fix: ensure assistant message content is never undefined for Gemini compatibility

[daniel-lxs]

Summary

Fixes ROO-425: OpenRouter + Gemini "parts field" validation error when assistant message has undefined content.

Problem

When assistant messages contain only tool_use blocks (no text), the content field was left as undefined. While OpenAI accepts content: null with tool_calls, Gemini (via OpenRouter) strictly requires every message to have content in the "parts" field.

This caused users to encounter a 400 Bad Request error:

ApiProviderError: Unable to submit request because it must include at least one parts field, which describes the prompt input.

Solution

Use empty string fallback (content ?? "") for assistant message content instead of allowing undefined.

Changes

• src/api/transform/openai-format.ts: Use content ?? "" instead of content
• src/api/transform/__tests__/openai-format.spec.ts: Add test case for Gemini compatibility

Testing

• ✅ All 24 tests pass in openai-format.spec.ts
• ✅ All 5179 tests pass across the project

Related

• Linear: https://linear.app/roocode/issue/ROO-425

---

Important

Fixes undefined content in assistant messages for Gemini compatibility by using empty string fallback in convertToOpenAiMessages().

• Behavior:
  • Fixes issue where assistant messages with only tool_use blocks had undefined content, causing errors with Gemini via OpenRouter.
  • Uses content ?? "" to ensure content is never undefined in convertToOpenAiMessages() in openai-format.ts.
• Testing:
  • Adds test case in openai-format.spec.ts to verify empty string content for Gemini compatibility.
  • All tests pass in openai-format.spec.ts and across the project.

^{This description was created by for f6304d4. You can customize this summary. It will automatically update as commits are pushed.}

Rooviewer   See task on Roo Cloud

Review complete. No issues found.

The fix correctly uses content ?? "" to ensure assistant messages never have undefined content, which resolves the Gemini compatibility issue (ROO-425). The implementation is minimal, well-documented, and properly tested.

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}