Flaky Test Diagnoser

Runs competing-hypothesis experiments to isolate the true root cause of a flaky test. Produces a diagnosis with reproducible evidence — or an honest inconclusive label. Never promotes a guess to "root cause."

This is a tracing skill, not a fixer. The goal is to explain why the test is intermittent with falsifiable evidence, not to jump into patching code.

Execution contracts

Subagent watchdog: any time this skill spawns a test-running subagent via run_in_background=true (multi-run protocols, ordering bisection, distinguishing experiments), it MUST be armed with a staleness monitor per _shared/subagent-watchdog.md. Use Flavor A (Monitor tail per spawn) with thresholds STALE=15 min, HUNG=45 min — test suites can legitimately take a long time, especially with N≥10 reruns, but a 45-min silent window is pathological. TaskOutput status reports PID liveness only; output-file mtime is the ground truth for progress. Contract inheritance: timed_out_heartbeat joins this skill's termination vocabulary as a peer of blocked_by_environment (watchdog kill is a different failure class — the test-runner subagent hung, not the test environment). A watchdog-killed experiment contributes zero runs to the N≥10 count; the skill must retry or escalate.

Workflow