AI-281: add @temporalio/langsmith samples

Four scenarios demonstrating the @temporalio/langsmith plugin:
activity-tracing, workflow-tracing, agent-pipeline, and message-handlers.
Each includes a worker, client, workflows, a mocha test, and a README.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: xumaple

langsmith/.eslintignore

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+node_modules
+lib
+.eslintrc.js

langsmith/.eslintrc.js

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+const { builtinModules } = require('module');
+
+const ALLOWED_NODE_BUILTINS = new Set(['assert']);
+
+module.exports = {
+  root: true,
+  parser: '@typescript-eslint/parser',
+  parserOptions: {
+    project: './tsconfig.json',
+    tsconfigRootDir: __dirname,
+  },
+  plugins: ['@typescript-eslint', 'deprecation'],
+  extends: [
+    'eslint:recommended',
+    'plugin:@typescript-eslint/eslint-recommended',
+    'plugin:@typescript-eslint/recommended',
+    'prettier',
+  ],
+  rules: {
+    // recommended for safety
+    '@typescript-eslint/no-floating-promises': 'error', // forgetting to await Activities and Workflow APIs is bad
+    'deprecation/deprecation': 'warn',
+
+    // code style preference
+    'object-shorthand': ['error', 'always'],
+
+    // relaxed rules, for convenience
+    '@typescript-eslint/no-unused-vars': [
+      'warn',
+      {
+        argsIgnorePattern: '^_',
+        varsIgnorePattern: '^_',
+      },
+    ],
+    '@typescript-eslint/no-explicit-any': 'off',
+  },
+  overrides: [
+    {
+      files: ['src/workflows.ts', 'src/workflows-*.ts', 'src/workflows/*.ts'],
+      rules: {
+        'no-restricted-imports': [
+          'error',
+          ...builtinModules.filter((m) => !ALLOWED_NODE_BUILTINS.has(m)).flatMap((m) => [m, `node:${m}`]),
+        ],
+      },
+    },
+  ],
+};

langsmith/.gitignore

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+lib
+node_modules

langsmith/.npmrc

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+package-lock=false

langsmith/.nvmrc

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+22

langsmith/.post-create

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+To begin development, install the Temporal CLI:
+
+Mac: {cyan brew install temporal}
+Other: Download and extract the latest release from https://github.com/temporalio/cli/releases/latest
+
+Start Temporal Server:
+
+{cyan temporal server start-dev}
+
+Use Node version 18+ (v22.x is recommended):
+
+Mac: {cyan brew install node@22}
+Other: https://nodejs.org/en/download/
+
+To see live traces in LangSmith, enable tracing:
+
+{cyan export LANGSMITH_TRACING=true}
+{cyan export LANGSMITH_API_KEY=...}
+
+Then run a scenario by path (see each scenario's README).

langsmith/.prettierignore

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+lib

langsmith/.prettierrc

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+printWidth: 120
+singleQuote: true

langsmith/README.md

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+# LangSmith Tracing for Temporal
+
+These samples use the `@temporalio/langsmith` integration to add [LangSmith](https://docs.smith.langchain.com/) tracing to Temporal Workflows and Activities. Code you already instrument with LangSmith's native `traceable` keeps working unchanged when it runs inside a Workflow or Activity body — you only add the plugin to your Temporal Client and Worker, and the plugin threads a single trace across the `workflow → activity → child-workflow` boundaries.
+
+This is a single project: one `package.json` and one set of configs at the `langsmith/` root, with each scenario in its own subdirectory. Run `npm install` once here, then run any scenario by path (see each scenario's README). The integration package itself is documented in the [`@temporalio/langsmith` README](https://github.com/temporalio/sdk-typescript/tree/main/contrib/langsmith).
+
+## Prerequisites
+
+These apply to every sample in this directory:
+
+- A running Temporal dev server: `temporal server start-dev`.
+- Node 22 or later.
+- Dependencies installed once at the `langsmith/` root: `npm install`.
+
+Tracing is **off by default**, matching the `langsmith` library. To see live traces in LangSmith, enable it in the Worker and Client process environment:
+
+```bash
+export LANGSMITH_TRACING=true
+export LANGSMITH_API_KEY=...
+```
+
+With tracing off the plugin is a no-op. The tests enable it in-process and assert against an in-memory LangSmith Client, so they need no API key.
+
+## Samples
+
+| Sample                                  | Demonstrates                                                                           |
+| :-------------------------------------- | :------------------------------------------------------------------------------------ |
+| [`activity-tracing`](./activity-tracing) | A `traceable` model call inside an Activity, nested under the Workflow and Activity runs. |
+| [`workflow-tracing`](./workflow-tracing) | Replay-safe `traceable` calls in a Workflow body — emitted once, never duplicated on replay. |
+| [`agent-pipeline`](./agent-pipeline)    | A multi-step agent whose trace threads through Activities and a child Workflow.        |
+| [`message-handlers`](./message-handlers) | `traceable` calls inside Signal and Update handlers, nested under each handler's run.  |

langsmith/activity-tracing/README.md

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+# Activity Tracing
+
+A `traceable` model call runs inside an Activity body. The Client starts the Workflow from inside its own `traceable` (`user_pipeline`), so the whole call nests under one trace. With `addTemporalRuns: true` the plugin also emits the Temporal scaffolding runs (`StartWorkflow:`, `RunActivity:`, …).
+
+## Run
+
+Run these from the `langsmith/` root (run `npm install` there once first). To see live traces, `export LANGSMITH_TRACING=true` and `export LANGSMITH_API_KEY=...` in each terminal.
+
+```bash
+# In one terminal, start the Worker (requires a local Temporal server):
+npx ts-node activity-tracing/src/worker.ts
+
+# In another terminal, run the scenario:
+npx ts-node activity-tracing/src/client.ts
+```
+
+## Test
+
+```bash
+npx mocha --exit --require ts-node/register --require source-map-support/register "activity-tracing/src/mocha/*.test.ts"
+```
+
+The test runs a real Worker against `TestWorkflowEnvironment` with an in-memory LangSmith Client, so no API key is required.
+
+## Expected trace
+
+```
+user_pipeline
+  StartWorkflow:GreetingWorkflow
+  RunWorkflow:GreetingWorkflow
+    StartActivity:answer
+    RunActivity:answer
+      inner_llm_call
+```