> ## Documentation Index
> Fetch the complete documentation index at: https://docs.apiyi.com/llms.txt
> Use this file to discover all available pages before exploring further.

# New FAQ: Fixing Gemini Thought Signature 400 Errors in Tool Calls

> Gemini 3 and later models raise two kinds of signature 400s in multi-turn tool calls: missing a thought_signature means the client dropped the field when replaying history and must fix its replay logic; Invalid / Corrupted thought signature means the current route does not accept it, so resend as is or strip history signatures. The gateway handles signatures on chat/completions.

**2026/10/15 16:53 (UTC+8)** · Docs Update · Google

📚 **New FAQ [How Do I Fix Gemini Thought Signature 400 Errors in Tool Calls?](/en/faq/gemini-thought-signature-missing) separates the two errors**

For multi-turn function calling on Gemini 3 and later models (`gemini-4-argon`, `gemini-3.8-flash`, and others), the `thoughtSignature` in the previous turn's `functionCall` part must be returned unchanged. This is a hard rule from the model provider:

* `Function call is missing a thought_signature`: the request carries no signature because the client dropped the field when assembling history. Routes differ in leniency, so the problem may surface only on a strictly validating model. Fix the replay logic rather than switching models
* `Invalid` / `Corrupted thought signature`: a signature was sent but the current route does not accept it, typically after the conversation switched route, model, key, or group midway. Resend once as is; if it still fails, strip signatures from the history turns and resend, or start a new conversation

On the OpenAI-compatible `/v1/chat/completions` endpoint the gateway manages signatures; standard `tool_calls` / `tool` messages are enough. The Function Calling guide and the Argon page now carry both fixes in their signature sections.

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