Diagnose Bug

Diagnose Bug

Use this skill when something is broken, failing, flaky, slow, or otherwise not understood.

Native Integration

Use native debug skills, subagents, terminal output, browser tools, traces, and logs when they exist. This skill defines the evidence standard: reproduce the real symptom, minimize it, test ranked hypotheses, instrument deliberately, and lock down the fix.

Workflow

1. Build a feedback loop.

   • Prefer a failing test.
   • Otherwise use a CLI command, HTTP script, browser script, captured trace, replay fixture, or minimal harness.
   • For flaky bugs, raise the reproduction rate with repetition, stress, seeds, or narrowed timing.

2. Reproduce the reported symptom.

   • Confirm the loop shows the same failure the user described.
   • Capture the exact error, wrong output, timing, or visible symptom.
   • Do not fix a nearby but different issue.

3. Minimize.

   • Reduce the input, scenario, or code path until the signal is as small and fast as practical.
   • Keep the minimized repro connected to the real failure mode.

4. Hypothesize.

   • Write three to five ranked, falsifiable hypotheses.
   • Each hypothesis must predict what evidence would confirm or reject it.

5. Instrument deliberately.

   • Change one variable at a time.
   • Prefer debugger or targeted probes over broad logging.
   • Tag temporary logs or probes so they are easy to remove.

6. Fix and lock down.

   • Convert the repro into a regression test when a correct seam exists.
   • Apply the smallest correct fix.
   • Re-run the minimized repro, the original repro, and relevant broader checks.

7. Clean up.

   • Remove temporary instrumentation.
   • Delete throwaway harnesses unless they became useful tests.
   • State the root cause in the final response or PR.

Done When

• The original symptom no longer reproduces.
• A regression check exists or the lack of a correct seam is documented.
• Temporary debugging artifacts are removed.
• The root cause and verification evidence are clear.