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This guide shows you how to trace Pi coding agent sessions to LangSmith using the @langchain/langsmith-pi-extension extension. Once configured, each Pi session sends traces to LangSmith. Each trace includes user messages, assistant responses, tool calls, and individual LLM invocations, which gives you full observability into your Pi coding agent runs.

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Prerequisites

Before setting up tracing, ensure you have:

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Installation

Install the extension via Pi: 
pi install npm:@langchain/langsmith-pi-extension

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Quick start

Tracing is disabled by default. Set the following environment variables to enable tracing and connect to your LangSmith account: 
export TRACE_TO_LANGSMITH=true
export LANGSMITH_PI_API_KEY="<your-langsmith-api-key>"
Run Pi as usual. When a session starts, the extension reports whether LangSmith tracing is enabled. You can also check the current tracing state at any time from within Pi: 
/langsmith-tracing
By default, traces are written to the pi-coding-agent LangSmith project.

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Configuration

Configuration can come from environment variables or JSON config files. Values are merged in the following order, with later sources taking precedence:
  1. Defaults
  2. ~/.pi/langsmith.json (global config)
  3. <current-working-directory>/.pi/langsmith.json (project config)
  4. Environment variables

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Environment variables

VariableDescription
TRACE_TO_LANGSMITHEnables tracing when set to true, 1, yes, or on. Disables tracing when set to false, 0, no, or off.
LANGSMITH_PI_API_KEYLangSmith API key. Falls back to LANGSMITH_API_KEY.
LANGSMITH_PI_ENDPOINTLangSmith API URL for self-hosted or custom deployments. Falls back to LANGSMITH_ENDPOINT.
LANGSMITH_PI_PROJECTLangSmith project name. Falls back to LANGSMITH_PROJECT. Defaults to pi-coding-agent.
LANGSMITH_PI_METADATAJSON object added to the root run metadata. Falls back to LANGSMITH_METADATA.
LANGSMITH_PI_RUNS_ENDPOINTSJSON array of replica run destinations. Falls back to LANGSMITH_RUNS_ENDPOINTS.
Example: 
export TRACE_TO_LANGSMITH=true
export LANGSMITH_PI_API_KEY="<your-langsmith-api-key>"
export LANGSMITH_PI_PROJECT="pi-coding-agent-dev"
export LANGSMITH_PI_METADATA='{"team":"infra","environment":"local"}'

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Config file

Create ~/.pi/langsmith.json for global settings or .pi/langsmith.json in a project directory for local overrides: 
{
  "enabled": true,
  "api_key": "<your-langsmith-api-key>",
  "api_url": "https://api.smith.langchain.com",
  "project": "pi-coding-agent",
  "metadata": { "environment": "local" }
}
Config file fields:
FieldRequiredDefaultDescription
enabledYesfalseSet to true to enable tracing from the config file.
api_keyNo*—LangSmith API key. Required unless provided by environment variable or replicas.
api_urlNoLangSmith SDK defaultLangSmith API URL, usually https://api.smith.langchain.com.
projectNopi-coding-agentLangSmith project name.
metadataNo—Object merged into root trace metadata.
replicasNo—Array of additional LangSmith destinations to replicate traces to.

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Replicas

Use replicas to send traces to multiple LangSmith destinations simultaneously. This is useful for forwarding traces to both a personal workspace and a shared team project, or to a self-hosted LangSmith instance alongside the cloud. 
{
  "enabled": true,
  "api_key": "<primary-api-key>",
  "project": "pi-coding-agent",
  "replicas": [
    {
      "api_key": "<replica-api-key>",
      "api_url": "https://replica-langsmith.example.com",
      "project": "pi-coding-agent-replica",
      "updates": {
        "tags": ["replica"]
      }
    }
  ]
}
Each replica entry can include an updates object to override metadata or tags on the replicated runs.
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