ai-debug — find the cause, not the symptom

What it produces

A named cause at file:line, a check that fails because of it, and only then a fix.

Steps

1. Reproduce it. If you cannot reproduce it, say so plainly and stop guessing: the next useful thing is a way to reproduce it, not a change.
2. Read the failing output in full. The first error is usually the real one and the rest are its consequences; the last error is the one people paste.
3. Name a cause you can point at. file:line, and one sentence on why that line produces this symptom. "Probably a race" is not a cause. If two causes are plausible, say which observation would tell them apart, then go and make that observation.
4. Before the fix, write the check that fails for this reason. A fix with no failing check before it is a change with an opinion attached.
5. Fix the cause, at the place all the callers go through. Patching the one path the report named leaves every sibling caller broken, and the shared fix is usually the smaller diff.
6. Run the check. Then run the suite. Then say what you changed and why it fixes the cause you named, not the symptom that was reported.
7. If you are two attempts in and it is still not fixed, stop and say so. That is a rule, not a suggestion: the third attempt is where the guessing starts.

Conflicts

Read both sides for intent before touching either. Lock files and generated files are regenerated, never merged by hand. Migrations are ordered, not combined. If two people meant different things, that is a conversation, not a resolution.

What this is not

• "I know what the bug is even though I cannot reproduce it" — a cause you cannot reproduce is a guess: the next useful thing is a way to reproduce it, not a change.

Done when

• The cause is named at file:line and a person could disagree with it.
• A check exists that fails without the fix and passes with it.
• You said what you changed, in a sentence somebody could act on.

The ai-engineering seam

1. The failing check you write in step 4 is a gate candidate: if the milestone's .ai-engineering/spec.html exists, add it there (G<n>: <what holds> + CHECK:) so ai-eng spec run proves the fix forever, not just today.
2. Long-running reproduction (a dev server, a watcher, a REPL) belongs to a supervised process, never a fire-and-forget shell call: name it, drive it over stdin, and stop it when the diagnosis ends.
3. Grounding duty (§11.6): every file:line you name was opened in this session. Never cite from memory — /ai-explore and /ai-read-docs are the lenses for that.
4. The verdict on WHY it broke belongs to the decide tier of the model pin (.ai-engineering/config.toml); mechanical checks (does it fail, does it pass) belong to verify.

Routing

In scope: broken behaviour with a reproducible symptom, failing CI, merge/rebase conflicts. Not for: no diagnosis yet but the fault must reach outsiders (/ai-issue-report), the fix needs a design (/ai-plan), the fix needs proof it satisfies the ask (/ai-verify), a recurring pitfall worth remembering (/ai-note).

Lifecycle

Lane: any Writes: nothing Read by: n/a Dies: n/a — the check it writes becomes a gate in spec.html and outlives it Next: none

Source: ai-engineering v1 (own), Apache-2.0.