| name | diagnosing-bugs |
| description | Diagnose hard bugs and performance regressions: build a red-capable feedback loop, reproduce, hypothesise, instrument, and fix at the root cause with a regression test. Use when debugging, hunting a root cause, asking 'why is this failing', or chasing flaky behavior. Do NOT use for post-incident reviews (incident-postmortem). |
| type | component |
| domain | 3-delivery |
Diagnosing Bugs
A discipline for hard bugs. Skip phases only when explicitly justified.
When exploring the codebase, read the project's agent/context docs (AGENTS.md / CLAUDE.md / CONTEXT.md, if present) to get a clear mental model of the relevant modules, and check ADRs in the area you're touching.
Outputs
Artifact: a root-cause diagnosis with evidence — the red-capable repro command and its output, the confirmed hypothesis, the fix, and a regression test (or a documented finding that no correct test seam exists)
Format: code fix + test in the target repo; diagnosis stated in the commit/PR message
Location: target repo (regression test at the correct seam; instrumentation removed)
Audience: reviewers and the next person to debug this area
The iron rule — root cause before fix
Never patch a symptom. A fix may only be written once you can state the root cause and the evidence for it (the confirmed hypothesis from Phase 4). If a change makes the loop go green but you cannot explain why the bug happened, you have hidden the bug, not fixed it — revert the change and keep diagnosing. Fix at the source, not at the layer where the symptom surfaces. This holds especially when the issue "seems simple" or you're under time pressure — simple bugs have root causes too, and systematic is faster than thrashing.
Phase 1 — Build a feedback loop
This is the skill. Everything else is mechanical. If you have a tight pass/fail signal for the bug — one that goes red on this bug — you will find the cause; bisection, hypothesis-testing, and instrumentation all just consume it. If you don't have one, no amount of staring at code will save you.
Spend disproportionate effort here. Be aggressive. Be creative. Refuse to give up.
Ways to construct one — try them in roughly this order
- Failing test at whatever seam reaches the bug — unit, integration, e2e.
- Curl / HTTP script against a running dev server.
- CLI invocation with a fixture input, diffing stdout against a known-good snapshot.
- Headless browser script (Playwright / Puppeteer) — drives the UI, asserts on DOM/console/network.
- Replay a captured trace. Save a real network request / payload / event log to disk; replay it through the code path in isolation.
- 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.
- Property / fuzz loop. If the bug is "sometimes wrong output", run 1000 random inputs and look for the failure mode.
- 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.
- Differential loop. Run the same input through old-version vs new-version (or two configs) and diff outputs.
- HITL bash script. Last resort. If a human must click, drive them with
scripts/hitl-loop.template.sh so the loop is still structured. Captured output feeds back to you.
Build the right feedback loop, and the bug is 90% fixed.
Tighten the loop
Treat the loop as a product. Once you have a loop, tighten it:
- 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.)
A 30-second flaky loop is barely better than no loop; a 2-second deterministic one is tight — a debugging superpower.
Non-deterministic bugs
The goal is not a clean repro but a higher reproduction rate. Loop the trigger 100×, parallelise, add stress, narrow timing windows, inject sleeps. A 50%-flake bug is debuggable; 1% is not — keep raising the rate until it's debuggable.
When you genuinely cannot build a loop
Stop and say so explicitly. List what you tried. Ask the user for: (a) access to whatever environment reproduces it, (b) a captured artifact (HAR file, log dump, core dump, screen recording with timestamps), or (c) permission to add temporary production instrumentation. Do not proceed to hypothesise without a loop.
Completion criterion — a tight loop that goes red
Phase 1 is done when the loop is tight and red-capable: you can name one command — a script path, a test invocation, a curl — that you have already run at least once (paste the invocation and its output), and that is:
If you catch yourself reading code to build a theory before this command exists, stop — jumping straight to a hypothesis is the exact failure this skill prevents. No red-capable command, no Phase 2.
Phase 2 — Reproduce + minimise
Run the loop. Watch it go red — the bug appears.
Confirm:
Minimise
Once it's red, shrink the repro to the smallest scenario that still goes red. Cut inputs, callers, config, data, and steps one at a time, re-running the loop after each cut — keep only what's load-bearing for the failure.
Why bother: a minimal repro shrinks the hypothesis space in Phase 3 (fewer moving parts left to suspect) and becomes the clean regression test in Phase 5.
Done when every remaining element is load-bearing — removing any one of them makes the loop go green.
Do not proceed until you have reproduced and minimised.
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."
If you cannot state the prediction, the hypothesis is a vibe — discard or sharpen it.
Show the ranked list to the user before testing. They often have domain knowledge that re-ranks instantly ("we just deployed a change to #3"), or know hypotheses they've already ruled out. Cheap checkpoint, big time saver. Don't block on it — proceed with your ranking if the user is AFK.
Phase 4 — Instrument
Each probe must map to a specific prediction from Phase 3. Change one variable at a time.
Tool preference:
- Debugger / REPL inspection if the env supports it. One breakpoint beats ten logs.
- Targeted logs at the boundaries that distinguish hypotheses.
- 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. Untagged logs survive; tagged logs die.
Perf branch. For performance regressions, logs are usually wrong. Instead: establish a baseline measurement (timing harness, performance.now(), profiler, query plan), then bisect. Measure first, fix second.
Phase 5 — Fix + regression test
The iron rule applies here: you may only write the fix once the root cause is confirmed by evidence — a fix that addresses the symptom rather than the cause is a failure, even if the loop goes green.
Write the regression test before the fix — but only if there is a correct seam for it.
A correct seam is one where the test exercises the real bug pattern as it occurs at the call site. If the only available seam is too shallow (single-caller test when the bug needs multiple callers, unit test that can't replicate the chain that triggered the bug), a regression test there gives false confidence.
If no correct seam exists, that itself is the finding. Note it. The codebase architecture is preventing the bug from being locked down. Flag this for the next phase.
If a correct seam exists:
- Turn the minimised repro into a failing test at that seam.
- Watch it fail.
- Apply the fix — at the root cause, not the symptom.
- Watch it pass.
- Re-run the Phase 1 feedback loop against the original (un-minimised) scenario.
Phase 6 — Cleanup + post-mortem
Required before declaring done:
Then ask: what would have prevented this bug? If the answer involves architectural change (no good test seam, tangled callers, hidden coupling), recommend an architecture-improvement follow-up with the specifics — and record the decision via adr if one is taken. Make the recommendation after the fix is in, not before — you have more information now than when you started.
