diagnose

name	diagnose
type	reference
description	Multi-agent diagnostic pipeline for complex/intermittent bugs. Orchestrates Investigator → Verifier → Solver → Lead Programmer with enforced handoff contracts. Use ONLY for non-obvious failures (root cause unclear, reproduction unstable, fixes reverted). NOT for trivial bugs with known cause — fix them directly.
paths	[]
effort	4
allowed-tools	Read, Glob, Grep, Write, Edit, Bash, Task
user-invocable	true
when_to_use	When a bug is reproducible but cause is unknown, when a 'fix' has been reverted 2+ times, or when a symptom appears in unfamiliar code. Do NOT use for typos, obvious nulls, or one-line logic errors.

Skill: /diagnose — Complex Bug Diagnostic Pipeline

When to invoke (and when NOT to)

✅ Use /diagnose when:

• Bug reproduces but root cause is unclear after one read-pass of the failing code
• Previous fix attempts have been reverted ≥ 2 times (symptoms return)
• Failure is intermittent (flaky test, race condition, timing-dependent)
• Failure occurs in unfamiliar code (agent has no prior context)
• User has explicitly requested /diagnose or "deep investigation"
• Circuit Breaker (Rule 14) tripped on the specialist agent that normally handles this domain

❌ Do NOT use /diagnose when:

• Cause is obvious (null ref, typo, missing import, incorrect import path)
• Fix is < 10 LOC and has a clear success check
• Bug is in code you just wrote this session (read-pass + local reasoning is faster)
• User wants a quick patch and has accepted the tradeoff

Pipeline overview

┌─────────────────┐     ┌─────────────────┐     ┌──────────────┐     ┌──────────────────┐
│  Investigator   │ ──► │    Verifier     │ ──► │    Solver    │ ──► │ Lead Programmer  │
│  (hypothesis)   │     │ (devil's adv.)  │     │  (tradeoffs) │     │  (assign + exec) │
└─────────────────┘     └─────────────────┘     └──────────────┘     └──────────────────┘
       │                       │                       │                       │
       ▼                       ▼                       ▼                       ▼
  investigation.json      verification.json      solution.json          implementation
  (root_cause,           (status: confirmed |    (3 options:           (delegates to
   evidence[],            refuted | inconclusive, Quick/Strategic/     backend-developer,
   confidence)            reproduction_steps)    Future-Proof)         qa-tester, etc.)

Each stage produces a required artifact saved to .investigations/<task_id>/ and a handoff contract (per Rule 16) to the next agent.

Stage 1 — Investigation

Agent: investigator Goal: Produce a falsifiable root-cause hypothesis backed by empirical evidence.

Inputs

• Symptom description (from user or TODO.md bug ID)
• Reproduction steps (or "cannot reproduce" + environment)
• Relevant log lines, stack traces, error IDs

Required output — investigation.json

{
  "task_id": "BUG-417",
  "symptom": "POST /api/orders returns 500 when cart has ≥10 items",
  "reproduction": {
    "steps": ["...", "..."],
    "frequency": "100% | intermittent (~30%) | once",
    "environment": "staging-eu-west-1"
  },
  "hypothesis": {
    "root_cause": "OrderService.calculateTotal() N+1 query exhausts pool when cart.items.length > 9",
    "confidence": "high | medium | low",
    "falsifiable_by": "Run with pool_size=50; if error disappears, cause confirmed"
  },
  "evidence": [
    {"type": "log", "ref": ".investigations/BUG-417/pg-pool-exhausted.log", "summary": "..."},
    {"type": "code", "ref": "src/services/order.service.ts:142", "summary": "Unbounded .map+await"}
  ],
  "unknowns": ["Why only eu-west-1?", "When did this start?"],
  "next_agent": "verifier"
}

Quality gate (Lead Programmer rejects if):

• hypothesis.falsifiable_by is vague ("check if it works")
• evidence has fewer than 2 items (unverifiable)
• unknowns is empty but confidence: low (contradictory)

Stage 2 — Verification

Agent: verifier Goal: Attempt to refute the hypothesis. Only confirmed if refutation fails.

Inputs

• investigation.json (from Stage 1)
• Access to staging/test environment

Required output — verification.json

{
  "task_id": "BUG-417",
  "status": "confirmed | refuted | inconclusive",
  "triangulation": [
    {"method": "reproduce_with_fix_applied", "result": "Error gone with pool_size=50"},
    {"method": "reproduce_without_fix", "result": "Error returns at 10 items"},
    {"method": "adjacent_test_case", "result": "9 items = OK, 10 items = fail → threshold confirmed"}
  ],
  "counter_hypotheses_ruled_out": [
    "DB slowness (ruled out: p99 < 50ms)",
    "Network flaps (ruled out: no packet loss in window)"
  ],
  "confidence": "high",
  "recommendation": "Proceed to solver — cause confirmed necessary AND sufficient"
}

Decision flow

status	Next action
confirmed	Hand off to solver
refuted	Return to investigator with counter-evidence. Max 2 round-trips.
inconclusive	STOP. Surface to user with all evidence. Do NOT proceed to solver.

Stage 3 — Solution

Agent: solver Goal: Generate 3 solution options with explicit tradeoffs; never pick silently.

Required output — solution.json

{
  "task_id": "BUG-417",
  "options": [
    {
      "name": "Quick",
      "description": "Increase pool_size from 20 → 50 in db.ts",
      "scope_loc": 1,
      "risk_tier": "Low",
      "tradeoff": "Masks root cause; higher RAM; future growth hits same wall"
    },
    {
      "name": "Strategic",
      "description": "Rewrite calculateTotal() to batch via IN-clause",
      "scope_loc": 40,
      "risk_tier": "Medium",
      "tradeoff": "Fixes N+1 permanently; requires regression test on discount logic"
    },
    {
      "name": "Future-Proof",
      "description": "Introduce DataLoader pattern across service layer",
      "scope_loc": 300,
      "risk_tier": "High",
      "tradeoff": "Eliminates entire class of N+1 bugs; 2-3 day refactor; needs ADR"
    }
  ],
  "recommendation": "Strategic — best risk/value ratio. Quick only if release is < 24h."
}

Quality gate

• All 3 options must have distinct scope (not three flavors of the same fix)
• tradeoff must state what is sacrificed, not just "takes longer"
• recommendation must cite a criterion (time budget, risk tier, blast radius)

Stage 4 — Finalization

Agent: lead-programmer Goal: Select option, assign specialist, track execution.

Actions

1. Review solution.json with user (if risk_tier: High or scope_loc > 100)
2. Select option → write selection to .investigations/<task_id>/decision.md
3. Create A2A handoff contract (Rule 16) via /handoff:
   • lead-programmer → backend-developer (or frontend-developer, data-engineer)
   • Acceptance criteria derived from investigation.hypothesis.falsifiable_by
4. Append ledger entry (Rule 15) to production/traces/decision_ledger.jsonl:

{"ts":"2026-04-17T14:22:00Z","session":"main","agent_id":"lead-programmer","task_id":"BUG-417","request":"/diagnose BUG-417","reasoning":"Verified N+1 as necessary+sufficient; selected Strategic per solver recommendation","choice":"Strategic refactor of calculateTotal()","outcome":"pass","risk_tier":"Medium","duration_s":1840}

Artifact storage

All intermediate reports MUST be saved to .investigations/<task_id>/:

.investigations/
└── BUG-417/
    ├── investigation.json     # Stage 1 output
    ├── verification.json      # Stage 2 output
    ├── solution.json          # Stage 3 output
    ├── decision.md            # Stage 4 — human-readable rationale
    ├── evidence/              # logs, screenshots, traces referenced in reports
    └── handoffs/              # A2A contracts (copied from .tasks/handoffs/)

Retention: Keep until bug is closed + 30 days, then archive to .investigations/archive/.

Escalation paths

Trigger	Escalate to
investigator fails 3× (Rule 14 OPEN)	Fallback to solver with raw symptom
verifier returns inconclusive twice	Surface to user; request manual reproduction
solver cannot produce 3 distinct options	Escalate to technical-director — scope unclear
User rejects all 3 options	Return to investigator; hypothesis likely wrong
Bug reoccurs after fix merges	Restart /diagnose with new task_id; link prior investigation in unknowns[]

Integration with coordination rules

• Rule 6 (Layered Recovery): If any stage fails once, retry with fresh context before escalating
• Rule 14 (Circuit Breaker): Read production/session-state/circuit-state.json before invoking each agent
• Rule 15 (Decision Ledger): Every stage transition logs to decision_ledger.jsonl
• Rule 16 (A2A Handoff): Stage 1→2, 2→3, 3→4 each require a handoff contract in .tasks/handoffs/

Concrete example — "Flaky checkout test"

Symptom: checkout.e2e.test.ts fails ~20% of CI runs; local always passes.

/diagnose flaky-checkout-e2e
  ↓
Stage 1 → investigator
  hypothesis: "Test clicks #submit before React hydration completes on slow CI runners"
  evidence: [CI traces showing hydration marker missing, local has DevTools overhead masking timing]
  confidence: medium (cannot reproduce locally)
  ↓
Stage 2 → verifier
  triangulation:
    - Inject 500ms delay before click → test passes 50/50 runs ✓
    - Remove delay → fails 9/50 ✗
    - Check for hydration marker instead of fixed delay → passes 50/50 ✓
  status: confirmed
  ↓
Stage 3 → solver
  Quick: add sleep(500ms)              [masks issue]
  Strategic: waitFor hydration marker  [addresses root cause]
  Future-Proof: custom test util that always waits for RSC boundary [reusable]
  recommendation: Strategic
  ↓
Stage 4 → lead-programmer
  selects Strategic; assigns to qa-tester
  handoff contract: "qa-tester updates checkout.e2e.test.ts to use waitFor(hydrationMarker)"
  acceptance_criteria: ["10 CI runs in a row pass", "no sleep() in test"]
  ledger entry written

Common pitfalls

Pitfall	Fix
Skipping Verification ("cause is obvious")	Verifier exists specifically to catch "obvious but wrong" hypotheses
Investigator produces only 1 hypothesis	Reject — require counter_hypotheses_ruled_out[] list in Stage 2
Solver picks Quick fix without naming tradeoff	Reject — all 3 options required for explicit tradeoff comparison
No artifact written to .investigations/	Reject — verbal diagnosis is not auditable
Running /diagnose in parallel on same bug	Only one active investigation per task_id; concurrent runs create race

Output to user

After Stage 4 completes, summarize in ≤ 5 lines:

/diagnose BUG-417 complete.
Root cause: Unbounded .map+await in OrderService.calculateTotal() exhausts pg pool.
Selected: Strategic (batch via IN-clause, ~40 LOC, Medium risk).
Assigned: @backend-developer; acceptance = load test with 50 items passes.
Artifacts: .investigations/BUG-417/