gstack-investigate

Tandem vendoring note. Vendored from gstack (MIT, (c) Garry Tan, https://github.com/garrytan/gstack @ a3259400). The upstream Claude-Code-specific boilerplate (telemetry, session tracking, update checks, plan-mode hooks, learnings/brain sync) was removed; the workflow itself is unchanged. Paths below are relative to this skill's directory; sibling skills live at ../<id>/ (ids are prefixed gstack-). $GSTACK_ROOT is an optional upstream gstack install (default ~/.claude/skills/gstack) needed only for optional helpers such as the browse binary; skip any step that needs it when it is absent. "Ask the user" means a plain chat question when no structured question tool exists.

When to invoke this skill

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.

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.

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 ask-the-user.
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:

   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.

Scope Lock

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

_FREEZE_SCRIPT="${CLAUDE_SKILL_DIR}/../freeze/bin/check-freeze.sh"
[ -x "$_FREEZE_SCRIPT" ] || _FREEZE_SCRIPT="${CLAUDE_SKILL_DIR}/../gstack-freeze/bin/check-freeze.sh"
[ -x "$_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:

eval "$($GSTACK_ROOT/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.

Phase 2: Pattern Analysis

Check if this bug matches a known pattern:

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, WebSearch for:

• "{framework} {generic error type}" — sanitize first: strip hostnames, IPs, file paths, SQL, customer data. Search the error category, not the raw message.
• "{library} {component} known issues"

If WebSearch is unavailable, 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. 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 WebSearch is unavailable, skip and proceed. Then return to Phase 1. Gather more evidence. Do not guess.
3. 3-strike rule: If 3 hypotheses fail, STOP. Use ask-the-user:

   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 ask-the-user 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:

$GSTACK_ROOT/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"]}'

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 → ask-the-user 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