fix(ai): fold gateway-namespaced Anthropic tool references onto the request's tools

[code-yeongyu]

Summary

A native Anthropic tool search can hand back tool_reference names under a wire-path namespace (mcp__<id>__<tool>) that the request never defined and that does not survive across requests. The tool_search_tool_result block replays verbatim on the next same-model request, and Anthropic rejects the whole request:

invalid_request_error: Tool reference 'mcp__925c__memory' not found in available tools

Observed live on 2026-09-08 (senpi 4adba7a, omo desktop, claude-fable-5-1): the turn hard-errored and fell back to another model. The same session's later native results carried a different namespace id (mcp__5153__), and other sessions carried 8e8c/7439/cdd2, so the namespace is not something senpi can predict or replay.

Root cause

demoteUnavailableToolReferences in packages/ai/src/api/anthropic-messages.ts collected the namespaced names as dangling but only rewrote references inside tool_result content; native tool_search_tool_result blocks fell through untouched (:974), and a dangling discovery still exempted a later tool_use from demotion (:900-901).

Fix

Availability is decided by the request's final tools array alone, and every reference site is repaired:

• a tool_reference (client tool_result content or native tool_search_tool_result) under a gateway namespace is folded onto the request's own tool name when that tool is defined; one that still does not resolve is dropped;
• a native search pair whose every reference stopped resolving is demoted to text together with its server_tool_use;
• a history tool_use under a gateway namespace is renamed to the request's tool name; a tool_use whose only justification was a dangling discovery is demoted like any other unavailable call.

Evidence

• RED (test patch on eab8c4b30): packages/ai bunx vitest --run test/anthropic-tool-reference-integrity.test.ts -> 5 failed | 5 passed (e.g. expected ['mcp__925c__memory'] to deeply equal ['memory']). GREEN with the fix: 10 passed. Run on a remote node (mengmotaMac, bun 1.4.0).
• Full packages/ai suite with the fix: 266 files / 2591 tests passed; the single failing file (test/codex-apply-patch-wire-schema.test.ts, Failed to resolve entry for package "@earendil-works/pi-tui") fails identically on unpatched eab8c4b30 in the same fresh worktree.
• coding-agent adjacent suites (test/mcp/native-anthropic.test.ts, test/tool-search/native-anthropic.test.ts, test/suite/regressions/0000-anthropic-native-tool-search-contract.test.ts): 3 files / 37 tests passed.
• Incident replay harness (the real transcript's server_tool_use + tool_search_tool_result blocks through streamAnthropic with a capturing fake client): baseline emits five dangling references mcp__925c__{memory,task_send,todo,eval,web_search}; fixed emits memory, task_send, todo, eval, web_search with zero dangling references.
• bun run check green; changelog gate PASS (packages/ai/src/changes.md entry + [Unreleased] Fixed entry).

Out of scope (filed separately)

• A real HTTP 400 never reaches after_provider_response, so the native-search adapter's permanent-400 fallback is unreachable on the live error path.
• The failed turn is not recovered after such a 400 (the session falls back to another model instead of retrying without native injection).

---

Summary by cubic

Fixes a 400 error on Anthropic requests after a native tool search: tool references that come back under a gateway namespace (mcp__<id>__<tool>) are now folded onto the request's own tool names, and unresolvable references are dropped instead of replaying verbatim.

Bug Fixes

• demoteUnavailableToolReferences now decides availability solely from the request's tools array.
• Namespaced tool_reference items in both tool_result and native tool_search_tool_result blocks are rewritten to the request's tool names.
• A native search pair with no resolvable references is demoted to text along with its server_tool_use.
• History tool_use blocks with namespaced names are renamed, and a dangling discovery no longer exempts a call from demotion.

^{Written for commit e9d914c. Summary will update on new commits.}