diff --git a/packages/pi/README.md b/packages/pi/README.md
index 81c7e3e9..3dd5ef4c 100644
--- a/packages/pi/README.md
+++ b/packages/pi/README.md
@@ -2,7 +2,7 @@
 
 Pi package for CortexKit Anthropic OAuth support. It overrides Pi's built-in `anthropic` provider with a CortexKit provider extension backed by the shared `@cortexkit/anthropic-auth-core` package.
 
-The Pi provider catalog includes Claude Fable 5 (`claude-fable-5`), limited-access Claude Mythos 5 (`claude-mythos-5`), Claude Opus 4.8, Claude Opus 4.5, Claude Sonnet 4.5, and Claude Sonnet 5 (`claude-sonnet-5`). Fable/Mythos reasoning uses Anthropic adaptive thinking with `thinking.display: "summarized"` and `output_config.effort`; the package does not send rejected manual `thinking.budget_tokens` for those models.
+The Pi provider catalog includes Claude Fable 5 (`claude-fable-5`), limited-access Claude Mythos 5 (`claude-mythos-5`), Claude Opus 5 (`claude-opus-5`), Claude Opus 4.8, Claude Opus 4.5, Claude Sonnet 4.5, and Claude Sonnet 5 (`claude-sonnet-5`). Fable/Mythos reasoning uses Anthropic adaptive thinking with `thinking.display: "summarized"` and `output_config.effort`; the package does not send rejected manual `thinking.budget_tokens` for those models.
 
 This package is part of the CortexKit Anthropic Auth monorepo, which supports both OpenCode (`@cortexkit/opencode-anthropic-auth`) and Pi (`@cortexkit/pi-anthropic-auth`) through the same shared core logic.
 
diff --git a/packages/pi/src/convert.ts b/packages/pi/src/convert.ts
index f7a9fdd1..f505e1f4 100644
--- a/packages/pi/src/convert.ts
+++ b/packages/pi/src/convert.ts
@@ -391,7 +391,36 @@ export async function buildAnthropicRequest(
     { type: 'text', text: CLAUDE_CODE_IDENTITY },
   ]
   if (context.systemPrompt?.trim()) {
-    system.push({ type: 'text', text: sanitize(context.systemPrompt) })
+    // Pi's prompt cannot sit in the top-level system[] array: two lines of its
+    // documentation paragraph (the docs/*.md enumeration and the "follow .md
+    // cross-references" instruction) are each independently sufficient to make
+    // Anthropic reject the request with 400 "You're out of extra usage". Entry
+    // count and payload size are ruled out — 2697 bytes of neutral filler in the
+    // same position is accepted. The same text is accepted inside messages[], so
+    // carry the prompt as a role: "system" message and keep system[] to the
+    // billing header and identity block.
+    //
+    // cache_control is set here rather than left to addEphemeralCacheControl,
+    // whose message-level breakpoint only fires when a message's content is an
+    // array — Pi sends plain strings, so it never fires. Without this marker the
+    // prompt sits outside the cached prefix and is reprocessed every turn.
+    const prompt = sanitize(context.systemPrompt)
+    const firstUserIndex = messages.findIndex((m) => m.role === 'user')
+    if (firstUserIndex !== -1) {
+      messages.splice(firstUserIndex + 1, 0, {
+        role: 'system',
+        content: [
+          {
+            type: 'text',
+            text: prompt,
+            cache_control: { type: 'ephemeral' },
+          },
+        ],
+      })
+    }
+    // No else: with no user message, messages[] is necessarily empty
+    // (convertMessages emits only user/assistant, and trailing assistants are
+    // stripped above), so the request is already invalid.
   }
 
   const body: AnthropicRequestBody = {
diff --git a/packages/pi/src/index.ts b/packages/pi/src/index.ts
index 5023fc68..738846e3 100644
--- a/packages/pi/src/index.ts
+++ b/packages/pi/src/index.ts
@@ -80,6 +80,15 @@ export default function cortexKitPiAnthropicAuth(pi: ExtensionAPI) {
         contextWindow: CLAUDE_FABLE_MYTHOS_5_CONTEXT_WINDOW,
         maxTokens: CLAUDE_FABLE_MYTHOS_5_MAX_OUTPUT_TOKENS,
       })),
+      {
+        id: 'claude-opus-5',
+        name: 'Claude Opus 5',
+        reasoning: true,
+        input: textImageInput(),
+        cost: { input: 5, output: 25, cacheRead: 0.5, cacheWrite: 6.25 },
+        contextWindow: 1_000_000,
+        maxTokens: 128_000,
+      },
       {
         id: 'claude-opus-4-8',
         name: 'Claude Opus 4.8',
diff --git a/packages/pi/src/tests/convert.test.ts b/packages/pi/src/tests/convert.test.ts
index 148731c7..a37067d2 100644
--- a/packages/pi/src/tests/convert.test.ts
+++ b/packages/pi/src/tests/convert.test.ts
@@ -33,10 +33,14 @@ function toolResultMsg(toolCallId: string, text: string): Message {
 
 const defaultCache = { enabled: false, mode: 'hybrid' as const }
 
-async function buildMessages(messages: Message[]) {
+// systemPrompt is opt-in. buildAnthropicRequest carries a non-empty prompt as a
+// separate role: "system" message, so tests that assert raw conversion output pass
+// no prompt and observe messages unchanged. The relocation itself is covered by
+// the "Claude Code system[] shape" block below.
+async function buildMessages(messages: Message[], systemPrompt?: string) {
   const context = {
     messages,
-    systemPrompt: 'test',
+    systemPrompt,
     tools: [],
   }
   const { body } = await buildAnthropicRequest(
@@ -326,6 +330,87 @@ describe('convertMessages — empty base64 image guard', () => {
   })
 })
 
+describe('buildAnthropicRequest — Claude Code system[] shape', () => {
+  // Pi's prompt cannot sit in the top-level system[] array — see the note in
+  // convert.ts. These cases pin the resulting shape: system[] holds only the
+  // billing header and the identity block, and Pi's prompt is carried as a
+  // role: "system" message immediately after the first user message.
+  async function buildBody(messages: Message[], systemPrompt?: string) {
+    const { body } = await buildAnthropicRequest(
+      'claude-sonnet-4-20250514',
+      { messages, systemPrompt, tools: [] } as any,
+      undefined,
+      defaultCache,
+    )
+    return body
+  }
+
+  test('keeps system[] to the billing header and identity block', async () => {
+    const body = await buildBody([userMsg('hello')], 'PI PROMPT')
+    expect(body.system).toHaveLength(2)
+    expect(JSON.stringify(body.system)).not.toContain('PI PROMPT')
+  })
+
+  test('carries the prompt as a system message after the first user message', async () => {
+    const body = await buildBody([userMsg('hello')], 'PI PROMPT')
+    expect(body.messages[0]).toEqual({ role: 'user', content: 'hello' })
+    expect(body.messages[1]).toEqual({
+      role: 'system',
+      content: [
+        {
+          type: 'text',
+          text: 'PI PROMPT',
+          // Set explicitly: addEphemeralCacheControl's message-level breakpoint
+          // only fires for array content on the last user message, which Pi's
+          // plain-string messages never satisfy.
+          cache_control: { type: 'ephemeral' },
+        },
+      ],
+    })
+  })
+
+  test('leaves a structured first user message untouched', async () => {
+    const body = await buildBody(
+      [
+        {
+          role: 'user',
+          content: [
+            { type: 'text', text: 'see image' },
+            { type: 'image', mimeType: 'image/png', data: 'aGVsbG8=' },
+          ],
+          timestamp: 0,
+        } as Message,
+      ],
+      'PI PROMPT',
+    )
+    const content = body.messages[0]?.content as Array<Record<string, unknown>>
+    expect(content).toHaveLength(2)
+    expect(content[0]).toMatchObject({ type: 'text', text: 'see image' })
+    expect(body.messages[1]).toMatchObject({ role: 'system' })
+  })
+
+  test('drops the prompt when there is no user message to carry it', async () => {
+    const body = await buildBody([assistantMsg('only assistant')], 'PI PROMPT')
+    // convertMessages emits only user/assistant and trailing assistants are
+    // stripped, so a conversation with no user message converts to empty.
+    // system[] must stay at two entries even on this path.
+    expect(body.messages).toHaveLength(0)
+    expect(body.system).toHaveLength(2)
+    expect(JSON.stringify(body.system)).not.toContain('PI PROMPT')
+  })
+
+  test('leaves the identity block as the last system entry for cache anchoring', async () => {
+    const body = await buildBody([userMsg('hello')], 'PI PROMPT')
+    expect(String(body.system?.at(-1)?.text)).toContain('Claude Code')
+  })
+
+  test('leaves system[] and messages untouched when no prompt is set', async () => {
+    const body = await buildBody([userMsg('hello')])
+    expect(body.system).toHaveLength(2)
+    expect(body.messages[0]).toEqual({ role: 'user', content: 'hello' })
+  })
+})
+
 describe('buildAnthropicRequest — Fable/Mythos thinking', () => {
   test('maps Pi reasoning to output_config effort for Claude Fable 5', async () => {
     const { body } = await buildAnthropicRequest(
diff --git a/packages/pi/src/tests/index.test.ts b/packages/pi/src/tests/index.test.ts
index c6acf867..05c58f8c 100644
--- a/packages/pi/src/tests/index.test.ts
+++ b/packages/pi/src/tests/index.test.ts
@@ -44,4 +44,23 @@ describe('cortexKitPiAnthropicAuth provider registration', () => {
       maxTokens: 128_000,
     })
   })
+
+  test('exposes Claude Opus 5 in the Pi Anthropic catalog', () => {
+    const { pi, providers } = mockPi()
+
+    cortexKitPiAnthropicAuth(pi)
+
+    const opus5 = providers
+      .get('anthropic')
+      ?.models?.find((model) => model.id === 'claude-opus-5')
+    expect(opus5).toMatchObject({
+      id: 'claude-opus-5',
+      name: 'Claude Opus 5',
+      reasoning: true,
+      input: ['text', 'image'],
+      cost: { input: 5, output: 25, cacheRead: 0.5, cacheWrite: 6.25 },
+      contextWindow: 1_000_000,
+      maxTokens: 128_000,
+    })
+  })
 })