Codebase Documentation Skill

Generate and maintain a structured set of markdown files in docs/ that give AI agents (and humans) a fast, accurate understanding of a codebase without needing to search through source files.

Philosophy

The docs you produce are a map, not the territory. They should let an AI agent answer questions like "where does authentication happen?", "how does data flow from API to database?", or "what are the key abstractions?" — without reading every file. Optimize for navigability and accuracy over exhaustiveness.

When to Use This Skill

• User asks to document a codebase or project
• User wants to update existing docs after making changes
• User wants AI-friendly project documentation
• User mentions docs/ directory maintenance
• User wants architecture or structural documentation

Step 1: Assess the Codebase

Before writing anything, understand what you're documenting.

1. Read the project root — check for existing README, package.json/Cargo.toml/pyproject.toml, config files, and any existing docs/ directory
2. Map the directory structure — identify source directories, test directories, config, scripts, assets
3. Identify the tech stack — languages, frameworks, key dependencies, build tools
4. Find entry points — main files, route definitions, CLI entry points, exported modules
5. Detect patterns — architecture style (MVC, microservices, monolith, plugin-based), state management, data layer

If a docs/ directory already exists, read it first. You'll be updating, not starting from scratch.

Step 2: Generate the Documentation Set

Create the docs/ directory with the files described below. Not every project needs every file — use judgment. A small CLI tool doesn't need DATA_MODEL.md, and a pure library doesn't need DEPLOYMENT.md.

Refer to references/doc-templates.md for the exact templates and structure for each file. Read that file before writing any documentation.

Required Files (always generate)

Situational Files (generate when relevant)

Index File

Always generate docs/INDEX.md as the last file. This is a table of contents linking all other docs with one-line descriptions. An AI agent uses this to decide which file to read for a given question.

Step 3: Writing Guidelines

Follow these principles for every file:

Structure for Scannability

• Start each file with a 2-3 sentence summary of what it covers
• Use headers (##, ###) to create clear sections
• Keep paragraphs short — 2-4 sentences max
• Use code blocks for file paths, commands, and type signatures
• Use tables for structured comparisons or listings

Write for AI Agents

• Be explicit about relationships — "Module A calls Module B's process() method when..."
• Include file paths — always reference actual paths like src/auth/middleware.ts
• Name functions and classes that are key entry points or interfaces
• Describe data flow with clear directionality — "User request → Router → Controller → Service → Repository → Database"
• State assumptions and constraints — "This service assumes a PostgreSQL connection is available"

Keep It Maintainable

• Include a Last updated date at the top of each file
• Add a brief ## Changes Log section at the bottom of each file with space for noting updates
• Reference specific file paths so staleness is detectable — if a referenced file moves, the docs are clearly wrong
• Prefer describing intent and architecture over implementation details that change frequently

What NOT to Document

• Don't duplicate what's already in README.md — reference it instead
• Don't document every function — focus on public interfaces and key internal abstractions
• Don't copy-paste code — describe what it does and where to find it
• Don't document generated files or node_modules

Step 4: Updating Existing Docs

When updating rather than creating from scratch:

1. Read all existing files in docs/ first
2. Re-scan the codebase to detect changes since the docs were last updated
3. Update affected files only — don't rewrite everything
4. Update the Last updated date and add a note to the Changes Log
5. Update docs/INDEX.md if any files were added or removed
6. If a documented file has been deleted or moved, update or remove the reference

Step 5: Validation

After generating or updating docs, do a quick sanity check:

1. Verify all file paths mentioned in docs actually exist
2. Confirm the directory tree in STRUCTURE.md matches reality
3. Ensure INDEX.md links to all docs that were created
4. Check that cross-references between doc files are correct

Output

All files go in the docs/ directory at the project root. Use the exact filenames specified above (UPPERCASE with .md extension). Present the full set to the user when done, summarizing what was created and any areas where the docs might need human review (e.g., business logic descriptions, architectural decisions that aren't obvious from code alone).