fix: correct Gemini 3 thought signature injection format via OpenRouter

[daniel-lxs]

Summary

Fix 'Thought signature is not valid' error when using Gemini 3 models (e.g., google/gemini-3-flash-preview) via OpenRouter with native tool calling.

Problem

Users encounter error 400 "Unable to submit request because Thought signature is not valid" when switching models mid-conversation to Gemini 3 via OpenRouter.

PostHog Issue: https://us.posthog.com/error_tracking/019bb2c8-0b6c-71c3-9ef8-a2c43cd045fd

Root Cause

The original code was creating multiple reasoning_details entries (one per tool call), but OpenRouter/Gemini expects exactly ONE entry per assistant turn with:

• id set to the first tool call's ID
• index set to 0

This format mismatch caused Google's API to reject the signatures as invalid.

Solution

Fixed the injection logic in src/api/providers/openrouter.ts to create ONE reasoning.encrypted block per assistant message with tool calls, matching the documented format from OpenRouter (conversation with Toven, Nov 2025).

Changes

• Create ONE reasoning.encrypted block per assistant message with tool calls
• Use first tool call's ID instead of each tool call's individual ID
• Always set index to 0 instead of incrementing indices

References

• Native Gemini provider handles this correctly in gemini-format.ts: "keep the signature only on the first functionCall part to match Gemini 3 parallel-calling behavior"
• OpenRouter documentation confirms ONE entry per assistant turn with first tool call's ID

Testing

All 24 OpenRouter tests pass.

Closes ROO-494

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Important

Fixes 'Thought signature is not valid' error for Gemini 3 models in openrouter.ts by correcting reasoning.encrypted block injection.

• Behavior:
  • Fixes 'Thought signature is not valid' error for Gemini 3 models in openrouter.ts by correcting reasoning.encrypted block injection.
  • Ensures one reasoning.encrypted block per assistant message with tool calls, using the first tool call's ID and setting index to 0.
• Testing:
  • All 24 OpenRouter tests pass.

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[roomote]

Rooviewer   See task on Roo Cloud

Review complete. No issues found.

The fix correctly addresses the 'Thought signature is not valid' error by creating ONE reasoning.encrypted block per assistant message with tool calls, using the first tool call's ID and setting index to 0. This approach is consistent with the OpenRouter documentation and the native Gemini provider's implementation in gemini-format.ts.

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