Gemini Thought Signature Validator

It's an encrypted, opaque string the Gemini 3 API attaches to a functionCall part to preserve the model's internal reasoning state across a multi-turn tool-use conversation. When you replay conversation history back to the API — including that functionCall and its result — you must include the thoughtSignature exactly as it was returned, unmodified. Leaving it out (or stripping it during your own serialization) makes the next request invalid.

The most common cause is a missing thoughtSignature on a functionCall part being replayed into history. There are two signature rules: for parallel function calls in one response, only the first functionCall part carries a signature; for sequential function calls across separate turns, each turn's functionCall needs its own. This tool checks both cases and tells you exactly which turn is missing one.

No — thought signatures are encrypted by Google and not decodable client-side (or server-side, by anyone but Gemini itself). This tool only checks structure: whether a signature is present where the API requires one, whether functionResponse parts match their functionCall in order and count, and whether the Gemini 3 functionCall id is echoed back correctly. It can't tell you if a signature's contents are valid, only whether one is missing where it shouldn't be.

A full request body ({ contents: [...] }), a bare contents array, a raw generateContent/streamGenerateContent response ({ candidates: [...] }), or a single content object ({ role, parts }). This tool detects which shape you gave it. Response-only shapes are checked for a missing signature but skip the ordering checks, since there's no conversation history to check against.