Claude thought_signature forwarded to Gemini causes Base64 decode error

[heyhuynhgiabuu]

Bug Description

When switching between Claude and Gemini models in the same session, Claude's thought_signature (with \claude# prefix) gets forwarded to Gemini API, causing a Base64 decode error.

Error Message

failed to decode thinking signature, please report this bug to google: illegal base64 data at input byte 0

Steps to Reproduce

1. Start a conversation using a Claude model
2. Claude generates a response with thought_signature
3. Switch to a Gemini model (via Antigravity OAuth)
4. Make a request to Gemini
5. Gemini rejects the request due to invalid Base64

Expected Behavior

The proxy should:

• Strip/sanitize thought_signature when forwarding requests to a different provider
• Not reuse cached signatures from Claude for Gemini requests

Additional Context

• Reported via ProxyPal: [BUG] Gemini API Error 400: Invalid 'thought_signature' containing "claude#" string (Base64 decoding failed) heyhuynhgiabuu/proxypal#146
• Affects Antigravity OAuth users who switch between Claude and Gemini