sc30gsw/sample/harness/next-js-pages/skills/fallow

Fallow: codebase intelligence for JavaScript and TypeScript

Codebase intelligence for JavaScript and TypeScript. The free static layer finds unused code, circular dependencies, code duplication, complexity hotspots, architecture boundary violations, and feature flag patterns. Runtime coverage merges production execution data into the same fallow health report for hot-path review, cold-path deletion confidence, and stale-flag evidence: a single local capture is free, while continuous/cloud runtime monitoring is paid. 90 framework plugins, zero configuration, sub-second static analysis.

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, --production, --production-dead-code, --production-health, --production-dupes, --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, --changed-workspaces, --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, --workspace, --changed-workspaces, --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 (also covers Angular templates as synthetic <template> findings: external .html files via templateUrl AND inline @Component({ template: \...` })literals; suppress external with<!-- fallow-ignore-file complexity -->at the top of the.htmlfile, suppress inline with// fallow-ignore-next-line complexitydirectly above the@Componentdecorator) |--complexity, --max-cyclomatic, --max-cognitive, --max-crap, --top, --sort, --file-scores, --hotspots, --ownership, --ownership-emails, --targets, --effort, --score, --min-score, --since, --min-commits, --save-snapshot, --trend, --coverage-gaps, --coverage, --coverage-root, --runtime-coverage, --min-invocations-hot, --min-observation-volume, --low-traffic-threshold, --workspace, --changed-workspaces, --baseline, --save-baseline| |audit | Combined dead-code + complexity + duplication for changed files |--base, --gate, --production, --production-dead-code, --production-health, --production-dupes, --workspace, --changed-workspaces, --ci, --fail-on-issues, --explain, --dead-code-baseline, --health-baseline, --dupes-baseline, --max-crap | |flags | Detect feature flag patterns (env vars, SDK calls, config objects) |--top | |explain | Explain one issue type without running analysis |<issue-type>, --format json | |license | Manage the local license JWT for continuous/cloud runtime monitoring (activate, status, refresh, deactivate) |activate --trial --email <addr>, activate --from-file, activate --stdin, status, refresh, deactivate | |coverage | Runtime coverage setup, focused analysis, and cloud inventory workflow helper |setup, setup --yes, setup --non-interactive, analyze --runtime-coverage <path>, analyze --cloud --repo owner/repo, upload-inventory | |coverage upload-source-maps| Upload build source maps from CI so bundled runtime coverage resolves to original source paths |--dir dist, --git-sha <sha>, --repo <name>, --strip-path=false, --dry-run | |schema | Dump CLI definition as JSON | | |config | Show the loaded config path and resolved config (verifies which.fallowrc.jsonis 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 | | Private type leaks | --private-type-leaks | Opt-in API hygiene check (default off) for exported signatures whose type references a same-file private type | | Unused dependencies | --unused-deps | Packages in dependencies, devDependencies, optionalDependencies, type-only production deps, and test-only production deps. In monorepos, internal workspace package names (e.g., @repo/ui) declared in another workspace's package.json but never imported are reported here too. | | 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. In monorepos, importing a workspace package from a workspace whose own package.json does not list it is reported here too; self-references stay allowed without requiring a package to depend on itself. | | 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 | n/a | 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 (unused files/exports/types/dependencies/members + circular dependencies + boundary violations + stale suppressions). Private type leaks are an opt-in API hygiene check via issue_types: ["private-type-leaks"]. Set boundary_violations: true as a convenience alias for issue_types: ["boundary-violations"]. Set group_by to "owner", "directory", "package", or "section" to partition results. The section mode reads GitLab CODEOWNERS [Section] headers and emits owners metadata per group | | 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. Set group_by to owner, directory, package, or section for per-group vital_signs / health_score; SARIF results gain properties.group, CodeClimate issues gain a top-level group field | | check_runtime_coverage | Merge V8 or Istanbul runtime-coverage data into the health report. One local capture is free; continuous/cloud or multi-capture runtime monitoring is paid. Required coverage param (V8 dir, V8 JSON, or Istanbul coverage-final.json). Tuning knobs: min_invocations_hot (default 100), min_observation_volume (default 5000), low_traffic_threshold (default 0.001), max_crap (default 30.0), top, group_by. Long dumps may exceed the 120s MCP timeout; raise FALLOW_TIMEOUT_SECS. Pick this over check_health when you have a coverage dump. | | get_hot_paths | Runtime-context slice over the same runtime coverage pipeline. Same params as check_runtime_coverage; read runtime_coverage.hot_paths for production hot paths. | | get_blast_radius | Runtime-context slice for blast-radius review. Same params as check_runtime_coverage; until runtime_coverage.blast_radius ships, combine file_scores[].fan_in, runtime_coverage.hot_paths, and runtime_coverage.findings. | | get_importance | Runtime-context slice for production-importance review. Same params as check_runtime_coverage; until runtime_coverage.importance ships, combine runtime_coverage.hot_paths, file_scores, hotspots, and targets. | | get_cleanup_candidates | Runtime-context slice for cleanup review. Same params as check_runtime_coverage; read runtime_coverage.findings for safe_to_delete, review_required, low_traffic, and coverage_unavailable. | | audit | Combined dead-code + complexity + duplication for changed files, returns verdict. Set gate to "new-only" or "all" | | fallow_explain | Explain one issue type without running analysis. Required issue_type; returns rationale, examples, fix guidance, and docs URL | | 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 | | feature_flags | Detect feature flag patterns (env vars, SDK calls, config objects). Set top to limit results | | trace_export | Trace why an export is used or unused (fallow dead-code --trace FILE:EXPORT_NAME --format json). Required file and export_name. Returns file reachability, entry-point status, direct references, re-export chains, and a reason string. Use before deleting a supposedly-unused export | | trace_file | Trace all graph edges for a file (fallow dead-code --trace-file PATH --format json). Required file. Returns reachability, exports, imports-from, imported-by, and re-exports. Use to decide whether a file is isolated, barrel-only, or imported by live entry points | | trace_dependency | Trace where a dependency is imported (fallow dead-code --trace-dependency PACKAGE --format json). Required package_name. Returns importing files, type-only importers, total import count, used_in_scripts (true when invoked from package.json scripts or CI configs), and is_used (combined import + script signal; mirrors the unused-deps detector so build tools like microbundle or vitest are not falsely flagged as unused). Use before removing a dependency or moving between dependencies and devDependencies | | trace_clone | Trace duplicate-code groups at a location (fallow dupes --trace FILE:LINE --format json). Required file and line. Returns the matched clone instance plus every clone group containing it. Supports mode, min_tokens, min_lines, threshold, skip_local, cross_language, ignore_imports. Use to consolidate duplication when you need the exact sibling locations |

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.

Node.js Bindings

When embedding fallow inside a Node.js process (editor extensions, long-running servers, custom tooling), prefer the NAPI bindings over spawning the CLI. Same analysis engine, same JSON envelopes, no subprocess or JSON parsing overhead.

npm install @fallow-cli/fallow-node

import { detectDeadCode, detectDuplication, computeHealth } from '@fallow-cli/fallow-node'

const deadCode = await detectDeadCode({ root: process.cwd(), explain: true })
const dupes = await detectDuplication({ root: process.cwd(), mode: 'mild', minTokens: 30 })
const health = await computeHealth({ root: process.cwd(), score: true, ownershipEmails: 'handle' })

Six async functions: detectDeadCode, detectCircularDependencies, detectBoundaryViolations, detectDuplication, computeComplexity, computeHealth. Each returns the same JSON envelope the CLI emits for --format json. Rejected promises throw a FallowNodeError with message, exitCode, and optional code, help, context fields that mirror the CLI's structured error surface.

Enum-like fields take lowercase CLI-style literals ("mild", "cyclomatic", "handle", "low"). Write-path commands (fix, init, setup-hooks, license activate, coverage setup) are not exposed; use the CLI for those.

See https://docs.fallow.tools/integrations/node-bindings for the full field reference.

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). For dependency findings, a non-empty used_in_workspaces array means the package is imported elsewhere in the monorepo; treat it as a workspace placement issue and do not auto-remove it.

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 (91 built-in: Next.js, Vite, Jest, Storybook, Tailwind, PandaCSS, etc.).

Production-only analysis

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

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

Analyze specific workspaces

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

# Multiple packages
fallow dead-code --format json --quiet --workspace web,admin

# Glob (matched against package name AND workspace path)
fallow dead-code --format json --quiet --workspace 'apps/*'

# Exclude one workspace from a set
fallow dead-code --format json --quiet --workspace 'apps/*,!apps/legacy'

# Monorepo CI: auto-scope to workspaces containing any file changed since origin/main
# (replaces hand-written --workspace lists that drift as the repo evolves)
fallow dead-code --format json --quiet --changed-workspaces origin/main

Scopes output while keeping the full cross-workspace graph. Patterns are tested against BOTH the package name (from package.json) AND the workspace path relative to the repo root; either match counts. Use !-prefixed patterns to exclude.

--changed-workspaces <REF> auto-derives the set from git diff. It's the CI primitive: point it at the PR base branch (e.g. origin/main) and fallow reports only on workspaces touched by the change. Mutually exclusive with --workspace. A missing ref or non-git directory is a hard error (exit 2) rather than a silent full-scope fallback, so CI never quietly widens back to the whole monorepo.

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 > .fallowrc.jsonc > fallow.toml > .fallow.toml. Both .fallowrc.json and .fallowrc.jsonc accept JSON-with-comments syntax (same parser); the .jsonc extension lets editors auto-detect JSONC syntax highlighting. 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"],
  "ignoreExportsUsedInFile": true,
  "publicPackages": ["@myorg/shared-lib"],
  "dynamicallyLoaded": ["plugins/**/*.ts"],
  "rules": {
    "unused-files": "error",
    "unused-exports": "warn",
    "unused-types": "off",
    "private-type-leaks": "warn",
  },
}

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

Config fields:

• ignoreExportsUsedInFile: knip-compatible; suppress unused-export findings when the exported symbol is referenced inside the file that declares it. Boolean (true covers all kinds) or { "type": true, "interface": true } object form for knip parity. Fallow groups type aliases and interfaces under the same unused-types issue, so both type-kind fields behave identically. References inside the export specifier itself (export { foo }, export default foo) do not count as same-file uses; those exports are still reported when no other in-file expression references the binding
• 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. Each entry is a plain string (global suppression) or a scoped object { extends?, implements?, members } matching only classes with the given heritage. Use scoped rules for common names like refresh or execute to avoid false negatives on unrelated classes; global strings for unique names like agInit. Example: ["agInit", { "implements": "ICellRendererAngularComp", "members": ["refresh"] }, { "extends": "BaseCommand", "members": ["execute"] }]. An unconstrained scoped rule (no extends or implements) is rejected at load time. Use plugin-level usedClassMembers in a .fallow/plugins/*.jsonc file for library-specific allowlists
• resolve.conditions: additional package.json exports / imports condition names to honor during module resolution. Baseline conditions (development, import, require, default, types, node, plus react-native / browser under RN/Expo) are always included; user entries prepend ahead of them. Use for community conditions like worker, edge-light, deno, or custom bundler conditions. Example: { "resolve": { "conditions": ["worker", "edge-light"] } }

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.