TDD

Logic changes here — bug fixes, features, refactors — follow t-wada style Red-Green-Refactor. For unfamiliar APIs, prototypes, and data exploration, execute-inspect-adjust first; not every probe needs a test.

The testing skill owns the ccusage-specific side: fixtures, adapter coverage, pricing, model names, snapshots, and CLI output.

Cycle

1. Sketch the behaviors as placeholders — it.todo(...) in Node test, #[ignore] in Rust — then take them one at a time, simplest first. Bug fixes start with a regression test that reproduces the bug.
2. Red — write the failing test and confirm it fails for the expected reason.
3. Green — write the minimum production code that passes. Fake it, make it real, then triangulate with more tests.
4. Refactor — clean up test and production code while everything stays green. Restructure only while green; when a test is red, fix the production code first.
5. Run the affected tests after each green and each refactor step. Full-suite runs are for final verification and CI.

Keep each test on one observable behavior, named after that behavior and asserted through a public interface; never weaken a valid test to get a green build. A test that asserts a document contains certain wording freezes text rather than proving a contract.

Three failure modes worth naming:

• Horizontal slicing. One seam, one test, one minimal implementation, then the next — not every test first and the implementation afterwards.
• Tautological expectations. The expected value comes from an independent source, not from rerunning the production algorithm inside the test.
• Mocking inside the system. Substitute only real boundaries — the pricing fetch and the log directories — through the seams the code already has: --offline, CLAUDE_CONFIG_DIR and its siblings, and ccusage-test-support. Collaborators the workspace owns are exercised through their public interface.

Focused runs

Rust tests live in the rust/Cargo.toml workspace. Prefix with direnv exec . when cargo is not already on PATH.

direnv exec . cargo test --manifest-path rust/Cargo.toml --workspace <name-filter>
direnv exec . cargo test --manifest-path rust/Cargo.toml --workspace -- --ignored

node --test --test-name-pattern '<name-filter>' apps/ccusage/src/cli.test.ts

just test runs both suites, just rust::test the Rust one, just test-node the Node one.