> ## Documentation Index
> Fetch the complete documentation index at: https://docs.trynexus.io/llms.txt
> Use this file to discover all available pages before exploring further.

# LangHandler

> Nexus reference for LangChain callback tracing

## `LangHandler(nexus_client, dev_mode=False, session_id=None, db_url=None)`

Callback handler for LangChain event instrumentation.

* `nexus_client`: required `NexusClient` source for `user_id`
* `dev_mode`: disables root-chain trigger calls when true
* `session_id`: optional metadata value
* `db_url`: optional DB override

## Callback hooks implemented

### LLM and chat model

* `on_llm_start`, `on_llm_end`, `on_llm_error`
* `on_chat_model_start`
* `on_llm_new_token` (present but not persisted by default)

### Chains

* `on_chain_start`, `on_chain_end`, `on_chain_error`
* root `on_chain_end` and root `on_chain_error` may trigger `/catch_route_manual`

### Tools

* `on_tool_start`, `on_tool_end`, `on_tool_error`

### Agents

* `on_agent_action`, `on_agent_finish`

### Retrieval and misc

* `on_retriever_start`, `on_retriever_end`, `on_retriever_error`
* `on_text`
* `on_retry`

## Event normalization

Incoming callback names are normalized into category-style values before persistence:

* `llm_start` -> `llm`
* `tool_error` -> `tool`
* `retriever_end` -> `retriever`

## Persistence behavior

Each callback writes to `agent_logs` with:

* run identifiers and parent linkage
* normalized event category
* serialized JSON for `input` and `output`
* nullable `error` text