Systematic Debugging

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 any symptom remains, stop and diagnose it separately.

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.

Quick bug lane

For a low-risk, 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. 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

1. Read the complete error/stack and record inputs, environment, versions, and success criteria.
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.
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 intent, normalized state, contract, or another source-of-truth exists;
• artifact/download/export/readback/cache patch without producer/owner proof.

PatchShape:
CanonicalOwner:
UpwardDrillSignal:
Decision: fix owner | continue investigation | escalate

A locally green test does not erase triage; a renamed carrier is not a new direction.

When a repair may reinterpret or retire existing semantics, responsibility, contract, or relationship, name the behavior to preserve, highest-risk counterexample, and material unknown. For each known explicit anchor or upstream/downstream reference, state its role and disposition: preserve, rebind to the canonical owner, retire with reason, or reject because of conflict. Leave unresolved relationships unknown; do not re-infer them downstream. Bind role before value and retire invalid responsibility, not evidenced carrier capability. This bounded reminder is not a behavior matrix, relationship graph, referential-integrity proof, or exhaustive discovery claim. It adds no artifact, TDD risk signal, or regression scope; the existing TDD route owner and configured/default mode still apply.

If the diagnosis crosses L3, a patch-shape signal fires, a user disputes the root claim, a prior fix leaves a symptom, compound/root topology is plausible, two or more anchored manifestations of one incident exist, reproduction conditions diverge across occurrences, or an upstream producer/config/default/contract/spec remains unexcluded, read root-cause-claim-contract.md before claiming a root cause. It is the sole owner of the Pre-Claim Gate, causal-closure/falsifier proof, layer-ceiling proof, and Causal Topology Gate.

Change Necessity

This decision is behavior-triggered, not prompt-triggered. It applies to any new source-code path. Before that path or a non-trivial source edit, expose the Change Necessity decision (no-change | docs/config-only | code-change | needs-clarification); field detail lives in advanced-debugging-governance.md.

Minimality and owner fit

For any proposed branch, fallback, adapter, compatibility path, or new owner, run Minimality Check (fields in advanced-debugging-governance.md) with verdict sufficient repair | local patch | needs first-principles review, and retire invalid responsibility: a local patch needs a retention reason and retirement trigger. For a new non-ordinary repair surface, run the Existence Check in docs/current/AEGIS_MINIMALITY_REFERENCE.md. If retirement involves old code, external compatibility, or persistent-state risk, compose anti-entropy-governance; it chooses the retirement path but never grants destructive authority.

Before editing an overloaded or mixed-purpose owner, complete Pre-Edit Complexity Check and Pre-Edit Owner-Fit Decision (templates in advanced-debugging-governance.md).

Use using-aegis/references/complexity-governance.md for pressure signals. Do not add new-responsibility in place by default. If the safer boundary changes the approved shape, update the plan/spec first.

Repair and proportional verification

Implement one owner fix; no bundled “while here” work. Under strict TDD, create the smallest failing test first. With TDD off, a reproduction is diagnostic evidence, not a RED gate or a prerequisite for production edits.

Verification must match the risk:

• local single-owner repair: original reproduction plus focused regression;
• shared/contract/cross-module repair: canonical owner plus affected consumers and compatibility boundary;
• fallback/owner retirement: main-path, lingering-reference, negative, and boundary checks;
• timing/concurrency repair: read condition-based-waiting.md only when evidence identifies polling, sleeps, or race timing as part of the cause;
• invalid state crossing several trusted boundaries: read defense-in-depth.md only after the root repair is known and evidence shows a second independent validation boundary is required.

Read advanced-debugging-governance.md before another fix for failed/ persistent / divergent repair or three failures; for unclear/disputed stop / Layer Stop Card / intervention; or plausible compound root. Closeout triggers: repair-added patch-shape; multi-site/one-regression; remaining pattern/anomaly/duplicate/wrong-owner/downstream repair; uninspected same-symptom fix; open recurrence/unsupported root status; missing compound topology-specific member/anti-disguise proof; outside-repo authority; unmigrated published-contract break; undefined spec; missing permission/info. They route H/T/D; detail is not causal proof.

For non-trivial debugging with configured workspace support:

python <aegis-workspace-helper> init --root <target-project-root>
python <aegis-workspace-helper> new-work --root <target-project-root> ...
python <aegis-workspace-helper> add-evidence --root <target-project-root> --work <YYYY-MM-DD-slug> ...
python <aegis-workspace-helper> check --root <target-project-root>

Failed attempts use <aegis-workspace-helper> add-attempt; add-evidence is terminal-only.

Fast bug fix or quick bug fix pressure does not skip this: if Ripple Signal Triage fires, record it before editing and verify the canonical owner plus affected downstream path. Records are advisory, not completion authority.

Closure

Always report:

• Repair — cause, owner, smallest change, compatibility, verification.
• Retirement — invalid responsibility status, carrier/capability disposition, retention reason/trigger, removal check.

Confirm the reproduction, same-pattern handling, authority, complexity, and retirement. Prefix debug logs (e.g. [DEBUG-a4f2]); confirm one-grep removal before close. Confidence: A = direct regression evidence; B = strong evidence with bounded unknowns; C = partial and not resolved.

Trace Digest may summarize audit evidence; never expose chain-of-thought or replace root-cause, rule-effect, and verification evidence.