fix: ensure reasoning_content is always present for DeepSeek thinking mode

[roomote]

Related GitHub Issue

Closes: #10175

Description

This PR attempts to address Issue #10175 where DeepSeek V3.2 API returns a 400 error after context condense.

Root Cause: DeepSeek's reasoner model (deepseek-reasoner) requires ALL assistant messages to have a reasoning_content field when using thinking mode. When context is condensed, the summary message is created without this field, causing the API to reject the request.

Fix: Modified src/api/transform/r1-format.ts to ensure that when converting messages for DeepSeek's thinking mode (indicated by mergeToolResultText: true), ALL assistant messages include the reasoning_content field, even if empty.

Changes made:

1. Array content handling (lines 193-208): Added logic to always include reasoning_content (empty string if none) when mergeToolResultText is true
2. String content handling (lines 240-252): Same logic applied for simple string content assistant messages

Test Procedure

1. Added 4 new unit tests in src/api/transform/__tests__/r1-format.spec.ts:

   • Test that summary messages get empty reasoning_content when mergeToolResultText: true
   • Test that non-thinking models remain unaffected (no reasoning_content added)
   • Test array content handling with mergeToolResultText: true
   • Test that existing reasoning_content is preserved when present

2. All 24 tests pass: cd src && npx vitest run api/transform/__tests__/r1-format.spec.ts

3. User @BowerMotorsport confirmed the fix works in their local testing

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes (if applicable).
• Documentation Impact: I have considered if my changes require documentation updates (see "Documentation Updates" section below).
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

Feedback and guidance are welcome!

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Rooviewer   See task on Roo Cloud

Reviewed and found no issues. The fix correctly ensures reasoning_content is always present on assistant messages when using DeepSeek's thinking mode, addressing the reported 400 error after context condense.

• No issues found

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