Cleanup Legacy

Find code marked deprecated/legacy/old/v1 and verify it's truly unused before deletion. Also find unreachable fallback branches (e.g., feature flag defaults that have flipped, version checks for unsupported runtimes).

Preflight

1. Language detect — applies to all.
2. Git state: refuse on dirty tree.
3. Report dir: ensure exists.
4. Read deprecation markers the project uses. Common ones:

• @deprecated JSDoc/TSDoc
• # deprecated Python comments, warnings.warn(DeprecationWarning)
• // Deprecated: Go convention
• #[deprecated] Rust attribute
• File/dir naming: legacy/, old/, v1/, _old.ts

1. Read feature flag config if present — flags that are 100% on with no opposite tests can have their else branches deleted.

Detect

Marker grep (multi-language)

grep -rn --include="*.ts" --include="*.tsx" --include="*.js" --include="*.py" --include="*.go" --include="*.rs" \
  -E "(@deprecated|# ?deprecated|// ?Deprecated|#\[deprecated|warnings\.warn.*Deprecation|TODO.*remove|FIXME.*legacy)" \
  --exclude-dir=node_modules --exclude-dir=dist . > /tmp/deprecated.txt

# Files/dirs with legacy naming
find . -type d \( -name "legacy" -o -name "old" -o -name "v1" -o -name "_archive" \) -not -path "*/node_modules/*" > /tmp/legacy-dirs.txt
find . -type f \( -name "*_old.*" -o -name "*-legacy.*" -o -name "*.deprecated.*" \) > /tmp/legacy-files.txt

Caller verification (TS/JS)

For each deprecated symbol, count references using ts-server / LSP if available, fall back to grep:

# For function `oldFn` exported from `lib/old.ts`:
grep -rn "oldFn" --include="*.ts" --include="*.tsx" . | grep -v "lib/old.ts" | wc -l

Caller verification (Python)

grep -rn "from .*old_mod import\|import old_mod" --include="*.py" .

Fallback branches

Look for patterns:

• if (process.env.NEW_FEATURE === 'true') { /* new */ } else { /* old */ } — if env always true in all configs, old branch is dead.
• if version >= 2: /* new */ else: /* old */ — if min version bumped past threshold.
• Browser/runtime checks for unsupported environments (if (typeof window === 'undefined') in a browser-only package).

Assess

Write .claude/cleanup-reports/cleanup-legacy-{YYYY-MM-DD}.md:

# Legacy Code Assessment — YYYY-MM-DD

## Summary
- Deprecated symbols found: N
  - HIGH (zero callers): X — safe to delete
  - MEDIUM (1-5 callers): Y — needs migration
  - LOW (heavy use): Z — deprecated in name but actively used
- Legacy directories: M
- Dead fallback branches: K

## Findings

### HIGH — `packages/utils/src/format-old.ts`
- Marked `@deprecated` in an older commit and unused by current callers.
- Exports: `formatV1`, `parseV1`. Repo grep shows zero usages outside the file.
- Action: delete file.

### HIGH — `apps/app/lib/feature-flags.ts:45-60`
- Branch: `if (NEW_DASHBOARD_ENABLED) { ... } else { renderOldDashboard() }`.
- Flag is `true` in all envs (`.env`, `.env.staging`, `.env.production`) and has been for 6+ months per git log.
- Action: remove the else branch + the flag check + the `renderOldDashboard` function (cascade).

### MEDIUM — `domains/billing/legacy.ts`
- 8 callers across 3 packages.
- Marked deprecated 3 months ago. Migration path documented in inline comment to use `domains/billing/v2`.
- Recommendation: do NOT delete. Provide a migration list to the human; this needs sequencing.

### LOW — `lib/utils/oldHelper.ts`
- Marked `@deprecated` but has 47 active callers.
- Either the deprecation is aspirational with no migration plan, or it was incorrectly marked. Flag for human review of the deprecation.

## Critical Assessment
[2-3 paragraphs: what's the pattern of deprecation in this codebase? Are deprecations followed by deletion? Are there feature flags that should have been cleaned up months ago? Are "legacy" directories accumulating but not draining?]

Apply

Auto-delete HIGH only.

Confidence rubric

HIGH (auto-delete):

• Marked deprecated AND zero references in the repo (verified by grep across all relevant extensions, NOT just the file's own).
• Fallback branch where the condition is provably always-true or always-false in all environments AND has been so for 90+ days (per git blame on the env file).
• Files in legacy/ or _archive/ directories with zero references in non-legacy code.

MEDIUM (report only):

• Deprecated with 1-N callers — needs migration plan.
• Fallback branch where condition varies by env or is recent.
• Symbols deprecated &1 || npx tsc --noEmit && npx eslint .

bun test 2>&1 pytest 2>&1 go test ./... 2>&1 cargo test 2>&1

If knip is installed, run it — newly-orphaned exports may surface

bunx knip 2>&1 || true

If verify fails: revert and downgrade. The cascade step is the most likely failure source — a "dead" helper might have been used by something the initial grep missed.

## Output

- "Removed N deprecated items, M dead branches. K items deferred for migration planning."
- Report path.
- Verify status.

## NEVER

- Delete a `@deprecated` symbol while it has any callers, even if the migration looks "obvious."
- Remove a feature flag branch without checking ALL config environments (`.env*`, `config/*.json`, flag service settings).
- Delete files in `legacy/` directories that other production code still imports — verify first.
- Remove version-check branches in libraries that support multiple runtime versions (e.g., a polyfill).
- Delete migration files, even if they're "old."
- Remove a fallback branch protecting against runtime conditions (network failure, missing env, etc.) — those aren't dead code.
- Auto-remove a feature flag definition while CI/CD or infrastructure references it (check Terraform, GitHub Actions, deployment scripts).

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [raintree-technology](https://github.com/raintree-technology)
- **Source:** [raintree-technology/agent-starter](https://github.com/raintree-technology/agent-starter)
- **License:** MIT
- **Homepage:** https://claude.raintree.technology

Install and usage instructions live in the source repository linked above.