> ## 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.

# Introduction

> Python API surface for Nexus instrumentation

Nexus exposes a Python-first API for trace logging.

## Core modules

* `NexusClient` and `Span` for manual instrumentation.
* `LangHandler` for LangChain callback instrumentation.
* `config` helpers for DB and trigger URLs.

## Data model written to storage

Both manual spans and LangChain callbacks map to rows with these key fields:

* `run_id`
* `user_id`
* `trace_id`
* `parent_run_id`
* `event_type`
* `name`
* `input`
* `output`
* `error`

## Reference pages

<CardGroup cols={2}>
  <Card title="NexusClient + Span" icon="square-code" href="/api-reference/python-client">
    Constructor behavior, span lifecycle, flush, and tree helpers.
  </Card>

  <Card title="LangHandler" icon="brain" href="/api-reference/langchain-handler">
    Callback hooks, event mapping, and root-chain trigger behavior.
  </Card>
</CardGroup>