Flaky Test Detector (.NET)

---
name: flaky-test-detector
description: "Detect flaky tests by scanning recent AzDo CI builds for test failures recurring across multiple unrelated PRs. Use when investigating intermittent failures, CI instability, deciding which tests to quarantine, or checking if RunTestCasesInSequence no-ops are causing parallel-safety issues."
metadata:
  origin: .NET
  author: fsharp-team
  version: "1.0"
---

# Flaky Test Detector

Identifies tests that fail intermittently across unrelated PRs — a strong signal of flakiness rather than a genuine regression. Also cross-references with existing fix PRs.

## When to Use

- Investigating CI instability ("is this test failure my fault or flaky?")
- Periodic hygiene: finding tests to quarantine or fix
- Before marking a test as `Skip = "Flaky"` — confirm it actually is flaky
- Checking if `RunTestCasesInSequence` (a no-op in xUnit 2) is masking parallelism bugs

## How It Works

1. Queries Azure DevOps builds API directly for recent failed fsharp-ci PR builds
2. Extracts test failures from each build via `Get-BuildErrors.ps1`
3. Aggregates by test name across distinct PRs
4. Cross-references with GitHub PRs that may address the flaky tests
5. Tests failing in **3+ distinct PRs** are flagged as flaky

## Usage

### Quick scan (last 14 days, 50 builds, threshold = 3)

```bash
pwsh .github/skills/flaky-test-detector/scripts/Get-FlakyTests.ps1
```

### Custom parameters

```bash
# More aggressive: 2+ PRs over 7 days
pwsh .github/skills/flaky-test-detector/scripts/Get-FlakyTests.ps1 -MinPRFailures 2 -DaysBack 7

# Wider net: 100 builds over 30 days  
pwsh .github/skills/flaky-test-detector/scripts/Get-FlakyTests.ps1 -MaxBuilds 100 -DaysBack 30
```

### Parameters

| Parameter | Default | Description |
|-----------|---------|-------------|
| `-MaxBuilds` | 50 | Maximum number of failed builds to scan from AzDo |
| `-MinPRFailures` | 3 | Min distinct PRs a test must fail in to be flagged |
| `-DaysBack` | 14 | Only consider builds within this time window |
| `-DefinitionId` | 90 | AzDo pipeline definition ID (90 = fsharp-ci) |
| `-Org` | dnceng-public | Azure DevOps organization |
| `-Project` | public | Azure DevOps project |

## Output

The script produces:
1. **Console report** with ranked flaky tests, PR numbers, job names, and sample errors
2. **Structured objects** (PowerShell) for programmatic consumption

## Interpreting Results

- **DistinctPRs ≥ 5**: Almost certainly flaky — consider quarantining immediately
- **DistinctPRs = 3–4**: Likely flaky — investigate root cause
- **DistinctPRs = 2**: Possibly flaky or a shared dependency issue — monitor

## Follow-up Actions

After identifying a flaky test:
1. Check if there's already a GitHub issue for it
2. If not, file one with the `Area-flaky-test` label
3. Consider marking with `[<Fact(Skip = "Flaky: #ISSUE")>]` if it blocks CI
4. Fix the root cause (timing, file locking, thread safety, etc.)