gstack/investigate/SKILL.md.tmpl

---
name: investigate
preamble-tier: 2
version: 1.0.0
description: |
  Systematic debugging with root cause investigation. Four phases: investigate,
  analyze, hypothesize, implement. Iron Law: no fixes without root cause.
  Use when asked to "debug this", "fix this bug", "why is this broken",
  "investigate this error", or "root cause analysis".
  Proactively invoke this skill (do NOT debug directly) when the user reports
  errors, 500 errors, stack traces, unexpected behavior, "it was working
  yesterday", or is troubleshooting why something stopped working. (gstack)
allowed-tools:
  - Bash
  - Read
  - Write
  - Edit
  - Grep
  - Glob
  - AskUserQuestion
  - WebSearch
triggers:
  - debug this
  - fix this bug
  - why is this broken
  - root cause analysis
  - investigate this error
hooks:
  PreToolUse:
    - matcher: "Edit"
      hooks:
        - type: command
          command: 'bash -c ''S="$HOME/.claude/skills/gstack/freeze/bin/check-freeze.sh"; [ -x "$S" ] && exec bash "$S"; exit 0'''
          statusMessage: "Checking debug scope boundary..."
    - matcher: "Write"
      hooks:
        - type: command
          command: 'bash -c ''S="$HOME/.claude/skills/gstack/freeze/bin/check-freeze.sh"; [ -x "$S" ] && exec bash "$S"; exit 0'''
          statusMessage: "Checking debug scope boundary..."
gbrain:
  schema: 1
  context_queries:
    - id: prior-investigations
      kind: list
      filter:
        type: timeline
        tags_contains: "repo:{repo_slug}"
        content_contains: "investigate"
      sort: updated_at_desc
      limit: 5
      render_as: "## Prior investigations in this repo"
    - id: project-learnings
      kind: filesystem
      glob: "~/.gstack/projects/{repo_slug}/learnings.jsonl"
      tail: 10
      render_as: "## Recent learnings (patterns + pitfalls)"
    - id: recent-eureka
      kind: filesystem
      glob: "~/.gstack/analytics/eureka.jsonl"
      tail: 5
      render_as: "## Recent eureka moments (cross-project)"
---

{{PREAMBLE}}

# Systematic Debugging

## Iron Law

**NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST.**

Fixing symptoms creates whack-a-mole debugging. Every fix that doesn't address root cause makes the next bug harder to find. Find the root cause, then fix it.

---

{{GBRAIN_CONTEXT_LOAD}}

## Phase 1: Root Cause Investigation

Gather context before forming any hypothesis.

1. **Collect symptoms:** Read the error messages, stack traces, and reproduction steps. If the user hasn't provided enough context, ask ONE question at a time via AskUserQuestion.

2. **Read the code:** Trace the code path from the symptom back to potential causes. Use Grep to find all references, Read to understand the logic.

3. **Check recent changes:**
   ```bash
   git log --oneline -20 -- <affected-files>
   ```
   Was this working before? What changed? A regression means the root cause is in the diff.

4. **Reproduce:** Can you trigger the bug deterministically? If not, gather more evidence before proceeding.

5. **Check investigation history:** Search prior learnings for investigations on the same files. Recurring bugs in the same area are an architectural smell. If prior investigations exist, note patterns and check if the root cause was structural.

{{LEARNINGS_SEARCH:query=debug investigation root cause hypothesis bug fix}}

Output: **"Root cause hypothesis: ..."** — a specific, testable claim about what is wrong and why.

### Refresh learnings for the hypothesis you just named

The top-of-skill learnings pull above is keyed to "debug investigation" broadly. Now that you have a specific hypothesis, re-pull learnings keyed to that hypothesis so prior fixes for the same problem-shape surface.

Pick ONE keyword from the hypothesis. The keyword should be a noun: the failing component name, the basename of the file you suspect (without extension), or the bug noun. The keyword MUST be alphanumeric or hyphen only — no quotes, slashes, dots, colons, or whitespace. If your candidate has any of those, simplify to just the alphanumeric stem.

Worked examples (investigate-specific): good keywords are `auth-cookie`, `session-expiry`, `redirect-loop`. Bad: `auth.ts:47`, `fix the auth bug`, `<hypothesis-keyword>`.

```bash
~/.claude/skills/gstack/bin/gstack-learnings-search --query "<your-keyword>" --limit 5 2>/dev/null || true
```

If any learnings come back, name which one applies to your investigation in one sentence. If none come back, continue without reference — the absence of a matching prior learning is itself useful information.

---

## Scope Lock

After forming your root cause hypothesis, lock edits to the affected module to prevent scope creep.

```bash
# $HOME-anchored like the careful/freeze frontmatter hooks (#1871): frontmatter
# hooks and early skill bash run before any runtime var like CLAUDE_SKILL_DIR
# exists, so a ${CLAUDE_SKILL_DIR}-relative path silently never resolves (#2469).
_FREEZE_SCRIPT="$HOME/.claude/skills/gstack/freeze/bin/check-freeze.sh"
[ -x "$_FREEZE_SCRIPT" ] && echo "FREEZE_AVAILABLE" || echo "FREEZE_UNAVAILABLE"
```

**If FREEZE_AVAILABLE:** Identify the narrowest directory containing the affected files. Write it to the freeze state file:

```bash
eval "$(~/.claude/skills/gstack/bin/gstack-paths)"
STATE_DIR="$GSTACK_STATE_ROOT"
mkdir -p "$STATE_DIR"
echo "<detected-directory>/" > "$STATE_DIR/freeze-dir.txt"
echo "Debug scope locked to: <detected-directory>/"
```

Substitute `<detected-directory>` with the actual directory path (e.g., `src/auth/`). Tell the user: "Edits restricted to `<dir>/` for this debug session. This prevents changes to unrelated code. Run `/unfreeze` to remove the restriction."

If the bug spans the entire repo or the scope is genuinely unclear, skip the lock and note why.

**If FREEZE_UNAVAILABLE:** Skip scope lock. Edits are unrestricted.

---

{{ASIDE_RESEARCH}}

## Phase 2: Pattern Analysis

Check if this bug matches a known pattern:

| Pattern | Signature | Where to look |
|---------|-----------|---------------|
| Race condition | Intermittent, timing-dependent | Concurrent access to shared state |
| Nil/null propagation | NoMethodError, TypeError | Missing guards on optional values |
| State corruption | Inconsistent data, partial updates | Transactions, callbacks, hooks |
| Integration failure | Timeout, unexpected response | External API calls, service boundaries |
| Configuration drift | Works locally, fails in staging/prod | Env vars, feature flags, DB state |
| Stale cache | Shows old data, fixes on cache clear | Redis, CDN, browser cache, Turbo |

Also check:
- `TODOS.md` for related known issues
- `git log` for prior fixes in the same area — **recurring bugs in the same files are an architectural smell**, not a coincidence

**External pattern search:** If the bug doesn't match a known pattern above, research through Aside (Web research runs in Aside, above). **Sanitize first:** strip hostnames, IPs, file paths, SQL, customer data. Search the error category, not the raw message:
- "{framework} {generic error type}"
- "{library} {component} known issues"

```bash
{{ASIDE_EXEC_PRELUDE}}
_aside_exec "Search the web for {framework} {generic error type} and {library} {component} known issues. Read-only: do not sign in, submit, or change anything. Reply with up to 6 bullets, each with its source URL, then stop."
```

If the Aside check did not print `READY`, run the same searches with the WebSearch tool when the host provides it; with neither, skip this search and proceed with hypothesis testing. If a documented solution or known dependency bug surfaces, present it as a candidate hypothesis in Phase 3.

---

## Phase 3: Hypothesis Testing

Before writing ANY fix, verify your hypothesis.

1. **Confirm the hypothesis:** Add a temporary log statement, assertion, or debug output at the suspected root cause. Run the reproduction. Does the evidence match?

2. **If the hypothesis is wrong:** Before forming the next hypothesis, consider searching for the error through Aside, as in Phase 2. **Sanitize first** — strip hostnames, IPs, file paths, SQL fragments, customer identifiers, and any internal/proprietary data from the error message. Search only the generic error type and framework context: "{component} {sanitized error type} {framework version}". If the error message is too specific to sanitize safely, skip the search. If the Aside check did not print `READY`, use the WebSearch tool when the host provides it; with neither, skip and proceed. Then return to Phase 1. Gather more evidence. Do not guess.

3. **3-strike rule:** If 3 hypotheses fail, **STOP**. Use AskUserQuestion:
   ```
   3 hypotheses tested, none match. This may be an architectural issue
   rather than a simple bug.

   A) Continue investigating — I have a new hypothesis: [describe]
   B) Escalate for human review — this needs someone who knows the system
   C) Add logging and wait — instrument the area and catch it next time
   ```

**Red flags** — if you see any of these, slow down:
- "Quick fix for now" — there is no "for now." Fix it right or escalate.
- Proposing a fix before tracing data flow — you're guessing.
- Each fix reveals a new problem elsewhere — wrong layer, not wrong code.

---

## Phase 4: Implementation

Once root cause is confirmed:

1. **Fix the root cause, not the symptom.** The smallest change that eliminates the actual problem.

2. **Minimal diff:** Fewest files touched, fewest lines changed. Resist the urge to refactor adjacent code.

3. **Write a regression test** that:
   - **Fails** without the fix (proves the test is meaningful)
   - **Passes** with the fix (proves the fix works)

4. **Run the full test suite.** Paste the output. No regressions allowed.

5. **If the fix touches >5 files:** Use AskUserQuestion to flag the blast radius:
   ```
   This fix touches N files. That's a large blast radius for a bug fix.
   A) Proceed — the root cause genuinely spans these files
   B) Split — fix the critical path now, defer the rest
   C) Rethink — maybe there's a more targeted approach
   ```

---

## Phase 5: Verification & Report

**Fresh verification:** Reproduce the original bug scenario and confirm it's fixed. This is not optional.

Run the test suite and paste the output.

Output a structured debug report:
```
DEBUG REPORT
════════════════════════════════════════
Symptom:         [what the user observed]
Root cause:      [what was actually wrong]
Fix:             [what was changed, with file:line references]
Evidence:        [test output, reproduction attempt showing fix works]
Regression test: [file:line of the new test]
Related:         [TODOS.md items, prior bugs in same area, architectural notes]
Status:          DONE | DONE_WITH_CONCERNS | BLOCKED
════════════════════════════════════════
```

Log the investigation as a learning for future sessions. Use `type: "investigation"` and include the affected files so future investigations on the same area can find this:

```bash
~/.claude/skills/gstack/bin/gstack-learnings-log '{"skill":"investigate","type":"investigation","key":"ROOT_CAUSE_KEY","insight":"ROOT_CAUSE_SUMMARY","confidence":9,"source":"observed","files":["affected/file1.ts","affected/file2.ts"]}'
```

{{LEARNINGS_LOG}}

{{GBRAIN_SAVE_RESULTS}}

---

## Important Rules

- **3+ failed fix attempts → STOP and question the architecture.** Wrong architecture, not failed hypothesis.
- **Never apply a fix you cannot verify.** If you can't reproduce and confirm, don't ship it.
- **Never say "this should fix it."** Verify and prove it. Run the tests.
- **If fix touches >5 files → AskUserQuestion** about blast radius before proceeding.
- **Completion status:**
  - DONE — root cause found, fix applied, regression test written, all tests pass
  - DONE_WITH_CONCERNS — fixed but cannot fully verify (e.g., intermittent bug, requires staging)
  - BLOCKED — root cause unclear after investigation, escalated