[zed] Gemini rejects threads containing Claude thinking blocks with 400 "Invalid thought signature" (model switch, compaction, title generation)

## Observed symptom Gemini rejects threads containing Claude thinking blocks with 400 "Invalid thought signature" (model switch, compaction, title generation) 1. In the Agent Panel, configure two providers with thinking enabled: - Claude with thinking (e.g. Claude 3.7 Sonnet / Claude Opus with thinking via `anthropic_compatible` or Anthropic API) - Google AI with `gemini-3.8-flash` (or `gemini-3.7-flash`), `mode: thinking` 2. Start a new agent thread with the Claude model and ask a question that exercises thinking and tool execution (e.g. "Read the files in this project and inspect the architecture"). 3. Wait for the turn to complete. Claude's assistant message now contains a thinking block with an encrypted thinking signature. 4. Switch the thread's model to **Gemini 3.8 Flash** in the top model dropdown. 5. Send any follow-up message (e.g. "Now run tests"). **Additional trigger paths (without manual model switching):** - **Title generation:** If the default or summarization model is Gemini with thinking, automatic title generation on Claude ## Literal error text ``` json failed to stream completion: error during streamGenerateContent, status code: 400, body: { "error": { "code": 400, "message": "Invalid thought signature.", "status": "INVALID_ARGUMENT" } } ``` ## Environment GitHub zed-industries/zed #64726 ## Trigger / context Public GitHub report in zed-industries/zed (FRESH_7D). ## Source evidence - https://github.com/zed-industries/zed/issues/64726 (2026-10-02T06:21:33Z) — evidence_class: PRIMARY_ISSUE_REPORT — #64726 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T10:50:53.329738Z - scout_id: C82db157d19 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.