---
description: Verify each behavior before building the next. One test, one implementation, one commit. No bulk-writing tests for behavior that does not yet exist.
globs:
alwaysApply: true
---

# Incremental Verification

## Principle

Behavior you have not written cannot be tested honestly. Writing many tests
before any implementation produces tests shaped by **imagined** behavior,
not actual behavior — they assert on function signatures and data shapes,
survive real regressions silently, and break on harmless refactors.

## Rule

For every new behavior: write one failing assertion, write the minimum
implementation that makes it pass, commit, repeat. No batched test
authoring. No scaffolding dozens of test cases before a single green run.

## Checklist

- [ ] Is there exactly one failing test right now? If more, split the commit.
- [ ] Does the test exercise a public interface a caller would actually hit?
- [ ] Would the test still make sense after a full internal refactor?
- [ ] Was the implementation written **after** seeing the test fail?

## Failure modes to catch

- "Shape tests": asserting on object keys, array length, or types rather
  than observable output. Rewrite against a caller's perspective.
- "Mock-driven tests": the test mostly configures mocks then asserts the
  mock was called. Move the mock to a system boundary or delete it.
- "Green before red": if every test you wrote passed on the first run,
  the test did not drive the design — treat it as suspect.

## When to break the rule

Exploratory spikes where you are probing an unknown API. Mark the spike
branch as throwaway and do not merge its tests.