fix(core): add on_tool_error to _AstreamEventsCallbackHandler

[vbarda]

Fixes #30708

[vercel]

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Project	Deployment	Preview	Comments	Updated (UTC)
langchain	Ready	Preview	Comment	Sep 11, 2025 1:41am

[deershark]

@vbarda @eyurtsev Because of this problem, some functions of our project cannot be implemented at present :( ..... May I ask if there are any problems with this PR? It has been two weeks.

[ben-gineer]

@vbarda - I'm also keen to use this to help handling tool errors.

[eyurtsev]

@ben-gineer and @deershark we'll try to get this in soon sorry the delay.

But you should also be favoring the .stream() or .astream() APIs and orchestrating using langgraph. astream_events API should largely be unnecessary these days for development

[ben-gineer]

Thanks. The only reason I’m using it is to allow me to stream events within a Streamlit app using LangGraph MCP tools without introducing a FastAPI intermediate service.

If you know of alternative solutions, I’d be happy to switch.

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #30709 will not alter performance

_{Comparing vb/add-on-tool-error (1a09da7) with wip-v1.0 (ced9fc2)}

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

✅ 13 untouched benchmarks

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #30709 will not alter performance

_{Comparing vb/add-on-tool-error (1a09da7) with wip-v1.0 (311aa94)¹}

Summary

✅ 14 untouched benchmarks

Footnotes

1. No successful run was found on wip-v1.0 (ced9fc2) during the generation of this report, so d1c5ff1 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[Copilot]

Pull Request Overview

This PR adds the missing on_tool_error callback handler to the _AstreamEventsCallbackHandler class to fix issue #30708. The change ensures that tool error events are properly captured and emitted in the streaming events API.

Key Changes

• Added on_tool_error method implementation to the event stream callback handler
• Added error field to the EventData schema to support error information in streaming events

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
libs/core/langchain_core/tracers/event_stream.py	Implements the missing on_tool_error callback method to handle tool errors in the streaming events
libs/core/langchain_core/runnables/schema.py	Adds the error field to EventData TypedDict to support error information in streaming events