LangSmith

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Setup

pnpm add braintrust langsmith

npm install braintrust langsmith

export BRAINTRUST_API_KEY=your-braintrust-api-key
export LANGSMITH_TRACING=true

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Tracing

• Auto-instrumentation (recommended): Run your app with node --import braintrust/hook.mjs to patch langsmith at startup without code changes.
• Manual instrumentation: Wrap langsmith namespaces explicitly using Braintrust’s wrapper functions.

import { initLogger } from "braintrust";
import { traceable } from "langsmith/traceable";

initLogger({
  projectName: "langsmith-example", // Replace with your project name
  apiKey: process.env.BRAINTRUST_API_KEY,
});

const retrieveContext = traceable(
  async ({ question }: { question: string }) => [
    { content: "Refunds accepted within 30 days.", title: "Refund policy" },
  ],
  { name: "retrieve-context", run_type: "retriever" },
);

const result = await retrieveContext({ question: "What is the refund window?" });
console.log(result);

node --import braintrust/hook.mjs trace-langsmith-auto.ts

import { initLogger, wrapLangSmithTraceable } from "braintrust";
import * as traceableNs from "langsmith/traceable";

initLogger({
  projectName: "langsmith-example", // Replace with your project name
  apiKey: process.env.BRAINTRUST_API_KEY,
});

const { traceable } = wrapLangSmithTraceable(traceableNs);

const retrieveContext = traceable(
  async ({ question }: { question: string }) => [
    { content: "Refunds accepted within 30 days.", title: "Refund policy" },
  ],
  { name: "retrieve-context", run_type: "retriever" },
);

const result = await retrieveContext({ question: "What is the refund window?" });
console.log(result);

​

What Braintrust traces

• traceable-wrapped function calls as spans, with inputs and outputs
• RunTree operations and Client API calls (createRun, updateRun, batchIngestRuns) as spans
• Run type mapped to span type: llm and embedding → LLM spans, tool and retriever → Tool spans, other run types → Task spans
• Token usage (prompt_tokens, completion_tokens, total_tokens, prompt_cached_tokens, prompt_cache_creation_tokens)
• time_to_first_token from new_token events in run history
• Model name and provider from ls_model_name and ls_provider metadata fields
• LLM settings (temperature, top_p, max_tokens, and related fields) from run metadata
• Tags and errors

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Resources

• LangSmith documentation
• braintrust on npm