Analyze an unfamiliar codebase and generate a structured onboarding guide with architecture map, key entry points, conventions, and a starter CLAUDE.md. Use when joining a new project or setting up Claude Code for the first time in a repo.

Codebase Onboarding Systematically analyze an unfamiliar codebase and produce a structured onboarding guide. Designed for developers joining a new project or setting up Claude Code in an existing repo for the first time. When to Use First time opening a project with Claude Code Joining a new team or repository User asks "help me understand this codebase" User asks to generate a CLAUDE.md for a project User says "onboard me" or "walk me through this repo" How It Works Phase 1: Reconnaissance Gather raw signals about the project without reading every file. Run these checks in parallel: Phase 2: Architecture Mapping From the reconnaissance data, identify: Tech Stack Language(s) and version constraints Framework(s) and major libraries Database(s) and ORMs Build tools and bundlers CI/CD platform Architecture Pattern Monolith, monorepo, microservices, or serverless Frontend/backend split or full stack API style: REST, GraphQL, gRPC, tRPC Key Directories Map the top level directories to their purpose: <! Example for a React project — replace with detected directories Data Flow Trace one request from entry to response: Where does a request enter? (router, handler, controller) How is it validated? (middleware, schemas, guards) Where is business logic? (services, models, use cases) How does it reach the database? (ORM, raw queries, repositories) Phase 3: Convention Detection Identify patterns the codebase already follows: Naming Conventions File naming: kebab case, camelCase, PascalCase, snake case Component/class naming patterns Test file naming: .test.ts , .spec.ts , test.go Code Patterns Error handling style: try/catch, Result types, error codes Dependency injection or direct imports State management approach Async patterns: callbacks, promises, async/await, channels Git Conventions Branch naming from recent branches Commit message style from recent commits PR workflow (squash, merge, rebase) If the repo has no commits yet or only a shallow history (e.g. git clone depth 1 ), skip this section and note "Git history unavailable or too shallow to detect conventions" Phase 4: Generate Onboarding Artifacts Produce two outputs: Output 1: Onboarding Guide Output 2: Starter CLAUDE.md Generate or update a project specific CLAUDE.md based on detected conventions. If CLAUDE.md already exists, read it first and enhance it — preserve existing project specific instructions and clearly call out what was added or changed. Best Practices 1. Don't read everything — reconnaissance should use Glob and Grep, not Read on every file. Read selectively only for ambiguous signals. 2. Verify, don't guess — if a framework is detected from config but the actual code uses something different, trust the code. 3. Respect existing CLAUDE.md — if one already exists, enhance it rather than replacing it. Call out what's new vs existing. 4. Stay concise — the onboarding guide should be scannable in 2 minutes. Details belong in the code, not the guide. 5. Flag unknowns — if a convention can't be confidently detected, say so rather than guessing. "Could not determine test runner" is better than a wrong answer. Anti Patterns to Avoid Generating a CLAUDE.md that's longer than 100 lines — keep it focused Listing every dependency — highlight only the ones that shape how you write code Describing obvious directory names — src/ doesn't need an explanation Copying the README — the onboarding guide adds structural insight the README lacks Examples Example 1: First time in a new repo User : "Onboard me to this codebase" Action : Run full 4 phase workflow → produce Onboarding Guide + Starter CLAUDE.md Output : Onboarding Guide printed directly to the conversation, plus a CLAUDE.md written to the project root Example 2: Generate CLAUDE.md for existing project User : "Generate a CLAUDE.md for this project" Action : Run Phases 1 3, skip Onboarding Guide, produce only CLAUDE.md Output : Project specific CLAUDE.md with detected conventions Example 3: Enhance existing CLAUDE.md User : "Update the CLAUDE.md with current project conventions" Action : Read existing CLAUDE.md, run Phases 1 3, merge new findings Output : Updated CLAUDE.md with additions clearly marked

codebase-onboarding