fallow

Files (5)

• plugin.json · .claude-plugin/
• SKILL.md
• cli-reference.md · references/
• gotchas.md · references/
• patterns.md · references/

SKILL.md (first lines)

# Fallow: Codebase Analyzer

The codebase analyzer for JavaScript and TypeScript. Finds unused code, circular dependencies, code duplication, complexity hotspots, architecture boundary violations, and feature flag patterns. 90 framework plugins, zero configuration, sub-second performance.

## When to Use

- Finding dead code (unused files, exports, types, enum/class members)
- Finding unused or unlisted dependencies
- Detecting code duplication and clones
- Checking code health and complexity hotspots
- Cleaning up a codebase before a release or refactor
- Auditing a project for structural issues
- Setting up CI checks for dead code or duplication thresholds
- Auto-fixing unused exports and dependencies
- Detecting feature flag patterns (environment gates, SDK calls, config objects)
- Investigating why a specific export or file appears unused

## When NOT to Use

- Runtime error analysis or debugging
- Type checking (use `tsc` for that)
- Linting style or formatting issues (use ESLint, Biome, Prettier)
- Security vulnerability scanning
- Bundle size analysis
- Projects that are not JavaScript or TypeScript

## Prerequisites

Fallow must be installed. If not available, install it:

```bash
npm install -g fallow          # prebuilt binaries (fastest)
# or
npx fallow dead-code               # run without installing
# or
cargo install fallow-cli        # build from source
```

## Agent Rules

1. **Always use `--format json --quiet 2>/dev/null`** for machine-readable output. The `2>/dev/null` discards stderr so progress messages and threshold warnings don't corrupt the JSON on stdout. Never use `2>&1`
2. **Always append `|| true`** to every fallow command. Exit code 1 means "issues found" (normal), not a runtime error. Without `|| true`, the Bash tool treats exit 1 as failure and cancels parallel commands. Only exit code 2 is a real error (invalid config, parse failure)
3. **Use `--explain`** to include a `_meta` object in JSON output with metric definitions, ranges, and interpretation hints
4. **Use issue type filters** (`--unused-exports`, `--unused-files`, etc.) to limit output scope
5. **Always `--dry-run` before `fix`**, then `fix --yes` to apply
6. **All output paths are relative** to the project root
7. **Never run `fallow watch`**. It is interactive and never exits

## Commands

| Command | Purpose | Key Flags |
|---------|---------|-----------|
| `fallow` | Run all analyses: dead code + duplication + complexity (default) | `--only`, `--skip`, `--ci`, `--fail-on-issues`, `--group-by`, `--summary`, `--fail-on-regression`, `--tolerance`, `--regression-baseline`, `--save-regression-baseline`, `--score`, `--trend`, `--save-snapshot` |
| `dead-code` | Dead code analysis (`check` is an alias) | `--unused-exports`, `--changed-since`, `--production`, `--file`, `--include-entry-exports`, `--stale-suppressions`, `--ci`, `--group-by`, `--summary`, `--fail-on-regression`, `--tolerance`, `--regression-baseline`, `--save-regression-baseline` |
| `dupes` | Code duplication detection | `--mode`, `--threshold`, `--top`, `--changed-since`, `--skip-local`, `--cross-language`, `--ignore-imports`, `--fail-on-regression`, `--tolerance`, `--regression-baseline`, `--save-regression-baseline` |
| `fix` | Auto-remove unused exports/deps | `--dry-run`, `--yes` (required in non-TTY) |
| `init` | Generate config file or pre-commit hook | `--toml`, `--hooks`, `--branch` |
| `migrate` | Convert knip/jscpd config | `--dry-run`, `--from PATH` |
| `list` | Inspect project structure | `--files`, `--entry-points`, `--plugins`, `--boundaries` |
| `health` | Function complexity analysis | `--complexity`, `--max-cyclomatic`, `--max-cognitive`, `--top`, `--sort`, `--file-scores`, `--hotspots`, `--targets`, `--effort`, `--score`, `--min-score`, `--since`, `--min-commits`, `--save-snapshot`, `--trend`, `--coverage-gaps`, `--workspace`, `--baseline`, `--save-baseline` |
| `audit` | Combined dead-code + complexity + duplication for changed files | `--base`, `--production`, `--workspace`, `--ci`, `--fail-on-issues`, `--explain` |
| `flags` | Detect feature flag patterns (env vars, SDK calls, config objects) | `--top` |
| `schema` | Dump CLI definition as JSON | |
| `config` | Show the loaded config path and resolved config (verifies which `.fallowrc.json` is in effect) | `--path` |

## Issue Types

| Type | Filter Flag | Description |
|------|-------------|-------------|
| Unused files | `--unused-files` | Files unreachable from entry points |
| Unused exports | `--unused-exports` | Symbols never imported elsewhere |
| Unused types | `--unused-types` | Type aliases and interfaces |
| Unused dependencies | `--unused-deps` | Packages in `dependencies`, `devDependencies`, `optionalDependencies`, type-only production deps, and test-only production deps |
| Unused enum members | `--unused-enum-members` | Enum values never referenced |
| Unused class members | `--unused-class-members` | Methods and properties |
| Unresolved imports | `--unresolved-imports` | Imports that can't be resolved |
| Unlisted dependencies | `--unlisted-deps` | Used packages missing from package.json |
| Duplicate exports | `--duplicate-exports` | Same symbol exported from multiple modules |
| Circular dependencies | `--circular-deps` | Import cycles in the module graph |
| Boundary violations | `--boundary-violations` | Imports crossing architecture zone boundaries. Presets: `layered`, `hexagonal`, `feature-sliced`, `bulletproof` |
| Stale suppressions | `--stale-suppressions` | `fallow-ignore` comments or `@expected-unused` JSDoc tags that no longer match any issue |
| Test-only dependencies | — | Production deps only imported from test files (should be devDependencies) |

## MCP Tools

When using fallow via MCP (`fallow-mcp`), the following tools are available:

| Tool | Description |
|------|-------------|
| `analyze` | Full dead code analysis. Set `boundary_violations: true` as a convenience alias for `issue_types: ["boundary-violations"]`. Set `group_by` to `"owner"` or `"directory"` to partition results |
| `check_changed` | Incremental analysis of files changed since a git ref |
| `find_dupes` | Code duplication detection. Set `changed_since` to scope to changed files since a git ref |
| `fix_preview` | Dry-run auto-fix preview |
| `fix_apply` | Apply auto-fixes (destructive) |
| `check_health` | Complexity metrics, health scores, hotspots, and refactoring targets |
| `audit` | Combined dead-code + complexity + duplication for changed files, returns verdict |
| `project_info` | Project metadata. Set `entry_points`, `files`, `plugins`, or `boundaries` to `true` to request specific sections |
| `list_boundaries` | Architecture boundary zones and access rules. Returns `{"configured": false}` if no boundaries configured |
| `detect_flags` | Detect feature flag patterns (env vars, SDK calls, config objects). Set `top` to limit results |

All tools accept `root`, `config`, `no_cache`, and `threads` params. The MCP server subprocess timeout defaults to 120s, configurable via `FALLOW_TIMEOUT_SECS`.

All JSON responses include structured `actions` arrays on every finding (dead code, health, duplication), enabling programmatic fix application or suppression.

## References

- [CLI Reference](references/cli-reference.md): complete command and flag specifications
- [Gotchas](references/gotchas.md): common pitfalls, edge cases, and correct usage patterns
- [Patterns](references/patterns.md): workflow recipes for CI, monorepos, migration, and incremental adoption

## Common Workflows

### Audit a project for all dead code

```bash
fallow dead-code --format json --quiet
```

Parse the JSON output. It contains arrays for each issue type (`unused_files`, `unused_exports`, `unused_types`, `unused_dependencies`, etc.) plus `total_issues` and `elapsed_ms` metadata. Each issue object includes an `actions` array with structured fix suggestions (action type, `auto_fixable` flag, description, and optional suppression comment).

From stolinski/s-stack, MIT.

More from stolinski/s-stack