feat(langchain): add last trace id property to CallbackHandler

[hassiebp]

Important

Add last_trace_id property to LangchainCallbackHandler to track the last run's trace ID in CallbackHandler.py.

• Feature:
  • Add last_trace_id property to LangchainCallbackHandler in CallbackHandler.py.
  • Update last_trace_id in on_chain_start() and __on_llm_action() to store the trace ID of the last run.

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Greptile Summary

Disclaimer: Experimental PR review

Added last_trace_id property to langchain CallbackHandler for easier access to the most recent trace identifier without parsing the runs dictionary.

• Added new property last_trace_id in langfuse/langchain/CallbackHandler.py that tracks the most recently used trace ID
• Updated property during chain start, LLM generation start, and LLM action events
• Implementation maintains existing error handling patterns and integrates with current tracing functionality
• Feature supports better tracing/debugging in LangChain integration as shown in test cases

[greptile-apps]

LGTM

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[chai3]

very nice for me

I was in trouble because CallbackHandler.get_trace_url() was removed in v3.
I think that client.get_trace_url(callback_handler.last_trace_id) will be available.

[hassiebp]

very nice for me

I was in trouble because CallbackHandler.get_trace_url() was removed in v3.

I think that client.get_trace_url(callback_handler.last_trace_id) will be available.

thanks for your feedback @chai3 !