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Function call is missing a thought_signature - Claude Code

Quick fix

Use the community cc-switch-gemini-fixer patch, or inject skip_thought_signature_validator into history messages, to bypass the signature validation.

Symptom

Section titled “Symptom”
Error output
API Error: 400 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:Bash` , position 64. Please refer to https://ai.google.dev/gemini-api/docs/thought-signatures for more details.

Cause

Section titled “Cause”

Google Gemini 3.x and Thinking-capable models use a stateless reasoning chain for tool calls: a functionCall carries an encrypted credential, thought_signature, and requires it to be sent back verbatim with the subsequent functionResponse. Translating the Anthropic protocol into the Gemini format, CC Switch drops that field while passing the message context through, so the Google API rejects the request with a 400. Separately, the User location is not supported some users see is an API region restriction — a different 400 variant.

Fix

Section titled “Fix”

  1. Download and run the community stopgap patch cc-switch-gemini-fixer, which handles passing the Gemini response signature through automatically.

  2. Alternatively, when building history messages containing a functionCall in the proxy conversion layer, inject "thought_signature": "skip_thought_signature_validator" by default to bypass the official validation.

Affected Versions

Section titled “Affected Versions”
ToolClaude Code / Codex
Version3.16.1 - 3.19.2
PlatformsWindowsmacOS

Source Issues

Section titled “Source Issues”

This page is distilled from 3 real issues

FAQ

Section titled “FAQ”
Can I use this patch on macOS?
Yes — community developers provide a macOS build of the cc-switch-gemini-fixer patch.
Is User location is not supported also a signature problem?
No. That is the Google Gemini API restricting your current network region, unrelated to a missing thought_signature. Switch to a proxy node in a supported region.
Section titled “Related problems”
Body text appears in thinking blocks, visible text nearly emptyUpgrade cc-switch to a build with PR #4210. Reactive thinking-placeholder handling fixes long-context freezes and body-text collapse.1 issues mergedGtk-CRITICAL: gtk_widget_get_scale_factor failedOn a GTK assertion failure running cc-switch under Linux or WSL2, edit settings.json to turn silent startup off, or clean the leftovers and reinstall.3 issues mergedHTTP 401 Unauthorized - CC Switch AnthropicUpgrade cc-switch to a fixed build so it sends the x-api-key and anthropic-version headers Anthropic requires.2 issues merged

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