LangSmith Exporter

LangSmith is LangChain's platform for monitoring and evaluating LLM applications. The LangSmith exporter sends your traces to LangSmith, providing insights into model performance, debugging capabilities, and evaluation workflows.

Installation

npm install @mastra/langsmith@beta

Configuration

Prerequisites

1. LangSmith Account: Sign up at smith.langchain.com
2. API Key: Generate an API key in LangSmith Settings → API Keys
3. Environment Variables: Set your credentials

.env

# Required
LANGSMITH_API_KEY=ls-xxxxxxxxxxxx

# Optional
LANGCHAIN_PROJECT=my-project  # Default project for traces
LANGSMITH_BASE_URL=https://api.smith.langchain.com  # For self-hosted

Zero-Config Setup

With environment variables set, use the exporter with no configuration:

src/mastra/index.ts

import { Mastra } from "@mastra/core";
import { Observability } from "@mastra/observability";
import { LangSmithExporter } from "@mastra/langsmith";

export const mastra = new Mastra({
  observability: new Observability({
    configs: {
      langsmith: {
        serviceName: "my-service",
        exporters: [new LangSmithExporter()],
      },
    },
  }),
});

Explicit Configuration

You can also pass credentials directly (takes precedence over environment variables):

src/mastra/index.ts

import { Mastra } from "@mastra/core";
import { Observability } from "@mastra/observability";
import { LangSmithExporter } from "@mastra/langsmith";

export const mastra = new Mastra({
  observability: new Observability({
    configs: {
      langsmith: {
        serviceName: "my-service",
        exporters: [
          new LangSmithExporter({
            apiKey: process.env.LANGSMITH_API_KEY,
          }),
        ],
      },
    },
  }),
});

Configuration Options

Complete Configuration

new LangSmithExporter({
  // Required credentials
  apiKey: process.env.LANGSMITH_API_KEY!,

  // Optional settings
  apiUrl: process.env.LANGSMITH_BASE_URL, // Default: https://api.smith.langchain.com
  projectName: "my-project", // Project to send traces to (overrides LANGCHAIN_PROJECT env var)
  callerOptions: {
    // HTTP client options
    timeout: 30000, // Request timeout in ms
    maxRetries: 3, // Retry attempts
  },
  logLevel: "info", // Diagnostic logging: debug | info | warn | error

  // LangSmith-specific options
  hideInputs: false, // Hide input data in UI
  hideOutputs: false, // Hide output data in UI
});

Environment Variables

Variable	Description
LANGSMITH_API_KEY	Your LangSmith API key (required)
LANGCHAIN_PROJECT	Default project name for traces (optional, defaults to "default")
LANGSMITH_BASE_URL	API URL for self-hosted instances (optional)

The projectName config option takes precedence over the LANGCHAIN_PROJECT environment variable, allowing you to programmatically route traces to different projects.

Related

• Tracing Overview
• LangSmith Documentation