fix: override end_turn stop reason when streaming response contains toolUse blocks#1827
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…oolUse Some models (e.g., Sonnet 4.x on Bedrock) return end_turn as the stop reason even when the response contains toolUse content blocks. In streaming mode, the stop_reason from messageStop is used directly without checking for toolUse blocks, causing the event loop to skip tool execution. Add a post-stream check: if stop_reason is end_turn but the assembled message contains toolUse blocks, override to tool_use. This matches the existing non-streaming behavior in event_loop.py. Fixes strands-agents#1810
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pgrayy
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src/strands/event_loop/streaming.py
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| # contains toolUse blocks. Some models (e.g., Sonnet 4.x) can return | ||
| # end_turn as stop_reason even when tool calls are present, which prevents | ||
| # the event loop from processing those tool calls. | ||
| # See: https://github.com/strands-agents/sdk-python/issues/1810 |
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Few call outs:
- I wouldn't worry about mentioning the ticket here in comments. That is more for the PR itself.
- We should try to fix this before the
yield ModelStreamChunkEventabove. This way the event is yielded with the correct stop reason. - This I would say warrants a warn instead of a debug.
- With this change, we should be able to remove the override logic in https://github.com/strands-agents/sdk-python/blob/main/src/strands/models/bedrock.py#L826.
- We should add tests.
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Thanks for the detailed review! Pushed updates addressing all points:
- Removed ticket reference from code comments
- Moved the override into the
messageStophandler so the event is yielded with the correct stop reason - Changed from
logger.debugtologger.warning - Removed the duplicate override logic in
bedrock.py— now handled centrally instreaming.py - Will add tests — working on it
…, remove bedrock override - Move stop_reason override before yield so ModelStreamChunkEvent has correct stop reason - Change log level from debug to warning - Remove duplicate override logic in bedrock.py (now handled centrally) - Remove ticket reference from code comments (belongs in PR)
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| content = state["message"].get("content", []) | ||
| if any("toolUse" in item for item in content): | ||
| logger.warning( | ||
| "stop_reason override: end_turn -> tool_use (response contains toolUse blocks)" |
pgrayy
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Mar 8, 2026
| callback(chunk) | ||
| # The stop_reason override for end_turn -> tool_use is now | ||
| # handled centrally in streaming.py so no model-specific | ||
| # fixup is needed here. |
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I wouldn't worry about adding this comment here.
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Problem
Some models (e.g., Sonnet 4.x on Bedrock) return
end_turnas the stop reason inmessageStopeven when the response containstoolUsecontent blocks. In streaming mode, thestop_reasonfrommessageStopis used directly without checking fortoolUseblocks, causing the event loop to skip tool execution entirely.This results in the agent returning prematurely with unexecuted tool calls.
Root Cause
In
stream_messages_async()(streaming.py), thestop_reasonfromhandle_message_stop()is yielded as-is. Unlike the non-streaming path inevent_loop.py(which checks_has_tool_use_in_latest_message), the streaming path has no corresponding override.Fix
After the streaming loop completes, check if the assembled message content contains
toolUseblocks. Ifstop_reasonisend_turnbuttoolUseblocks are present, override totool_use.Fixes #1810