[GitHub Copilot Agent / Gemini] Tool execution fails with 400 INVALID_ARGUMENT: Function call missing thought_signature

## Observed symptom [Copilot Agent] Tool execution fails with 400 INVALID_ARGUMENT: Function call missing thought_signature Type: <b>Bug</b> Window: Agents ### Environment / Context - Extension: GitHub Copilot / Copilot Chat - Feature: Copilot Agent Mode (Tool / PowerShell integration) ### Issue Summary When running Copilot Agent commands that trigger tool calling (e.g., `default_api:powershell`), the request fails after 5 retries due to an invalid API request payload. The underlying Gemini API requires a `thought_signature` parameter in `functionCall` parts, which is currently missing from the generated request. ### Error Message Reason: Failed to get response from the AI model; retried 5 times (total retry wait time: 14.79 seconds) Last error: 502 {"error":{"message":"{\n "error": {\n "code": 400,\n "message": "Function call is missing a thought_signature in functionCall parts. This is required for tools ## Literal error text ``` 400 INVALID_ARGUMENT: Function call missing thought_signature ``` ## Environment GitHub issue microsoft/vscode #339255; state=open ## Trigger / context Type: <b>Bug</b> Window: Agents ### Environment / Context - Extension: GitHub Copilot / Copilot Chat - Feature: Copilot Agent Mode (Tool / PowerShell integration) ### Issue Summary When running Copilot Agent commands that trigger tool calling (e.g., `default_api:powershell`), the request fails after 5 retries due to an invalid API request payload. The underlying Gemini API requires a `thought_signature` parameter in `functionCall` parts, which is currently missing from the generated request. ### Error Message Reason: Failed to get response from the AI model; retried 5 times (total retry wait time: 14.79 seconds) Last error: 502 {"error":{"message":"{\n "error": {\n "code": 400,\n "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 model performance. Additional data, function call default_api:powershell , position 2. Please refer to https://ai.google.dev/gemini-api/docs/thought-signatures for more details.",\n "status": "INVALID_ARGUMENT"\n }\n}\n","code":400,"status":"Bad Request"}} ### Steps to Reproduce 1. Trigger Copilot Agent to execute a workspace action or PowerShell command. 2. The agent attempts tool invocation (`default_api:powershell`). 3. Request fails with 502/400 Bad Request. ### Expected Behavior The agent payload should conform to the latest Gemini API specification by injecting the `thought_signature` into `functionCall` parts whe ## Source evidence - https://github.com/microsoft/vscode/issues/339255 (2026-10-02T09:14:56Z) — GitHub issue #339255 (open) ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:40:08.726088Z - scout_id: C52c4ffd33e ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome.