langchain-local-dev-loop

# LangChain Local Dev Loop ## Overview Set up a productive local development workflow for LangChain: project structure, mocked LLMs for unit tests (no API calls), integration tests with real providers, and dev tooling. ## Project Structure ``` my-langchain-app/ ├── src/ │ ├── chains/ # LCEL chain definitions │ │ ├── summarize.ts │ │ └── rag.ts │ ├── tools/ # Tool definitions │ │ └── calculator.ts │ ├── agents/ # Agent configurations │ │ └── assistant.ts │ └── index.ts ├── tests/ │ ├── unit/ # Mocked tests (no API calls) │ │ └── chains.test.ts │ └── integration/ # Real API tests (CI gated) │ └── rag.test.ts ├── .env # API keys (git-ignored) ├── .env.example # Template for required vars ├── package.json ├── tsconfig.json └── vitest.config.ts ``` ## Step 1: Dev Dependencies ```bash set -euo pipefail npm install @langchain/core @langchain/openai langchain zod npm install -D vitest @types/node tsx dotenv typescript ``` ## Step 2: Vitest Configuration ```typescript // vitest.config.ts import { defineConfig } from "vitest/config"; export default defineConfig({ test: { include: ["tests/**/*.test.ts"], environment: "node", setupFiles: ["./tests/setup.ts"], testTimeout: 30000, }, }); ``` ```typescript // tests/setup.ts import "dotenv/config"; ``` ## Step 3: Unit Tests with Mocked LLM ```typescript // tests/unit/chains.test.ts import { describe,...