Gemini 400 — Function call missing a thought_signature

What this error means

Gemini 2.5 returns 400 INVALID_ARGUMENT when a functionCall part is replayed without the thought_signature it originally issued.

How to fix it

• Preserve thought_signature on every functionCall part you send back
• Keep function turns immediately after the matching user or tool turn
• Do not hand-edit or truncate prior tool messages

{
  "error": {
    "code": 400,
    "message": "Function call is missing a thought_signature in functionCall parts.",
    "status": "INVALID_ARGUMENT"
  }
}