diagnose

name	diagnose
description	Disciplined diagnosis loop for hard bugs and performance regressions. Reproduce → minimise → hypothesise → instrument → fix → regression-test. Use when user says "diagnose this" / "debug this", reports a bug, says something is broken/throwing/failing, or describes a performance regression.

Diagnose 🔬

A discipline for hard bugs. Skip phases only when explicitly justified.

Phase 1 — Build a feedback loop

This is the skill. Everything else is mechanical. If you have a fast, deterministic, agent-runnable pass/fail signal for the bug, you will find the cause.

Spend disproportionate effort here. Be aggressive. Be creative. Refuse to give up.

Ways to construct one — try in roughly this order

1. Failing test at whatever seam reaches the bug — unit, integration, e2e.
2. Curl / HTTP script against a running dev server.
3. CLI invocation with a fixture input, diffing stdout against a known-good snapshot.
4. Headless browser script (Playwright / Puppeteer) — drives the UI, asserts on DOM/console/network.
5. Replay a captured trace. Save a real network request / payload / event log to disk; replay it through the code path in isolation.
6. Throwaway harness. Spin up a minimal subset of the system (one service, mocked deps) that exercises the bug code path with a single function call.
7. Property / fuzz loop. If the bug is "sometimes wrong output", run 1000 random inputs and look for the failure mode.
8. Bisection harness. If the bug appeared between two known states (commit, dataset, version), automate "boot at state X, check, repeat" so you can git bisect run it.
9. Differential loop. Run the same input through old-version vs new-version (or two configs) and diff outputs.
10. HITL bash script. Last resort. If a human must click, drive them with a structured script so the loop is still structured.

Iterate on the loop itself

• Can I make it faster? (Cache setup, skip unrelated init, narrow the test scope.)
• Can I make the signal sharper? (Assert on the specific symptom, not "didn't crash".)
• Can I make it more deterministic? (Pin time, seed RNG, isolate filesystem, freeze network.)

Non-deterministic bugs

Goal is not a clean repro but a higher reproduction rate. Loop the trigger 100×, parallelise, add stress, narrow timing windows, inject sleeps.

When you genuinely cannot build a loop

Stop and say so explicitly. List what you tried. Ask the user for:

• Access to whatever environment reproduces it
• A captured artifact (HAR file, log dump, core dump, screen recording with timestamps)
• Permission to add temporary production instrumentation

Do not proceed to Phase 2 until you have a loop you believe in.

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Phase 2 — Reproduce

Run the loop. Watch the bug appear.

Confirm:

• The loop produces the failure mode the user described
• The failure is reproducible across multiple runs
• You have captured the exact symptom (error message, wrong output, slow timing)

Do not proceed until you reproduce the bug.

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Phase 3 — Hypothesise

Generate 3–5 ranked hypotheses before testing any of them. Single-hypothesis generation anchors on the first plausible idea.

Each hypothesis must be falsifiable: state the prediction it makes.

Format: "If is the cause, then will make the bug disappear / will make it worse."

Show the ranked list to the user before testing. They often have domain knowledge that re-ranks instantly.

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Phase 4 — Instrument

Each probe must map to a specific prediction from Phase 3. Change one variable at a time.

Tool preference:

1. Debugger / REPL inspection if the env supports it
2. Targeted logs at the boundaries that distinguish hypotheses
3. Never "log everything and grep"

Tag every debug log with a unique prefix, e.g. [DEBUG-a4f2]. Cleanup at the end becomes a single grep.

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Phase 5 — Fix + regression test

Write the regression test before the fix — but only if there is a correct seam for it.

If a correct seam exists:

1. Turn the minimised repro into a failing test at that seam.
2. Watch it fail.
3. Apply the fix.
4. Watch it pass.
5. Re-run the Phase 1 feedback loop against the original scenario.

If no correct seam exists, that itself is the finding. Note it. The codebase architecture is preventing the bug from being locked down.

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Phase 6 — Cleanup + post-mortem

Required before declaring done:

• Original repro no longer reproduces
• Regression test passes (or absence of seam is documented)
• All [DEBUG-...] instrumentation removed
• Throwaway prototypes deleted
• The hypothesis that turned out correct is stated in the commit / PR message

Then ask: what would have prevented this bug?

If the answer involves architectural change (no good test seam, tangled callers, hidden coupling) hand off to the improve-codebase-architecture skill with the specifics.