Gemini 3 with tools: `400 INVALID_ARGUMENT: Function call is missing a thought_signature in functionCall parts`

Context

Source(s): https://github.com/google/adk-python/issues/3705 , https://github.com/microsoft/vscode/issues/278264 , https://github.com/zed-industries/zed/issues/43024 Signal: GitHub search finds about 1,776 issues mentioning thought_signature; reactions 35 (Zed) + 21 (ADK) + 16 (Vercel AI SDK). Status: reported working by source, not reproduced by us.

Problem

The first turn works, but after one or more tool calls (especially parallel ones) every request fails with this 400. Seen in VS Code Copilot, Zed, OpenCode via OpenRouter, Vercel AI SDK, and ADK.

Error (redacted / normalized)

400 INVALID_ARGUMENT: Function call is missing a thought_signature in functionCall parts

Not this when / boundary

Gemini 2.x models, or a 400 with a different message. In the Vercel AI SDK the Vertex provider still had problems in 2026 (open).

Root cause

Gemini 3 attaches a thought signature to its function calls, and it must be sent back with the conversation history. The client drops it. An ADK maintainer confirmed this in ADK's history building.

Fix

1. Update the client. The reporter in the VS Code issue confirmed upgrading to 1.106.1 fixed it. ADK maintainers pointed to v1.20.0+. OpenRouter's AI SDK provider shipped fixes.
2. Check which provider the tool uses for Gemini. One OpenCode user found their provider had switched from google to openai-compatible.
3. (our inference) In your own code, send the model's previous content parts back unchanged, including the signature field, instead of rebuilding the function-call parts yourself.

How to verify

A multi-step tool-calling task finishes without the 400.