TDD

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Wrote implementation before the test? Delete it. Not "keep it as reference", not "adapt it while the tests catch up" — you will end up testing what you built instead of what was required. Delete means delete; implement again from the test. Violating the letter of this law is violating the spirit of it.

Exceptions exist only with the user's explicit permission: throwaway prototypes, generated code, pure configuration. "Skip it just this once" is not an exception; it is the rationalization this skill exists to block.

Where tests go: pre-agreed seams

A seam is a public surface where behavior is observable without reaching into internals. Tests live only at seams listed in the feature's design.md under Seams for testing. Refuse to write a test at a seam that table does not confirm. No design.md for this work (tier 0/1 change, legacy area)? Propose the seam(s) to the user and get agreement before the first test. Testing effort is a budget — agreed seams put it on critical paths instead of on every internal detail.

The loop

Work one vertical slice at a time: one behavior → one test → one minimal implementation → repeat. Each cycle responds to what the previous one taught you. Never write all the tests up front and then all the code — bulk-written tests verify imagined behavior.