bug: Invalid thinking block signature when switching from Gemini CLI to Claude OAuth mid-conversation

[CF-Developers]

Description

When using CLIProxyAPI with both Gemini CLI OAuth and Claude OAuth providers, switching from a Gemini model to a Claude model mid-conversation causes a 400 invalid_request_error due to invalid thinking block signatures.

Steps to Reproduce

1. Configure CLIProxyAPI with both Gemini CLI OAuth (cliproxyapi -login) and Claude OAuth (cliproxyapi -claude-login)
2. Launch Claude Code with ANTHROPIC_BASE_URL pointing to CLIProxyAPI (http://127.0.0.1:8317)
3. Start a conversation using a Gemini model (e.g., /model gemini-3-pro-preview)
4. Send a few messages to build up conversation history
5. Switch to a Claude model mid-conversation (e.g., /model claude-opus-4-6)
6. Send a message

Expected Behavior

The conversation should continue seamlessly with the Claude model, with CLIProxyAPI handling the provider transition transparently.

Actual Behavior

Claude's API returns a 400 error:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.3.content.0: Invalid `signature` in `thinking` block"},"request_id":"req_011CY5rpLDkzLmojWXnJH1ML"}

Root Cause

When Gemini responds through CLIProxyAPI, the proxy wraps responses in Claude-format thinking blocks with proxy-generated signatures. These signatures are not cryptographically valid (only Anthropic's servers can produce valid ones). When the user switches to a real Claude model, the conversation history containing these fake-signed thinking blocks is sent to Anthropic's API, which validates the signatures and rejects the request.

Suggested Fix

When routing a request to Claude and the conversation history contains thinking blocks from a non-Claude provider (Gemini CLI, etc.), CLIProxyAPI should either:

1. Convert to redacted_thinking type — Claude's API accepts redacted thinking blocks without signature validation
2. Strip thinking blocks from previous turns that originated from non-Claude providers
3. Remove signatures from thinking blocks that were not generated by Claude's API

Option 1 (redacted_thinking) is likely the cleanest approach as it preserves the conversation structure while avoiding signature validation.

Environment

• CLIProxyAPI Version: 6.8.15 (Homebrew)
• OS: macOS (Darwin 25.2.0, arm64)
• Providers: Gemini CLI OAuth + Claude OAuth (Max plan)
• Models tested: gemini-3-pro-preview → claude-opus-4-6
• Client: Claude Code CLI

Related Issues

• Gemini-3-pro-high Corrupted thought signature #1538 — Gemini-3-pro-high corrupted thought signature (Antigravity-specific)
• bug: Invalid signature in thinking block (API 400) on follow-up requests #1398 — Invalid signature in thinking block on follow-up requests (closed/fixed)
• fix(antigravity): prevent corrupted thought signature when switching models #994 — Fix corrupted signature when switching models (closed/fixed)

Those fixes addressed Antigravity-specific scenarios but do not cover the Gemini CLI OAuth → Claude OAuth switching case.