fix(core): add on_tool_error to _AstreamEventsCallbackHandler

[vbarda]

Fixes #30708

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
langchain	⬜️ Ignored (Inspect)	Visit Preview		Jul 16, 2025 3:17pm

[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 (0b2075c) with master (12d370a)}

⚠️ 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 (0b2075c) with master (12d370a)}

Summary

✅ 14 untouched benchmarks