## TL;DR
The test only fails on main because PRs never run it: it is a merge-queue-only regression, not flakiness. Confirm the coverage gap (which suites run where), bisect the merged commits since it last passed on main, and fix the regression. Labeling it flaky hides a real break.

## The symptom
```text
agent labeled a test flaky that only fails on main  -  PRs never run it, so the flake is a merge-queue-only regression
```

## Steps
### 1. Confirm the coverage gap
Check which suites run on PRs vs main. The test runs only on main; PRs never execute it.
Expected: The gap documented: main-only test.

### 2. Bisect the merge range
Find the last green main run and bisect the merged commits since. The culprit is a merged change, not randomness.
Expected: The breaking commit identified.

### 3. Fix the regression
Address the actual break the commit introduced for this test's scenario.
Expected: Test green on main.

### 4. Close the coverage gap
Either run the test on PRs too, or accept main-only status explicitly with a fast main-failure alert. Do not leave it in the gap unlabeled.
Expected: The test is covered where it runs.

## When to use
- Test fails on main but PRs are green
- The test does not run on PRs
- Someone labeled it flaky without a bisect

## When not to use
- The test fails on PRs too (then investigate normally)
- The main failure is genuinely intermittent (bisect first to be sure)
- Main is green and the label is historical (re-evaluate)

## Why it happens
If PRs never run a test, regressions in its scenario merge silently and only surface on main. 'Fails on main, passes on PRs' looks like flakiness if you squint, but the pattern is deterministic: every main run fails. It is a regression with a coverage gap, not a flake.

## Edge cases
- Merge-queue batching: bisect the batch, not just the branch.
- Main-only infrastructure differences: rule those out during the bisect.
- Scheduled main runs: use their history as the bisect bounds.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_shs1l8xyNw6x8rhOywGgMQ