Cursor rule

1---
2description: "Investigate a bug or issue by exploring the codebase to find root cause, then write a TDD-based fix plan to specs/bugs/BUG-*.md. Use when user reports a bug, wants to investigate a problem, mentions \"triage\", or wants to plan a fix."
3alwaysApply: false
4---
5 
6# Investigate Bug
7 
8**Boundary**: End-to-end bug entry point — history check → RCA (via `diagnose-root`) → fix approach → TDD plan → bug file. Delegates the 4-phase RCA to `diagnose-root`; does not re-implement it.
9 
10Investigate a reported problem, find its root cause, and write a TDD fix plan to `specs/bugs/BUG-*.md`. This is a mostly hands-off workflow — minimize questions to the user.
11 
12## Process
13 
14### 0. Read previous bug history
15 
16Before starting diagnosis:
17 
181. Read `specs/bugs/registry.yaml` (if it exists) — check for prior bugs in the same `scope` or with similar symptoms.
192. If a relevant prior bug is found, read the corresponding `specs/bugs/BUG-*.md` file to understand previous root cause analysis and fix approach.
203. Note in your investigation whether this is a recurrence, a related issue, or novel.
21 
22### 1. Capture the problem
23 
24Get a brief description of the issue from the user. If they haven't provided one, ask ONE question: "What's the problem you're seeing?"
25 
26Do NOT ask follow-up questions yet. Start investigating immediately.
27 
28> **Security-impact assessment** — After capturing the problem, assess and document: `Security impact: NONE / LOW / MEDIUM / HIGH / CRITICAL`. If HIGH or CRITICAL, assign bug severity HIGH and document the exploit path in findings. If MEDIUM+, include exploit path in the bug file. Document "no security exploit path identified" for NONE/LOW.
29 
30### 2. Explore and diagnose (4-phase RCA)
31 
32Run the 4-phase root-cause analysis via the `diagnose-root` skill (Reproduce → Isolate → Hypothesize → Verify). That skill is the canonical RCA engine — do not re-implement the phases here.
33 
34Also look at:
35- Recent changes to affected files (`git log --oneline <file>`)
36- Existing tests (what's tested, what's missing)
37- Similar patterns elsewhere in the codebase that work correctly
38 
39> **HARD GATE** — Do NOT proceed to Step 3 (Fix Approach) until `diagnose-root` Phase 4 produces a verified root cause. "It probably is X" is not verified.
40 
41### 3. Identify the fix approach
42 
43Based on your investigation, determine:
44 
45- The minimal change needed to fix the root cause
46- Which modules/interfaces are affected
47- What behaviors need to be verified via tests
48- Whether this is a regression, missing feature, or design flaw
49- Risk level: Low / Medium / High
50 
51### 4. Design TDD fix plan
52 
53Create a concrete, ordered list of RED-GREEN cycles. Each cycle is one vertical slice:
54 
55- **RED**: Describe a specific test that captures the broken/missing behavior
56- **GREEN**: Describe the minimal code change to make that test pass
57 
58Rules:
59- Tests verify behavior through public interfaces, not implementation details
60- One test at a time, vertical slices (NOT all tests first, then all code)
61- Each test should survive internal refactors
62- Include a final refactor step if needed
63- **Durability**: Only suggest fixes that would survive radical codebase changes. Tests assert on observable outcomes (API responses, UI state, user-visible effects), not internal state.
64 
65### 5. Write the bug file
66 
67Save the investigation and fix plan to `specs/bugs/BUG-NNN-slug.md`. Create the `specs/bugs/` directory if it doesn't exist.
68 
69After writing, append a row to `specs/bugs/registry.yaml` with: bug_id (same timestamp), date, severity, priority, scope, summary, and file path. Create `specs/bugs/registry.yaml` if it doesn't exist.
70 
71<diagnosis-template>
72 
73# BUG-YYYY-MM-DDTHHMMSS: [short title]
74 
75## Problem
76 
77A clear description of the bug or issue, including:
78- What happens (actual behavior)
79- What should happen (expected behavior)
80- How to reproduce (if applicable)
81 
82## Root Cause Analysis
83 
84Describe what you found during investigation:
85- The code path involved
86- Why the current code fails
87- Any contributing factors
88- Risk level: Low / Medium / High
89 
90Do NOT include specific file paths, line numbers, or implementation details that couple to current code layout. Describe modules, behaviors, and contracts instead.
91 
92## TDD Fix Plan
93 
94A numbered list of RED-GREEN cycles:
95 
961. **RED**: Write a test that [describes expected behavior]
97   **GREEN**: [Minimal change to make it pass]
98   **verify**: [runnable command]
99 
1002. **RED**: Write a test that [describes next behavior]
101   **GREEN**: [Minimal change to make it pass]
102   **verify**: [runnable command]
103 
104**REFACTOR**: [Any cleanup needed after all tests pass]
105 
106## Acceptance Criteria
107 
108- [ ] Criterion 1
109- [ ] Criterion 2
110- [ ] All new tests pass
111- [ ] Existing tests still pass
112 
113## Resolution
114 
115<!-- filled in by validate-fix -->
116 
117</diagnosis-template>
118 
119After writing the bug file, print a one-line summary of the root cause and suggest running `kickoff-branch` next to create a fix branch.
120 
121## References
122 
123<!-- story: e35s10 -->
124 
125- [Feathers' Seams & Characterization Tests](../../docs/references/feathers.md) — seam types, characterization tests, and the legacy code change algorithm
126- [Fowler's Code Smells](../../docs/references/fowler.md) — identifying structural problems by smell before investigating
127