fix: preserve DeepSeek reasoning_content through condense and API transmission

[hannesrudolph]

Problem

DeepSeek's interleaved thinking mode requires reasoning_content to be passed back during multi-turn tool call sequences. After context condensing, this field was being lost, causing 400 API errors with the message:

Missing reasoning_content field in the assistant message at message index 1

Root Cause

The buildCleanConversationHistory() function in Task.ts was stripping reasoning_content from assistant messages before sending them to the API. According to the DeepSeek API documentation:

Since the tool invocation process in thinking mode requires users to pass back reasoning_content to the API, if your code does not correctly pass back reasoning_content, the API will return a 400 error.

Changes

1. Added reasoning_content field to ApiMessage type - This ensures the type system properly tracks this field through the codebase.

2. Updated condense logic to preserve reasoning_content - Added getReasoningContent() helper function and ensured messages retained after condensing preserve their reasoning content.

3. Fixed turn alternation - After condensing, the summary message (assistant) followed by kept messages could create consecutive assistant messages. Added logic to inject an empty user message when needed to maintain proper turn alternation.

4. Updated buildCleanConversationHistory() - Now preserves reasoning_content on assistant messages when sending to the API.

5. Added comprehensive tests - Tests verify reasoning content preservation and turn alternation fixes.

Testing

• All 41 condense tests pass
• All 20 r1-format tests pass
• Full test suite passes (4923 tests)

Documentation Reference

• DeepSeek Thinking Mode - Tool Calls

---

Important

Preserve reasoning_content in DeepSeek API messages to prevent errors and ensure proper turn alternation in message condensing.

• Behavior:
  • Preserve reasoning_content in getKeepMessagesWithToolBlocks() and buildCleanConversationHistory() to prevent DeepSeek API errors.
  • Ensure turn alternation by extending keep range backwards in getKeepMessagesWithToolBlocks().
• Types:
  • Add reasoning_content to ApiMessage type in apiMessages.ts.
• Functions:
  • Update getKeepMessagesWithToolBlocks() in index.ts to handle reasoning_content and turn alternation.
  • Modify buildCleanConversationHistory() in Task.ts to include reasoning_content in API messages.
• Tests:
  • Add tests in index.spec.ts to verify reasoning_content preservation and turn alternation logic.

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