write-tests

name	write-tests
description	Write or revise Marin tests with an emphasis on behavior, regression coverage, pytest style, and avoiding "slop tests." Use when adding tests, fixing failing tests, reviewing test quality, or deciding what test would catch a bug.

Write Tests

Use this skill when a change needs tests or when existing tests look too coupled to implementation details.

Read root TESTING.md before writing or reviewing non-trivial tests. It is the shared project-wide testing policy used by write-tests, review-pr, and commit.

Also read the relevant module-specific docs before choosing test style, fixtures, mocks, markers, or commands:

• root AGENTS.md
• root TESTING.md
• the nearest subproject AGENTS.md for files under lib/*
• package testing docs referenced from that AGENTS.md, such as lib/iris/TESTING.md

Workflow

1. Find existing tests for the touched behavior before creating a new file.
2. Check module-specific testing rules for commands, markers, fakes, mocks, and numerical tolerances.
3. Name the behavior that should fail if the code is wrong.
4. Write the smallest test that observes that behavior through a public API, structured output, persisted state, or real side effect.
5. Prefer a regression test before the fix when fixing a bug. Ensure the test fails before implementing the bug fix.
6. Keep test setup realistic but small. Use fixtures and parameterization to remove duplication.
7. Run the narrow test first, then the relevant package test command. Before a PR, run the repo lint entry point required by AGENTS.md.

Default Commands

• Run a narrow test path while editing, then run uv run --no-project infra/ci/run_tests.py. Do not override the repository's default marker expression.
• For package-specific commands, use the relevant lib/*/AGENTS.md or testing doc.

For PR preparation, run ./infra/pre-commit.py --changed-files --fix or ./infra/pre-commit.py --all-files --fix as appropriate. Do not replace it with uv run pre-commit.