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-pest — When writing, running, fixing, or reviewing tests. Covers the markhuot/craft-pest-core harness, database isolation (its rollback is opt-in and its env overrides are cwd-bound — both cause silent writes to the dev database), factories, HTTP/queue/DB assertions, and CI test jobs.
• 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.
• Naming a route or query param token — it collides with Craft's reserved tokenParam and the request is rejected with a 400 before your controller runs. See controllers.md (Reserved request params).
• Any non-underscore-prefixed template in a plugin's templates/ dir is directly routable in the CP, bypassing your controller's beforeAction() gates. Underscore-prefix every template that isn't an intentional direct route. See cp.md (CP template routing bypasses controllers).

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)
• "Register a custom element or field type" → read events.md (Registration Pattern → Registration scope — never gate by request context)
• "Trashed custom elements never get deleted / gc not purging / rows piling up in the elements table" → read events.md (Registration Pattern → Registration scope)
• "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" → load craft-pest skill → isolation.md
• "Write a test for a controller action" → load craft-pest skill → patterns.md
• "Tests are writing to my dev database / created thousands of stray elements" → load craft-pest skill → isolation.md
• "Create or delete sites from code (importer, provisioning command, fixture)" → read architecture.md (Creating or deleting sites at runtime)
• "Element queries stopped filtering by siteId after I created a site" → read architecture.md (Creating or deleting sites at runtime — refreshSites() doesn't invalidate the variant queries read)
• "Deleted a site but sections/category groups still reference it" → read architecture.md (Creating or deleting sites at runtime — deleteSite()'s prune vs memoized service caches)
• "Compare a record's dateUpdated against a cutoff / 'changed since' filter is off by hours" → read architecture.md (Those strings are naive UTC)
• "Read a datetime column out of an ActiveRecord into a model" → read architecture.md (Record-to-Model Hydration Boundary)
• "My element query with ['like', ...] returns nothing" → read architecture.md (Element-query param setters don't take Yii operator tuples)
• "Match elements by title/slug prefix" → read architecture.md (Element-query param setters don't take Yii operator tuples)
• "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 ($MYAPIKEY)" → 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)
• "Build a fluent render builder / craft.plugin.widget().render()" → read architecture.md (Render builders: attribute merging, option allowlists)
• "Site can't override my plugin's front-end CSS / plugin styles win" → read architecture.md (Plugin-registered CSS loads after the site's stylesheet)
• "Plugin JS works with shipped templates but not user-written ones" → read architecture.md (The JS-to-markup contract is public API)
• "Generate docs/schemas/types from a registry / generator output differs per install or edition" → read quality.md (Generated Artifacts Must Not Read Runtime Registries)
• "Cut a release / tag a version / Packagist isn't serving the new version" → load the craft-plugin-release skill
• "Deleted a site but my plugin's per-site rows are still there" → read architecture.md (Site deletion is a soft delete)
• "Cached false / cache returns false and I can't tell if it's a miss" → read caching.md (Data Caching → boolean sentinel idiom)
• "Where should plugin/operational settings (thresholds, notification routing, workflow mappings) live — project config or DB?" → read architecture.md (Settings belong in project config)
• "Should thresholds be DB-only to avoid cross-environment churn?" → read architecture.md (Settings belong in project config) — this is a misconception to correct
• "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)
• "Install fails with 'Too many keys specified; max 64 keys allowed' / duplicate indexes piling up" → read migrations.md (Re-runnable index creation and MySQL's 64-key ceiling)
• "Run a module's migrations / --track=module: rejected / library-shipped module schema never applies" → read migrations.md (Modules and library-shipped modules have no CLI track)
• "Fix a bad stored project-config value from a plugin upgrade" → read migrations.md (Never call ProjectConfig::flush() inside a migration)
• "Unsuspend/restore a user programmatically / user still locked out after unsuspend" → read elements.md (getStatus() hides co-existing states)
• "Do I need to override settingsAttributes()?" → read field-types-custom.md (What settingsAttributes() actually includes)
• "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)
• "Plugin supports a config/<handle>.php override — how should the settings screen behave?" → read cp.md (Settings Pages → Config-file overrides: warn on the field, don't disable it)
• "Setting saves but nothing changes / config file silently wins" → read cp.md (Settings Pages → Config-file overrides)
• "Why doesn't my URL tab navigate / tabs vs sidebar nav for separate pages?" → read cp.md (Tabs switch panes — they never navigate)
• "Build a CP index/list screen: columns, copy chips, row actions" → read cp-ui-patterns.md (Index-screen column scheme) + cp-components.md (VueAdminTable)
• "VueAdminTable clips headings / where do intro copy and the New button go?" → read cp-components.md (VueAdminTable → sole pane content)
• "Utility icon not showing / icon() path" → read cp-components.md (Utility Pages)
• "Hide the Visibility/Editability condition builders on a layout element" → read fields.md (Suppressing the condition builders)
• "Show an 'overrides global' warning on a field" → read cp-ui-patterns.md (Field Warning Parameter)
• "What CSS variables / design tokens does Craft CP use?" → read cp-ui-patterns.md (Craft CSS Custom Properties — color/status/spacing/radius/control/focus/font tokens)
• "Lightswitch or checkbox for a boolean setting?" → read cp.md (Form Macros Reference → Lightswitch vs checkbox)
• "Render a copytext / API-key / webhook-URL field" → read cp.md (Form Macros Reference → copytextField)
• "Which button class for a Save / Delete / Add button?" → read cp.md (Form Macros Reference → Button modifier classes)
• "Add an inner sidebar sub-nav to a CP pane" → read cp.md (Form Macros Reference → Inner sidebar navigation)
• "Add an inline editable table to a settings page" → read cp-components.md (Editable Table Field)
• "Show a modal or confirmation dialog in the CP (there's no Craft.confirm())" → read cp-components.md (Modals) + load craft-garnish skill for behavior
• "What relation-field view modes / previewMode / maxRelations exist?" → read element-index.md (Relation Field Display/Props)
• "Add a Tip / Heading / built-in UI element to a field layout" → read fields.md (Field-Layout UI Elements Reference)
• "Add a panel to an element edit screen's sidebar" → read cp-ui-patterns.md (Element Edit Screen) + elements.md (EVENTDEFINESIDEBAR_HTML)
• "Add a split button to an element's top toolbar" → read cp-ui-patterns.md (Element Edit Screen → toolbar split button) + elements.md (EVENTDEFINEADDITIONAL_BUTTONS)
• "Why does my sidebar panel render unstyled / look wrong?" → read cp-ui-patterns.md (Element Edit Screen → .meta vs .meta read-only)
• "Override metaFieldsHtml() on a custom element" → read cp-ui-patterns.md (Element Edit Screen → metaFieldsHtml override)
• "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 + fields.md
• "Build a relation field type" → read field-types-custom.md (Relation Fields)
• "Add a condition rule to the entry index" → read conditions.md + element-index.md
• "Build a custom condition rule" → read conditions.md
• "Send email from a plugin" → read email.md
• "Register a custom system message" → read email.md (Registering Custom System Messages)
• "Configure SMTP transport" → read config-app.md + email.md
• "Deploy Craft CMS to production" → read deployment.md
• "Set up CI/CD for a Craft project" → read deployment.md (CI/CD Patterns)
• "Zero-downtime deploy" → read deployment.md (Zero-Downtime)
• "Roll back a failed deploy" → read deployment.md (Rollback Strategies)
• "Work with drafts and revisions" → read drafts-revisions.md
• "Create a draft programmatically" → read drafts-revisions.md (Creating Drafts)
• "Skip side effects for drafts in afterSave" → read drafts-revisions.md (Plugin Considerations)
• "Add generated fields to a custom element" → read elements.md (Generated Fields)
• "Customize how my element appears as a chip or card" → read element-index.md (Element Display Modes)
• "Add a screen to the User edit page" → read elements.md (Extending User Edit Screens)
• "Make plugin settings read-only when allowAdminChanges is off" → read cp.md (Read-Only Mode)
• "Add tabs to a plugin's settings page" → read cp.md (Settings Pages → With tabs or custom actions). settingsHtml() is single-pane only — tabs require a custom controller and a template extending _layouts/cp directly.
• "Render plugin settings inside its own CP section (not the global settings/plugins screen)" → read cp.md (Settings Pages → Keep settings inside the plugin's own CP section)
• "Make a plugin Cloud-compatible" → load craft-cloud skill → plugin-development.md (ephemeral filesystem, asset-bundle constraints, queue cap, CSRF function, cookie-free design)
• "Deploy a Craft project to Cloud" → load craft-cloud skill → config-file.md + deploy-pipeline.md + extension.md
• "Migrate a self-hosted Craft site to Cloud" → load craft-cloud skill → migration.md
• "Why does my plugin's file write silently fail on Cloud?" → load craft-cloud skill → plugin-development.md (Ephemeral filesystem) + extension.md (App::isEphemeral)
• "Build an SSO/OIDC login or SCIM provisioning feature" → read identity-protocols.md + sessions-and-auth.md
• "Handle single logout / logout_token / provider-initiated deprovisioning" → read identity-protocols.md

Load only the reference files your task needs — each file costs input tokens on every turn.

Plugin vs Module Differences

Plugins and modules share the same architecture patterns. The differences are in bootstrapping and registration:

Module Template Root Registration

use craft\events\RegisterTemplateRootsEvent;
use craft\web\View;

Event::on(View::class, View::EVENT_REGISTER_CP_TEMPLATE_ROOTS,
    function(RegisterTemplateRootsEvent $event) {
        $event->roots['my-module'] = __DIR__ . '/templates';
    }
);

Module Translation Registration

Craft::$app->i18n->translations['my-module'] = [
    'class' => \craft\i18n\PhpMessageSource::class,
    'sourceLanguage' => 'en',
    'basePath' => __DIR__ . '/translations',
    'allowOverrides' => true,
];

Module Console Command Registration

public function init()
{
    Craft::setAlias('@mymodule', __DIR__);

    if (Craft::$app->getRequest()->getIsConsoleRequest()) {
        $this->controllerNamespace = 'modules\\mymodule\\console\\controllers';
    } else {
        $this->controllerNamespace = 'modules\\mymodule\\controllers';
    }

    parent::init(); // MUST come after setting controllerNamespace
}

The module must be bootstrapped in config/app.php for console commands to be discoverable.