bug(core): 400 Bad Request: "Function call is missing a thought_signature in functionCall parts" during parallel tool calls

[LazyCountry]

Describe the Bug

When running the Gemini CLI with Gemini 3+ or Vertex AI models, conversations fail with a 400 Bad Request/INVALID_ARGUMENT API Error under two common scenarios:

1. Executing parallel tool calls in a single turn.
2. Hardening/scrubbing history where previous function calls lack signatures.

Error 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.

[gemini-cli]

This issue reports a critical 400 Bad Request error when using Gemini 3+ models with parallel tool calls, significantly impacting core functionality. Please provide your CLI version.

[asomegs]

This issue reports a critical 400 Bad Request error when using Gemini 3+ models with parallel tool calls, significantly impacting core functionality. Please provide your CLI version.

I encounter the same error when trying to use a skill. It started happening after updating to version 0.53.0. The issue does not occur when I downgrade to version 0.52.0.

[LazyCountry]

This issue reports a critical 400 Bad Request error when using Gemini 3+ models with parallel tool calls, significantly impacting core functionality. Please provide your CLI version.

I used 0.53.0.

[valvin1]

some users of gemini-cli through our Gemini Enterprise subscription are reporting the same issue after upgrading from 0.52 to 0.53

[dimpavloff]

This is affecting my company on 0.53.0. The problem does not exist on 0.52.x