fallow: codebase intelligence for TypeScript and JavaScript

Detect feature flag patterns in your codebase. Fallow identifies environment variable flags (process.env.FEATURE_*), SDK calls from common providers (LaunchDarkly, Statsig, Unleash, GrowthBook, Split, PostHog, Vercel Flags, ConfigCat, Flagsmith, Optimizely, Eppo), and config object patterns (opt-in). Reports flag locations, detection confidence, and cross-references with dead code findings.

Use fallow flags --format json for structured output that agents and scripts can parse. Combine with --top N to focus on the most common flags.

fallow flags

Options

Output

Flag	Description
`--top <N>`	Show only the top N flags
`-f, --format <FORMAT>`	Output format: `human` (default), `json`, `sarif`, `compact`, `markdown`, `codeclimate`, `gitlab-codequality`, `pr-comment-github`, `pr-comment-gitlab`, `review-github`, `review-gitlab`
`-q, --quiet`	Suppress progress output
`--explain`	Add metric explanations. In JSON format, adds a `_meta` object with descriptions and docs links.
`--summary`	Print a one-line summary of flag counts at the end of the run. In JSON format, adds a `summary` counts object.

Scoping

Flag	Description
`-r, --root <PATH>`	Project root directory (default: current working directory)
`-c, --config <PATH>`	Path to config file (default: auto-detected)
`-w, --workspace <NAME>`	Scope output to a single workspace package
`--changed-since <REF>`	Only report flags in files changed since a git ref
`--production`	Production mode: exclude test/story/dev files
`--no-production`	Force production mode off, overriding a project config’s `production: true` (conflicts with `--production`)

Performance

Flag	Description
`--no-cache`	Disable incremental caching (force full re-parse)
`--threads <N>`	Number of parser threads (default: available parallelism)
`--performance`	Show pipeline timing breakdown

CI

Flag	Description
`--ci`	CI mode: equivalent to `--format sarif --fail-on-issues --quiet`
`--fail-on-issues`	Exit with code 1 if flags are found
`--sarif-file <PATH>`	Write SARIF output to a file (in addition to `--format`)
`--group-by <MODE>`	Group output by `owner` (CODEOWNERS), `directory` (first path component), `package` (workspace package), or `section` (GitLab CODEOWNERS `[Section]` headers). See global flags.

Regression

Flag	Description
`--baseline <PATH>`	Compare against a previously saved baseline file
`--save-baseline <PATH>`	Save current results as a baseline file
`--fail-on-regression`	Fail if flag count increased beyond tolerance vs a regression baseline
`--tolerance <N>`	Allowed increase: `"2%"` (percentage) or `"5"` (absolute). Default: `"0"`
`--regression-baseline <PATH>`	Path to regression baseline file (default: `.fallow/regression-baseline.json`)
`--save-regression-baseline <PATH>`	Save current flag counts as a regression baseline

Detection categories

Fallow detects three categories of feature flag patterns:

Category	What it finds	Examples
Environment variable flags	`process.env.*` checks used as feature gates	`process.env.FEATURE_NEW_UI`, `process.env.ENABLE_DARK_MODE`
SDK calls	Feature flag SDK method calls from known providers	LaunchDarkly `variation()`, Statsig `checkGate()`, PostHog `useFeatureFlagEnabled()`, Vercel Flags `flag({ key })`
Config object patterns	Object property lookups used as boolean guards (opt-in)	`config.featureX`, `settings.enableNewFlow`

Each detected flag includes a confidence level indicating how certain fallow is that the pattern represents a feature flag.

Configuration

Fallow ships with built-in detectors for the common providers and env-var conventions, so most projects need no configuration. If fallow flags reports No feature flags detected but your project does use flags, you are likely on an SDK or env prefix fallow does not recognize by default. Extend detection with the flags section in .fallowrc.json, .fallowrc.jsonc, fallow.toml, or .fallow.toml. When you run fallow flags on full defaults and nothing is found, the CLI lists the built-in env prefixes and SDKs it scanned for, so you can tell a true negative from a missing detector.

Schema

Field	Type	Default	Description
`flags.sdkPatterns`	array	`[]`	Custom SDK call patterns, merged with the built-in providers.
`flags.sdkPatterns[].function`	string	required	Function name to match (e.g. `"useFlag"`).
`flags.sdkPatterns[].nameArg`	integer	`0`	Zero-based index of the argument holding the flag name.
`flags.sdkPatterns[].provider`	string	(none)	Optional provider label shown in output.
`flags.envPrefixes`	array	`[]`	Extra `process.env.*` prefixes treated as flags, merged with the built-ins.
`flags.configObjectHeuristics`	boolean	`false`	Detect property accesses on `feature`/`flag`/`toggle` objects as low-confidence flags. Opt-in (higher false-positive rate).

Custom sdkPatterns and envPrefixes are merged with the built-in sets, never replace them.

Examples

{
  "flags": {
    "sdkPatterns": [
      { "function": "isFeatureEnabled", "nameArg": 0, "provider": "PostHog" }
    ]
  }
}

{
  "flags": {
    "sdkPatterns": [
      { "function": "getValueAsync", "nameArg": 0, "provider": "ConfigCat" }
    ]
  }
}

{
  "flags": {
    "sdkPatterns": [
      { "function": "flag", "nameArg": 0, "provider": "Vercel Flags" }
    ],
    "envPrefixes": ["NEXT_PUBLIC_FEATURE_", "MYAPP_ENABLE_"]
  }
}

{
  "flags": {
    "sdkPatterns": [
      { "function": "useMyFlag", "nameArg": 0, "provider": "InHouse" }
    ],
    "configObjectHeuristics": true
  }
}

Several providers shown above (PostHog, ConfigCat, Vercel Flags) are already built in, so these examples are illustrative. Reach for sdkPatterns when fallow does not recognize your wrapper function or you use an in-house SDK.

Examples

# Detect all feature flags
fallow flags

# Show only the top 10 flags
fallow flags --top 10

# JSON output for pipelines
fallow flags --format json --quiet

# SARIF for GitHub Code Scanning
fallow flags --format sarif > flags.sarif

# Compact format (one line per flag)
fallow flags --format compact

# Markdown for PR comments
fallow flags --format markdown | gh pr comment --body-file -

# CodeClimate for GitLab CI
fallow flags --format codeclimate > gl-code-quality-report.json

# Only flags in changed files
fallow flags --changed-since main

# Scope to a workspace package
fallow flags --workspace @app/core

# Production code only
fallow flags --production

JSON output

$ fallow flags --format json

{
  "schema_version": 8,
  "version": "3.17.0",
  "elapsed_ms": 116,
  "feature_flags": [],
  "total_flags": 0
}

Key fields

Field	Type	Description
`schema_version`	integer	JSON schema version (currently 3)
`version`	string	Fallow version that produced the output
`elapsed_ms`	integer	Analysis duration in milliseconds
`feature_flags`	array	Detected feature flag patterns
`total_flags`	integer	Total number of detected flags

MCP tool

The feature_flags MCP tool wraps fallow flags --format json --quiet --explain:

Example request

{
  "tool": "feature_flags",
  "arguments": {
    "top": 10
  }
}

The response always includes _meta explanatory metadata (the MCP wrapper enables --explain by default). Returns the same JSON envelope as the CLI. See MCP integration for setup instructions.

Dead code analysis

Cross-references feature flags with dead code findings.

Global flags

Flags available on every fallow command.

MCP integration

Use fallow tools from AI coding agents.

Commands

fallow flags

Options

Output

Scoping

Performance

CI

Regression

Detection categories

Configuration

Schema

Examples

Examples

JSON output

Key fields

MCP tool

See also

Dead code analysis

Global flags

MCP integration

​Options

​Output

​Scoping

​Performance

​CI

​Regression

​Detection categories

​Configuration

​Schema

​Examples

​Examples

​JSON output

​Key fields

​MCP tool

​See also

Dead code analysis

Global flags

MCP integration

Options

Output

Scoping

Performance

CI

Regression

Detection categories

Configuration

Schema

Examples

Examples

JSON output

Key fields

MCP tool

See also