fix(mcp-apps): defer _meta.ui strip to per-request RegisterTools

[mattdholloway]

Problem

The MCP Apps _meta.ui strip lives in Builder.Build(), which calls checkFeatureFlag(context.Background()). The HTTP feature checker (createHTTPFeatureChecker) reads insiders mode from the request context — a background context never has it set, so the FF reports MCP Apps off and the strip runs eagerly at server startup. Per-request inventory factories then serve pre-stripped tools regardless of whether the request actually arrived on the /insiders route.

Symptom

github/github-mcp-server-remote returns 0 tools with _meta.ui over HTTP /insiders, despite the source unconditionally setting it on get_me, issue_write, and create_pull_request. VS Code only renders MCP App UIs because of its persistent tool cache from earlier deploys; new clients see no UIs at all.

Reproduction (10 lines, no remote dependency)

go build -o /tmp/ghmcp ./cmd/github-mcp-server
GITHUB_PERSONAL_ACCESS_TOKEN=$TOK GITHUB_INSIDERS=true /tmp/ghmcp http --insiders &
# initialize then tools/list against http://localhost:8082/insiders
# → 41 tools, 0 with _meta (broken)

# Same binary, stdio:
GITHUB_PERSONAL_ACCESS_TOKEN=$TOK GITHUB_INSIDERS=true /tmp/ghmcp stdio --insiders
# → 41 tools, 3 with _meta (works — confirms the bug is HTTP-specific)

Fix

Drop the strip from Build(). Apply it in RegisterTools(ctx, …) where the per-request context is in scope and the HTTP feature checker can correctly detect insiders mode (or the remote checker can correctly read user identity for Statsig flag lookup).

The same root cause affects github/github-mcp-server-remote — its featureflags.NewComposedFeatureFlagChecker reads requestctx.User(ctx), which background context lacks, so the remote_mcp_ui_apps Statsig flag always returned false. The fix here covers both downstreams since RegisterTools is the single entry point for tool registration.

Stdio is unaffected: it uses a closure-captured insiders mode flag (createFeatureChecker) that doesn't depend on context, and the per-request strip in RegisterTools produces the same outcome.

Verification

End-to-end against the deployed remote tool definitions:

Mode	Route	_meta.ui count	Notes
HTTP	/insiders	3	was 0 — fixed
HTTP	/	0	correct (no insiders)
stdio --insiders	—	3	unchanged
stdio	—	0	correct

Tools that re-appear with _meta.ui over HTTP /insiders after the fix:
get_me, issue_write, create_pull_request.

Tests

• pkg/http: new TestInsidersRoutePreservesUIMeta pins the regression — verifies the FF check returns true on a context with insiders mode set, and false on a bare context.
• pkg/inventory: existing TestWithMCPApps_DisabledStripsUIMetadata and TestWithMCPApps_UIOnlyMetaBecomesNil updated to use the new RegisterTools-as-strip-site contract via a captureRegisteredTools helper.

Full go test ./... passes.

[Copilot]

Pull request overview

This PR fixes an HTTP-only regression where MCP Apps UI metadata (_meta.ui) was being stripped at server startup due to feature flag evaluation running against context.Background(), causing /insiders requests to incorrectly receive tools without UI metadata.

Changes:

• Moved _meta.ui stripping from Builder.Build() to Inventory.RegisterTools(ctx, ...) so the per-request context is used for feature-flag evaluation.
• Updated inventory tests to validate “wire-visible” tool metadata at registration time.
• Added an HTTP regression test targeting the /insiders context/feature-flag behavior.

Show a summary per file

File	Description
pkg/inventory/registry.go	Applies MCP Apps UI meta stripping during per-request tool registration instead of at build time.
pkg/inventory/builder.go	Removes build-time stripping so inventories aren’t pre-stripped at startup.
pkg/inventory/registry_test.go	Updates tests to validate registration-time stripping and adds a helper to inspect registered tool definitions.
pkg/http/handler_test.go	Adds a regression test for the insiders-route feature-flag behavior related to _meta.ui.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2