fallow

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:

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: complete command and flag specifications
• Gotchas: common pitfalls, edge cases, and correct usage patterns
• Patterns: workflow recipes for CI, monorepos, migration, and incremental adoption

Common Workflows

Audit a project for all dead code

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).

Find only unused exports (smaller output)

fallow dead-code --format json --quiet --unused-exports

Check if a PR introduces dead code

fallow dead-code --format json --quiet --changed-since main --fail-on-issues

Exit code 1 if new dead code is introduced. Only analyzes files changed since the main branch.

Find code duplication

fallow dupes --format json --quiet
fallow dupes --format json --quiet --mode semantic

The semantic mode detects renamed variables. Other modes: strict (exact), mild (default, syntax normalized), weak (different literals).

Safe auto-fix cycle

# 1. Preview what will be removed
fallow fix --dry-run --format json --quiet

# 2. Review the output, then apply
fallow fix --yes --format json --quiet

# 3. Verify the fix worked
fallow dead-code --format json --quiet

The --yes flag is required in non-TTY environments (agent subprocesses). Without it, fix exits with code 2.

Discover project structure

fallow list --entry-points --format json --quiet
fallow list --plugins --format json --quiet

Shows detected entry points and active framework plugins (90 built-in: Next.js, Vite, Jest, Storybook, Tailwind, etc.).

Production-only analysis

fallow dead-code --format json --quiet --production

Excludes test/dev files (*.test.*, *.spec.*, *.stories.*) and only analyzes production scripts.

Analyze a single workspace package

fallow dead-code --format json --quiet --workspace my-package

Scopes output to one package while keeping the full cross-workspace graph.

Scope to specific files (lint-staged)

fallow dead-code --format json --quiet --file src/utils.ts --file src/helpers.ts

Only reports issues in the specified files. Project-wide dependency issues are suppressed. Warns on non-existent paths.

Catch typos in entry file exports

fallow dead-code --format json --quiet --include-entry-exports

Reports unused exports in entry files (package.json main/exports, framework pages). By default, exports in entry files are assumed externally consumed. This flag catches typos like meatdata instead of metadata.

Debug why something is flagged

# Trace an export's usage chain
fallow dead-code --format json --quiet --trace src/utils.ts:myFunction

# Trace all edges for a file
fallow dead-code --format json --quiet --trace-file src/utils.ts

# Trace where a dependency is used
fallow dead-code --format json --quiet --trace-dependency lodash

Migrate from knip or jscpd

# Preview migration
fallow migrate --dry-run

# Apply migration (creates .fallowrc.json)
fallow migrate

# Migrate to TOML (creates fallow.toml)
fallow migrate --toml

Auto-detects knip.json, .knip.json, .jscpd.json, and package.json embedded configs.

Initialize a new config

fallow init              # creates .fallowrc.json, adds .fallow/ to .gitignore
fallow init --toml       # creates fallow.toml, adds .fallow/ to .gitignore
fallow init --hooks      # scaffold a pre-commit git hook
fallow init --hooks --branch develop  # hook using custom base branch

Exit Codes

Code	Meaning
0	Success, no error-severity issues
1	Error-severity issues found
2	Runtime error (invalid config, parse failure, or fix without --yes in non-TTY)

When --format json is active and exit code is 2, errors are emitted as JSON on stdout:

{"error": true, "message": "invalid config: ...", "exit_code": 2}

Configuration

Fallow reads config from project root: .fallowrc.json > fallow.toml > .fallow.toml. Most projects work with zero configuration thanks to 90 auto-detecting framework plugins.

{
  "$schema": "https://raw.githubusercontent.com/fallow-rs/fallow/main/schema.json",
  "entry": ["src/index.ts"],
  "ignorePatterns": ["**/*.generated.ts"],
  "ignoreDependencies": ["autoprefixer"],
  "publicPackages": ["@myorg/shared-lib"],
  "dynamicallyLoaded": ["plugins/**/*.ts"],
  "rules": {
    "unused-files": "error",
    "unused-exports": "warn",
    "unused-types": "off"
  }
}

Rules: "error" (fail CI), "warn" (report only), "off" (skip detection).

Config fields:

• publicPackages: workspace packages that are public libraries; exports from these packages are not flagged as unused
• dynamicallyLoaded: glob patterns for files loaded at runtime (plugin dirs, locale files); treated as always-used
• usedClassMembers: class method/property names that extend the built-in Angular/React lifecycle allowlist with framework-invoked names (ag-Grid agInit/refresh, TypeORM up/down, Web Components connectedCallback, etc.). Use plugin-level usedClassMembers in a .fallow/plugins/*.jsonc file for library-specific allowlists

Inline suppression

// fallow-ignore-next-line
export const keepThis = 1;

// fallow-ignore-next-line unused-export
export const keepThisToo = 2;

// fallow-ignore-file
// fallow-ignore-file unused-export

// Mark as intentionally unused (tracked for staleness)
/** @expected-unused */
export const deprecatedHelper = () => {};

Key Gotchas

• fix --yes is required in non-TTY (agent) environments. Without it, fix exits with code 2
• Zero config by default. 90 framework plugins auto-detect. Don't create config unless customization is needed
• Syntactic analysis only. No TypeScript compiler, so fully dynamic import(variable) is not resolved
• Function overloads are deduplicated. TypeScript function overload signatures are merged into a single export (not reported as separate unused exports)
• Re-export chains are resolved. Exports through barrel files are tracked, not falsely flagged
• --changed-since is additive. Only new issues in changed files, not all issues in the project

For the full list with examples, see references/gotchas.md.

Instructions

1. Identify the task from the user's request (audit, fix, find dupes, set up CI, migrate, debug)
2. Run the appropriate command with --format json --quiet
3. Use filter flags to limit output when the user asks about specific issue types
4. Always dry-run before fix. Show the user what will change, then apply
5. Report results clearly. Summarize issue counts, list specific findings, suggest next steps
6. For false positives, suggest inline suppression comments or config rule adjustments

If $ARGUMENTS is provided, use it as the --root path or pass it as the target for the appropriate fallow command.