fallow flags

CLI reference for fallow flags. Detect feature flag patterns, environment variable gates, SDK calls, and config-based toggles across your codebase.

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

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

PostHog

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

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

Basic analysis

# Detect all feature flags
fallow flags

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

JSON output

$ fallow flags --format json

{
  "schema_version": 3,
  "version": "2.88.3",
  "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.

See also

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.