Debug Workflow — Root-Cause First

The gap between weak and strong debugging is not intelligence — it is
patience. Weak models guess and patch. Strong models trace and understand.

The Core Rule

You may not modify any file until you have identified the root cause.

A symptom is not a root cause. A guess is not a root cause. You need to point at a specific line and explain the precise mechanism by which it produces the wrong behavior.

Phase 1 — REPRODUCE

Goal: Establish the concrete gap between actual and expected behavior.

"I read the bug report" ≠ "I reproduced the bug." The report gives the user's interpretation. Reproduction gives the actual behavior. These often differ.

Minimum bar to proceed — you must have EITHER:

• Run the failing code and observed the exact output, OR
• Traced the code mentally with exact inputs and confirmed the failure path

Output:

REPRODUCTION
Input:    [exact trigger]
Actual:   [exact wrong behavior]
Expected: [what should happen]
Deterministic: YES / NO — [conditions if NO]

Cannot reproduce → STOP. Ask for a minimal reproduction. Do not proceed on a description alone — you will build hypotheses on sand.

Phase 2 — LOCATE

Goal: Find the first point where execution diverges from correctness.

This is where most debugging fails. The mistake is almost always stopping too early — you find where the error manifests and assume the bug lives there.

Work backward in layers:

1. Start at the symptom: identify the line producing the error/wrong output
2. Ask "why is this value wrong?": trace where the bad value came from — a

parameter? a return value? a module-level variable?

1. Cross the boundary: follow the bad value to its origin — often a

different file or layer. This is the step weak models skip.

1. Keep going until you reach where the INTENT is wrong, not just the VALUE.

The root cause is where the programmer's assumption broke, not where the runtime complained.

Common mistakes: stopping at the crash site instead of tracing why the value was wrong three calls earlier; looking only in the reported file; blaming the library before ruling out your own code; grep-ing the error message instead of tracing data flow.

Output:

LOCATION
Symptom location:    [file:line — where the error surfaces]
Root cause location: [file:line — where the actual mistake is]
Causal chain:        [A → B → C — how the bad value propagates]

If symptom and root cause are the same line, you probably haven't traced far enough.

Phase 3 — HYPOTHESIZE

Goal: State a precise, falsifiable explanation of why the bug exists.

Good hypothesis: names a specific mechanism, makes a testable prediction, explains ALL observed symptoms (not just some).

Bad hypothesis: vague attribution ("state management issue"); blaming external code without evidence; explaining the symptom not the cause ("it crashes because the value is null" — WHY is it null?); fitting only some symptoms.

Confidence calibration:

• HIGH: read the code path end-to-end, can point at the exact line,

mechanism explains all symptoms

• MEDIUM: plausible mechanism, one unverified link in the chain
• LOW: have a direction, haven't traced the full path

HYPOTHESIS
Root cause: [specific mechanism]
Evidence:   [what supports this]
Predicts:   [what else should be true if correct]
Confidence: HIGH / MEDIUM / LOW

LOW → go back to Phase 2. MEDIUM → acceptable only if further tracing is impractical. Prefer HIGH before fixing.

Phase 4 — FIX

Goal: Apply the minimal change that addresses the root cause. Nothing more.

Scope-creep signals (stop immediately if you notice these):

• Renaming variables not involved in the bug
• Restructuring control flow for "clarity" beyond the fix
• Adding features ("this function should also handle...")
• Diff touches files not in the causal chain
• You think "while I'm at it"

Hard constraints:

• NO suppressing errors (catch-and-ignore, ?. chains hiding null origins)
• NO defensive fallbacks masking the bug (|| default without understanding

why the value is missing)

• NO any / @ts-ignore / equivalent type escapes
• If behavior changes for other callers, document it explicitly
• Non-obvious fix → add comment: // Fix: [mechanism in one line]

Phase 5 — VERIFY

Goal: Confirm the fix is correct, not just that the symptom disappeared.

"It works now" ≠ "it is fixed." A suppressed error also makes symptoms vanish. Verification must confirm the MECHANISM is addressed.

Mandatory checks:

1. Original failure from Phase 1 → confirm it passes
2. Two adjacent cases (boundary values, the "other branch" of your fix)
3. Existing tests (or manual check of normal behavior if no tests)
4. Call-site audit: other callers affected by the behavior change?

VERIFICATION
Original case  : PASS / FAIL
Adjacent case 1: PASS / FAIL — [what]
Adjacent case 2: PASS / FAIL — [what]
Existing tests : PASS / FAIL / NOT AVAILABLE
Other callers  : UNAFFECTED / AFFECTED — [details]
VERDICT: PASS / FAIL

FAIL → return to Phase 2, not Phase 4. Failed verification means your understanding is wrong, not just your fix.

Failure Recovery

After 2 failed fixes on the same hypothesis

Your hypothesis is wrong. The most common trap: you found a real problem, but it's not THE problem. The thing you're fixing is genuinely imperfect, but it doesn't cause this specific symptom. This is the "real but irrelevant bug" trap.

Required: abandon the hypothesis entirely.

HYPOTHESIS INVALIDATED
Tried:         [what you attempted]
Why wrong:     [why it didn't fix the symptom]
New direction: [hypothesis from a DIFFERENT root cause category]

After 3 cumulative failures

Stop all edits. Three failures means: missing context (runtime state you can't see), wrong layer (build system, bundler, environment), or fundamentally wrong mental model of the system.

Required: revert all changes, report:

BLOCKED — 3 attempts failed
  1. [approach] → [result]
  2. [approach] → [result]
  3. [approach] → [result]
What I know:      [confirmed facts]
What I'm missing: [specific unknowns]
To proceed:       [concrete ask]

Few-Shot Reference

Example: "User profile updates are lost intermittently"

❌ Weak model: reads updateProfile, sees db.save(user) (already awaited), adds error logging, declares fixed. Bug persists.

✅ This workflow:

Phase 1: Update returns 200 with correct data, but old data reappears on refresh. ~1 in 3 updates lost. API response is correct. Deterministic: NO.

Phase 2: GET returns stale data. Trace: GET reads from userCache.get(id) first, falls back to DB. updateProfile writes to DB then invalidates cache. But a refreshCache background job runs every 5s. The DB write uses a deferred transaction — commits AFTER the handler returns. So the sequence is: (1) write to DB buffer, (2) invalidate cache, (3) transaction commits. If refreshCache fires between steps 2 and 3, it reads uncommitted (old) state and populates cache with stale data.

LOCATION
Symptom:     GET /api/user/:id returns stale cached data
Root cause:  cache invalidation runs BEFORE transaction commit
Chain:       write DB buffer → invalidate cache → refreshCache reads
             uncommitted old data → repopulates cache → commit (too late)

Phase 3: Cache invalidation fires before DB commit; the background refresh job can read uncommitted state during this window. Explains intermittency (depends on refreshCache timing) and correct API response (reads write buffer, not cache). Confidence: HIGH.

Phase 4: Move cache invalidation into the transaction's afterCommit hook. One line moved, one hook added.

Phase 5: 50 rapid updates, 0 lost. Concurrent sessions: both reflected. Server restart mid-update: cache rebuilt correctly. Tests: PASS. VERDICT: PASS.

Why weak models fail here: updateProfile looks correct in isolation — it writes and invalidates properly. The bug lives in the interaction between three components (handler, transaction middleware, background job). You only find it by tracing across module boundaries.