## TL;DR

An agent that runs the full suite for every change burns time and money and teaches everyone to ignore test results. Scope it: map the changed files to tests with the runner's own selection (`--changedSince`, affected graphs, or a file-to-test map), run that subset for fast feedback, and reserve the full suite for nightly runs or pre-merge. Shared-code changes widen by dependency graph, not to everything.

## Error

```text
[agent log] change: 1 file (src/utils/format.ts)
[agent log] running full suite: 4,200 tests, est. 38 min
The change touches one pure function with 12 direct tests.
```

## Steps

1. List what changed: `git diff --name-only origin/main...HEAD`. Expected: a short file list, usually a handful of paths.
2. Map changed files to tests: `jest --changedSince=origin/main`, `nx affected --target=test`, or a checked-in file-to-test map. Expected: a test subset, typically orders of magnitude smaller than the suite.
3. Run the subset first and report the result. Expected: feedback in minutes, not tens of minutes.
4. If the change touches shared code, widen the subset by the dependency graph (direct dependents), not to the full suite. Expected: shared-module changes run their dependents only.
5. Set a budget: if the affected set exceeds N tests or M minutes, the agent states the scope and asks before going wider. Expected: no surprise 40-minute runs.

## When to use

- The agent runs the full suite for small, scoped changes.
- Test feedback takes so long the agent (or developer) stops waiting for it.
- CI minutes or compute cost spike after agent-driven development starts.

## When not to use

- Release or pre-merge validation where the full suite is the requirement.
- The change touches test infrastructure itself (conftest, setup files, global mocks); scope is genuinely unclear there.

## Tool compatibility

- Any coding agent (Claude Code, Cursor, etc.) running Jest, Vitest, pytest, Go test, or Nx/Turborepo affected commands.
- Needs git history for --changedSince style selection.

## Variant phrasings

### agent runs 4000 tests for a one-line change

The headline symptom. Changed-file selection cuts it to the dozen that matter.

### test feedback takes 40 minutes

Nobody waits 40 minutes. The subset runs in minutes; the full suite moves to nightly.

### CI costs spiked after agents started committing

Full-suite-per-change multiplied by agent velocity. Scoping is the cost control.

## Why it happens

"Run the tests" is ambiguous and agents resolve ambiguity toward safety: the full suite cannot miss anything. Nothing in the loop prices the 38 minutes, so the agent never learns that a scoped run plus a nightly full run catches the same regressions cheaper.

## Edge cases

- Test selection can miss integration effects; that is what the nightly full run is for, not a reason to scope nothing.
- New test files with no history are invisible to --changedSince; always include untracked and new test files in the subset.
- Flaky tests in the subset will still flake; scoping does not fix flakiness, it just makes the signal cheaper to get.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_SvgxHuqzk_XrAd67eYDfsw