Orphaned advisor_tool_result after /compact makes the session permanently un-continuable (400)

Summary

After /compact, a session that had previously used the advisor tool becomes permanently un-continuable: every subsequent request fails with a 400 naming an orphaned advisor_tool_result block. The only escape is /advisor off.

Error

API Error: 400 messages.3.content.0: unexpected `tool_use_id` found in `advisor_tool_result` blocks:
srvtoolu_01RWdWPc9bkfPqFmB8bwC5K4. Each `advisor_tool_result` block must have a corresponding
`server_tool_use` block before it.

Steps to reproduce

1. Start a long session with the advisor enabled.
2. Call the advisor a few times (it succeeds normally).
3. Run /compact.
4. Continue working.

From that point on, every request fails with the 400 above, always naming the same srvtoolu_… id and the same message index.

What appears to be happening

The advisor is a server-side tool, so each call stores a server_tool_use block paired with an advisor_tool_result. Compaction rebuilds the message history, and it looks like the advisor_tool_result survives while the server_tool_use that produced it is dropped or summarised away — leaving an orphaned result block at a fixed index. The API validates the pairing on every subsequent request, so the failure is deterministic rather than transient.

Impact

• Not recoverable from inside the session. The orphan is in the replayed history, so every retry fails identically. Retrying, waiting, or rephrasing does nothing.
• An active /goal loop retries on a timer and burns its attempts against the same 400, reporting only "the API returned an unexpected response", which does not point at the advisor at all. Diagnosing it takes reading the raw error.
• Work in flight is not lost, but the session cannot continue until the user finds /advisor off — which is not an obvious remedy from the error text.

Workaround

/advisor off, then continue. The session recovers immediately.

Expected

Compaction should keep an advisor call and its result together, or drop both — never emit an advisor_tool_result whose server_tool_use it discarded. Failing that, the client could detect the orphan and strip it rather than sending a request that cannot succeed.

Environment

• Claude Code v2.1.273, macOS (darwin 27.0.0), zsh
• Model: Opus 5 (1M context)
• Advisor tool enabled, backed by Fable 5.1