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"Function call is missing a thought_signature in functionCall parts" when using Gemini BYOK. #1381

Description

Feedback source

JetBrains Marketplace

GitHub Copilot plugin version

1.5.65-243

JetBrains IDE and version

IntelliJ IDEA 2025.3.2

Platform

Microsoft Windows NT 10.0.26200.0 x64

Steps to reproduce

  • Get a Gemini API key from Google AI Studio
  • Open "Manage Models" on Copilot Chat sidebar.
  • In the Gemini provider page, paste the Gemini key, then select Gemini models.
  • Back to the Copilot Chat page, select Gemini 3 Flash Preview (others may have the same issue?) in Other Models category.
  • Select Agent mode.
  • Send any chat that can lead to the agent calls tools. (like "Read the file and explain" or "fix errors on this file")

Logs (optional)

No response

Expected behavior

The chat/agent can response and calls tools.

Actual behavior

After one tool call complete, the following error will be displayed:

Sorry, an error occurred while generating a response. Details: unhandled status from server: 400 [{ "error": { "code": 400, "message": "Function call is missing a thought_signature in functionCall parts. This is required for tools to work correctly, and missing thought_signature may lead to degraded ... Read more from logs. Request ID: {uuid}

Additional information

Thought Signatures | Gemini API

Metadata

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Labels

1.5.65bugSomething isn't workingcomponent: agent-modeAgent mode execution, tools, and workflowscomponent: model-selectionModel picker, model availabilityimpact: highSignificantly affects user productivitypriority: highMajor functionality broken, significant impactstatus: needs-triageNewly opened, needs categorization

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