version: "1.0.0" name: onboarding-and-archaeology description: Ramp up quickly on an unfamiliar codebase by reading strategically, mapping invariants, and making safe first changes. Use when joining a project, reviewing a legacy system, or orienting in a large monorepo. license: Apache-2.0 compatibility: Works with coding agents that support Agent Skills. metadata: agentpacks.version: "0.1.0"

Onboarding and Code Archaeology

An unfamiliar codebase is an archaeological site. Read the strata before you dig.

Orientation Pass (First Hour)

• Start with the entry point: main, index, app.py, cmd/. Follow the startup sequence to understand what the process does before it handles any request.
• Read CLAUDE.md, CONTRIBUTING.md, README.md, and any docs/ folder. These record decisions that are no longer visible in the code.
• Check the CI definition (.github/workflows/, .circleci/, Makefile). The build steps tell you what the maintainers consider essential.
• git log --oneline -50 gives you recent momentum: what changed, at what frequency, and by whom.
• git log --oneline --follow -- <file> on the most-changed file shows what's been fought over.

Mapping the System

• Draw a box diagram of the major components from git log and directory structure before reading any code deeply. Hypothesize, then verify.
• Identify the seams: where do modules call each other? Grep for import statements and function call sites to find the dependency graph.
• Find the data model first — it's the stable substrate everything else is built on. Schema files, ORM models, and proto definitions are high-leverage reads.
• Identify the top-level invariants: what must always be true? Look for comments marked // INVARIANT, // IMPORTANT, // NOTE, panics, and assertions — they mark the load-bearing assumptions.

Reading Code Strategically

• Read tests before implementation. Tests document intended behavior and reveal edge cases the author thought about. Missing tests reveal gaps.
• Read the error paths as carefully as the happy path. Error handling often contains the most domain knowledge.
• When a function is surprising, read its git history (git log -p -- <file>): the patch context explains why, not just what.
• Don't read everything. Spend the first day identifying which 20% of the code is responsible for 80% of the behavior — focus there.

Safe First Changes

• Before touching anything, add a test for the behavior you're about to change. If no test exists, write one that passes today. This is your safety net.
• Make the smallest possible change first: a one-line fix, a renamed variable, a new test. Running the full test suite on a trivial change validates your local setup.
• Prefer additive changes (new function, new flag) over modifications. Modifications break callers you haven't found yet.
• When changing behavior, use the expand-contract pattern: add the new path, migrate callers, delete the old path — three separate, reviewable steps.
• Flag your assumptions in PR descriptions: "I believe X is always true here because Y. If that's wrong, this change will break Z."

Asking the Right Questions

• Don't ask "what does this code do?" — read it. Ask "why does it do it this way?" and "what would break if it did it differently?"
• When something looks wrong, assume you're missing context before assuming it's a bug. Check git blame, linked issues, and ADRs first.
• Document what you learn in the code or in ADRs as you go — future contributors (including you in 6 months) will have the same questions.

Checklist

• [ ] Entry point and startup sequence read.
• [ ] Data model and top-level invariants identified.
• [ ] git log reviewed for recent momentum and hotspots.
• [ ] Tests read before implementation for each area being modified.
• [ ] First change covered by a test before modifying code.
• [ ] Surprising code traced through git history for context.