debug

Do not fix a bug before you know its root cause. The search starts at the symptom, but it does not end there.

Fast path: when the error output shows the first cause directly, skip the hypothesis steps. An example is a stack line that points at the cause. Do not skip the reproduction, the regression test, or the verify skill in step 8. If the visible fix does not make the reproduction pass on the first try, the cause was not visible. Then do all the steps.

1. Before you reproduce the bug, create a task list with these steps as its items. If a task list already exists, add the steps to it. Each step is one item. When you skip a step, mark it done and add skip: <reason> to its text. The fast path skips the hypothesis steps this way. To go back to an earlier step, add that step and each step after it again as new items. Then delete the unfinished items that the new items replace.
2. Reproduce the bug first. Build the smallest reproduction that fails every time. If you cannot reproduce the bug, collect evidence until you can: logs, inputs, and versions. Do not guess.
3. Read the actual error. Find the first cause in the chain of causes, not the nearest symptom.
4. Make one hypothesis. Write it with the observation that would confirm it or refute it. Get that observation before you start a fix: a log line, an assertion, or a probe. If the evidence does not exist yet, add instrumentation. Keep the instrumentation small and easy to remove, and remove it when it has given its answer.
5. When two hypotheses compete, design one observation that shows which one is true. Do not try fixes one after another.
6. Fix the root cause, one fix at a time. Flag another problem that you find, such as a second bug, but do not fix it. Dead code is not another problem. Before the fix, delete the dead code in each file that the fix changes, as implement does. Do this also when the bug did not touch that code. Dead code is functions, imports, and parameters that no caller uses, and branches that no caller reaches. It is also stale flags, and validators that repeat a check that a boundary already did. It is also comments that repeat the code or excuse a workaround. Keep dead code that a non-goal puts out of scope. Read fix root causes before a fix that adds a guard, a catch, or a comment that defends it. Read it also when the same cause can occur elsewhere, or when the failure shows only after a restart.
7. Add the regression test that would have caught this bug. It fails on the old code and passes with the fix.
8. Before you report the fix, read the verify skill and follow it. Your report claims that the bug is fixed, and verify proves that claim. When the bug appeared inside a slice, the full gate waits for the slice's commit.

Rules: