nexus-testing

name	nexus-testing
description	Use for flaky or nondeterministic tests, especially local-vs-CI inconsistencies. Trigger on intermittent failures, race/timing symptoms, retry-only greens, or unexplained skip/xfail markers. Output should isolate reproducible cause, narrowest durable fix, and prevention guidance. When in doubt, use this skill.

Nexus Flaky Test Root Cause Analyzer

Structured protocol for diagnosing, isolating, and permanently eliminating non-deterministic test failures. Not a retry wrapper — treats flakiness as a first-class defect.

name:             nexus-testing
category:         testing / quality
required_context: test file path, failure frequency, CI log or local output, stack trace if available
expected_inputs:  test name, framework, failure pattern, environment (local/CI/both), reproduction rate
expected_outputs: flakiness type, reproduction steps, root cause (one sentence), narrowest fix +
                  verification command, prevention recommendation

Core Principles

1. A test failing 1-in-20 runs is still failing — it is a false alarm factory.
2. A retry without investigation ships unknown risk.
3. Fix the root cause, not the symptom — masking a race condition makes it fail more often.
4. Never mark xfail/skip without a linked issue and expiry date.
5. Test flakiness often mirrors a real production concurrency or data integrity hazard.

---

Step 1 — Collect Context

Required before any investigation:

Signal	How
Failure rate	Estimate from CI history (1/5? 1/100?)
Stack trace	Full verbatim trace from a failing run
Test path	Exact: tests/users/test_create.py::test_create_user
Framework + version	pytest --version, jest --version, go version
CI vs local	Fails only in CI, only locally, or both?
Parallelism config	-n auto, --workers, t.Parallel()?
Recent changes	git log --oneline -10 on test file and its imports

If CI-only failure, diff these before reading code: runtime version, OS/arch, parallelism, TZ / NTP, network access, env vars (missing vars silently produce defaults), I/O speed, Docker layer cache.

---

Step 2 — Classify Flakiness Type

Type	Signature	Mechanism
Ordering	Fails after specific other tests; passes alone	Shared mutable state not reset
Timing	Fails on slow machines or under load	Hard-coded delays, no backoff, wall-clock assertions
Resource	Fails parallel, passes single-threaded	Port conflicts, shared temp dirs, DB row locks
Environment	Fails in CI only	Runtime delta, missing env var, OS behavior
External	Network errors, connection refused, timeouts	Real HTTP/DB calls in test
Concurrency	Assertion error varies each run; stack trace differs	Thread/async race, missing lock

Heuristic:

• Fails when run alone → Timing / Resource / External / Concurrency
• Passes when run alone → Ordering (shared state from another test)
• Fails only in CI → Environment delta
• Failure message varies → Concurrency or External
• Fails more under parallel load → Resource contention or Concurrency

Record: Primary: <type> | Secondary: <type or none>

---

Step 3 — Reproduce

Run at minimum 5 times before investigating. Confirm flaky, not consistently failing.

pytest tests/path/to/test.py::test_fn -xvs                          # alone, verbose
pytest tests/ --randomly-seed=last -x                                # ordering check
pytest tests/ -n auto -x                                             # contention check
for i in {1..20}; do pytest tests/path/to/test.py::test_fn -x --tb=no -q; done | grep -c FAILED

go test ./pkg/... -run TestFn -count=20 -v 2>&1 | grep -E "PASS|FAIL"  # Go
jest --testNamePattern "name" --runInBand --verbose                       # Jest

Record: X/N failed, fails alone or suite-only, rate under parallelism, verbatim stack trace.

---

Step 4 — Identify Root Cause

Ordering: find unreset shared state — grep -n "global\|@classmethod\|setUp\|tearDown\|autouse"; look for DB rows not rolled back, in-memory caches, module-level singletons, scope="session" fixtures.

Timing: grep -rn "time.sleep\|asyncio.sleep\|setTimeout" and polling loops without timeout; look for fixed delays insufficient on slow machines, wall-clock assertions.

Resource: grep -rn "port.*=[0-9]\{4,5\}\|localhost:[0-9]\{4,5\}" and grep -rn "tmp\|tempfile"; look for hardcoded ports, shared temp dirs, DB connections without rollback.

Environment: cat .github/workflows/*.yml | grep -E "python-version|node-version|go-version"; compare env vars between local and CI.

External: grep -rn "requests\.\|httpx\.\|fetch(\|http.Get" and real DB connection strings in tests.

Concurrency: grep -rn "threading\.\|asyncio\.\|goroutine" and missing Lock/Mutex/await.

State root cause as exactly one sentence:

"test_X fails intermittently because Y when Z."

If you cannot state it in one sentence, root cause is not yet identified.

---

Step 5 — Fix

Apply the narrowest fix. Do not improve unrelated code.

Type	Wrong fix	Correct fix
Ordering	Delete the interfering test	Add teardown resetting shared state after each test
Timing	Increase time.sleep(N)	Replace with wait_for(condition, timeout=N)
Resource	Manually assign different port	Use port=0 or a free_port() fixture
Environment	Hard-code CI env locally	Parametrize via env var; test both modes in CI
External	Add retry in test	Mock external call; test real integration separately
Concurrency	Add sleep before assertion	Use lock, event, barrier, or join

Verification — required consecutive passes by failure rate:

Rate	Required passes
>20%	20 consecutive passes
5–20%	30–50 consecutive passes
1–5%	50 consecutive passes
<1%	100 passes or 30-day CI monitoring

for i in {1..20}; do pytest tests/path/to/test.py::test_fn -x --tb=short -q; done

---

Step 6 — Prevention

• Ordering: add pytest --randomly-seed=random to every PR; fixture teardown in PR template.
• Timing: add wait_for(condition, timeout, poll_interval) helper; flag new time.sleep in CI.
• Resource: add free_port() and tmp_dir() fixtures to conftest.py.
• External: enforce --block-network in unit tests; separate @pytest.mark.integration.
• Concurrency: shared AsyncEventLoop fixture; asyncio_mode = "auto" in pytest.ini.

---

Output Contract

Every investigation closes with this report. Fill every field.

## Flaky Test Report

**Test:**             [full path + test name]
**Framework:**        [pytest / jest / go test / other]
**Failure Rate:**     [X/N runs]
**Primary Type:**     [Ordering | Timing | Resource | Environment | External | Concurrency]
**Secondary Type:**   [same options, or "none"]
**Root Cause:**       [one sentence — "test_X fails because Y when Z"]
**Why It Happens:**   [2–3 sentences — mechanism]
**Fix Applied:**      [file path + line numbers + change description]
**Verification:**     [command + result — "20/20 passes"]
**Prevention:**       [guardrail that stops this class from returning]
**Follow-up Needed:** [yes/no — if yes, describe]

---

Anti-Patterns (Quick Reference)

• Do not add time.sleep() — masks the race, breaks on slower machines.
• Do not mark xfail/skip without a linked issue and target fix date.
• Do not retry in CI without investigating — retries hide failures.
• Do not close without a reproduction command in the ticket.
• Do not declare fixed after < required consecutive passes.

---

Sub-documents

Document	Purpose
checklists/investigation-checklist.md	Pre/during/post checklists for structured investigation
anti-patterns/common-mistakes.md	Common wrong fixes and why they fail
validation/output-validation.md	Fix confirmation, confidence scoring, escalation