fix(ai): demote unavailable Anthropic tool references in request payloads

[code-yeongyu]

Problem

Resuming a session whose history contains calls to tools that no longer exist in the request — e.g. an MCP server absent after senpi --session <id> resume, a deactivated extension tool, or an onPayload hook stripping a definition — fails the entire turn:

400 invalid_request_error: Tool reference 'mcp_computer_use_drag' not found in available tools

Anthropic validates that every tool referenced by the message history is available in the same request: defined in tools, or discovered through a tool_reference block. The codebase already repaired pairing (orphan tool_results, dangling calls) but never referential integrity between history and the tool definitions, so one missing MCP server bricked the whole resumed session.

Fix

demoteUnavailableToolReferences() in packages/ai/src/api/anthropic-messages.ts — a final payload pass applied after sanitizeUnsupportedNativeTools() on every request, right before send:

1. Collects defined tool names (params.tools) and discovered names (any nested tool_reference, including replayed server-side tool-search results).
2. Demotes tool_use blocks referencing unavailable tools to plain text (name + input preserved).
3. Demotes their tool_results in lockstep (original result text preserved) so no orphan-pairing 400 replaces the original one.
4. Strips tool_reference entries whose definition vanished (e.g. removed by a payload hook), never leaving empty tool_result content.

Deferred-tool discovery (splitDeferredTools + tool_reference) is untouched and covered by a guard test.

Evidence

• New packages/ai/test/anthropic-tool-reference-integrity.test.ts: 5 tests driving the full request path offline through a fake Anthropic client — single/mixed-turn demotion, still-available tools intact, deferred tool_reference discovery intact, dangling-reference stripping. Red before, green after.
• packages/ai full suite: 1458 passed (the one initial suite failure was an unbuilt packages/tui/dist in the fresh worktree; green after npm run build there — unrelated to this change).
• Root npm run check: exit 0.
• senpi-qa (local-ignore/qa-evidence/20260728-anthropic-tool-ref-integrity/):
  • mock-loop.mjs --self-test --api anthropic-messages: 20/20 passed, zero real provider calls, real auth unchanged.
  • cli-smoke.mjs --self-test: 8/8 passed.

---

Summary by cubic

Prevents Anthropic 400s when history references tools that aren’t in the current request by demoting those tool calls/results to text and removing dangling tool_reference entries so the turn can continue.

• Bug Fixes
  • Added demoteUnavailableToolReferences() in packages/ai/src/api/anthropic-messages.ts, run after sanitizeUnsupportedNativeTools() on every request.
  • Collects available tool names from tools and nested tool_reference blocks; converts missing tool_use to text (keeps name + input) and demotes paired tool_result (keeps result text) to avoid orphan errors.
  • Strips tool_reference items whose definitions were removed (e.g., by an onPayload hook) without leaving empty tool_result content; deferred discovery remains intact.
  • Added packages/ai/test/anthropic-tool-reference-integrity.test.ts to cover mixed turns, still-available tools, deferred tool_reference flows, and dangling-reference stripping.

^{Written for commit 5ecb304. Summary will update on new commits.}