swiftlint

SKILL.md

SwiftLint

SwiftLint enforces Swift style and conventions by linting source files against a configurable rule set. It is the most widely adopted Swift linter. This skill covers setup, configuration, rule selection, suppression, CI integration, and rollout strategy.

SwiftLint is a style enforcement tool, not a style guide. For underlying Swift naming and design conventions, see swift-api-design-guidelines. For architecture patterns, see swift-architecture.

Contents

Recommended Setup

Configuration

Rule Selection Strategy

Suppressions

Baselines

Autocorrect

CI Integration

Integration Decision Tree

Multiple Configurations

Common Mistakes

Review Checklist

References

Recommended Setup

Default: build tool plugin via SimplyDanny/SwiftLintPlugins.

Add the plugin package to Package.swift or via Xcode's package dependencies:

// Package.swift
dependencies: [
  .package(url: "https://github.com/SimplyDanny/SwiftLintPlugins", from: "<reviewed-version>")
]

For SwiftPM targets, apply the plugin:

.target(
    name: "MyApp",
    plugins: [.plugin(name: "SwiftLintBuildToolPlugin", package: "SwiftLintPlugins")]
)

For Xcode projects without a Package.swift, add the package dependency in the project settings, then enable the plugin under the target's Build Phases or the package's plugin trust dialog.

The build tool plugin runs SwiftLint automatically on every build. No run script required.

First build: Xcode prompts to trust the plugin. Select "Trust & Enable All" for the SwiftLintPlugins package.

For alternatives (run scripts, command plugin, Homebrew CLI), see references/plugins-run-scripts-and-integrations.md.

Configuration

Create .swiftlint.yml at the project root. SwiftLint discovers this file by walking up from each source file's directory.

# .swiftlint.yml — conservative starter config
disabled_rules:
  - trailing_whitespace
  - todo

opt_in_rules:
  - empty_count
  - closure_spacing
  - force_unwrapping
  - sorted_imports
  - vertical_whitespace_opening_braces
  - private_swiftui_state
  - unhandled_throwing_task
  - accessibility_label_for_image

included:
  - Sources
  - Tests

excluded:
  - .build
  - DerivedData
  - "**/.build"
  - "**/Generated"

line_length:
  warning: 140
  error: 200

type_body_length:
  warning: 300
  error: 500

file_length:
  warning: 500
  error: 1000

Key configuration options:

Key
Purpose

disabled_rules
Turn off default-enabled rules

opt_in_rules
Turn on rules not enabled by default

only_rules
Use only the listed rules (mutually exclusive with disabled_rules/opt_in_rules)

analyzer_rules
Rules requiring compiler logs (run via swiftlint analyze)

baseline
Path to an existing baseline file used to suppress known violations

write_baseline
Path where SwiftLint should write a new baseline file

included
Paths to lint (default: current directory)

excluded
Paths to skip

strict
Elevate all warnings to errors

lenient
Downgrade all errors to warnings

allow_zero_lintable_files
Suppress the error when no Swift files are found

reporter
Output format: xcode (default), json, checkstyle, sarif, csv, emoji, etc.

For full configuration details including severity tuning, environment-variable interpolation, and nested/remote configs, see references/adoption-and-configuration.md.

Rule Selection Strategy

SwiftLint ships with three rule categories:

Default rules — enabled automatically, cover widely accepted conventions

Opt-in rules — disabled by default, enable selectively via opt_in_rules

Analyzer rules — require compiler logs, enabled via analyzer_rules

Browse the full categorized list at https://realm.github.io/SwiftLint/rule-directory.html.

Recommended approach for new projects:

Start with defaults. Run swiftlint rules to see which rules are enabled.

Disable rules that conflict with your team's established conventions.

Add opt-in rules one at a time. Review violations before committing each addition.

Do not use only_rules unless you have a specific reason to start from zero.

Recommended approach for existing codebases:

Start with the default rule set.

Create a baseline (see Baselines) to suppress all existing violations.

Enforce zero new violations in CI.

Burn down baseline violations incrementally.

Do not transcribe or memorize the rule directory. Look up rule identifiers and configuration options at the official rule directory when needed.

Suppressions

Suppress SwiftLint for specific lines when a rule produces a false positive or when the violation is intentional and reviewed.

// swiftlint:disable:next force_cast
let view = object as! UIView

let legacy = try! JSONDecoder().decode(T.self, from: data) // swiftlint:disable:this force_try

// swiftlint:disable:previous large_tuple

Disable for a region:

// swiftlint:disable cyclomatic_complexity
func complexRouter(...) { ... }
// swiftlint:enable cyclomatic_complexity

Disable all rules (use sparingly):

// swiftlint:disable all
// ... generated or legacy code ...
// swiftlint:enable all

Policy:

Prefer targeted single-rule suppressions over all.

Always re-enable after the region ends.

For generated code, prefer excluded paths in .swiftlint.yml over inline suppressions.

For test targets with different tolerance, use a child configuration (see Multiple Configurations).

For full suppression syntax, see references/rules-suppressions-and-baselines.md.

Baselines

Baselines let you adopt SwiftLint in an existing codebase without fixing every legacy violation first.

Create a baseline:

swiftlint --write-baseline .swiftlint.baseline

This records all current violations. Future runs compare against this baseline and only report new violations.

Use the baseline:

swiftlint --baseline .swiftlint.baseline

In CI, pass --baseline so only new violations fail the build. Burn down the baseline over time by fixing legacy violations and regenerating.

For baseline workflows and rollout strategy, see references/rules-suppressions-and-baselines.md.

Autocorrect

SwiftLint can fix some violations automatically:

swiftlint --fix
# or the legacy alias:
swiftlint --autocorrect

Warnings:

Never run --fix as a pre-compile build phase. Auto-fixes modify source files. If run automatically on every build, this creates an unpredictable edit-build loop and can mask real issues.

Run --fix manually or in a dedicated CI step, then review the diff.

Not all rules support autocorrect. Check swiftlint rules — the "Correctable" column shows which rules can auto-fix.

Always commit or stash before running --fix.

CI Integration

CI is the primary enforcement surface. A CI check ensures no one merges code that increases the violation count.

Recommended CI pattern:

# GitHub Actions example
- name: Lint
  run: |
    brew install swiftlint
    swiftlint --strict --reporter sarif > swiftlint.sarif

Key CI options:

Flag
Effect

--strict
Exits non-zero on warnings (not just errors)

--reporter sarif
GitHub Advanced Security compatible output

--reporter json
Machine-readable output

--reporter checkstyle
Jenkins/SonarQube compatible

--baseline .swiftlint.baseline
Only fail on new violations

For SARIF upload to GitHub code scanning, add github/codeql-action/upload-sarif after the lint step.

For full CI recipes and reporter details, see references/plugins-run-scripts-and-integrations.md.

Integration Decision Tree

Choose how to run SwiftLint based on project shape:

Scenario
Recommended integration

SwiftPM package or Xcode project with Package.swift
Build tool plugin via SwiftLintPlugins

SwiftPM project needing CLI flags (--fix, --baseline)
Command plugin: swift package plugin swiftlint

Xcode project without SwiftPM, team uses Homebrew
Run script build phase

CI/CD pipeline
Homebrew or Docker install, run swiftlint directly

Pre-commit hook
Homebrew install + .pre-commit-config.yaml or git hook script

The build tool plugin is preferred for local development because it requires no PATH configuration, pins the SwiftLint version via package resolution, and runs automatically on build.

For detailed setup instructions for each integration, see references/plugins-run-scripts-and-integrations.md.

Multiple Configurations

SwiftLint supports layered configuration files. A .swiftlint.yml in a subdirectory inherits from and overrides the parent config.

Common patterns:

Relaxed test config: place a .swiftlint.yml in Tests/ that disables force_unwrapping and raises file_length

Strict module config: place a stricter .swiftlint.yml in a shared module directory

Remote config: use parent_config with an HTTPS URL to pull a shared team config (caching supported)

# Tests/.swiftlint.yml — child config
disabled_rules:
  - force_unwrapping
  - force_try

file_length:
  warning: 800

You can also pass multiple configs on the CLI:

swiftlint --config .swiftlint.yml --config .swiftlint-extra.yml

Later configs override earlier ones for overlapping keys.

For nested config resolution, remote configs, and CLI multi-config details, see references/adoption-and-configuration.md.

Common Mistakes

Running --fix in a build phase. Auto-fixing on every build creates unpredictable source modifications. Run --fix manually.

Using only_rules without understanding the implication. This disables all rules except those listed. Most teams should use disabled_rules + opt_in_rules instead.

Suppressing with // swiftlint:disable all and forgetting to re-enable. This silently disables all linting for the rest of the file.

Not pinning the SwiftLint version. Different versions have different default rules. Use the build tool plugin (version pinned via SPM) or pin in your Brewfile / CI config.

Excluding too broadly. Excluding Tests/ entirely means test code gets no linting. Use a child config with relaxed rules instead.

Ignoring the toolchain mismatch. SwiftLint must be built with (or compatible with) the same Swift toolchain used to compile your project. Mismatches cause parsing errors. See references/plugins-run-scripts-and-integrations.md for multi-toolchain guidance.

Adopting too many opt-in rules at once in a large codebase. This creates an overwhelming number of violations. Add rules incrementally and use baselines.

Not configuring included paths. Without included, SwiftLint scans the working directory recursively, which may pick up vendored or generated code.

Review Checklist

 .swiftlint.yml exists at the project root with explicit included/excluded paths

 SwiftLint version is pinned (via SPM plugin resolution, Brewfile, or CI config)

 Build tool plugin is enabled for each target that should be linted

 CI runs swiftlint --strict (or with --baseline for incremental adoption)

 No --fix / --autocorrect in build phases

 Inline suppressions target specific rules, not all

 Inline suppressions include a comment explaining why

 Test targets have appropriate config (relaxed rules via child config, not excluded entirely)

 Autocorrect changes are reviewed in a separate commit

 New opt-in rules are added one at a time with team consensus

References

references/adoption-and-configuration.md — Installation paths, .swiftlint.yml deep dive, severity tuning, environment variables, nested/remote configs, rollout strategy

references/plugins-run-scripts-and-integrations.md — Build tool plugin, command plugin, run scripts, CI recipes, multi-toolchain guidance, VS Code, Fastlane, Docker, pre-commit

references/rules-suppressions-and-baselines.md — Default vs opt-in vs analyzer rules, suppression syntax, baseline workflows, false-positive handling

references/rule-reference.md — Bundled exhaustive rule index for local lookup; verify current details with swiftlint rules or the official rule directory

references/custom-rules-and-analyze.md — Regex custom rules, Swift custom rules (brief), swiftlint analyze, compiler-log workflow

SwiftLint documentation — Official docs

SwiftLint rule directory — Full categorized rule list

SimplyDanny/SwiftLintPlugins — Recommended plugin package