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Use PrintingHandler to capture LangChain callback events without manually instrumenting every call site.

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Set up the handler

from nexus_library.nexus_core import NexusClient
from nexus_library.langchain import PrintingHandler

nexus_key = "your_nexus_api_key"
nexus_client = NexusClient(api_key=nexus_key)
callback_handler = PrintingHandler(nexus_client)
Attach callback_handler to your runnable or agent callback configuration.

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What gets logged

The handler listens to standard callback hooks, including:
  • LLM and chat model lifecycle (start, end, error)
  • chain lifecycle (start, end, error)
  • tool lifecycle (start, end, error)
  • agent actions and finish events
  • retriever lifecycle (start, end, error)
  • text and retry events
Each event is persisted with parent linkage so you can reconstruct execution trees.

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Event category mapping

The handler normalizes callback names into category-style event_type values.
Example: llm_start and llm_end are stored as llm.

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Root-chain trigger behavior

When a root chain ends (or errors), the handler can trigger:
  • POST {trigger_url}/catch_route_manual
  • query params: user_id and trace_id
Set dev_mode=True to disable this external trigger while testing locally. Get your nexus_api_key from the Nexus platform: Nexus platform.

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Best practices

  • Reuse one NexusClient for one logical run.
  • Keep dev_mode=True in local and CI environments.
  • Treat trigger endpoints as production infrastructure and monitor failures.