Code craft Prove what the types can, then test only what they cannot. Every invariant has one owner: the strongest rung of the [proof ladder](principles/proof ladder.md) that can actually prove it. In order: types and linters, effects, proofs, database constraints, unit tests, integration tests, end to end tests. Use the repository's current instructions and established conventions. Apply these defaults where the requested change needs a design decision; they do not authorize unrelated restructuring or new tooling. Read the reference for the decision in scope: Decision Reference Decide which rung owns an invariant [Proof ladder](principles/proof ladder.md) Represent states, workflows, and domain identities [Illegal states](principles/illegal states.md) Decode external input into useful types [Parse, don't validate](principles/parse dont validate.md) Type dependencies, failures, and resource lifetimes [Effects](principles/effects.md) Model recoverable errors and required gates [Errors as values](principles/errors as values.md) Guard a sensitive precondition [Proof parameters](principles/proof parameters.md) Choose an abstraction or investigate performance [Simplicity](principles/simplicity.md) Maintain module boundaries and their documentation [Architecture docs](principles/architecture docs.md) Read a language reference only when its idioms, concurrency, or tooling details matter: [TypeScript](languages/typescript.md), [Rust](languages/rust.md), [Go](languages/go.md), or [Python](languages/python.md). For new TypeScript code and the full project template, Jess's stack is Effect, Effect Schema or Zod, xstate, and gdp ts. For new Rust servers it is tokio, axum, snafu, aerosol, and sqlx. In an existing repository, use its equivalents; adding any of these is an architecture change to ask about first. Use [database craft](../database craft/SKILL.md) for schemas, constraints, queries, and jobs, and [testing craft](../testing craft/SKILL.md) for the test rungs. Repository setup belongs to [project bootstrap](../project bootstrap/SKILL.md) when that setup is requested. Follow the project's required verification; reviews do not automatically require a full test suite. Adapted from leonardomso/rust skills (MIT), Matklad's Rust100k series, Alexis King's writing on parsing and type safety, and Matt Noonan's Ghosts of Departed Proofs.

code-craft