: string
+  pkce: PkceChallenge
+}
+
+export function buildAuthUrl(params: AuthUrlParams): string {
+  const url = new URL(params.authorization_endpoint)
+  url.searchParams.set('response_type', 'code')
+  url.searchParams.set('client_id', params.client_id)
+  url.searchParams.set('redirect_uri', params.redirect_uri)
+  url.searchParams.set('scope', params.scopes.join(' '))
+  url.searchParams.set('state', params.state)
+  url.searchParams.set('code_challenge', params.pkce.code_challenge)
+  url.searchParams.set('code_challenge_method', params.pkce.code_challenge_method)
+  return url.toString()
+}
+
+export function resolveScopes(requestedScopes: string[], idpScopesSupported?: string[]): string[] {
+  const scopes = [...requestedScopes]
+  // Add offline_access only if the IdP advertises it
+  if (idpScopesSupported?.includes('offline_access') && !scopes.includes('offline_access')) {
+    scopes.push('offline_access')
+  }
+  return scopes
+}
+
+// --- Callback Server ---
+
+export interface CallbackResult {
+  code: string
+  port: number
+}
+
+export function startCallbackServer(
+  expectedState: string,
+  timeoutMs: number = 300_000, // 5 minutes
+  ports: readonly number[] = CALLBACK_PORTS, // tests pass [0] for an ephemeral port
+): { promise: Promise<CallbackResult>; ready: Promise<number>; server: Server } {
+  let resolvedPort = 0
+  let server: Server
+  let readyResolve!: (port: number) => void
+  let readyReject!: (err: Error) => void
+  const ready = new Promise<number>((resolve, reject) => {
+    readyResolve = resolve
+    readyReject = reject
+  })
+
+  const promise = new Promise<CallbackResult>((resolve, reject) => {
+    // Fully shut down: stop listening AND destroy lingering keep-alive
+    // sockets. Without this, an HTTP client's pooled connection keeps the
+    // dead server alive and can swallow requests meant for a later server
+    // on the same port.
+    const shutdown = () => {
+      try { server.close() } catch { /* already closed */ }
+      try { server.closeAllConnections() } catch { /* Node < 18.2 */ }
+    }
+
+    const timer = setTimeout(() => {
+      shutdown()
+      reject(new AuthError('Login timed out after 5 minutes. Please try again.'))
+    }, timeoutMs)
+
+    server = createServer((req, res) => {
+      const url = new URL(req.url ?? '/', `http://127.0.0.1`)
+      const code = url.searchParams.get('code')
+      const state = url.searchParams.get('state')
+      const error = url.searchParams.get('error')
+
+      // Connection: close on every response — the callback server is
+      // single-purpose and must never leave pooled keep-alive sockets behind.
+      if (error) {
+        res.writeHead(400, { 'Content-Type': 'text/html', 'Connection': 'close' })
+        res.end('<html><body><h1>Login failed</h1><p>You can close this tab.</p></body></html>')
+        clearTimeout(timer)
+        shutdown()
+        reject(new AuthError(`IdP returned error: ${error}`))
+        return
+      }
+
+      if (state !== expectedState) {
+        res.writeHead(400, { 'Content-Type': 'text/plain', 'Connection': 'close' })
+        res.end('Invalid state parameter')
+        return // don't close — might be a stale request
+      }
+
+      if (!code) {
+        res.writeHead(400, { 'Content-Type': 'text/plain', 'Connection': 'close' })
+        res.end('Missing authorization code')
+        return
+      }
+
+      res.writeHead(200, { 'Content-Type': 'text/html', 'Connection': 'close' })
+      res.end('<html><body><h1>✓ Login successful</h1><p>You can close this tab.</p></body></html>')
+      clearTimeout(timer)
+      shutdown()
+      resolve({ code, port: resolvedPort })
+    })
+
+    // Try ports in order. The error handler is guarded on `resolvedPort` so a
+    // late error event can never trigger a second listen() after a successful
+    // bind (which would silently move the server off the advertised port).
+    const tryListen = (ports: readonly number[], idx: number) => {
+      if (idx >= ports.length) {
+        clearTimeout(timer)
+        const err = new AuthError(`All callback ports (${ports.join(', ')}) are in use. Close other codeburn instances and retry.`)
+        readyReject(err)
+        reject(err)
+        return
+      }
+      const port = ports[idx]!
+      server.once('error', (err: NodeJS.ErrnoException) => {
+        if (resolvedPort !== 0) return // already bound — never rebind
+        if (err.code === 'EADDRINUSE') {
+          tryListen(ports, idx + 1)
+        } else {
+          clearTimeout(timer)
+          const authErr = new AuthError(`Callback server error: ${err.message}`)
+          readyReject(authErr)
+          reject(authErr)
+        }
+      })
+      server.listen(port, '127.0.0.1', () => {
+        // port 0 = OS-assigned ephemeral port — read the real one back
+        const addr = server.address()
+        resolvedPort = typeof addr === 'object' && addr ? addr.port : port
+        readyResolve(resolvedPort)
+      })
+    }
+
+    tryListen(ports, 0)
+  })
+
+  // `ready` resolves with the actually-bound port once listening — callers
+  // must await it before building the redirect URI (port fallback means the
+  // first port in CALLBACK_PORTS is not guaranteed).
+  return { promise, ready, server: server! }
+}
+
+// --- Token Exchange ---
+
+export async function exchangeCode(
+  tokenEndpoint: string,
+  code: string,
+  codeVerifier: string,
+  redirectUri: string,
+  clientId: string,
+): Promise<TokenResponse> {
+  assertHttps(tokenEndpoint, 'token_endpoint')
+  const body = new URLSearchParams({
+    grant_type: 'authorization_code',
+    code,
+    code_verifier: codeVerifier,
+    redirect_uri: redirectUri,
+    client_id: clientId,
+  })
+
+  const response = await fetch(tokenEndpoint, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+    body: body.toString(),
+  })
+
+  if (!response.ok) {
+    let detail = ''
+    try { detail = await response.text() } catch {}
+    throw new AuthError(`Token exchange failed (HTTP ${response.status}): ${detail}`)
+  }
+
+  const data = await response.json() as Record<string, unknown>
+  if (typeof data.access_token !== 'string') {
+    throw new AuthError('Token response missing access_token')
+  }
+
+  return {
+    access_token: data.access_token,
+    refresh_token: typeof data.refresh_token === 'string' ? data.refresh_token : undefined,
+    expires_in: typeof data.expires_in === 'number' ? data.expires_in : undefined,
+    token_type: typeof data.token_type === 'string' ? data.token_type : 'Bearer',
+  }
+}
+
+export async function refreshToken(
+  tokenEndpoint: string,
+  refreshTokenValue: string,
+  clientId: string,
+): Promise<TokenResponse> {
+  assertHttps(tokenEndpoint, 'token_endpoint')
+  const body = new URLSearchParams({
+    grant_type: 'refresh_token',
+    refresh_token: refreshTokenValue,
+    client_id: clientId,
+  })
+
+  const response = await fetch(tokenEndpoint, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+    body: body.toString(),
+  })
+
+  if (!response.ok) {
+    let detail = ''
+    try { detail = await response.text() } catch {}
+    if (response.status === 400 || response.status === 401) {
+      throw new AuthError('Sync auth expired. Run `codeburn sync setup` to re-authenticate.')
+    }
+    throw new AuthError(`Token refresh failed (HTTP ${response.status}): ${detail}`)
+  }
+
+  const data = await response.json() as Record<string, unknown>
+  if (typeof data.access_token !== 'string') {
+    throw new AuthError('Refresh response missing access_token')
+  }
+
+  return {
+    access_token: data.access_token,
+    refresh_token: typeof data.refresh_token === 'string' ? data.refresh_token : undefined,
+    expires_in: typeof data.expires_in === 'number' ? data.expires_in : undefined,
+    token_type: typeof data.token_type === 'string' ? data.token_type : 'Bearer',
+  }
+}
+
+export async function revokeToken(
+  revocationEndpoint: string,
+  token: string,
+  clientId: string,
+): Promise<void> {
+  try {
+    await fetch(revocationEndpoint, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+      body: new URLSearchParams({ token, client_id: clientId }).toString(),
+    })
+  } catch {
+    // Best-effort — don't fail logout if revocation endpoint is unavailable
+  }
+}
diff --git a/src/sync/cli.ts b/src/sync/cli.ts
new file mode 100644
index 00000000..9136dbfd
--- /dev/null
+++ b/src/sync/cli.ts
@@ -0,0 +1,327 @@
+/**
+ * codeburn sync — CLI commands.
+ *
+ * Registers: sync setup | push | status | logout | reset
+ */
+
+import type { Command } from 'commander'
+import { randomBytes } from 'crypto'
+
+import { fetchDiscoveryDoc } from './discovery.js'
+import {
+  fetchOidcConfig,
+  generatePkce,
+  buildAuthUrl,
+  resolveScopes,
+  startCallbackServer,
+  exchangeCode,
+  refreshToken,
+  revokeToken,
+  CALLBACK_PORTS,
+} from './auth.js'
+import { createCredentialStore } from './credentials.js'
+import { readSyncConfig, writeSyncConfig, deleteSyncConfig, updateLastSync } from './config.js'
+import { collectUnsentCalls, sendBatches, batchCalls, MAX_PER_PUSH } from './push.js'
+
+export function registerSyncCommands(program: Command): void {
+  const sync = program
+    .command('sync')
+    .description('Sync AI usage telemetry to a remote OTLP endpoint')
+
+  // --- setup ---
+  sync
+    .command('setup <url>')
+    .description('Configure sync with a remote endpoint (one-time)')
+    .action(async (url: string) => {
+      const baseUrl = url.replace(/\/$/, '')
+      process.stderr.write(`Fetching discovery doc from ${baseUrl}...\n`)
+
+      // 1. Fetch codeburn discovery doc
+      const discovery = await fetchDiscoveryDoc(baseUrl)
+      process.stderr.write(`  Issuer: ${discovery.issuer}\n`)
+      process.stderr.write(`  Client: ${discovery.client_id}\n`)
+
+      // 2. Fetch OIDC configuration from the issuer
+      const oidc = await fetchOidcConfig(discovery.issuer)
+      process.stderr.write(`  Auth endpoint: ${oidc.authorization_endpoint}\n`)
+
+      // 3. Resolve scopes
+      const scopes = resolveScopes(discovery.scopes, oidc.scopes_supported)
+
+      // 4. Generate PKCE
+      const pkce = generatePkce()
+      const state = randomBytes(16).toString('hex')
+
+      // 5. Start callback server — await the actually-bound port (port
+      // fallback means it may not be the first in CALLBACK_PORTS)
+      const { promise: callbackPromise, ready } = startCallbackServer(state)
+      const port = await ready
+      const redirectUri = `http://127.0.0.1:${port}/callback`
+
+      // 6. Build auth URL and open browser
+      const authUrl = buildAuthUrl({
+        authorization_endpoint: oidc.authorization_endpoint,
+        client_id: discovery.client_id,
+        redirect_uri: redirectUri,
+        scopes,
+        state,
+        pkce,
+      })
+
+      process.stderr.write(`\nOpening browser for login...\n`)
+      process.stderr.write(`If the browser doesn't open, visit:\n  ${authUrl}\n\n`)
+
+      // Open browser (best-effort, platform-specific).
+      // execFileSync with args array — authUrl comes from the remote discovery
+      // doc so it must never be shell-interpolated. Scheme is also validated.
+      try {
+        if (!/^https:\/\//.test(authUrl)) {
+          throw new Error('auth URL must be https')
+        }
+        const { execFileSync } = await import('child_process')
+        if (process.platform === 'darwin') {
+          execFileSync('open', [authUrl], { stdio: 'ignore' })
+        } else if (process.platform === 'win32') {
+          // `start` is a cmd builtin; empty first arg is the window title
+          execFileSync('cmd', ['/c', 'start', '', authUrl], { stdio: 'ignore' })
+        } else {
+          execFileSync('xdg-open', [authUrl], { stdio: 'ignore' })
+        }
+      } catch {
+        // Browser open failed — user sees the URL above
+      }
+
+      // 7. Wait for callback
+      process.stderr.write(`Waiting for login (5 min timeout)...\n`)
+      const callback = await callbackPromise
+
+      // 8. Exchange code for tokens
+      const tokenRedirectUri = `http://127.0.0.1:${callback.port}/callback`
+      const tokens = await exchangeCode(
+        oidc.token_endpoint,
+        callback.code,
+        pkce.code_verifier,
+        tokenRedirectUri,
+        discovery.client_id,
+      )
+
+      if (!tokens.refresh_token) {
+        process.stderr.write(`Warning: IdP did not return a refresh token. You may need to re-authenticate frequently.\n`)
+      }
+
+      // 9. Store credentials
+      const store = createCredentialStore()
+      if (tokens.refresh_token) {
+        store.store(tokens.refresh_token)
+      }
+
+      // 10. Write config
+      writeSyncConfig({
+        baseUrl,
+        clientId: discovery.client_id,
+        tracesPath: discovery.traces_path,
+        issuer: discovery.issuer,
+      })
+
+      process.stderr.write(`\n✓ Sync configured successfully.\n`)
+      process.stderr.write(`  Endpoint: ${baseUrl}\n`)
+      process.stderr.write(`  Token stored in: ${store.method()}\n`)
+      process.stderr.write(`\nRun \`codeburn sync push\` to send telemetry data.\n`)
+    })
+
+  // --- status ---
+  sync
+    .command('status')
+    .description('Show sync configuration and auth status')
+    .action(async () => {
+      const config = readSyncConfig()
+      if (!config) {
+        process.stderr.write('Sync not configured. Run `codeburn sync setup <url>` first.\n')
+        process.exit(1)
+      }
+
+      const store = createCredentialStore()
+      const token = store.retrieve()
+
+      process.stdout.write(`Endpoint: ${config.baseUrl}\n`)
+      process.stdout.write(`Traces path: ${config.tracesPath}\n`)
+      process.stdout.write(`Issuer: ${config.issuer}\n`)
+      process.stdout.write(`Auth: ${token ? 'configured' : 'missing (run sync setup)'}\n`)
+      process.stdout.write(`Token storage: ${store.method()}\n`)
+      process.stdout.write(`Last sync: ${config.lastSync ?? 'never'}\n`)
+    })
+
+  // --- logout ---
+  sync
+    .command('logout')
+    .description('Remove stored credentials and revoke token')
+    .action(async () => {
+      const config = readSyncConfig()
+      const store = createCredentialStore()
+      const token = store.retrieve()
+
+      // Revoke if we have a token and know the revocation endpoint
+      if (token && config) {
+        try {
+          const oidc = await fetchOidcConfig(config.issuer)
+          if (oidc.revocation_endpoint) {
+            await revokeToken(oidc.revocation_endpoint, token, config.clientId)
+            process.stderr.write('Token revoked at IdP.\n')
+          }
+        } catch {
+          // Best-effort revocation
+        }
+      }
+
+      store.delete()
+      deleteSyncConfig()
+      process.stderr.write('Sync credentials and config removed.\n')
+    })
+
+  // --- reset ---
+  sync
+    .command('reset')
+    .description('Clear the sent-ledger (next push re-sends all calls in window)')
+    .option('--confirm', 'Required to confirm reset')
+    .action(async (opts: { confirm?: boolean }) => {
+      if (!opts.confirm) {
+        process.stderr.write('This will clear the sent-ledger, causing the next push to re-send all data.\n')
+        process.stderr.write('Run with --confirm to proceed.\n')
+        process.exit(1)
+      }
+
+      const { clearLedger } = await import('./ledger.js')
+      const removed = clearLedger()
+      if (removed > 0) {
+        process.stderr.write(`Ledger cleared (${removed} entries). Next push will re-send all calls in window.\n`)
+      } else {
+        process.stderr.write('No ledger entries found (nothing to reset).\n')
+      }
+    })
+
+  // --- push (placeholder for Step 2) ---
+  sync
+    .command('push')
+    .description('Push unsent telemetry data to the configured endpoint')
+    .option('--since <period>', 'Time window: today, 7d, 30d, month, all (max 6 months)', '7d')
+    .option('--dry-run', 'Show what would be sent without sending')
+    .action(async (opts: { since: string; dryRun?: boolean }) => {
+      const config = readSyncConfig()
+      if (!config) {
+        process.stderr.write('Sync not configured. Run `codeburn sync setup <url>` first.\n')
+        process.exit(1)
+      }
+
+      const store = createCredentialStore()
+      const rt = store.retrieve()
+      if (!rt) {
+        process.stderr.write('No auth token found. Run `codeburn sync setup` to authenticate.\n')
+        process.exit(1)
+      }
+
+      // Refresh token
+      try {
+        const oidc = await fetchOidcConfig(config.issuer)
+        const tokens = await refreshToken(oidc.token_endpoint, rt, config.clientId)
+
+        // Store rotated token if present
+        if (tokens.refresh_token && tokens.refresh_token !== rt) {
+          store.store(tokens.refresh_token)
+        }
+
+        if (opts.dryRun) {
+          process.stderr.write(`[dry-run] Auth: valid (Bearer token obtained)\n`)
+        }
+
+        // Collect data
+        const { parseAllSessions } = await import('../parser.js')
+        const { getDateRange } = await import('../cli-date.js')
+
+        // Map --since to a parser period. Strict: unknown values are an error.
+        const sinceToPeriod: Record<string, string> = {
+          'today': 'today',
+          '7d': 'week', 'week': 'week',
+          '30d': '30days', '30days': '30days',
+          'month': 'month',
+          'all': 'all', // up to 6 months (parser retention limit)
+        }
+        const period = sinceToPeriod[opts.since]
+        if (!period) {
+          process.stderr.write(`Unknown --since value "${opts.since}". Valid: today, 7d, 30d, month, all.\n`)
+          process.exit(1)
+        }
+        const { range } = getDateRange(period)
+        const projects = await parseAllSessions(range)
+
+        // Flatten + filter against sent-ledger
+        const { allCalls, unsent } = collectUnsentCalls(projects)
+
+        if (opts.dryRun) {
+          const toPushCount = Math.min(unsent.length, MAX_PER_PUSH)
+          const cost = unsent.slice(0, MAX_PER_PUSH).reduce((s, c) => s + c.call.costUSD, 0)
+          process.stderr.write(`[dry-run] Window: ${opts.since} — ${allCalls.length} calls total, ${allCalls.length - unsent.length} already synced\n`)
+          process.stderr.write(`[dry-run] Would push ${toPushCount} calls ($${cost.toFixed(2)}) to ${config.baseUrl}${config.tracesPath}\n`)
+          if (unsent.length > MAX_PER_PUSH) {
+            process.stderr.write(`[dry-run] ${unsent.length - MAX_PER_PUSH} more calls exceed the ${MAX_PER_PUSH} safety limit — a second push would be needed\n`)
+          }
+          return
+        }
+
+        if (unsent.length === 0) {
+          process.stderr.write(`Nothing to push (${allCalls.length} calls already synced).\n`)
+          updateLastSync()
+          return
+        }
+
+        // Safety valve (not a routine cap — pushes run to completion)
+        const toPush = unsent.slice(0, MAX_PER_PUSH)
+        if (unsent.length > MAX_PER_PUSH) {
+          process.stderr.write(`${unsent.length} unsent calls exceed the ${MAX_PER_PUSH} safety limit. Pushing first ${MAX_PER_PUSH}; run again to continue.\n`)
+        }
+
+        // Batch and send (loops until done; waits out 429 rate limits)
+        const discoveryDoc = await fetchDiscoveryDoc(config.baseUrl)
+        const batches = batchCalls(toPush, discoveryDoc.max_batch_size)
+        const endpoint = `${config.baseUrl}${config.tracesPath}`
+
+        const result = await sendBatches({
+          endpoint,
+          accessToken: tokens.access_token,
+          batches,
+          log: msg => process.stderr.write(`${msg}\n`),
+        })
+
+        if (result.outcome === 'auth-rejected') {
+          process.stderr.write('Auth rejected by server. Run `codeburn sync setup` to re-authenticate.\n')
+          process.exit(1)
+        }
+        if (result.outcome === 'rate-limited') {
+          process.stderr.write(`Rate limited — gave up after repeated retries. Remaining calls will be sent on the next push.\n`)
+        }
+        if (result.outcome === 'server-error') {
+          process.stderr.write(`Server error (HTTP ${result.httpStatus}). Remaining calls will be sent on the next push.\n`)
+        }
+
+        // Update lastSync
+        updateLastSync()
+
+        // Summary
+        process.stderr.write(`\nSynced ${result.totalSent} calls ($${result.totalCostSent.toFixed(2)}) to ${config.baseUrl}\n`)
+        if (result.totalRejected > 0) {
+          process.stderr.write(`  ${result.totalRejected} spans rejected (will retry on next push)\n`)
+        }
+        if (unsent.length > MAX_PER_PUSH) {
+          process.stderr.write(`  ${unsent.length - MAX_PER_PUSH} calls remaining (safety limit). Run \`codeburn sync push\` again.\n`)
+        }
+
+        // Non-zero exit when the push did not complete, so cron/scripts can
+        // detect it. Ledgered progress is kept; next push resumes.
+        if (result.outcome !== 'complete') {
+          process.exitCode = 1
+        }
+      } catch (err) {
+        process.stderr.write(`${(err as Error).message}\n`)
+        process.exit(1)
+      }
+    })
+}
diff --git a/src/sync/config.ts b/src/sync/config.ts
new file mode 100644
index 00000000..6ddfcec0
--- /dev/null
+++ b/src/sync/config.ts
@@ -0,0 +1,66 @@
+/**
+ * codeburn sync — config file management.
+ *
+ * Stores non-secret sync configuration at ~/.config/codeburn/sync.json
+ */
+
+import { existsSync, mkdirSync, readFileSync, writeFileSync, unlinkSync } from 'fs'
+import { join } from 'path'
+import { homedir } from 'os'
+
+export interface SyncConfig {
+  baseUrl: string
+  clientId: string
+  tracesPath: string
+  issuer: string
+  lastSync?: string
+}
+
+function configDir(): string {
+  return join(homedir(), '.config', 'codeburn')
+}
+
+function configPath(): string {
+  return join(configDir(), 'sync.json')
+}
+
+export function readSyncConfig(): SyncConfig | null {
+  const path = configPath()
+  if (!existsSync(path)) return null
+
+  try {
+    const raw = readFileSync(path, 'utf-8')
+    const data = JSON.parse(raw) as Record<string, unknown>
+
+    if (typeof data.baseUrl !== 'string' || typeof data.clientId !== 'string') {
+      return null
+    }
+
+    return {
+      baseUrl: data.baseUrl,
+      clientId: data.clientId,
+      tracesPath: typeof data.tracesPath === 'string' ? data.tracesPath : '/v1/traces',
+      issuer: typeof data.issuer === 'string' ? data.issuer : '',
+      lastSync: typeof data.lastSync === 'string' ? data.lastSync : undefined,
+    }
+  } catch {
+    return null
+  }
+}
+
+export function writeSyncConfig(config: SyncConfig): void {
+  const dir = configDir()
+  mkdirSync(dir, { recursive: true })
+  writeFileSync(configPath(), JSON.stringify(config, null, 2) + '\n')
+}
+
+export function updateLastSync(): void {
+  const config = readSyncConfig()
+  if (!config) return
+  config.lastSync = new Date().toISOString()
+  writeSyncConfig(config)
+}
+
+export function deleteSyncConfig(): void {
+  try { unlinkSync(configPath()) } catch { /* may not exist */ }
+}
diff --git a/src/sync/credentials.ts b/src/sync/credentials.ts
new file mode 100644
index 00000000..9f802599
--- /dev/null
+++ b/src/sync/credentials.ts
@@ -0,0 +1,218 @@
+/**
+ * codeburn sync — OS credential storage.
+ *
+ * Stores refresh tokens in the OS keychain.
+ * Falls back to a 0600 file when no keychain is available.
+ */
+
+import { execFileSync } from 'child_process'
+import { existsSync, mkdirSync, readFileSync, writeFileSync, unlinkSync, chmodSync } from 'fs'
+import { join } from 'path'
+import { homedir } from 'os'
+
+const SERVICE_NAME = 'codeburn-sync'
+const ACCOUNT_NAME = 'refresh-token'
+
+export type StorageMethod = 'keychain' | 'secret-tool' | 'dpapi' | 'file'
+
+export interface CredentialStore {
+  store(token: string): void
+  retrieve(): string | null
+  delete(): void
+  method(): StorageMethod
+}
+
+// --- macOS Keychain ---
+
+class KeychainStore implements CredentialStore {
+  store(token: string): void {
+    // Delete existing first (add-generic-password fails if entry exists)
+    try {
+      execFileSync('security', ['delete-generic-password', '-s', SERVICE_NAME, '-a', ACCOUNT_NAME], { stdio: 'pipe' })
+    } catch { /* may not exist */ }
+
+    // Token passed as arg to execFileSync (no shell interpolation).
+    // Note: still briefly visible in process args on macOS; `security` has no
+    // stdin mode for -w with non-interactive use, so this is the best available.
+    execFileSync('security', ['add-generic-password', '-s', SERVICE_NAME, '-a', ACCOUNT_NAME, '-w', token], { stdio: 'pipe' })
+  }
+
+  retrieve(): string | null {
+    try {
+      const result = execFileSync(
+        'security',
+        ['find-generic-password', '-s', SERVICE_NAME, '-a', ACCOUNT_NAME, '-w'],
+        { encoding: 'utf-8', stdio: ['pipe', 'pipe', 'pipe'] }
+      )
+      return result.trim() || null
+    } catch {
+      return null
+    }
+  }
+
+  delete(): void {
+    try {
+      execFileSync('security', ['delete-generic-password', '-s', SERVICE_NAME, '-a', ACCOUNT_NAME], { stdio: 'pipe' })
+    } catch { /* may not exist */ }
+  }
+
+  method(): StorageMethod { return 'keychain' }
+}
+
+// --- Linux libsecret ---
+
+class SecretToolStore implements CredentialStore {
+  store(token: string): void {
+    // Token passed via stdin — never appears in argv or shell string
+    execFileSync(
+      'secret-tool',
+      ['store', `--label=${SERVICE_NAME}`, 'service', SERVICE_NAME, 'account', ACCOUNT_NAME],
+      { input: token, stdio: ['pipe', 'pipe', 'pipe'] }
+    )
+  }
+
+  retrieve(): string | null {
+    try {
+      const result = execFileSync(
+        'secret-tool',
+        ['lookup', 'service', SERVICE_NAME, 'account', ACCOUNT_NAME],
+        { encoding: 'utf-8', stdio: ['pipe', 'pipe', 'pipe'] }
+      )
+      return result.trim() || null
+    } catch {
+      return null
+    }
+  }
+
+  delete(): void {
+    try {
+      execFileSync('secret-tool', ['clear', 'service', SERVICE_NAME, 'account', ACCOUNT_NAME], { stdio: 'pipe' })
+    } catch { /* may not exist */ }
+  }
+
+  method(): StorageMethod { return 'secret-tool' }
+}
+
+// --- Windows DPAPI ---
+
+class DpapiStore implements CredentialStore {
+  private filePath: string
+
+  constructor() {
+    this.filePath = join(homedir(), '.config', 'codeburn', '.sync-token-dpapi')
+  }
+
+  store(token: string): void {
+    // Token passed via environment variable — never in argv or command string
+    const ps = `$s = ConvertTo-SecureString $env:CODEBURN_SYNC_TOKEN -AsPlainText -Force; ConvertFrom-SecureString $s`
+    const encrypted = execFileSync('powershell', ['-NoProfile', '-Command', ps], {
+      encoding: 'utf-8',
+      stdio: ['pipe', 'pipe', 'pipe'],
+      env: { ...process.env, CODEBURN_SYNC_TOKEN: token },
+    }).trim()
+
+    const dir = join(homedir(), '.config', 'codeburn')
+    mkdirSync(dir, { recursive: true })
+    writeFileSync(this.filePath, encrypted, { mode: 0o600 })
+  }
+
+  retrieve(): string | null {
+    if (!existsSync(this.filePath)) return null
+    try {
+      const encrypted = readFileSync(this.filePath, 'utf-8').trim()
+      const ps = `$s = ConvertTo-SecureString $env:CODEBURN_SYNC_BLOB; [Runtime.InteropServices.Marshal]::PtrToStringAuto([Runtime.InteropServices.Marshal]::SecureStringToBSTR($s))`
+      const result = execFileSync('powershell', ['-NoProfile', '-Command', ps], {
+        encoding: 'utf-8',
+        stdio: ['pipe', 'pipe', 'pipe'],
+        env: { ...process.env, CODEBURN_SYNC_BLOB: encrypted },
+      })
+      return result.trim() || null
+    } catch {
+      return null
+    }
+  }
+
+  delete(): void {
+    try { unlinkSync(this.filePath) } catch { /* may not exist */ }
+  }
+
+  method(): StorageMethod { return 'dpapi' }
+}
+
+// --- File Fallback ---
+
+class FileStore implements CredentialStore {
+  private filePath: string
+
+  constructor() {
+    this.filePath = join(homedir(), '.config', 'codeburn', '.sync-token')
+  }
+
+  store(token: string): void {
+    const dir = join(homedir(), '.config', 'codeburn')
+    mkdirSync(dir, { recursive: true })
+    writeFileSync(this.filePath, token, { mode: 0o600 })
+    // Ensure permissions (writeFile mode doesn't always work on existing files)
+    try { chmodSync(this.filePath, 0o600) } catch {}
+  }
+
+  retrieve(): string | null {
+    if (!existsSync(this.filePath)) return null
+    try {
+      return readFileSync(this.filePath, 'utf-8').trim() || null
+    } catch {
+      return null
+    }
+  }
+
+  delete(): void {
+    try { unlinkSync(this.filePath) } catch { /* may not exist */ }
+  }
+
+  method(): StorageMethod { return 'file' }
+}
+
+// --- Factory ---
+
+function isCommandAvailable(cmd: string): boolean {
+  try {
+    const probe = process.platform === 'win32' ? 'where' : 'which'
+    execFileSync(probe, [cmd], { stdio: 'pipe' })
+    return true
+  } catch {
+    return false
+  }
+}
+
+export function createCredentialStore(): CredentialStore {
+  // Test/CI escape hatch: force the file store (respects $HOME, so tests
+  // can fully isolate with a temp HOME). Without this, darwin machines
+  // would hit the real login keychain during the offline test suite.
+  if (process.env.CODEBURN_SYNC_TOKEN_STORE === 'file') {
+    return new FileStore()
+  }
+
+  if (process.platform === 'darwin') {
+    return new KeychainStore()
+  }
+
+  if (process.platform === 'win32') {
+    return new DpapiStore()
+  }
+
+  // Linux: try secret-tool, fall back to file
+  if (isCommandAvailable('secret-tool')) {
+    // Also verify the keyring daemon is running
+    try {
+      execFileSync('secret-tool', ['lookup', 'service', '__codeburn_probe__', 'account', '__probe__'], { stdio: 'pipe' })
+      return new SecretToolStore()
+    } catch (err) {
+      // Exit code 1 = not found (keyring works). Other errors = keyring not running.
+      if ((err as { status?: number }).status === 1) {
+        return new SecretToolStore()
+      }
+    }
+  }
+
+  return new FileStore()
+}
diff --git a/src/sync/discovery.ts b/src/sync/discovery.ts
new file mode 100644
index 00000000..abd97dc1
--- /dev/null
+++ b/src/sync/discovery.ts
@@ -0,0 +1,119 @@
+/**
+ * codeburn sync — discovery document parser.
+ *
+ * Fetches and validates {baseUrl}/.well-known/codeburn-export.json
+ */
+
+export interface CodeburnDiscoveryDoc {
+  version: number
+  issuer: string
+  client_id: string
+  scopes: string[]
+  traces_path: string
+  max_batch_size: number
+}
+
+export class DiscoveryError extends Error {
+  constructor(message: string) {
+    super(message)
+    this.name = 'DiscoveryError'
+  }
+}
+
+const LOOPBACK_HOSTS = new Set(['127.0.0.1', '[::1]', 'localhost'])
+
+/**
+ * Enforce https on remote endpoints (RFC 8252 §8.3). Refresh tokens and
+ * bearer tokens travel on these URLs — plaintext http is only permitted
+ * for loopback addresses (local development and offline tests).
+ */
+export function assertHttps(url: string, label: string): void {
+  let parsed: URL
+  try {
+    parsed = new URL(url)
+  } catch {
+    throw new DiscoveryError(`${label} is not a valid URL: ${url}`)
+  }
+  if (parsed.protocol === 'https:') return
+  if (parsed.protocol === 'http:' && LOOPBACK_HOSTS.has(parsed.hostname)) return
+  throw new DiscoveryError(
+    `${label} must use https (got ${parsed.protocol}//${parsed.host}). ` +
+    `Plain http is only allowed for loopback (127.0.0.1).`
+  )
+}
+
+const SUPPORTED_VERSION = 1
+
+export function parseDiscoveryDoc(raw: unknown): CodeburnDiscoveryDoc {
+  if (!raw || typeof raw !== 'object' || Array.isArray(raw)) {
+    throw new DiscoveryError('Discovery doc must be a JSON object')
+  }
+
+  const doc = raw as Record<string, unknown>
+
+  // Version check
+  const version = typeof doc.version === 'number' ? doc.version : 1
+  if (version > SUPPORTED_VERSION) {
+    throw new DiscoveryError(
+      `This endpoint requires codeburn sync v${version}. Please update codeburn.`
+    )
+  }
+
+  // Required fields
+  const issuer = doc.issuer
+  if (typeof issuer !== 'string' || !issuer) {
+    throw new DiscoveryError('Discovery doc missing required field: issuer')
+  }
+
+  const client_id = doc.client_id
+  if (typeof client_id !== 'string' || !client_id) {
+    throw new DiscoveryError('Discovery doc missing required field: client_id')
+  }
+
+  // Format validation after presence checks (clearer errors)
+  assertHttps(issuer, 'Issuer')
+
+  // Optional with defaults
+  const scopes = Array.isArray(doc.scopes)
+    ? doc.scopes.filter((s): s is string => typeof s === 'string')
+    : ['openid']
+
+  const traces_path = typeof doc.traces_path === 'string' ? doc.traces_path : '/v1/traces'
+
+  const max_batch_size = typeof doc.max_batch_size === 'number' && doc.max_batch_size > 0
+    ? doc.max_batch_size
+    : 1000
+
+  return { version, issuer, client_id, scopes, traces_path, max_batch_size }
+}
+
+export async function fetchDiscoveryDoc(baseUrl: string): Promise<CodeburnDiscoveryDoc> {
+  assertHttps(baseUrl, 'Base URL')
+  const url = `${baseUrl.replace(/\/$/, '')}/.well-known/codeburn-export.json`
+
+  let response: Response
+  try {
+    response = await fetch(url)
+  } catch (err) {
+    throw new DiscoveryError(`Cannot reach ${url}: ${(err as Error).message}`)
+  }
+
+  if (response.status === 404) {
+    throw new DiscoveryError(
+      `Server does not support codeburn sync.\n${url} returned 404.`
+    )
+  }
+
+  if (!response.ok) {
+    throw new DiscoveryError(`${url} returned HTTP ${response.status}`)
+  }
+
+  let body: unknown
+  try {
+    body = await response.json()
+  } catch {
+    throw new DiscoveryError(`${url} returned invalid JSON`)
+  }
+
+  return parseDiscoveryDoc(body)
+}
diff --git a/src/sync/index.ts b/src/sync/index.ts
new file mode 100644
index 00000000..347d03e4
--- /dev/null
+++ b/src/sync/index.ts
@@ -0,0 +1,8 @@
+export { registerSyncCommands } from './cli.js'
+export { fetchDiscoveryDoc, parseDiscoveryDoc, assertHttps, type CodeburnDiscoveryDoc } from './discovery.js'
+export { fetchOidcConfig, generatePkce, buildAuthUrl, resolveScopes, exchangeCode, refreshToken } from './auth.js'
+export { createCredentialStore, type CredentialStore, type StorageMethod } from './credentials.js'
+export { readSyncConfig, writeSyncConfig, deleteSyncConfig, type SyncConfig } from './config.js'
+export { readLedger, appendToLedger, ledgerKeySet, clearLedger } from './ledger.js'
+export { buildOtlpPayload, batchCalls, deriveSpanId, deriveTraceId, getDeviceId } from './otlp.js'
+export { collectUnsentCalls, sendBatches, parseRetryAfterMs, MAX_PER_PUSH, type PushResult, type PushOutcome } from './push.js'
diff --git a/src/sync/ledger.ts b/src/sync/ledger.ts
new file mode 100644
index 00000000..99ef8560
--- /dev/null
+++ b/src/sync/ledger.ts
@@ -0,0 +1,87 @@
+/**
+ * codeburn sync — sent-ledger.
+ *
+ * Client-side deduplication: tracks which calls have been successfully pushed.
+ * Push logic: window minus ledger = what to send.
+ */
+
+import { existsSync, mkdirSync, readFileSync, writeFileSync, unlinkSync, renameSync } from 'fs'
+import { join } from 'path'
+import { homedir } from 'os'
+
+export interface LedgerEntry {
+  key: string  // deduplicationKey
+  ts: string   // call timestamp (for pruning)
+}
+
+const SIX_MONTHS_MS = 180 * 24 * 60 * 60 * 1000
+
+function cacheDir(): string {
+  // Honor XDG_CACHE_HOME — the ledger is reconstructible state, not config
+  const xdg = process.env.XDG_CACHE_HOME
+  const base = xdg && xdg.trim() ? xdg : join(homedir(), '.cache')
+  return join(base, 'codeburn')
+}
+
+function ledgerPath(): string {
+  return join(cacheDir(), 'sync-ledger.json')
+}
+
+export function readLedger(): LedgerEntry[] {
+  const path = ledgerPath()
+  if (!existsSync(path)) return []
+  try {
+    const raw = readFileSync(path, 'utf-8')
+    const entries = JSON.parse(raw) as unknown
+    if (!Array.isArray(entries)) return []
+    return entries.filter(
+      (e): e is LedgerEntry => typeof e === 'object' && e !== null && typeof e.key === 'string'
+    )
+  } catch {
+    return []
+  }
+}
+
+export function writeLedger(entries: LedgerEntry[]): void {
+  const dir = cacheDir()
+  mkdirSync(dir, { recursive: true })
+  // Atomic write: a crash mid-write must not corrupt the ledger — a corrupt
+  // ledger reads as empty and the next push re-sends the whole window.
+  const path = ledgerPath()
+  const tmp = `${path}.tmp`
+  writeFileSync(tmp, JSON.stringify(entries))
+  renameSync(tmp, path)
+}
+
+/** Append new entries after a successful push. Also prunes entries older than 6 months. */
+export function appendToLedger(newEntries: LedgerEntry[]): void {
+  const existing = readLedger()
+  const cutoff = new Date(Date.now() - SIX_MONTHS_MS).toISOString()
+
+  // Prune old + dedupe
+  const keySet = new Set(existing.map(e => e.key))
+  const pruned = existing.filter(e => !e.ts || e.ts > cutoff)
+
+  for (const entry of newEntries) {
+    if (!keySet.has(entry.key)) {
+      pruned.push(entry)
+      keySet.add(entry.key)
+    }
+  }
+
+  writeLedger(pruned)
+}
+
+/** Get the set of already-sent deduplication keys for fast lookup. */
+export function ledgerKeySet(): Set<string> {
+  return new Set(readLedger().map(e => e.key))
+}
+
+/** Clear the ledger (for sync reset). Returns the number of entries removed. */
+export function clearLedger(): number {
+  const path = ledgerPath()
+  if (!existsSync(path)) return 0
+  const count = readLedger().length
+  unlinkSync(path)
+  return count
+}
diff --git a/src/sync/otlp.ts b/src/sync/otlp.ts
new file mode 100644
index 00000000..8a12c7e6
--- /dev/null
+++ b/src/sync/otlp.ts
@@ -0,0 +1,143 @@
+/**
+ * codeburn sync — OTLP payload builder.
+ *
+ * Converts ParsedApiCall[] into an ExportTraceServiceRequest (OTLP/HTTP JSON).
+ * Span and trace IDs are derived deterministically from deduplicationKey/sessionId.
+ */
+
+import { createHash } from 'crypto'
+import { hostname, userInfo } from 'os'
+import type { ParsedApiCall } from '../types.js'
+
+export interface OtlpSpan {
+  traceId: string
+  spanId: string
+  name: string
+  startTimeUnixNano: string
+  endTimeUnixNano: string
+  attributes: OtlpAttribute[]
+}
+
+export interface OtlpAttribute {
+  key: string
+  value: OtlpValue
+}
+
+export type OtlpValue =
+  | { stringValue: string }
+  | { intValue: string }
+  | { doubleValue: number }
+  | { boolValue: boolean }
+  | { arrayValue: { values: OtlpValue[] } }
+
+export interface OtlpPayload {
+  resourceSpans: Array<{
+    resource: { attributes: OtlpAttribute[] }
+    scopeSpans: Array<{
+      spans: OtlpSpan[]
+    }>
+  }>
+}
+
+// --- Device ID (pseudonymous, stable) ---
+
+let cachedDeviceId: string | null = null
+
+/** Pure derivation — exposed so the encoding can be golden-pinned in tests. */
+export function deriveDeviceId(host: string, username: string): string {
+  return createHash('sha256').update(`${host}:${username}`).digest('hex').slice(0, 16)
+}
+
+export function getDeviceId(): string {
+  if (cachedDeviceId) return cachedDeviceId
+  cachedDeviceId = deriveDeviceId(hostname(), userInfo().username)
+  return cachedDeviceId
+}
+
+// --- Span/Trace ID derivation (deterministic) ---
+
+export function deriveSpanId(deduplicationKey: string): string {
+  return createHash('sha256').update(deduplicationKey).digest('hex').slice(0, 16)
+}
+
+export function deriveTraceId(sessionId: string): string {
+  return createHash('sha256').update(sessionId).digest('hex').slice(0, 32)
+}
+
+// --- Timestamp conversion ---
+
+function toUnixNano(isoTimestamp: string): string {
+  const ms = new Date(isoTimestamp).getTime()
+  if (isNaN(ms)) return '0'
+  return (BigInt(ms) * 1_000_000n).toString()
+}
+
+// --- Payload construction ---
+
+export interface CallWithSession {
+  call: ParsedApiCall
+  sessionId: string
+  project: string
+}
+
+export function buildOtlpPayload(calls: CallWithSession[]): OtlpPayload {
+  const deviceId = getDeviceId()
+
+  const spans: OtlpSpan[] = calls.map(({ call, sessionId, project }) => {
+    const startNano = toUnixNano(call.timestamp)
+    // End time = start + 1ms (we don't have real duration, but OTLP requires both)
+    const endNano = (BigInt(startNano) + 1_000_000n).toString()
+
+    const attributes: OtlpAttribute[] = [
+      { key: 'ai.provider', value: { stringValue: call.provider } },
+      { key: 'ai.model', value: { stringValue: call.model } },
+      { key: 'ai.input_tokens', value: { intValue: String(call.usage.inputTokens) } },
+      { key: 'ai.output_tokens', value: { intValue: String(call.usage.outputTokens) } },
+      { key: 'ai.cost_usd', value: { doubleValue: call.costUSD } },
+      { key: 'ai.project', value: { stringValue: project } },
+      { key: 'ai.speed', value: { stringValue: call.speed } },
+    ]
+
+    if (call.tools.length > 0) {
+      attributes.push({
+        key: 'ai.tools',
+        value: { arrayValue: { values: call.tools.map(t => ({ stringValue: t })) } },
+      })
+    }
+
+    // cost_estimated = true when provider reports char-based estimates
+    const isEstimated = call.provider === 'kiro' || call.usage.inputTokens === 0
+    attributes.push({ key: 'ai.cost_estimated', value: { boolValue: isEstimated } })
+
+    return {
+      traceId: deriveTraceId(sessionId),
+      spanId: deriveSpanId(call.deduplicationKey),
+      name: `${call.provider}/${call.model}`,
+      startTimeUnixNano: startNano,
+      endTimeUnixNano: endNano,
+      attributes,
+    }
+  })
+
+  return {
+    resourceSpans: [{
+      resource: {
+        attributes: [
+          { key: 'codeburn.device_id', value: { stringValue: deviceId } },
+        ],
+      },
+      scopeSpans: [{
+        spans,
+      }],
+    }],
+  }
+}
+
+/** Split calls into batches of maxBatchSize. */
+export function batchCalls(calls: CallWithSession[], maxBatchSize: number): CallWithSession[][] {
+  const batches: CallWithSession[][] = []
+  for (let i = 0; i < calls.length; i += maxBatchSize) {
+    batches.push(calls.slice(i, i + maxBatchSize))
+  }
+  return batches
+}
diff --git a/src/sync/push.ts b/src/sync/push.ts
new file mode 100644
index 00000000..0444c718
--- /dev/null
+++ b/src/sync/push.ts
@@ -0,0 +1,176 @@
+/**
+ * codeburn sync — push orchestration.
+ *
+ * Extracted from the CLI action so the flatten/filter/batch/send/ledger
+ * pipeline is unit-testable without a full CLI invocation.
+ */
+
+import type { ProjectSummary } from '../types.js'
+import { assertHttps } from './discovery.js'
+import { ledgerKeySet, appendToLedger, type LedgerEntry } from './ledger.js'
+import { buildOtlpPayload, batchCalls, type CallWithSession } from './otlp.js'
+
+/**
+ * Safety valve, not a routine cap — pushes now loop until all batches are
+ * sent (429s are waited out). This only bounds a single push in pathological
+ * cases (e.g. corrupted ledger causing a full re-send of years of data).
+ */
+export const MAX_PER_PUSH = 50_000
+
+/** Flatten parsed projects into individual calls and filter out already-sent ones. */
+export function collectUnsentCalls(projects: ProjectSummary[]): {
+  allCalls: CallWithSession[]
+  unsent: CallWithSession[]
+} {
+  const allCalls: CallWithSession[] = []
+  for (const project of projects) {
+    for (const session of project.sessions) {
+      for (const turn of session.turns) {
+        for (const call of turn.assistantCalls) {
+          allCalls.push({
+            call,
+            sessionId: session.sessionId,
+            project: project.project,
+          })
+        }
+      }
+    }
+  }
+
+  const sent = ledgerKeySet()
+  const unsent = allCalls.filter(c => !sent.has(c.call.deduplicationKey))
+  return { allCalls, unsent }
+}
+
+export type PushOutcome = 'complete' | 'auth-rejected' | 'rate-limited' | 'server-error'
+
+export interface PushResult {
+  outcome: PushOutcome
+  totalSent: number
+  totalRejected: number
+  totalCostSent: number
+  retryAfter?: string
+  httpStatus?: number
+  /** Total milliseconds spent waiting on 429 Retry-After */
+  totalWaitMs?: number
+}
+
+export interface SendBatchesOptions {
+  endpoint: string
+  accessToken: string
+  batches: CallWithSession[][]
+  log?: (msg: string) => void
+  /** Injectable sleep for tests. Defaults to real setTimeout. */
+  sleep?: (ms: number) => Promise<void>
+  /** Max wait per 429 (caps Retry-After). Default 120s. */
+  maxWaitMs?: number
+  /** Consecutive 429 retries per batch before giving up. Default 3. */
+  max429Retries?: number
+}
+
+/** Parse Retry-After header: delta-seconds or HTTP-date. Returns ms, or null. */
+export function parseRetryAfterMs(value: string | null): number | null {
+  if (!value) return null
+  if (/^-?\d+$/.test(value.trim())) {
+    const seconds = Number(value)
+    return seconds >= 0 ? seconds * 1000 : null
+  }
+  const date = Date.parse(value)
+  if (!isNaN(date)) return Math.max(0, date - Date.now())
+  return null
+}
+
+/**
+ * Send batches sequentially until all are sent. Ledgers each fully-accepted
+ * batch. Partially-rejected batches are NOT ledgered (OTLP doesn't identify
+ * which spans were rejected; deterministic span IDs make full-batch retry safe).
+ *
+ * 429 responses are honored: waits Retry-After (capped at maxWaitMs, default
+ * backoff 5s when absent) and retries the same batch, up to max429Retries
+ * consecutive times before giving up. Stops on 401/5xx — unsent batches
+ * retry on the next push.
+ */
+export async function sendBatches(opts: SendBatchesOptions): Promise<PushResult> {
+  assertHttps(opts.endpoint, 'Traces endpoint')
+  const log = opts.log ?? (() => {})
+  const sleep = opts.sleep ?? ((ms: number) => new Promise<void>(r => setTimeout(r, ms)))
+  const maxWaitMs = opts.maxWaitMs ?? 120_000
+  const max429Retries = opts.max429Retries ?? 3
+
+  let totalSent = 0
+  let totalRejected = 0
+  let totalCostSent = 0
+  let totalWaitMs = 0
+
+  for (const batch of opts.batches) {
+    let attempts429 = 0
+
+    // Retry loop for the current batch (429 only)
+    for (;;) {
+      const payload = buildOtlpPayload(batch)
+
+      const response = await fetch(opts.endpoint, {
+        method: 'POST',
+        headers: {
+          'Content-Type': 'application/json',
+          'Authorization': `Bearer ${opts.accessToken}`,
+        },
+        body: JSON.stringify(payload),
+      })
+
+      if (response.status === 401) {
+        return { outcome: 'auth-rejected', totalSent, totalRejected, totalCostSent, totalWaitMs, httpStatus: 401 }
+      }
+
+      if (response.status === 429) {
+        const retryAfterHeader = response.headers.get('Retry-After')
+        attempts429++
+        if (attempts429 > max429Retries) {
+          return {
+            outcome: 'rate-limited', totalSent, totalRejected, totalCostSent, totalWaitMs,
+            retryAfter: retryAfterHeader ?? undefined, httpStatus: 429,
+          }
+        }
+        const waitMs = Math.min(parseRetryAfterMs(retryAfterHeader) ?? 5000, maxWaitMs)
+        log(`  Rate limited — waiting ${Math.round(waitMs / 1000)}s before retrying (attempt ${attempts429}/${max429Retries})`)
+        totalWaitMs += waitMs
+        await sleep(waitMs)
+        continue
+      }
+
+      if (!response.ok) {
+        return { outcome: 'server-error', totalSent, totalRejected, totalCostSent, totalWaitMs, httpStatus: response.status }
+      }
+
+      // Check for partial success
+      let rejected = 0
+      try {
+        const body = await response.json() as { partialSuccess?: { rejectedSpans?: number | string } }
+        // proto3 int64 JSON mapping: strict protojson servers send int64 as a
+        // string — Number() both so `totalRejected +=` never concatenates.
+        rejected = Number(body?.partialSuccess?.rejectedSpans ?? 0)
+        if (!Number.isFinite(rejected) || rejected < 0) rejected = 0
+      } catch { /* empty response = full success */ }
+
+      if (rejected > 0) {
+        // OTLP partial_success doesn't identify WHICH spans were rejected.
+        // Ledger nothing — the whole batch retries on the next push.
+        totalRejected += rejected
+        log(`  Batch: ${rejected}/${batch.length} spans rejected — whole batch will retry on next push`)
+      } else {
+        const entries: LedgerEntry[] = batch.map(c => ({
+          key: c.call.deduplicationKey,
+          ts: c.call.timestamp,
+        }))
+        appendToLedger(entries)
+        totalSent += batch.length
+        totalCostSent += batch.reduce((s, c) => s + c.call.costUSD, 0)
+      }
+      break // batch done (success or partial) — move to next batch
+    }
+  }
+
+  return { outcome: 'complete', totalSent, totalRejected, totalCostSent, totalWaitMs }
+}
+
+export { batchCalls }
diff --git a/tests/fixtures/mock-idp.ts b/tests/fixtures/mock-idp.ts
new file mode 100644
index 00000000..e873b6b6
--- /dev/null
+++ b/tests/fixtures/mock-idp.ts
@@ -0,0 +1,231 @@
+/**
+ * Mock OIDC Identity Provider for codeburn sync e2e tests.
+ *
+ * Serves:
+ * - /.well-known/codeburn-export.json (discovery doc)
+ * - /.well-known/openid-configuration (OIDC discovery)
+ * - /oauth2/authorize (redirect with code — not used directly in tests)
+ * - /oauth2/token (exchanges code for tokens)
+ * - /oauth2/revoke (token revocation)
+ */
+
+import { createHash } from 'crypto'
+import { createServer, type Server, type IncomingMessage, type ServerResponse } from 'http'
+
+export interface MockIdpOptions {
+  /** Port to listen on (0 = ephemeral) */
+  port?: number
+  /** Refresh token to issue */
+  refreshToken?: string
+  /** Access token to issue */
+  accessToken?: string
+  /** Simulate rotation: return a new refresh token on each exchange */
+  rotateTokens?: boolean
+}
+
+export interface MockIdp {
+  port: number
+  baseUrl: string
+  server: Server
+  close(): Promise<void>
+  /** Tokens issued so far */
+  issuedTokens: { access: string[]; refresh: string[] }
+  /** Revoked tokens */
+  revokedTokens: string[]
+  /** Authorization codes that have been exchanged */
+  exchangedCodes: string[]
+}
+
+export async function startMockIdp(opts: MockIdpOptions = {}): Promise<MockIdp> {
+  const refreshToken = opts.refreshToken ?? 'mock-refresh-token-v1'
+  const accessToken = opts.accessToken ?? 'mock-access-token-xyz'
+  let currentRefreshToken = refreshToken
+  let rotationCounter = 0
+  let codeCounter = 0
+  // code -> S256 code_challenge registered at /oauth2/authorize
+  const pendingCodes = new Map<string, string>()
+
+  const state: MockIdp = {
+    port: 0,
+    baseUrl: '',
+    server: null!,
+    issuedTokens: { access: [], refresh: [] },
+    revokedTokens: [],
+    exchangedCodes: [],
+    close: async () => {},
+  }
+
+  const server = createServer((req: IncomingMessage, res: ServerResponse) => {
+    const url = new URL(req.url ?? '/', `http://127.0.0.1:${state.port}`)
+    const path = url.pathname
+
+    // --- Discovery doc ---
+    if (path === '/.well-known/codeburn-export.json') {
+      res.writeHead(200, { 'Content-Type': 'application/json' })
+      res.end(JSON.stringify({
+        version: 1,
+        issuer: state.baseUrl,
+        client_id: 'mock-client-id',
+        scopes: ['openid', 'codeburn:write'],
+        traces_path: '/v1/traces',
+        max_batch_size: 100,
+      }))
+      return
+    }
+
+    // --- OIDC Discovery ---
+    if (path === '/.well-known/openid-configuration') {
+      res.writeHead(200, { 'Content-Type': 'application/json' })
+      res.end(JSON.stringify({
+        issuer: state.baseUrl,
+        authorization_endpoint: `${state.baseUrl}/oauth2/authorize`,
+        token_endpoint: `${state.baseUrl}/oauth2/token`,
+        revocation_endpoint: `${state.baseUrl}/oauth2/revoke`,
+        scopes_supported: ['openid', 'offline_access', 'codeburn:write'],
+        response_types_supported: ['code'],
+        code_challenge_methods_supported: ['S256'],
+      }))
+      return
+    }
+
+    // --- Authorize endpoint (records the PKCE challenge, issues a code,
+    //     redirects to the client's redirect_uri like a real IdP) ---
+    if (path === '/oauth2/authorize' && req.method === 'GET') {
+      const challenge = url.searchParams.get('code_challenge')
+      const method = url.searchParams.get('code_challenge_method')
+      const redirectUri = url.searchParams.get('redirect_uri')
+      const reqState = url.searchParams.get('state')
+      if (!challenge || method !== 'S256' || !redirectUri) {
+        res.writeHead(400, { 'Content-Type': 'application/json' })
+        res.end(JSON.stringify({ error: 'invalid_request', error_description: 'missing code_challenge/S256/redirect_uri' }))
+        return
+      }
+      codeCounter++
+      const code = `mock-code-${codeCounter}`
+      pendingCodes.set(code, challenge)
+      const location = `${redirectUri}?code=${encodeURIComponent(code)}&state=${encodeURIComponent(reqState ?? '')}`
+      res.writeHead(302, { Location: location })
+      res.end()
+      return
+    }
+
+    // --- Token endpoint ---
+    if (path === '/oauth2/token' && req.method === 'POST') {
+      let body = ''
+      req.on('data', chunk => { body += chunk })
+      req.on('end', () => {
+        const params = new URLSearchParams(body)
+        const grantType = params.get('grant_type')
+
+        if (grantType === 'authorization_code') {
+          const code = params.get('code')
+          if (!code) {
+            res.writeHead(400, { 'Content-Type': 'application/json' })
+            res.end(JSON.stringify({ error: 'invalid_request', error_description: 'missing code' }))
+            return
+          }
+
+          // PKCE S256 verification (RFC 7636 §4.6): the code must have been
+          // issued by /oauth2/authorize, and BASE64URL(SHA256(code_verifier))
+          // must equal the challenge registered with it.
+          const expectedChallenge = pendingCodes.get(code)
+          if (!expectedChallenge) {
+            res.writeHead(400, { 'Content-Type': 'application/json' })
+            res.end(JSON.stringify({ error: 'invalid_grant', error_description: 'unknown or reused code' }))
+            return
+          }
+          const verifier = params.get('code_verifier')
+          const computed = verifier
+            ? createHash('sha256').update(verifier).digest('base64url')
+            : ''
+          if (computed !== expectedChallenge) {
+            res.writeHead(400, { 'Content-Type': 'application/json' })
+            res.end(JSON.stringify({ error: 'invalid_grant', error_description: 'PKCE verification failed' }))
+            return
+          }
+          pendingCodes.delete(code) // single-use
+
+          state.exchangedCodes.push(code)
+          state.issuedTokens.access.push(accessToken)
+          state.issuedTokens.refresh.push(currentRefreshToken)
+
+          res.writeHead(200, { 'Content-Type': 'application/json' })
+          res.end(JSON.stringify({
+            access_token: accessToken,
+            refresh_token: currentRefreshToken,
+            token_type: 'Bearer',
+            expires_in: 3600,
+          }))
+          return
+        }
+
+        if (grantType === 'refresh_token') {
+          const rt = params.get('refresh_token')
+          if (rt !== currentRefreshToken) {
+            res.writeHead(400, { 'Content-Type': 'application/json' })
+            res.end(JSON.stringify({ error: 'invalid_grant' }))
+            return
+          }
+
+          // Simulate rotation if enabled
+          if (opts.rotateTokens) {
+            rotationCounter++
+            currentRefreshToken = `mock-refresh-token-v${rotationCounter + 1}`
+          }
+
+          const newAccess = `${accessToken}-refreshed-${Date.now()}`
+          state.issuedTokens.access.push(newAccess)
+          state.issuedTokens.refresh.push(currentRefreshToken)
+
+          res.writeHead(200, { 'Content-Type': 'application/json' })
+          res.end(JSON.stringify({
+            access_token: newAccess,
+            refresh_token: currentRefreshToken,
+            token_type: 'Bearer',
+            expires_in: 3600,
+          }))
+          return
+        }
+
+        res.writeHead(400, { 'Content-Type': 'application/json' })
+        res.end(JSON.stringify({ error: 'unsupported_grant_type' }))
+      })
+      return
+    }
+
+    // --- Revocation endpoint ---
+    if (path === '/oauth2/revoke' && req.method === 'POST') {
+      let body = ''
+      req.on('data', chunk => { body += chunk })
+      req.on('end', () => {
+        const params = new URLSearchParams(body)
+        const token = params.get('token')
+        if (token) state.revokedTokens.push(token)
+        res.writeHead(200, { 'Content-Type': 'application/json' })
+        res.end('{}')
+      })
+      return
+    }
+
+    // --- 404 ---
+    res.writeHead(404, { 'Content-Type': 'text/plain' })
+    res.end('Not Found')
+  })
+
+  await new Promise<void>((resolve, reject) => {
+    server.listen(opts.port ?? 0, '127.0.0.1', () => {
+      const addr = server.address()
+      if (typeof addr === 'object' && addr) {
+        state.port = addr.port
+        state.baseUrl = `http://127.0.0.1:${addr.port}`
+      }
+      resolve()
+    })
+    server.once('error', reject)
+  })
+
+  state.server = server
+  state.close = () => new Promise(resolve => server.close(() => resolve()))
+
+  return state
+}
diff --git a/tests/sync-e2e.test.ts b/tests/sync-e2e.test.ts
new file mode 100644
index 00000000..de1f5c4d
--- /dev/null
+++ b/tests/sync-e2e.test.ts
@@ -0,0 +1,265 @@
+/**
+ * End-to-end test for codeburn sync setup flow.
+ *
+ * Spins up a mock IdP, runs the auth flow programmatically
+ * (simulating the browser callback), and verifies tokens are
+ * stored and retrievable.
+ */
+
+import { describe, it, expect, beforeAll, afterAll, beforeEach, afterEach } from 'vitest'
+import { mkdtemp, rm } from 'fs/promises'
+import { join } from 'path'
+import { tmpdir } from 'os'
+
+import { startMockIdp, type MockIdp } from './fixtures/mock-idp.js'
+import { fetchDiscoveryDoc } from '../src/sync/discovery.js'
+import {
+  fetchOidcConfig,
+  generatePkce,
+  buildAuthUrl,
+  resolveScopes,
+  startCallbackServer,
+  exchangeCode,
+  refreshToken,
+  revokeToken,
+} from '../src/sync/auth.js'
+import { createCredentialStore } from '../src/sync/credentials.js'
+import { writeSyncConfig, readSyncConfig, deleteSyncConfig } from '../src/sync/config.js'
+
+let idp: MockIdp
+let tmpHome: string
+const originalHome = process.env.HOME
+const originalStore = process.env.CODEBURN_SYNC_TOKEN_STORE
+
+beforeAll(async () => {
+  idp = await startMockIdp({ rotateTokens: false })
+  // Force the file store so this suite never touches the real OS keychain
+  // (on darwin, createCredentialStore() would otherwise ignore HOME and
+  // read/write the login keychain under the real service/account names).
+  process.env.CODEBURN_SYNC_TOKEN_STORE = 'file'
+})
+
+afterAll(async () => {
+  await idp.close()
+  if (originalStore === undefined) delete process.env.CODEBURN_SYNC_TOKEN_STORE
+  else process.env.CODEBURN_SYNC_TOKEN_STORE = originalStore
+})
+
+beforeEach(async () => {
+  tmpHome = await mkdtemp(join(tmpdir(), 'codeburn-sync-e2e-'))
+  process.env.HOME = tmpHome
+})
+
+afterEach(async () => {
+  process.env.HOME = originalHome
+  await rm(tmpHome, { recursive: true, force: true })
+})
+
+describe('sync e2e (mock IdP)', () => {
+  it('full setup flow: discovery → OIDC → callback → token → store', async () => {
+    // 1. Fetch discovery doc
+    const discovery = await fetchDiscoveryDoc(idp.baseUrl)
+    expect(discovery.version).toBe(1)
+    expect(discovery.issuer).toBe(idp.baseUrl)
+    expect(discovery.client_id).toBe('mock-client-id')
+    expect(discovery.scopes).toContain('codeburn:write')
+
+    // 2. Fetch OIDC config
+    const oidc = await fetchOidcConfig(discovery.issuer)
+    expect(oidc.authorization_endpoint).toContain('/oauth2/authorize')
+    expect(oidc.token_endpoint).toContain('/oauth2/token')
+    expect(oidc.revocation_endpoint).toContain('/oauth2/revoke')
+    expect(oidc.scopes_supported).toContain('offline_access')
+
+    // 3. Resolve scopes (offline_access should be added since IdP supports it)
+    const scopes = resolveScopes(discovery.scopes, oidc.scopes_supported)
+    expect(scopes).toContain('offline_access')
+
+    // 4. Generate PKCE + state
+    const pkce = generatePkce()
+    const state = 'e2e-test-state'
+
+    // 5. Start callback server
+    const { promise: callbackPromise, ready } = startCallbackServer(state, 5000, [0])
+    const port = await ready
+    const redirectUri = `http://127.0.0.1:${port}/callback`
+
+    // 6. Build auth URL (verify it's well-formed)
+    const authUrl = buildAuthUrl({
+      authorization_endpoint: oidc.authorization_endpoint,
+      client_id: discovery.client_id,
+      redirect_uri: redirectUri,
+      scopes,
+      state,
+      pkce,
+    })
+    const parsedUrl = new URL(authUrl)
+    expect(parsedUrl.searchParams.get('code_challenge_method')).toBe('S256')
+    expect(parsedUrl.searchParams.get('client_id')).toBe('mock-client-id')
+
+    // 7. Drive the real authorize flow: hit the IdP's authorize endpoint
+    // (registers the PKCE challenge, issues a code), then follow its
+    // redirect to our local callback server — like a browser would.
+    const authResp = await fetch(authUrl, { redirect: 'manual' })
+    expect(authResp.status).toBe(302)
+    const location = authResp.headers.get('location')!
+    expect(location).toContain(`http://127.0.0.1:${port}/callback`)
+    await fetch(location)
+
+    const callbackResult = await callbackPromise
+    expect(callbackResult.code).toMatch(/^mock-code-/)
+
+    // 8. Exchange code for tokens
+    const tokens = await exchangeCode(
+      oidc.token_endpoint,
+      callbackResult.code,
+      pkce.code_verifier,
+      redirectUri,
+      discovery.client_id,
+    )
+    expect(tokens.access_token).toBe('mock-access-token-xyz')
+    expect(tokens.refresh_token).toBe('mock-refresh-token-v1')
+    expect(tokens.expires_in).toBe(3600)
+
+    // Verify the mock IdP received the code
+    expect(idp.exchangedCodes).toContain(callbackResult.code)
+
+    // 8b. PKCE negative checks: wrong verifier rejected; code is single-use
+    const authResp2 = await fetch(authUrl.replace(`state=${state}`, 'state=neg-test'), { redirect: 'manual' })
+    const loc2 = new URL(authResp2.headers.get('location')!)
+    const code2 = loc2.searchParams.get('code')!
+    await expect(
+      exchangeCode(oidc.token_endpoint, code2, 'wrong-verifier-aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa', redirectUri, discovery.client_id)
+    ).rejects.toThrow(/PKCE verification failed|Token exchange failed/)
+    await expect(
+      exchangeCode(oidc.token_endpoint, callbackResult.code, pkce.code_verifier, redirectUri, discovery.client_id)
+    ).rejects.toThrow(/unknown or reused code|Token exchange failed/)
+
+    // 9. Store refresh token
+    const store = createCredentialStore()
+    store.store(tokens.refresh_token!)
+    const retrieved = store.retrieve()
+    expect(retrieved).toBe('mock-refresh-token-v1')
+
+    // 10. Write config
+    writeSyncConfig({
+      baseUrl: idp.baseUrl,
+      clientId: discovery.client_id,
+      tracesPath: discovery.traces_path,
+      issuer: discovery.issuer,
+    })
+    const config = readSyncConfig()
+    expect(config).not.toBeNull()
+    expect(config!.baseUrl).toBe(idp.baseUrl)
+    expect(config!.clientId).toBe('mock-client-id')
+  }, 10000)
+
+  it('token refresh flow', async () => {
+    // Store a refresh token
+    const store = createCredentialStore()
+    store.store('mock-refresh-token-v1')
+
+    // Refresh it
+    const oidc = await fetchOidcConfig(idp.baseUrl)
+    const tokens = await refreshToken(oidc.token_endpoint, 'mock-refresh-token-v1', 'mock-client-id')
+
+    expect(tokens.access_token).toContain('mock-access-token-xyz-refreshed')
+    expect(tokens.refresh_token).toBe('mock-refresh-token-v1') // no rotation
+    expect(tokens.token_type).toBe('Bearer')
+  })
+
+  it('refresh with invalid token returns auth error', async () => {
+    const oidc = await fetchOidcConfig(idp.baseUrl)
+
+    await expect(
+      refreshToken(oidc.token_endpoint, 'wrong-token', 'mock-client-id')
+    ).rejects.toThrow('Sync auth expired')
+  })
+
+  it('logout revokes token at IdP', async () => {
+    const store = createCredentialStore()
+    store.store('token-to-revoke')
+
+    writeSyncConfig({
+      baseUrl: idp.baseUrl,
+      clientId: 'mock-client-id',
+      tracesPath: '/v1/traces',
+      issuer: idp.baseUrl,
+    })
+
+    // Revoke
+    const oidc = await fetchOidcConfig(idp.baseUrl)
+    await revokeToken(oidc.revocation_endpoint!, 'token-to-revoke', 'mock-client-id')
+
+    expect(idp.revokedTokens).toContain('token-to-revoke')
+
+    // Clean up
+    store.delete()
+    deleteSyncConfig()
+
+    expect(store.retrieve()).toBeNull()
+    expect(readSyncConfig()).toBeNull()
+  })
+
+  it('status shows correct info after setup', async () => {
+    const store = createCredentialStore()
+    store.store('status-test-token')
+
+    writeSyncConfig({
+      baseUrl: idp.baseUrl,
+      clientId: 'mock-client-id',
+      tracesPath: '/v1/traces',
+      issuer: idp.baseUrl,
+      lastSync: '2026-07-07T20:00:00Z',
+    })
+
+    const config = readSyncConfig()
+    const token = store.retrieve()
+
+    expect(config!.baseUrl).toBe(idp.baseUrl)
+    expect(config!.lastSync).toBe('2026-07-07T20:00:00Z')
+    expect(token).toBe('status-test-token')
+    expect(store.method()).toMatch(/keychain|secret-tool|dpapi|file/)
+
+    // Clean up
+    store.delete()
+    deleteSyncConfig()
+  })
+})
+
+describe('sync e2e — token rotation', () => {
+  let rotatingIdp: MockIdp
+
+  beforeAll(async () => {
+    rotatingIdp = await startMockIdp({ rotateTokens: true, refreshToken: 'rt-rotation-v1' })
+  })
+
+  afterAll(async () => {
+    await rotatingIdp.close()
+  })
+
+  it('stores rotated refresh token after refresh', async () => {
+    const store = createCredentialStore()
+    store.store('rt-rotation-v1')
+
+    const oidc = await fetchOidcConfig(rotatingIdp.baseUrl)
+
+    // First refresh — should get rt-rotation-v2
+    const tokens1 = await refreshToken(oidc.token_endpoint, 'rt-rotation-v1', 'mock-client-id')
+    expect(tokens1.refresh_token).toBe('mock-refresh-token-v2')
+
+    // Store the new one (as the client would)
+    store.store(tokens1.refresh_token!)
+
+    // Second refresh with the new token
+    const tokens2 = await refreshToken(oidc.token_endpoint, tokens1.refresh_token!, 'mock-client-id')
+    expect(tokens2.refresh_token).toBe('mock-refresh-token-v3')
+
+    // Old token should fail
+    await expect(
+      refreshToken(oidc.token_endpoint, 'rt-rotation-v1', 'mock-client-id')
+    ).rejects.toThrow('Sync auth expired')
+
+    store.delete()
+  })
+})
diff --git a/tests/sync-headless-e2e.test.ts b/tests/sync-headless-e2e.test.ts
new file mode 100644
index 00000000..2d40cdb6
--- /dev/null
+++ b/tests/sync-headless-e2e.test.ts
@@ -0,0 +1,143 @@
+/**
+ * E2E test: codeburn sync setup with headless browser against real Cognito.
+ *
+ * This test exercises the FULL PKCE flow:
+ * 1. Starts the callback server (simulating `codeburn sync setup`)
+ * 2. Builds the auth URL with PKCE challenge
+ * 3. Opens a headless Chromium to the Cognito Hosted UI
+ * 4. Fills the login form with test credentials
+ * 5. Submits → Cognito redirects to localhost callback
+ * 6. Callback server receives code + state
+ * 7. Exchanges code for tokens
+ *
+ * Requirements:
+ * - Playwright + Chromium installed (`npx playwright install chromium`)
+ * - Real Cognito endpoint deployed (CodeburnSyncBackend stack)
+ * - Test user created in the pool
+ *
+ * Run: npx vitest run tests/sync-headless-e2e.test.ts
+ * Skip in CI without infra: set SKIP_HEADLESS_E2E=1
+ */
+
+import { describe, it, expect, beforeAll, afterAll } from 'vitest'
+import { chromium, type Browser, type Page } from 'playwright'
+
+import { fetchDiscoveryDoc } from '../src/sync/discovery.js'
+import {
+  fetchOidcConfig,
+  generatePkce,
+  buildAuthUrl,
+  resolveScopes,
+  startCallbackServer,
+  exchangeCode,
+} from '../src/sync/auth.js'
+import { randomBytes } from 'crypto'
+
+// --- Configuration (from deployed stack outputs) ---
+const BASE_URL = process.env.CODEBURN_SYNC_URL
+const TEST_EMAIL = process.env.CODEBURN_SYNC_EMAIL
+const TEST_PASSWORD = process.env.CODEBURN_SYNC_PASSWORD
+
+// Only runs when ALL three env vars are set. Developer-only test.
+const SKIP = !BASE_URL || !TEST_EMAIL || !TEST_PASSWORD
+
+describe.skipIf(SKIP)('sync setup — headless browser PKCE flow', () => {
+  let browser: Browser
+
+  beforeAll(async () => {
+    browser = await chromium.launch({
+      headless: true,
+      args: ['--no-sandbox', '--disable-setuid-sandbox'],
+    })
+  })
+
+  afterAll(async () => {
+    await browser?.close()
+  })
+
+  it('full PKCE flow: callback server → Cognito login → token exchange', async () => {
+    // 1. Fetch discovery + OIDC config
+    const discovery = await fetchDiscoveryDoc(BASE_URL)
+    const oidc = await fetchOidcConfig(discovery.issuer)
+    const scopes = resolveScopes(discovery.scopes, oidc.scopes_supported)
+
+    // 2. PKCE + state
+    const pkce = generatePkce()
+    const state = randomBytes(16).toString('hex')
+
+    // 3. Start callback server
+    const { promise: callbackPromise, ready } = startCallbackServer(state, 30000)
+    const port = await ready
+    const redirectUri = `http://127.0.0.1:${port}/callback`
+
+    // 4. Build auth URL
+    const authUrl = buildAuthUrl({
+      authorization_endpoint: oidc.authorization_endpoint,
+      client_id: discovery.client_id,
+      redirect_uri: redirectUri,
+      scopes,
+      state,
+      pkce,
+    })
+
+    // 5. Open headless browser to Cognito Hosted UI
+    const page: Page = await browser.newPage()
+
+    try {
+      await page.goto(authUrl, { waitUntil: 'networkidle' })
+
+      // 6. Fill login form
+      // Cognito Hosted UI renders two tabbed forms (Sign In + Sign Up).
+      // The inputs exist but may not be CSS-visible. Use JS to fill the
+      // sign-in form directly, targeting inputs within the form that has
+      // the signInSubmitButton.
+      await page.evaluate((creds) => {
+        const forms = document.querySelectorAll('form')
+        for (const form of forms) {
+          if (!form.querySelector('input[name="signInSubmitButton"]')) continue
+          const username = form.querySelector('input[name="username"]') as HTMLInputElement
+          const password = form.querySelector('input[name="password"]') as HTMLInputElement
+          if (username) { username.value = creds.email; username.dispatchEvent(new Event('input', { bubbles: true })) }
+          if (password) { password.value = creds.password; password.dispatchEvent(new Event('input', { bubbles: true })) }
+        }
+      }, { email: TEST_EMAIL, password: TEST_PASSWORD })
+
+      // 7. Submit the form via JS
+      await page.evaluate(() => {
+        const btn = document.querySelector('input[name="signInSubmitButton"]') as HTMLInputElement
+        if (btn) btn.click()
+      })
+
+      // 8. Wait for redirect to our callback server
+      // Cognito will redirect to http://127.0.0.1:{port}/callback?code=...&state=...
+      // The page will load our callback server's "Login successful" response
+      await page.waitForURL(`http://127.0.0.1:${port}/callback*`, { timeout: 15000 })
+
+      // 9. Callback server should have received the code
+      const result = await callbackPromise
+      expect(result.code).toBeTruthy()
+      expect(result.code.length).toBeGreaterThan(10)
+
+      // 10. Exchange code for tokens
+      const tokens = await exchangeCode(
+        oidc.token_endpoint,
+        result.code,
+        pkce.code_verifier,
+        redirectUri,
+        discovery.client_id,
+      )
+
+      expect(tokens.access_token).toBeTruthy()
+      expect(tokens.refresh_token).toBeTruthy()
+      expect(tokens.token_type).toBe('Bearer')
+      expect(tokens.expires_in).toBeGreaterThan(0)
+
+      console.log('✅ Full PKCE flow completed successfully!')
+      console.log(`   Access token: ${tokens.access_token.slice(0, 30)}...`)
+      console.log(`   Refresh token: ${tokens.refresh_token!.slice(0, 30)}...`)
+      console.log(`   Expires in: ${tokens.expires_in}s`)
+    } finally {
+      await page.close()
+    }
+  }, 60000) // 60s timeout for the full browser flow
+})
diff --git a/tests/sync-infra-e2e.test.ts b/tests/sync-infra-e2e.test.ts
new file mode 100644
index 00000000..71265e7c
--- /dev/null
+++ b/tests/sync-infra-e2e.test.ts
@@ -0,0 +1,232 @@
+/**
+ * Integration test: codeburn sync push against real AWS infrastructure.
+ *
+ * Requires:
+ * - Deployed CodeburnSyncBackend stack
+ * - Environment variables: CODEBURN_SYNC_URL, CODEBURN_SYNC_EMAIL, CODEBURN_SYNC_PASSWORD
+ * - AWS credentials (for verifying CloudWatch logs)
+ *
+ * Run:
+ *   CODEBURN_SYNC_URL=https://xxx.execute-api.us-west-2.amazonaws.com \
+ *   CODEBURN_SYNC_EMAIL=andklee@amazon.com \
+ *   CODEBURN_SYNC_PASSWORD='password' \
+ *   AWS_PROFILE=andklee-dev \
+ *   npx vitest run tests/sync-infra-e2e.test.ts
+ */
+
+import { describe, it, expect, beforeAll, afterAll } from 'vitest'
+import { randomBytes, createHash } from 'crypto'
+
+import { fetchDiscoveryDoc } from '../src/sync/discovery.js'
+import { fetchOidcConfig, refreshToken } from '../src/sync/auth.js'
+import { buildOtlpPayload, deriveSpanId, type CallWithSession } from '../src/sync/otlp.js'
+import type { ParsedApiCall, TokenUsage } from '../src/types.js'
+
+const BASE_URL = process.env.CODEBURN_SYNC_URL
+const TEST_EMAIL = process.env.CODEBURN_SYNC_EMAIL
+const TEST_PASSWORD = process.env.CODEBURN_SYNC_PASSWORD
+const AWS_REGION = process.env.AWS_REGION || 'us-west-2'
+
+const SKIP = !BASE_URL || !TEST_EMAIL || !TEST_PASSWORD
+
+describe.skipIf(SKIP)('sync infra e2e — push + verify', () => {
+  let accessToken: string
+  let clientId: string
+  let tracesEndpoint: string
+  const testRunId = randomBytes(4).toString('hex')
+
+  beforeAll(async () => {
+    // Get tokens via admin API (Cognito USER_PASSWORD_AUTH)
+    const discovery = await fetchDiscoveryDoc(BASE_URL!)
+    clientId = discovery.client_id
+    tracesEndpoint = `${BASE_URL}${discovery.traces_path}`
+
+    // Direct auth (USER_PASSWORD_AUTH) for testing
+    const tokenEndpoint = `https://codeburn-sync.auth.${AWS_REGION}.amazoncognito.com/oauth2/token`
+
+    // Use Cognito initiateAuth via fetch to the Cognito service
+    const cognitoUrl = `https://cognito-idp.${AWS_REGION}.amazonaws.com/`
+    const userPoolId = discovery.issuer.split('/').pop()!
+
+    const authResp = await fetch(cognitoUrl, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/x-amz-json-1.1',
+        'X-Amz-Target': 'AWSCognitoIdentityProviderService.InitiateAuth',
+      },
+      body: JSON.stringify({
+        AuthFlow: 'USER_PASSWORD_AUTH',
+        ClientId: clientId,
+        AuthParameters: {
+          USERNAME: TEST_EMAIL,
+          PASSWORD: TEST_PASSWORD,
+        },
+      }),
+    })
+
+    const authResult = await authResp.json() as { AuthenticationResult?: { AccessToken?: string } }
+    accessToken = authResult.AuthenticationResult?.AccessToken ?? ''
+    expect(accessToken).toBeTruthy()
+  })
+
+  it('pushes a test span and receives 200', async () => {
+    const testDedup = `test:infra-e2e:${testRunId}:span1`
+    const usage: TokenUsage = {
+      inputTokens: 999,
+      outputTokens: 111,
+      cacheCreationInputTokens: 0,
+      cacheReadInputTokens: 0,
+      cachedInputTokens: 0,
+      reasoningTokens: 0,
+      webSearchRequests: 0,
+    }
+
+    const call: ParsedApiCall = {
+      provider: 'test',
+      model: 'test-model',
+      usage,
+      costUSD: 0.001,
+      tools: ['TestTool'],
+      mcpTools: [],
+      skills: [],
+      subagentTypes: [],
+      hasAgentSpawn: false,
+      hasPlanMode: false,
+      speed: 'standard',
+      timestamp: new Date().toISOString(),
+      bashCommands: [],
+      deduplicationKey: testDedup,
+    }
+
+    const callWithSession: CallWithSession = {
+      call,
+      sessionId: `test-session-${testRunId}`,
+      project: 'infra-e2e-test',
+    }
+
+    const payload = buildOtlpPayload([callWithSession])
+
+    const response = await fetch(tracesEndpoint, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        'Authorization': `Bearer ${accessToken}`,
+      },
+      body: JSON.stringify(payload),
+    })
+
+    expect(response.status).toBe(200)
+    const body = await response.json() as Record<string, unknown>
+    // No partial_success means full success
+    expect(body.partialSuccess?.rejectedSpans).toBeUndefined()
+  })
+
+  it('rejects unauthenticated requests', async () => {
+    const payload = buildOtlpPayload([])
+
+    const response = await fetch(tracesEndpoint, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        'Authorization': 'Bearer invalid-token',
+      },
+      body: JSON.stringify(payload),
+    })
+
+    expect(response.status).toBe(401)
+  })
+
+  it('pushes multiple spans in one batch', async () => {
+    const calls: CallWithSession[] = Array.from({ length: 5 }, (_, i) => {
+      const usage: TokenUsage = {
+        inputTokens: 100 * (i + 1),
+        outputTokens: 50 * (i + 1),
+        cacheCreationInputTokens: 0,
+        cacheReadInputTokens: 0,
+        cachedInputTokens: 0,
+        reasoningTokens: 0,
+        webSearchRequests: 0,
+      }
+      return {
+        call: {
+          provider: 'test',
+          model: 'test-batch-model',
+          usage,
+          costUSD: 0.01 * (i + 1),
+          tools: [],
+          mcpTools: [],
+          skills: [],
+          subagentTypes: [],
+          hasAgentSpawn: false,
+          hasPlanMode: false,
+          speed: 'standard' as const,
+          timestamp: new Date().toISOString(),
+          bashCommands: [],
+          deduplicationKey: `test:infra-e2e:${testRunId}:batch:${i}`,
+        },
+        sessionId: `test-batch-session-${testRunId}`,
+        project: 'infra-e2e-batch',
+      }
+    })
+
+    const payload = buildOtlpPayload(calls)
+
+    const response = await fetch(tracesEndpoint, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        'Authorization': `Bearer ${accessToken}`,
+      },
+      body: JSON.stringify(payload),
+    })
+
+    expect(response.status).toBe(200)
+  })
+
+  it('deterministic span IDs allow safe re-push', async () => {
+    const dedup = `test:infra-e2e:${testRunId}:idempotent`
+    const spanId = deriveSpanId(dedup)
+
+    // Push same data twice
+    const call: CallWithSession = {
+      call: {
+        provider: 'test',
+        model: 'test-idempotent',
+        usage: { inputTokens: 1, outputTokens: 1, cacheCreationInputTokens: 0, cacheReadInputTokens: 0, cachedInputTokens: 0, reasoningTokens: 0, webSearchRequests: 0 },
+        costUSD: 0,
+        tools: [],
+        mcpTools: [],
+        skills: [],
+        subagentTypes: [],
+        hasAgentSpawn: false,
+        hasPlanMode: false,
+        speed: 'standard',
+        timestamp: new Date().toISOString(),
+        bashCommands: [],
+        deduplicationKey: dedup,
+      },
+      sessionId: 'idem-session',
+      project: 'idem-project',
+    }
+
+    const payload = buildOtlpPayload([call])
+
+    // First push
+    const r1 = await fetch(tracesEndpoint, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json', 'Authorization': `Bearer ${accessToken}` },
+      body: JSON.stringify(payload),
+    })
+    expect(r1.status).toBe(200)
+
+    // Second push (identical payload)
+    const r2 = await fetch(tracesEndpoint, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json', 'Authorization': `Bearer ${accessToken}` },
+      body: JSON.stringify(payload),
+    })
+    expect(r2.status).toBe(200)
+
+    // Both succeed — server tolerates duplicates per OTLP spec
+  })
+})
diff --git a/tests/sync-ledger-otlp.test.ts b/tests/sync-ledger-otlp.test.ts
new file mode 100644
index 00000000..0b32f80d
--- /dev/null
+++ b/tests/sync-ledger-otlp.test.ts
@@ -0,0 +1,375 @@
+/**
+ * Unit tests for sync ledger and OTLP payload builder.
+ */
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest'
+import { mkdtemp, rm } from 'fs/promises'
+import { join } from 'path'
+import { tmpdir } from 'os'
+
+import {
+  deriveSpanId,
+  deriveTraceId,
+  buildOtlpPayload,
+  batchCalls,
+  getDeviceId,
+  deriveDeviceId,
+  type CallWithSession,
+} from '../src/sync/otlp.js'
+
+import type { ParsedApiCall, TokenUsage } from '../src/types.js'
+
+// ── Helpers ───────────────────────────────────────────────────────────
+
+function makeCall(overrides: Partial<ParsedApiCall> & { deduplicationKey: string }): ParsedApiCall {
+  const usage: TokenUsage = {
+    inputTokens: 1000,
+    outputTokens: 500,
+    cacheCreationInputTokens: 0,
+    cacheReadInputTokens: 0,
+    cachedInputTokens: 0,
+    reasoningTokens: 0,
+    webSearchRequests: 0,
+  }
+  return {
+    provider: 'kiro',
+    model: 'claude-sonnet-4-6',
+    usage,
+    costUSD: 0.05,
+    tools: ['Edit', 'Bash'],
+    mcpTools: [],
+    skills: [],
+    subagentTypes: [],
+    hasAgentSpawn: false,
+    hasPlanMode: false,
+    speed: 'standard',
+    timestamp: '2026-07-10T10:00:00.000Z',
+    bashCommands: [],
+    deduplicationKey: 'test:key:1',
+    ...overrides,
+  }
+}
+
+function makeCallWithSession(overrides?: Partial<ParsedApiCall> & { deduplicationKey?: string }): CallWithSession {
+  return {
+    call: makeCall({ deduplicationKey: overrides?.deduplicationKey ?? 'test:key:1', ...overrides }),
+    sessionId: 'session-abc',
+    project: 'my-project',
+  }
+}
+
+// ── OTLP Span/Trace ID Derivation ────────────────────────────────────
+
+describe('deriveSpanId', () => {
+  it('returns 16 hex chars', () => {
+    const id = deriveSpanId('cursor:bubble:abc123')
+    expect(id).toMatch(/^[0-9a-f]{16}$/)
+  })
+
+  it('is deterministic (same input = same output)', () => {
+    const a = deriveSpanId('my:dedup:key')
+    const b = deriveSpanId('my:dedup:key')
+    expect(a).toBe(b)
+  })
+
+  it('different inputs produce different IDs', () => {
+    const a = deriveSpanId('key-1')
+    const b = deriveSpanId('key-2')
+    expect(a).not.toBe(b)
+  })
+
+  // GOLDEN PIN — do not update this value. The idempotency contract depends
+  // on span IDs being stable across releases: if the hash input or encoding
+  // ever changes, every re-sent span gets a new identity and backends that
+  // key on span ID double-count history. If this test fails, revert the
+  // encoding change (or design an explicit migration).
+  it('golden: SHA-256(deduplicationKey) first 8 bytes as hex', () => {
+    expect(deriveSpanId('golden-dedup-key')).toBe('ec3ca28cceacf381')
+  })
+})
+
+describe('deriveTraceId', () => {
+  it('returns 32 hex chars', () => {
+    const id = deriveTraceId('session-xyz')
+    expect(id).toMatch(/^[0-9a-f]{32}$/)
+  })
+
+  it('is deterministic', () => {
+    const a = deriveTraceId('session-1')
+    const b = deriveTraceId('session-1')
+    expect(a).toBe(b)
+  })
+
+  // GOLDEN PIN — see deriveSpanId golden test for why this must not change.
+  it('golden: SHA-256(sessionId) first 16 bytes as hex', () => {
+    expect(deriveTraceId('golden-session-id')).toBe('ff1b1358ef64c52f80e50e7ae47ca176')
+  })
+})
+
+describe('getDeviceId', () => {
+  it('returns 16 hex chars', () => {
+    const id = getDeviceId()
+    expect(id).toMatch(/^[0-9a-f]{16}$/)
+  })
+
+  it('is stable across calls', () => {
+    expect(getDeviceId()).toBe(getDeviceId())
+  })
+
+  // GOLDEN PIN — device ID must be stable across releases so a developer's
+  // machine keeps one identity in the backend.
+  it('golden: SHA-256(hostname:username) first 8 bytes as hex', () => {
+    expect(deriveDeviceId('host.example', 'alice')).toBe('004d1a2fc048f575')
+  })
+})
+
+// ── OTLP Payload Builder ──────────────────────────────────────────────
+
+describe('buildOtlpPayload', () => {
+  it('builds valid OTLP structure with one span', () => {
+    const payload = buildOtlpPayload([makeCallWithSession()])
+
+    expect(payload.resourceSpans).toHaveLength(1)
+    expect(payload.resourceSpans[0]!.resource.attributes).toEqual([
+      { key: 'codeburn.device_id', value: { stringValue: expect.stringMatching(/^[0-9a-f]{16}$/) } },
+    ])
+
+    const spans = payload.resourceSpans[0]!.scopeSpans[0]!.spans
+    expect(spans).toHaveLength(1)
+
+    const span = spans[0]!
+    expect(span.traceId).toMatch(/^[0-9a-f]{32}$/)
+    expect(span.spanId).toMatch(/^[0-9a-f]{16}$/)
+    expect(span.name).toBe('kiro/claude-sonnet-4-6')
+    expect(span.startTimeUnixNano).toBe('1783677600000000000')
+  })
+
+  it('includes correct span attributes', () => {
+    const payload = buildOtlpPayload([makeCallWithSession()])
+    const attrs = payload.resourceSpans[0]!.scopeSpans[0]!.spans[0]!.attributes
+    const attrMap = Object.fromEntries(attrs.map(a => [a.key, a.value]))
+
+    expect(attrMap['ai.provider']).toEqual({ stringValue: 'kiro' })
+    expect(attrMap['ai.model']).toEqual({ stringValue: 'claude-sonnet-4-6' })
+    expect(attrMap['ai.input_tokens']).toEqual({ intValue: '1000' })
+    expect(attrMap['ai.output_tokens']).toEqual({ intValue: '500' })
+    expect(attrMap['ai.cost_usd']).toEqual({ doubleValue: 0.05 })
+    expect(attrMap['ai.project']).toEqual({ stringValue: 'my-project' })
+    expect(attrMap['ai.speed']).toEqual({ stringValue: 'standard' })
+  })
+
+  it('includes tools as array attribute', () => {
+    const payload = buildOtlpPayload([makeCallWithSession()])
+    const attrs = payload.resourceSpans[0]!.scopeSpans[0]!.spans[0]!.attributes
+    const toolsAttr = attrs.find(a => a.key === 'ai.tools')
+
+    expect(toolsAttr).toBeDefined()
+    expect(toolsAttr!.value).toEqual({
+      arrayValue: { values: [{ stringValue: 'Edit' }, { stringValue: 'Bash' }] },
+    })
+  })
+
+  it('omits tools attribute when empty', () => {
+    const call = makeCallWithSession({ tools: [] as string[] } as any)
+    const payload = buildOtlpPayload([call])
+    const attrs = payload.resourceSpans[0]!.scopeSpans[0]!.spans[0]!.attributes
+    const toolsAttr = attrs.find(a => a.key === 'ai.tools')
+    expect(toolsAttr).toBeUndefined()
+  })
+
+  it('multiple calls produce multiple spans', () => {
+    const calls = [
+      makeCallWithSession({ deduplicationKey: 'k1' }),
+      makeCallWithSession({ deduplicationKey: 'k2' }),
+      makeCallWithSession({ deduplicationKey: 'k3' }),
+    ]
+    const payload = buildOtlpPayload(calls)
+    const spans = payload.resourceSpans[0]!.scopeSpans[0]!.spans
+    expect(spans).toHaveLength(3)
+    // Each span has a unique spanId
+    const ids = new Set(spans.map(s => s.spanId))
+    expect(ids.size).toBe(3)
+  })
+
+  it('same deduplicationKey produces same spanId (idempotent re-send)', () => {
+    const call = makeCallWithSession({ deduplicationKey: 'stable-key' })
+    const p1 = buildOtlpPayload([call])
+    const p2 = buildOtlpPayload([call])
+    expect(p1.resourceSpans[0]!.scopeSpans[0]!.spans[0]!.spanId)
+      .toBe(p2.resourceSpans[0]!.scopeSpans[0]!.spans[0]!.spanId)
+  })
+})
+
+// ── Batching ──────────────────────────────────────────────────────────
+
+describe('batchCalls', () => {
+  it('returns single batch when under limit', () => {
+    const calls = Array.from({ length: 5 }, (_, i) =>
+      makeCallWithSession({ deduplicationKey: `k${i}` })
+    )
+    const batches = batchCalls(calls, 1000)
+    expect(batches).toHaveLength(1)
+    expect(batches[0]).toHaveLength(5)
+  })
+
+  it('splits into multiple batches at the limit', () => {
+    const calls = Array.from({ length: 2500 }, (_, i) =>
+      makeCallWithSession({ deduplicationKey: `k${i}` })
+    )
+    const batches = batchCalls(calls, 1000)
+    expect(batches).toHaveLength(3)
+    expect(batches[0]).toHaveLength(1000)
+    expect(batches[1]).toHaveLength(1000)
+    expect(batches[2]).toHaveLength(500)
+  })
+
+  it('empty input returns empty array', () => {
+    expect(batchCalls([], 1000)).toEqual([])
+  })
+})
+
+// ── Ledger ────────────────────────────────────────────────────────────
+
+describe('ledger', () => {
+  let tmpDir: string
+  const originalHome = process.env.HOME
+
+  beforeEach(async () => {
+    tmpDir = await mkdtemp(join(tmpdir(), 'codeburn-ledger-'))
+    process.env.HOME = tmpDir
+    // env-isolation.ts redirects XDG_CACHE_HOME to a per-worker sandbox shared
+    // across tests — the ledger honors XDG, so point it at the per-test dir.
+    process.env.XDG_CACHE_HOME = join(tmpDir, '.cache')
+  })
+
+  afterEach(async () => {
+    process.env.HOME = originalHome
+    await rm(tmpDir, { recursive: true, force: true })
+  })
+
+  it('readLedger returns empty array when no file', async () => {
+    const { readLedger } = await import('../src/sync/ledger.js')
+    expect(readLedger()).toEqual([])
+  })
+
+  it('writeLedger + readLedger round-trips', async () => {
+    const { writeLedger, readLedger } = await import('../src/sync/ledger.js')
+    const entries = [
+      { key: 'k1', ts: '2026-07-10T00:00:00Z' },
+      { key: 'k2', ts: '2026-07-11T00:00:00Z' },
+    ]
+    writeLedger(entries)
+    expect(readLedger()).toEqual(entries)
+  })
+
+  it('appendToLedger adds new entries and deduplicates', async () => {
+    const { writeLedger, appendToLedger, readLedger } = await import('../src/sync/ledger.js')
+    writeLedger([{ key: 'existing', ts: '2026-07-01T00:00:00Z' }])
+    appendToLedger([
+      { key: 'existing', ts: '2026-07-01T00:00:00Z' },  // duplicate
+      { key: 'new-one', ts: '2026-07-10T00:00:00Z' },
+    ])
+    const result = readLedger()
+    expect(result).toHaveLength(2)
+    expect(result.map(e => e.key).sort()).toEqual(['existing', 'new-one'])
+  })
+
+  it('appendToLedger prunes entries older than 6 months', async () => {
+    const { writeLedger, appendToLedger, readLedger } = await import('../src/sync/ledger.js')
+    const old = new Date(Date.now() - 200 * 24 * 60 * 60 * 1000).toISOString() // 200 days ago
+    writeLedger([{ key: 'old-entry', ts: old }])
+    appendToLedger([{ key: 'fresh', ts: new Date().toISOString() }])
+    const result = readLedger()
+    expect(result.map(e => e.key)).toEqual(['fresh'])
+  })
+
+  it('ledgerKeySet returns set of keys', async () => {
+    const { writeLedger, ledgerKeySet } = await import('../src/sync/ledger.js')
+    writeLedger([
+      { key: 'a', ts: '2026-07-01T00:00:00Z' },
+      { key: 'b', ts: '2026-07-02T00:00:00Z' },
+    ])
+    const keys = ledgerKeySet()
+    expect(keys.has('a')).toBe(true)
+    expect(keys.has('b')).toBe(true)
+    expect(keys.has('c')).toBe(false)
+  })
+
+  it('clearLedger removes the file and returns count', async () => {
+    const { writeLedger, clearLedger, readLedger } = await import('../src/sync/ledger.js')
+    writeLedger([{ key: 'x', ts: '2026-07-01T00:00:00Z' }])
+    const count = clearLedger()
+    expect(count).toBe(1)
+    expect(readLedger()).toEqual([])
+  })
+
+  it('clearLedger returns 0 when no file', async () => {
+    const { clearLedger } = await import('../src/sync/ledger.js')
+    expect(clearLedger()).toBe(0)
+  })
+
+  it('corrupt ledger file reads as empty (crash-safe recovery)', async () => {
+    const { readLedger } = await import('../src/sync/ledger.js')
+    const { mkdirSync, writeFileSync } = await import('fs')
+    const { join } = await import('path')
+    const dir = join(process.env.HOME!, '.cache', 'codeburn')
+    mkdirSync(dir, { recursive: true })
+    writeFileSync(join(dir, 'sync-ledger.json'), '{"truncated mid-wri')
+    expect(readLedger()).toEqual([])
+  })
+
+  it('writes are atomic — no .tmp file left behind', async () => {
+    const { writeLedger } = await import('../src/sync/ledger.js')
+    const { existsSync } = await import('fs')
+    const { join } = await import('path')
+    writeLedger([{ key: 'a', ts: '2026-07-01T00:00:00Z' }])
+    const dir = join(process.env.HOME!, '.cache', 'codeburn')
+    expect(existsSync(join(dir, 'sync-ledger.json'))).toBe(true)
+    expect(existsSync(join(dir, 'sync-ledger.json.tmp'))).toBe(false)
+  })
+
+  it('honors XDG_CACHE_HOME when set', async () => {
+    const { writeLedger, readLedger } = await import('../src/sync/ledger.js')
+    const { existsSync } = await import('fs')
+    const { join } = await import('path')
+    const xdgDir = join(process.env.HOME!, 'xdg-cache')
+    const original = process.env.XDG_CACHE_HOME
+    process.env.XDG_CACHE_HOME = xdgDir
+    try {
+      writeLedger([{ key: 'xdg-entry', ts: '2026-07-01T00:00:00Z' }])
+      expect(existsSync(join(xdgDir, 'codeburn', 'sync-ledger.json'))).toBe(true)
+      expect(readLedger().map(e => e.key)).toEqual(['xdg-entry'])
+    } finally {
+      if (original === undefined) delete process.env.XDG_CACHE_HOME
+      else process.env.XDG_CACHE_HOME = original
+    }
+  })
+})
+
+// ── assertHttps (RFC 8252 §8.3) ───────────────────────────────────────
+
+describe('assertHttps', () => {
+  it('accepts https URLs', async () => {
+    const { assertHttps } = await import('../src/sync/discovery.js')
+    expect(() => assertHttps('https://telemetry.example.com', 'Base URL')).not.toThrow()
+  })
+
+  it('accepts http on loopback (offline tests, local dev)', async () => {
+    const { assertHttps } = await import('../src/sync/discovery.js')
+    expect(() => assertHttps('http://127.0.0.1:8080/x', 'Base URL')).not.toThrow()
+    expect(() => assertHttps('http://localhost:3000', 'Base URL')).not.toThrow()
+    expect(() => assertHttps('http://[::1]:9999', 'Base URL')).not.toThrow()
+  })
+
+  it('rejects plain http on non-loopback hosts', async () => {
+    const { assertHttps } = await import('../src/sync/discovery.js')
+    expect(() => assertHttps('http://telemetry.example.com', 'Base URL')).toThrow(/must use https/)
+    expect(() => assertHttps('http://192.168.1.10', 'Issuer')).toThrow(/must use https/)
+  })
+
+  it('rejects non-http(s) schemes and garbage', async () => {
+    const { assertHttps } = await import('../src/sync/discovery.js')
+    expect(() => assertHttps('ftp://example.com', 'Base URL')).toThrow(/must use https/)
+    expect(() => assertHttps('not a url', 'Base URL')).toThrow(/not a valid URL/)
+  })
+})
diff --git a/tests/sync-push.test.ts b/tests/sync-push.test.ts
new file mode 100644
index 00000000..0fe5c9bd
--- /dev/null
+++ b/tests/sync-push.test.ts
@@ -0,0 +1,402 @@
+/**
+ * Unit tests for sync push orchestration (src/sync/push.ts).
+ *
+ * Covers the review gaps: partial-success handling, 429 rate limiting,
+ * 401 auth rejection, 5xx server errors, and the flatten→filter pipeline.
+ */
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest'
+import { createServer, type Server } from 'http'
+import { mkdtemp, rm } from 'fs/promises'
+import { join } from 'path'
+import { tmpdir } from 'os'
+
+import type { ParsedApiCall, TokenUsage, ProjectSummary } from '../src/types.js'
+import type { CallWithSession } from '../src/sync/otlp.js'
+
+// ── Helpers ───────────────────────────────────────────────────────────
+
+function makeUsage(): TokenUsage {
+  return {
+    inputTokens: 100,
+    outputTokens: 50,
+    cacheCreationInputTokens: 0,
+    cacheReadInputTokens: 0,
+    cachedInputTokens: 0,
+    reasoningTokens: 0,
+    webSearchRequests: 0,
+  }
+}
+
+function makeCall(key: string, costUSD = 0.01): ParsedApiCall {
+  return {
+    provider: 'test',
+    model: 'test-model',
+    usage: makeUsage(),
+    costUSD,
+    tools: [],
+    mcpTools: [],
+    skills: [],
+    subagentTypes: [],
+    hasAgentSpawn: false,
+    hasPlanMode: false,
+    speed: 'standard',
+    timestamp: '2026-07-10T10:00:00.000Z',
+    bashCommands: [],
+    deduplicationKey: key,
+  }
+}
+
+function makeCws(key: string, costUSD = 0.01): CallWithSession {
+  return { call: makeCall(key, costUSD), sessionId: 'sess-1', project: 'proj-1' }
+}
+
+/** Minimal mock OTLP server with scriptable responses per request. */
+type MockResponse = { status: number; body?: unknown; headers?: Record<string, string> }
+
+function startMockOtlp(responses: MockResponse[]): Promise<{
+  url: string
+  server: Server
+  requests: Array<{ auth: string | undefined; body: unknown }>
+}> {
+  const requests: Array<{ auth: string | undefined; body: unknown }> = []
+  let idx = 0
+
+  return new Promise(resolve => {
+    const server = createServer((req, res) => {
+      let raw = ''
+      req.on('data', c => { raw += c })
+      req.on('end', () => {
+        requests.push({ auth: req.headers.authorization, body: JSON.parse(raw || '{}') })
+        const r = responses[Math.min(idx, responses.length - 1)]!
+        idx++
+        res.writeHead(r.status, { 'Content-Type': 'application/json', ...r.headers })
+        res.end(r.body !== undefined ? JSON.stringify(r.body) : '{}')
+      })
+    })
+    server.listen(0, '127.0.0.1', () => {
+      const addr = server.address() as { port: number }
+      resolve({ url: `http://127.0.0.1:${addr.port}/v1/traces`, server, requests })
+    })
+  })
+}
+
+// ── Test env: isolated HOME so ledger writes go to a temp dir ─────────
+
+let tmpDir: string
+const originalHome = process.env.HOME
+
+beforeEach(async () => {
+  tmpDir = await mkdtemp(join(tmpdir(), 'codeburn-push-'))
+  process.env.HOME = tmpDir
+  // env-isolation.ts redirects XDG_CACHE_HOME to a per-worker sandbox shared
+  // across tests — the ledger honors XDG, so point it at the per-test dir.
+  process.env.XDG_CACHE_HOME = join(tmpDir, '.cache')
+})
+
+afterEach(async () => {
+  process.env.HOME = originalHome
+  await rm(tmpDir, { recursive: true, force: true })
+})
+
+// ── collectUnsentCalls ────────────────────────────────────────────────
+
+describe('collectUnsentCalls', () => {
+  it('flattens projects → sessions → turns → calls', async () => {
+    const { collectUnsentCalls } = await import('../src/sync/push.js')
+
+    const projects = [{
+      project: 'proj-a',
+      sessions: [{
+        sessionId: 's1',
+        turns: [
+          { assistantCalls: [makeCall('k1'), makeCall('k2')] },
+          { assistantCalls: [makeCall('k3')] },
+        ],
+      }],
+    }] as unknown as ProjectSummary[]
+
+    const { allCalls, unsent } = collectUnsentCalls(projects)
+    expect(allCalls).toHaveLength(3)
+    expect(unsent).toHaveLength(3)
+    expect(allCalls[0]!.project).toBe('proj-a')
+    expect(allCalls[0]!.sessionId).toBe('s1')
+  })
+
+  it('filters out calls already in the ledger', async () => {
+    const { collectUnsentCalls } = await import('../src/sync/push.js')
+    const { writeLedger } = await import('../src/sync/ledger.js')
+
+    writeLedger([{ key: 'k1', ts: '2026-07-10T00:00:00Z' }])
+
+    const projects = [{
+      project: 'p',
+      sessions: [{
+        sessionId: 's1',
+        turns: [{ assistantCalls: [makeCall('k1'), makeCall('k2')] }],
+      }],
+    }] as unknown as ProjectSummary[]
+
+    const { allCalls, unsent } = collectUnsentCalls(projects)
+    expect(allCalls).toHaveLength(2)
+    expect(unsent).toHaveLength(1)
+    expect(unsent[0]!.call.deduplicationKey).toBe('k2')
+  })
+})
+
+// ── sendBatches: success path ─────────────────────────────────────────
+
+describe('sendBatches — success', () => {
+  it('sends all batches, ledgers all calls, accumulates cost', async () => {
+    const { sendBatches } = await import('../src/sync/push.js')
+    const { readLedger } = await import('../src/sync/ledger.js')
+
+    const { url, server, requests } = await startMockOtlp([{ status: 200, body: {} }])
+    try {
+      const batches = [
+        [makeCws('a', 0.10), makeCws('b', 0.20)],
+        [makeCws('c', 0.30)],
+      ]
+      const result = await sendBatches({ endpoint: url, accessToken: 'tok-123', batches })
+
+      expect(result.outcome).toBe('complete')
+      expect(result.totalSent).toBe(3)
+      expect(result.totalRejected).toBe(0)
+      expect(result.totalCostSent).toBeCloseTo(0.60)
+
+      // Two HTTP requests with Bearer auth
+      expect(requests).toHaveLength(2)
+      expect(requests[0]!.auth).toBe('Bearer tok-123')
+
+      // All three keys ledgered
+      const keys = readLedger().map(e => e.key).sort()
+      expect(keys).toEqual(['a', 'b', 'c'])
+    } finally {
+      server.close()
+    }
+  })
+})
+
+// ── sendBatches: partial success ──────────────────────────────────────
+
+describe('sendBatches — partial success', () => {
+  it('does NOT ledger a partially-rejected batch (whole batch retries)', async () => {
+    const { sendBatches } = await import('../src/sync/push.js')
+    const { readLedger } = await import('../src/sync/ledger.js')
+
+    const { url, server } = await startMockOtlp([
+      { status: 200, body: { partialSuccess: { rejectedSpans: 1 } } }, // batch 1: partial
+      { status: 200, body: {} },                                       // batch 2: full success
+    ])
+    try {
+      const batches = [
+        [makeCws('p1'), makeCws('p2')],  // partially rejected — must NOT ledger
+        [makeCws('ok1')],                // fully accepted — must ledger
+      ]
+      const result = await sendBatches({ endpoint: url, accessToken: 't', batches })
+
+      expect(result.outcome).toBe('complete')
+      expect(result.totalSent).toBe(1)
+      expect(result.totalRejected).toBe(1)
+
+      const keys = readLedger().map(e => e.key)
+      expect(keys).toEqual(['ok1'])           // p1/p2 absent → they retry next push
+    } finally {
+      server.close()
+    }
+  })
+})
+
+// ── sendBatches: error paths ──────────────────────────────────────────
+
+describe('sendBatches — errors', () => {
+  it('401 → auth-rejected, stops immediately, ledgers nothing further', async () => {
+    const { sendBatches } = await import('../src/sync/push.js')
+    const { readLedger } = await import('../src/sync/ledger.js')
+
+    const { url, server, requests } = await startMockOtlp([{ status: 401 }])
+    try {
+      const result = await sendBatches({
+        endpoint: url, accessToken: 'bad',
+        batches: [[makeCws('x')], [makeCws('y')]],
+      })
+      expect(result.outcome).toBe('auth-rejected')
+      expect(result.httpStatus).toBe(401)
+      expect(result.totalSent).toBe(0)
+      expect(requests).toHaveLength(1)         // second batch never sent
+      expect(readLedger()).toEqual([])
+    } finally {
+      server.close()
+    }
+  })
+
+  it('429 → waits Retry-After and retries the same batch until it succeeds', async () => {
+    const { sendBatches } = await import('../src/sync/push.js')
+    const { readLedger } = await import('../src/sync/ledger.js')
+
+    const sleeps: number[] = []
+    const { url, server, requests } = await startMockOtlp([
+      { status: 200, body: {} },                            // batch 1: ok
+      { status: 429, headers: { 'Retry-After': '2' } },     // batch 2: limited
+      { status: 200, body: {} },                            // batch 2 retry: ok
+      { status: 200, body: {} },                            // batch 3: ok
+    ])
+    try {
+      const result = await sendBatches({
+        endpoint: url, accessToken: 't',
+        batches: [[makeCws('sent')], [makeCws('limited')], [makeCws('third')]],
+        sleep: async ms => { sleeps.push(ms) },
+      })
+      expect(result.outcome).toBe('complete')
+      expect(result.totalSent).toBe(3)                     // ALL batches sent
+      expect(result.totalWaitMs).toBe(2000)
+      expect(sleeps).toEqual([2000])                       // honored Retry-After: 2
+      expect(requests).toHaveLength(4)                     // 3 batches + 1 retry
+      expect(readLedger().map(e => e.key).sort()).toEqual(['limited', 'sent', 'third'])
+    } finally {
+      server.close()
+    }
+  })
+
+  it('persistent 429 → gives up after max retries, remaining deferred', async () => {
+    const { sendBatches } = await import('../src/sync/push.js')
+    const { readLedger } = await import('../src/sync/ledger.js')
+
+    const sleeps: number[] = []
+    const { url, server, requests } = await startMockOtlp([
+      { status: 429, headers: { 'Retry-After': '1' } },     // every request limited
+    ])
+    try {
+      const result = await sendBatches({
+        endpoint: url, accessToken: 't',
+        batches: [[makeCws('stuck')], [makeCws('never')]],
+        sleep: async ms => { sleeps.push(ms) },
+        max429Retries: 2,
+      })
+      expect(result.outcome).toBe('rate-limited')
+      expect(result.totalSent).toBe(0)
+      expect(sleeps).toEqual([1000, 1000])                 // 2 retries = 2 waits
+      expect(requests).toHaveLength(3)                     // initial + 2 retries; 2nd batch never sent
+      expect(readLedger()).toEqual([])
+    } finally {
+      server.close()
+    }
+  })
+
+  it('429 without Retry-After uses 5s default; wait capped at maxWaitMs', async () => {
+    const { sendBatches } = await import('../src/sync/push.js')
+
+    const sleeps: number[] = []
+    const { url, server } = await startMockOtlp([
+      { status: 429 },                                      // no Retry-After → 5s default
+      { status: 429, headers: { 'Retry-After': '999' } },   // 999s → capped
+      { status: 200, body: {} },
+    ])
+    try {
+      const result = await sendBatches({
+        endpoint: url, accessToken: 't',
+        batches: [[makeCws('x')]],
+        sleep: async ms => { sleeps.push(ms) },
+        maxWaitMs: 10_000,
+      })
+      expect(result.outcome).toBe('complete')
+      expect(sleeps).toEqual([5000, 10_000])                // default, then capped
+    } finally {
+      server.close()
+    }
+  })
+
+  it('5xx → server-error, batch not ledgered, remaining deferred', async () => {
+    const { sendBatches } = await import('../src/sync/push.js')
+    const { readLedger } = await import('../src/sync/ledger.js')
+
+    const { url, server, requests } = await startMockOtlp([{ status: 503 }])
+    try {
+      const result = await sendBatches({
+        endpoint: url, accessToken: 't',
+        batches: [[makeCws('a')], [makeCws('b')]],
+      })
+      expect(result.outcome).toBe('server-error')
+      expect(result.httpStatus).toBe(503)
+      expect(result.totalSent).toBe(0)
+      expect(requests).toHaveLength(1)
+      expect(readLedger()).toEqual([])
+    } finally {
+      server.close()
+    }
+  })
+
+  it('retry after failure re-sends the unledgered calls (idempotent recovery)', async () => {
+    const { sendBatches, collectUnsentCalls } = await import('../src/sync/push.js')
+
+    // First attempt: server error → nothing ledgered
+    const first = await startMockOtlp([{ status: 500 }])
+    try {
+      await sendBatches({ endpoint: first.url, accessToken: 't', batches: [[makeCws('r1')]] })
+    } finally {
+      first.server.close()
+    }
+
+    // Simulate the next push: the same call is still unsent
+    const projects = [{
+      project: 'p',
+      sessions: [{ sessionId: 's1', turns: [{ assistantCalls: [makeCall('r1')] }] }],
+    }] as unknown as ProjectSummary[]
+    const { unsent } = collectUnsentCalls(projects)
+    expect(unsent).toHaveLength(1)
+
+    // Second attempt succeeds and ledgers
+    const second = await startMockOtlp([{ status: 200, body: {} }])
+    try {
+      const result = await sendBatches({ endpoint: second.url, accessToken: 't', batches: [unsent] })
+      expect(result.outcome).toBe('complete')
+      expect(result.totalSent).toBe(1)
+    } finally {
+      second.server.close()
+    }
+
+    // Now filtered out
+    const { unsent: after } = collectUnsentCalls(projects)
+    expect(after).toHaveLength(0)
+  })
+})
+
+// ── MAX_PER_PUSH safety valve ─────────────────────────────────────────
+
+describe('MAX_PER_PUSH', () => {
+  it('is a 50K safety valve (pushes run to completion, not capped at 5K)', async () => {
+    const { MAX_PER_PUSH } = await import('../src/sync/push.js')
+    expect(MAX_PER_PUSH).toBe(50_000)
+  })
+})
+
+// ── parseRetryAfterMs ─────────────────────────────────────────────────
+
+describe('parseRetryAfterMs', () => {
+  it('parses delta-seconds', async () => {
+    const { parseRetryAfterMs } = await import('../src/sync/push.js')
+    expect(parseRetryAfterMs('30')).toBe(30_000)
+    expect(parseRetryAfterMs('0')).toBe(0)
+  })
+
+  it('parses HTTP-date', async () => {
+    const { parseRetryAfterMs } = await import('../src/sync/push.js')
+    const future = new Date(Date.now() + 10_000).toUTCString()
+    const ms = parseRetryAfterMs(future)
+    expect(ms).toBeGreaterThan(8_000)
+    expect(ms).toBeLessThanOrEqual(10_500)
+  })
+
+  it('past HTTP-date clamps to 0', async () => {
+    const { parseRetryAfterMs } = await import('../src/sync/push.js')
+    const past = new Date(Date.now() - 60_000).toUTCString()
+    expect(parseRetryAfterMs(past)).toBe(0)
+  })
+
+  it('returns null for missing or garbage values', async () => {
+    const { parseRetryAfterMs } = await import('../src/sync/push.js')
+    expect(parseRetryAfterMs(null)).toBeNull()
+    expect(parseRetryAfterMs('soon™')).toBeNull()
+    expect(parseRetryAfterMs('-5')).toBeNull()
+  })
+})
diff --git a/tests/sync.test.ts b/tests/sync.test.ts
new file mode 100644
index 00000000..5c512572
--- /dev/null
+++ b/tests/sync.test.ts
@@ -0,0 +1,260 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest'
+import { mkdtemp, rm, writeFile, readFile, mkdir } from 'fs/promises'
+import { join } from 'path'
+import { tmpdir } from 'os'
+import { createServer, type Server } from 'http'
+
+import { parseDiscoveryDoc, DiscoveryError } from '../src/sync/discovery.js'
+import {
+  generatePkce,
+  buildAuthUrl,
+  resolveScopes,
+  startCallbackServer,
+  CALLBACK_PORTS,
+} from '../src/sync/auth.js'
+
+// ── Discovery Doc Parser ──────────────────────────────────────────────
+
+describe('parseDiscoveryDoc', () => {
+  it('parses valid v1 doc', () => {
+    const doc = parseDiscoveryDoc({
+      version: 1,
+      issuer: 'https://cognito-idp.us-east-1.amazonaws.com/us-east-1_XXXX',
+      client_id: 'abc123',
+      scopes: ['openid', 'codeburn:write'],
+      traces_path: '/v1/traces',
+      max_batch_size: 500,
+    })
+    expect(doc.version).toBe(1)
+    expect(doc.issuer).toBe('https://cognito-idp.us-east-1.amazonaws.com/us-east-1_XXXX')
+    expect(doc.client_id).toBe('abc123')
+    expect(doc.scopes).toEqual(['openid', 'codeburn:write'])
+    expect(doc.traces_path).toBe('/v1/traces')
+    expect(doc.max_batch_size).toBe(500)
+  })
+
+  it('rejects version > 1', () => {
+    expect(() => parseDiscoveryDoc({ version: 2, issuer: 'https://idp.example', client_id: 'y' }))
+      .toThrow('Please update codeburn')
+  })
+
+  it('rejects missing issuer', () => {
+    expect(() => parseDiscoveryDoc({ version: 1, client_id: 'y' }))
+      .toThrow('missing required field: issuer')
+  })
+
+  it('rejects missing client_id', () => {
+    expect(() => parseDiscoveryDoc({ version: 1, issuer: 'https://idp.example' }))
+      .toThrow('missing required field: client_id')
+  })
+
+  it('defaults traces_path to /v1/traces when absent', () => {
+    const doc = parseDiscoveryDoc({ issuer: 'https://idp.example', client_id: 'y' })
+    expect(doc.traces_path).toBe('/v1/traces')
+  })
+
+  it('defaults max_batch_size to 1000 when absent', () => {
+    const doc = parseDiscoveryDoc({ issuer: 'https://idp.example', client_id: 'y' })
+    expect(doc.max_batch_size).toBe(1000)
+  })
+
+  it('defaults scopes to ["openid"] when absent', () => {
+    const doc = parseDiscoveryDoc({ issuer: 'https://idp.example', client_id: 'y' })
+    expect(doc.scopes).toEqual(['openid'])
+  })
+
+  it('treats absent version as v1', () => {
+    const doc = parseDiscoveryDoc({ issuer: 'https://idp.example', client_id: 'y' })
+    expect(doc.version).toBe(1)
+  })
+
+  it('rejects non-https issuer (RFC 8252 §8.3)', () => {
+    expect(() => parseDiscoveryDoc({ version: 1, issuer: 'http://idp.example', client_id: 'y' }))
+      .toThrow(/must use https/)
+  })
+
+  it('rejects non-object input', () => {
+    expect(() => parseDiscoveryDoc(null)).toThrow('must be a JSON object')
+    expect(() => parseDiscoveryDoc('string')).toThrow('must be a JSON object')
+    expect(() => parseDiscoveryDoc([1, 2])).toThrow('must be a JSON object')
+  })
+})
+
+// ── PKCE ──────────────────────────────────────────────────────────────
+
+describe('generatePkce', () => {
+  it('generates code_verifier of valid length (43+ chars)', () => {
+    const { code_verifier } = generatePkce()
+    expect(code_verifier.length).toBeGreaterThanOrEqual(43)
+  })
+
+  it('generates different values each time', () => {
+    const a = generatePkce()
+    const b = generatePkce()
+    expect(a.code_verifier).not.toBe(b.code_verifier)
+  })
+
+  it('code_challenge is base64url of SHA-256(verifier)', () => {
+    const { code_verifier, code_challenge } = generatePkce()
+    const { createHash } = require('crypto')
+    const expected = createHash('sha256').update(code_verifier).digest('base64url')
+    expect(code_challenge).toBe(expected)
+  })
+
+  it('uses S256 method', () => {
+    const { code_challenge_method } = generatePkce()
+    expect(code_challenge_method).toBe('S256')
+  })
+})
+
+// ── Auth URL ──────────────────────────────────────────────────────────
+
+describe('buildAuthUrl', () => {
+  const baseParams = {
+    authorization_endpoint: 'https://idp.example.com/oauth2/authorize',
+    client_id: 'test-client',
+    redirect_uri: 'http://127.0.0.1:19876/callback',
+    scopes: ['openid', 'codeburn:write'],
+    state: 'random-state-value',
+    pkce: generatePkce(),
+  }
+
+  it('includes all required parameters', () => {
+    const url = new URL(buildAuthUrl(baseParams))
+    expect(url.searchParams.get('response_type')).toBe('code')
+    expect(url.searchParams.get('client_id')).toBe('test-client')
+    expect(url.searchParams.get('redirect_uri')).toBe('http://127.0.0.1:19876/callback')
+    expect(url.searchParams.get('state')).toBe('random-state-value')
+    expect(url.searchParams.get('code_challenge_method')).toBe('S256')
+  })
+
+  it('joins scopes with space', () => {
+    const url = new URL(buildAuthUrl(baseParams))
+    expect(url.searchParams.get('scope')).toBe('openid codeburn:write')
+  })
+
+  it('uses 127.0.0.1 literal (not localhost)', () => {
+    const url = buildAuthUrl(baseParams)
+    expect(url).toContain('127.0.0.1')
+    expect(url).not.toContain('localhost')
+  })
+})
+
+// ── Scope Resolution ──────────────────────────────────────────────────
+
+describe('resolveScopes', () => {
+  it('appends offline_access when IdP supports it', () => {
+    const result = resolveScopes(['openid'], ['openid', 'offline_access', 'profile'])
+    expect(result).toContain('offline_access')
+  })
+
+  it('does not append offline_access when IdP does not support it', () => {
+    const result = resolveScopes(['openid'], ['openid', 'profile'])
+    expect(result).not.toContain('offline_access')
+  })
+
+  it('does not duplicate offline_access if already requested', () => {
+    const result = resolveScopes(['openid', 'offline_access'], ['openid', 'offline_access'])
+    expect(result.filter(s => s === 'offline_access')).toHaveLength(1)
+  })
+
+  it('passes through scopes unchanged when IdP scopes unknown', () => {
+    const result = resolveScopes(['openid', 'codeburn:write'], undefined)
+    expect(result).toEqual(['openid', 'codeburn:write'])
+  })
+})
+
+// ── Callback Server ──────────────────────────────────────────────────
+
+describe('startCallbackServer', () => {
+  it('accepts valid callback with matching state', async () => {
+    const state = 'test-state-123'
+    const { promise, ready } = startCallbackServer(state, 5000, [0])
+
+    // Wait for server to bind
+    const port = await ready
+
+    // Simulate IdP callback
+    await fetch(`http://127.0.0.1:${port}/callback?code=auth-code-xyz&state=${state}`)
+
+    const result = await promise
+    expect(result.code).toBe('auth-code-xyz')
+  }, 10000)
+
+  it('rejects callback with wrong state', async () => {
+    const state = 'correct-state'
+    const { promise, ready } = startCallbackServer(state, 5000, [0])
+    const port = await ready
+
+    // Send with wrong state — server stays running
+    const resp = await fetch(`http://127.0.0.1:${port}/callback?code=xxx&state=wrong-state`)
+    expect(resp.status).toBe(400)
+
+    // Now send correct one
+    await fetch(`http://127.0.0.1:${port}/callback?code=real-code&state=${state}`)
+    const result = await promise
+    expect(result.code).toBe('real-code')
+  }, 10000)
+
+  it('times out after configured duration', async () => {
+    const { promise } = startCallbackServer('state', 300, [0]) // 300ms timeout
+
+    await expect(promise).rejects.toThrow('timed out')
+  }, 5000)
+})
+
+// ── Config ────────────────────────────────────────────────────────────
+
+describe('syncConfig', () => {
+  let tmpDir: string
+  const originalHome = process.env.HOME
+
+  beforeEach(async () => {
+    tmpDir = await mkdtemp(join(tmpdir(), 'codeburn-sync-config-'))
+    process.env.HOME = tmpDir
+  })
+
+  afterEach(async () => {
+    process.env.HOME = originalHome
+    await rm(tmpDir, { recursive: true, force: true })
+  })
+
+  it('writes and reads config', async () => {
+    // Dynamically import to pick up new HOME
+    const { writeSyncConfig, readSyncConfig } = await import('../src/sync/config.js')
+
+    writeSyncConfig({
+      baseUrl: 'https://metrics.test.com',
+      clientId: 'test-client',
+      tracesPath: '/v1/traces',
+      issuer: 'https://idp.test.com',
+    })
+
+    const config = readSyncConfig()
+    expect(config).not.toBeNull()
+    expect(config!.baseUrl).toBe('https://metrics.test.com')
+    expect(config!.clientId).toBe('test-client')
+  })
+
+  it('returns null when no config exists', async () => {
+    const { readSyncConfig } = await import('../src/sync/config.js')
+    const config = readSyncConfig()
+    expect(config).toBeNull()
+  })
+
+  it('config file does not contain tokens', async () => {
+    const { writeSyncConfig } = await import('../src/sync/config.js')
+
+    writeSyncConfig({
+      baseUrl: 'https://metrics.test.com',
+      clientId: 'client',
+      tracesPath: '/v1/traces',
+      issuer: 'https://idp.test.com',
+    })
+
+    const raw = await readFile(join(tmpDir, '.config', 'codeburn', 'sync.json'), 'utf-8')
+    expect(raw).not.toContain('token')
+    expect(raw).not.toContain('secret')
+    expect(raw).not.toContain('password')
+  })
+})