/systematic-debugging

name: systematic-debugging
description: "Use when encountering a bug, test failure, or unexpected behavior, before proposing fixes"

Execute

Bug, failure, or unexpected behavior:

1. **Isolate** — read error, reproduce, inspect the diff, and drill upward through diagnostic layers: L1 symptom → L2 logic → L3 system → L4 architecture → L5 cross-system contract → L6 platform → L7 spec gap. Layers are observation altitudes, not one causal chain; the causal shape at the stop altitude is classified explicitly before any root claim. Stop only when causal proof accounts for the recurrence generator or reaches a T-class boundary. 2. **Identify owner** — compare working behavior, trace the bad value, locate the canonical owner, and treat duplicate owners as a finding. 3. **Decide before editing** — Before fixing, run Patch-Shape Triage and Ripple Signal Triage when shared logic, contracts, fallbacks, adapters, producer/consumer seams, or source-of-truth boundaries are involved. Surface Change Necessity for any new source-code path or non-trivial source edit. Run Minimality Check for a new branch, fallback, adapter, owner, or compatibility path, and Pre-Edit Complexity Check for an overloaded owner or complexity growth. After Change Necessity selects `code-change` and before the first repair edit, own the TDD Route for the repair slice per `test-driven-development` (`off` default; `strict` on behavior/bugfix/shared/contract/persistence/permission/migration risk). 4. **Prove** — test one hypothesis with the smallest reproduction or verification. A failing test first is required only by a recorded `TDD Route: strict`; with `TDD Mode: off`, do not require a failing test or RED/GREEN cycle. Three failed fixes means stop and question architecture. 5. **Repair and close** — fix minimally at the canonical owner, verify in proportion to risk, review architecture, and close both repair and retirement tracks. If a symptom remains or shifts, reopen the causal model; do not assume it is a separate defect.

Done: confidence ≥ B, causal status matches recurrence evidence or an external terminal, tracks explicit, no H signal, and required D evidence passes.

Core invariant

Find root cause and fix the bug class at its canonical owner. A minimal fix is not the smallest textual diff; it is the smallest sufficient owner-level repair. Apply the same causal-sufficiency standard to every bug. Trace relevant influences and justify exclusions; resolve repair-changing unknowns before retaining a solution. Experiments may precede closure. Evidence gaps, not apparent size or risk, determine investigation depth.

Quick bug lane

For an evidenced local, reproducible, single-owner bug with no patch-shape signal, keep the readback compact: `Symptom`, `Reproduction`, `Root Cause`, `Change Necessity`, `Fix Boundary`, and `Verification`. Skip the causal card only when the causal-proof owner's `Quick Exit Proof` passes. This reduces readback, not the causal coverage or evidence standard. Quick bug lane must surface Change Necessity before source edits. One sentence may cover the user-visible need, no-change/non-code option, why code must change, minimum boundary, and an explicit decision token such as `Decision: code-change`. If shared logic, a contract, fallback, duplicate owner, consumer patch, or cross-module behavior appears, leave this lane.

`Aegis Visibility` names the evidence/owner/patch-shape/verification effect. Pass root cause, avoided misfix, boundary, evidence, complexity, and risk to `verification-before-completion`; no separate receipt.

Diagnose before repair

For UI/interaction defects, compose `ui-ux-governance` for relevant state and recovery rules at the user-visible seam. Keep reproduction/root-cause ownership here; a proposed manual check is not executed evidence.

1. Read the complete error/stack and record inputs, environment, versions, and success criteria. Reproduce the user's failed outcome on the correct object, through final delivery; successful parsing or green internal checks may hide the failure. Verify the repaired behavior and a relevant neighboring case, not a hardcoded example. Read-only diagnosis does not authorize repair. 2. Reproduce consistently. If unstable, read `feedback-loop-construction.md` **only when evidence shows intermittent or timing-dependent reproduction** and build a bounded loop. Shrink the repro to load-bearing elements as the test input, never the fix scope: still drill upward; test at the correct seam. 3. Inspect recent changes and compare a working example. Code is evidence; if authority, glossary, code, and tests disagree, compose `establishing-project-context` rather than silently redefining a term. 4. Instrument component boundaries, then trace the bad value toward its source. Read `root-cause-tracing.md` **only when the observed bad value is several calls or components downstream from its origin**. For task distortion, follow source → understanding → expression → execution → delivery to the first evidenced change in meaning. This identifies where divergence is observed, not a proven root; reuse sufficient current evidence. 5. State one hypothesis and falsify it with one-variable evidence. Do not stack speculative fixes. End each loop with `Goal | DeeperCause | Evidence | Risk/Unknown | Decision`.

Canonical-owner and patch-shape gate

Before editing, continue upward unless evidence proves the local site is the canonical owner when the candidate is any of these signals:

• keyword, phrase, regex, negation-word list, or sample-text exception;
• local guard, extra conditional, `try`/`catch`, early return, or one-off branch;
• fallback, adapter, compatibility branch, prompt branch, or legacy path expansion;
• consumer/caller/readiness/presentation-layer patch;
• downstream logic re-parses raw text or re-infers action/state while typed inten