Debug

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Debug to root cause

The Iron Law

No fix without a root-cause investigation first. A patch applied to a symptom you don't understand usually moves the bug rather than removing it.

The feedback loop IS the skill

Everything else is mechanical. Your first job is a fast, deterministic pass/fail signal for the bug. Build the best one you can afford, preferring higher rungs:

1. A failing automated test that reproduces it (best — becomes your regression test)
2. A one-shot command: curl, a CLI invocation, a script with a snapshot diff
3. A tiny harness that exercises the path
4. Manual reproduction with logged output (last resort)

A 2-second deterministic loop is a debugging superpower; invest in it before theorizing. If you can't reproduce it, that's the bug to solve first. (Prefer the earliest rung you can manage — 1 is best.)

Hypotheses: plural and falsifiable

Don't anchor on the first idea. Write 3-5 ranked hypotheses, each with a prediction you can test ("if it's a caching issue, then clearing the cache fixes it; if it's a race, then adding a delay changes the outcome"). Instrument to discriminate between them, not just to confirm a favorite.

Tag every diagnostic probe you add with a marker like [DEBUG-a4f2] so cleanup at the end is a single grep — never leave instrumentation behind.

The 3-fixes rule

If you've tried 3 fixes and the problem keeps reappearing somewhere new, STOP. Do not attempt fix #4. Cascading symptom-fixes mean your mental model of the architecture is wrong — discard the current hypotheses and re-derive them against the structure, not the symptom. (Your human's tells — "stop guessing", "ultrathink this", "why does this keep happening" — are the same signal: restart at root cause.)

Name the cause before you fix

Before writing any fix, state it in one sentence: this symptom is caused by X, via mechanism Y. If you can't, you haven't found the root cause — keep instrumenting. The fix must target X, not the place the symptom surfaced.

Performance and flaky bugs use a different loop

• Performance regressions: measure, don't theorize. Capture a quantified baseline and a target delta, profile to find the real hot path, and use git bisect to find the commit that introduced it. "Feels slow" is not a signal; a number before/after is.
• Flaky / non-deterministic bugs: there's no 2-second deterministic signal yet, so manufacture one — run the case N times and measure the failure rate, then force it to reproduce reliably (loop the test, inject delays, fix the seed, enable a race detector) before fixing. A fix you can't show drives the failure rate to 0 over N runs is not verified.

Close the loop

Once you've found and fixed the root cause, the reproducing test becomes a permanent regression test (hand to forge:tdd if it isn't already a test). Remove all [DEBUG-] probes. Then forge:verify with the now-green test as evidence.