Summary

• _copy_reasoning_content_for_api had a step-ordering bug introduced by PR fix(agent): ordering fix in _copy_reasoning_content_for_api — cross-provider reasoning isolation #15749: the tool-call empty-string injection (old step 2) ran before the reasoning field promotion (old step 3)
• A tool-call message with a valid reasoning field but no reasoning_content would hit the empty-string path, get reasoning_content="" injected, and return early — the reasoning value was silently discarded
• Fix: move the reasoning → reasoning_content promotion to step 2, before the empty-string fallbacks

The bug

run_agent.py — _copy_reasoning_content_for_api (pre-fix):

# Step 1: explicit reasoning_content → preserve ✓
# Step 2: tool_calls + DeepSeek/Kimi → inject ""  ← fires here
#         RETURNS EARLY
# Step 3: reasoning field → promote               ← never reached

A message like {"role": "assistant", "reasoning": "thought trace", "tool_calls": [...]} on a DeepSeek session would return reasoning_content="" instead of reasoning_content="thought trace".

The fix

Reorder steps 2 and 3 so reasoning promotion runs first:

# Step 1: explicit reasoning_content → preserve
# Step 2: reasoning field present → promote to reasoning_content  ← moved up
# Step 3: tool_calls + DeepSeek/Kimi + no reasoning → inject ""
# Step 4: all other DeepSeek/Kimi + no reasoning → inject ""
# Step 5: non-string cleanup

Empty-string injection now only fires when neither reasoning_content nor reasoning is present — the poisoned-history case it was designed for.

Test plan

• Before: test_deepseek_reasoning_field_promoted → AssertionError: assert '' == 'thought trace'
• After: all 21 tests in test_deepseek_reasoning_content_echo.py pass
• Regression guard: reverted run_agent.py → test_deepseek_reasoning_field_promoted failed with '' != 'thought trace'; restored → 21 passing
• Broader suite: tests/run_agent/ — 1048 passed, 1 pre-existing baseline failure (test_inf_stays_string_for_integer_only reproduces on clean origin/main)

Related

• Fixes regression: #15749 breaks reasoning field promotion for DeepSeek/Kimi tool-call messages #15812
• Regression introduced by fix(agent): ordering fix in _copy_reasoning_content_for_api — cross-provider reasoning isolation #15749
• Related: [Bug] _copy_reasoning_content_for_api: cross-provider reasoning promotion leaks stale content to DeepSeek/Kimi #15748 (same code path)

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