HTTP 400 "invalid argument" was triggered when OmniRoute translated tool calls
to Gemini format, because thoughtSignature was injected onto every functionCall
part unconditionally. Affects two code paths:
1. openai-to-gemini.ts — OpenAI tool_calls → Gemini functionCall
2. claude-to-gemini.ts — Claude tool_use blocks → Gemini functionCall
thoughtSignature is only valid on thinking/reasoning parts (those with
thought: true or thoughtSignature as their primary field). When present on a
functionCall part, the Gemini API returns HTTP 400 'invalid argument'.
The thinking parts that legitimately carry thoughtSignature (emitted when a
message has reasoning_content / thinking blocks) are untouched.
Regression tests (T43) cover:
- single tool call: no thoughtSignature on functionCall part (openai path)
- multiple tool calls: none carry thoughtSignature (openai path)
- thinking regression guard: thoughtSignature still on thought parts
- claude-to-gemini path: tool_use blocks produce clean functionCall parts
Fixes#724
c5003665c3