When 429/500 errors occur during tool execution, empty assistant messages
with stopReason='error' and content=[] get persisted to the session file.
These break the tool_use -> tool_result chain that Claude/Gemini require:
- Claude expects every tool_use block to have a matching tool_result
- Empty error messages inserted mid-sequence violate this invariant
- Results in: 'tool_use ids were found without tool_result blocks'
This patch filters out empty error messages when building session context,
allowing sessions to recover gracefully from transient API errors.
Evidence from production:
- 113 of 170 sessions had empty error messages
- Session 30764430 demonstrated recovery: 429 at 14:30:11 IST,
resumed successfully at 14:30:22, completed at 14:30:34
Sorry Peter, one more patch! 🙈
1cfe409a09