Craft CMS 5 — Extending (Plugins & Modules)

Reference for extending Craft CMS 5 through plugins and modules. Covers everything from elements and services to controllers, migrations, fields, and events.

This skill is scoped to extending Craft — building plugins, modules, custom element types, field types, and backend integrations. For site/platform development (content modeling, sections, entry types, Twig templating, plugin selection), see the craft-site skill.

Companion Skills — Always Load Together

When this skill triggers, also load:

• craft-php-guidelines — PHPDoc standards, section headers, naming conventions, class organization, ECS/PHPStan, verification checklist. Required for any PHP code.
• ddev — All commands run through DDEV. Required for running ECS, PHPStan, scaffolding, and tests.
• craft-garnish — When working on CP JavaScript, asset bundles, or interactive CP components. Covers Garnish's class system, UI widgets (Modal, HUD, DisclosureMenu, Select), drag system, and the Craft.* JS class pattern.
• craft-cloud — When the project is hosted on Craft Cloud (detect via craft-cloud.yaml at the repo root or craftcms/cloud in composer.json). Required for plugin Cloud-compatibility constraints — App::isEphemeral() guards, asset-bundle CDN publishing, 15-minute queue-job cap, csrfInput() function over raw token output, and the cloud/up deploy lifecycle events.

Documentation

• Extend guide: https://craftcms.com/docs/5.x/extend/
• Class reference: https://docs.craftcms.com/api/v5/
• Generator: https://craftcms.com/docs/5.x/extend/generator.html

Use WebFetch on specific doc pages when a reference file doesn't cover enough detail.

Common Pitfalls (Cross-Cutting)

• Always use addSelect() in beforePrepare() — it's the Craft convention and safely additive when multiple extensions contribute columns.
• Queue workers run in primary site context — use ->site('*') for cross-site queries.
• Including id in getConfig() — project config uses UIDs, never database IDs.
• Business logic in models or controllers — services are where logic belongs.
• Modules need manual template root, translation, and controllerNamespace registration — nothing is automatic.
• DateTimeHelper in elements/queries, Carbon in services — never mix in the same class.
• Hardcoding /admin in CP URLs — cpTrigger is configurable. Use UrlHelper::cpUrl() in PHP, cpUrl() in Twig.
• Passing $request->getBodyParams() directly to savePluginSettings() on split-settings pages — only submitted keys persist, other settings are silently dropped. Load the full settings model first, update properties, then save.

Reference Files

Read the relevant reference file(s) for your task. Multiple files often apply together.

Task examples:

• "Build a custom element type" → read elements.md (Architecture section first) + element-index.md + fields.md + migrations.md + cp.md
• "Build a hierarchical/tree element type" → read elements.md (Architecture: One Element Class with Native Structure)
• "Add a webhook endpoint" → read controllers.md + events.md
• "Create a queue job that syncs elements" → read queue-jobs.md + elements.md + debugging.md
• "Add a settings page with form fields" → read controllers.md + cp.md + architecture.md
• "Register a custom field type" → read fields.md + events.md
• "Fix PHPStan errors" → read quality.md
• "Add a dashboard widget" → read cp-components.md (Dashboard Widgets) + events.md (Widget Types section)
• "Expose template variables for plugin users" → read events.md (Twig Extensions section)
• "Attach custom methods to entries" → read events.md (Behaviors section)
• "Build a CP utility page" → read cp-components.md (Utility Pages) + events.md (Utilities section)
• "Set up Vite for a plugin's CP assets" → read plugin-vite.md + load craft-garnish skill
• "Add drag-to-reorder or interactive JS to a CP page" → load craft-garnish skill
• "Write CP JavaScript for a custom field type" → read fields.md + load craft-garnish skill
• "Build a headless Craft API" → read graphql.md + load craft-site skill for headless.md
• "Configure preview for a Next.js front-end" → load craft-site skill for headless.md
• "Set up Pest tests for a plugin" → read testing.md
• "Write a test for a controller action" → read testing.md
• "Configure Redis for caching and sessions" → read config-app.md
• "Set up environment variables for production" → read config-bootstrap.md
• "Find a GeneralConfig setting" → read config-general.md
• "Read a config value in plugin code (App::env, parseEnv, GeneralConfig)" → read config-bootstrap.md + config-general.md
• "Check if allowAdminChanges is enabled in plugin code" → read config-general.md + cp.md (Read-Only Mode)
• "Resolve env vars in plugin settings ($MY_API_KEY)" → read config-bootstrap.md (App::parseEnv)
• "Understand CRAFT_* env var conventions" → read config-bootstrap.md
• "Configure mail transport / SMTP" → read config-app.md
• "Set up custom URL routes" → read config-bootstrap.md
• "Configure search to find short words" → read config-app.md
• "Set up GraphQL tokens and schemas" → read graphql.md + config-general.md
• "Set up caching for a high-traffic site" → read caching.md
• "Register custom permissions for my plugin" → read permissions.md
• "Check user permissions in templates" → read permissions.md
• "Set up plugin editions / feature gating" → read architecture.md (Plugin Editions section)
• "Upgrade a plugin from Craft 4 to 5" → read quality.md (Rector section)
• "Set up CI for a Craft plugin" → read quality.md (CI/CD Integration section)
• "Create sections or fields in a migration" → read migrations.md (Content Migrations section)
• "Set up database read replicas" → read config-app.md (Database Replicas section)
• "Register a module in app.php" → read config-app.md (Module Registration section)
• "Create a custom validator" → read architecture.md (Custom Validators section)
• "Create a custom filesystem type" → read events.md (Filesystem Types section)
• "Build a custom condition rule for an element index" → read cp-ui-patterns.md (Condition Builders)
• "Build a tri-state on/inherit/off control" → read cp-ui-patterns.md (Tri-State Inheritance Controls)
• "Add tabbed settings page to a plugin" → read cp.md (Tabbed Settings Pages)
• "Show an 'overrides global' warning on a field" → read cp-ui-patterns.md (Field Warning Parameter)
• "What CSS variables does Craft CP use?" → read cp-ui-patterns.md (Craft CSS Custom Properties)
• "Set up pre-commit hooks for code quality" → read quality.md (Pre-Commit Hooks section)
• "Restrict element access by user group" → read element-authorization.md + permissions.md
• "Scope CP element index by permission" → read element-authorization.md (Layer 3: Query Scoping)
• "Add authorization events to a custom element" → read element-authorization.md + elements.md
• "Build defense-in-depth for a security plugin" → read element-authorization.md (Defense Patterns)
• "Force-logout a user from all devices" → read sessions-and-auth.md (Plugin Patterns)
• "Understand how Craft sessions work" → read sessions-and-auth.md
• "Implement password reset required" → read sessions-and-auth.md (passwordResetRequired Gap)
• "Add a column to the Users element index" → read element-index.md (Extending Element Indexes via Events)
• "Add a bulk action to an element index" → read element-index.md (Adding a custom bulk action)
• "Add an action to the per-element edit-screen menu" → read element-index.md (Per-Element Edit-Screen Action Menu)
• "Render a status pill in a table column" → read element-index.md (Status Pills in Table Attributes)
• "Add a custom sidebar source to the element index" → read element-index.md (Adding a sidebar source)
• "Build a custom field type" → read field-types-custom.md +