Discourse Upcoming Changes Authoring

Upcoming Changes Framework — Authoring Guide

This skill is for working on the upcoming changes framework itself — the internal machinery that powers feature flag rollout in Discourse. For adding a new feature flag using the framework, see the discourse-upcoming-changes skill instead.

Architecture Overview

The upcoming changes system has three layers: a Ruby core that manages state and business logic, a services layer that orchestrates tracking/notifications/toggling, and an Ember frontend that renders the admin UI and applies per-user overrides.

Ruby Core

lib/upcoming_changes.rb — The central module. All business logic for resolving values, checking user eligibility, caching, and image handling lives here.

Key methods to understand:

• enabled?(setting_name) — Determines the effective value of a change. This is where auto-promotion logic lives: if a setting's status meets/exceeds promote_upcoming_changes_on_status, it resolves to true even if the DB default is false. Permanent settings always resolve to true (admins can't disable them). It returns false outright when the owning plugin is not configurable, or — for plugin-owned changes, unless they opt out with requires_plugin_enabled: false — when the owning plugin is disabled. Both take precedence over the above. See Requiring the Owning Plugin.
• enabled_for_user?(setting_name, user) — The primary access check. Considers: the resolved value from enabled?, group restrictions, anonymous users (only get access if no group restrictions).
• stats_for_user(user:, acting_guardian:) — Returns per-change status for a user including why they have/don't have access (the user_enabled_reasons enum).
• current_statuses / permanent_upcoming_changes — Cached lookups keyed by git version (one-time cost per deploy). Cleared by clear_caches! and automatically when TrackStatusChanges detects changes.
• settings_hidden_while_enabled — Returns the set of other site setting names that should be hidden from admins because an enabled change declares them in its hide_settings: metadata. Computed live (not toggled at opt-in time) so it tracks both opt-in paths and is multisite-safe. See Hiding Settings While Enabled below.

UpcomingChanges::ConditionalDisplay (defined inside lib/upcoming_changes.rb) — Hides individual upcoming changes from the admin UI when they don't make sense in the current context (e.g. a Horizon-related change on a site without Horizon installed). Core gates can define a should_display_<upcoming_change_name>? class method on it; plugin-owned upcoming changes should use Plugin::Instance#register_upcoming_change_conditional_display. If no rule is defined, the change is always displayed. See Conditional Display below.

app/models/upcoming_change_event.rb — Audit trail. Every lifecycle event (added, removed, status change, manual toggle, admin notification) is recorded here. Has unique indexes to prevent duplicate events of specific types per change.

lib/site_setting_extension.rb — Where upcoming_change: metadata in site_settings.yml gets parsed. When a setting is registered with this metadata, it stores the parsed result in @upcoming_change_metadata and defines a {name}_groups_map method. The impact string is split into impact_type and impact_role. The hide_settings: array (if present) is normalized to an array of symbols. Also handles upcoming_change_default_override: metadata — see Default Overrides below.

lib/site_settings/hidden_provider.rb — HiddenProvider#all (the source of SiteSetting.hidden_settings) unions in UpcomingChanges.settings_hidden_while_enabled before applying the :hidden_site_settings plugin modifier, so plugins can still explicitly un-hide a setting. The union is skipped entirely when no change opts in, to avoid allocating a new Set on every read. See Hiding Settings While Enabled below.

lib/site_settings/defaults_provider.rb — Manages the default values for all settings, including upcoming change default overrides. Tracks which overrides are active via @active_upcoming_change_overrides and applies them when resolving defaults. Provides upcoming_change_override_metadata for the frontend to display warnings about changed defaults.

app/models/site_setting_group.rb — Stores group restrictions for settings. Group IDs are pipe-separated strings ("1|2|3"). The setting_group_ids class method returns a hash used for in-memory caching.

Services Layer

All services use Service::Base. They're organized under app/services/upcoming_changes/:

SiteSetting::UpsertGroups — Manages group assignments for settings (upserts SiteSettingGroup, refreshes caches, notifies clients).

Scheduled Jobs

app/jobs/scheduled/check_upcoming_changes.rb — Runs every 20 minutes inside a DistributedMutex. Calls Track (to log added/removed/status_changed events) then NotifyPromotions (which sends auto-promotion notifications immediately). Supports verbose logging via the upcoming_change_verbose_logging setting. Does not send "change available" notifications to admins — that is the weekly job below.

app/jobs/scheduled/notify_admins_of_available_upcoming_changes.rb — Runs once a week. Collects every change that was either added at, or status-changed to, promote_upcoming_changes_on_status - 1 within the last week and has no prior admins_notified_available_change or admins_notified_automatic_promotion event. Notifies all admins who have enable_upcoming_change_available_notifications enabled, consolidating into a single notification per admin via NotificationDataMerger (merging with existing unread notifications if present, otherwise creating one new notification per admin covering all newly-available changes). Gated on UpcomingChanges.should_notify_admins? so new sites are suppressed. Writes admins_notified_available_change events and a log_upcoming_change_available staff log entry per change.

Frontend

Admin page — admin/templates/admin-config/upcoming-changes.gjs renders the page header, admin/components/admin-config-areas/upcoming-changes.gjs is the container with filtering, and admin/components/admin-config-areas/upcoming-change-item.gjs renders each row.

Key frontend patterns:

• Filtering by status, impact type, impact role, and enabled/disabled state via DFilterControls
• Group selection uses a multi-select dropdown with debounced API saves
• Toast notifications for all toggle/group changes
• Lightbox integration for preview images

Site settings service (app/services/site-settings.js) — Loads upcoming changes from PreloadStore, applies them as overrides to site settings, and stores them in settings.currentUserUpcomingChanges.

Body CSS classes — app/controllers/application.js generates uc-{dasherized-key} classes on <body> for each enabled upcoming change that opts in via body_class: true, allowing CSS-based feature gating. The controller intersects siteSettings.currentUserUpcomingChanges (enabled-for-this-user changes) with site.upcoming_changes_with_css (changes that opted into CSS) — a change must be in both to get a body class. The opt-in list comes from SiteSerializer#upcoming_changes_with_css, which returns the change names whose metadata has body_class: true. See CSS Opt-In below.

Notifications — Two notification types (upcoming-change-available, upcoming-change-automatically-promoted) handle singular/dual/many change descriptions and link to the admin page with filter params.

Sidebar — Badge notification dot appears on the upcoming changes link when currentUser.hasNewUpcomingChanges is true.

MessageBus — Subscribes to /client_settings and updates both siteSettings and currentUserUpcomingChanges in real time.

Controller

admin/config/upcoming_changes_controller.rb — Three endpoints:

• GET index — List changes (with filter_statuses param)
• PUT update_groups — Set group restrictions for a setting
• PUT toggle_change — Enable/disable a setting

Problem Check

app/services/problem_check/upcoming_change_stable_opted_out.rb — Warns admins hourly if they've opted out of a stable/permanent change.

Default Overrides

Upcoming changes can override the default value of a different site setting when enabled. This allows feature rollouts to change related setting defaults without breaking admin customizations.

Metadata Format

A setting declares a default override with the upcoming_change_default_override key in config/site_settings.yml:

# The upcoming change setting (the "trigger")
increase_suggested_topics_max_days_old_default:
  default: false
  type: bool
  upcoming_change:
    status: experimental
    impact: "site_setting_default,all_members"

# The setting whose default changes (the "target")
suggested_topics_max_days_old:
  default: 365
  type: integer
  upcoming_change_default_override:
    upcoming_change: increase_suggested_topics_max_days_old_default
    new_default: 1000

When increase_suggested_topics_max_days_old_default is enabled (either manually by admin or via auto-promotion), the default value of suggested_topics_max_days_old changes from 365 to 1000. The impact field on the trigger setting should include site_setting_default as its impact_type.

How It Works

1. Registration — lib/site_setting_extension.rb parses upcoming_change_default_override during setting registration and stores it in upcoming_change_default_overrides (a hash keyed by setting name).

2. Activation — During SiteSetting.refresh!, each override is checked: if UpcomingChanges.enabled?(override[:upcoming_change]) returns true, the override is activated via defaults.activate_upcoming_change_override. The setting's current value is updated to new_default only if the admin has not manually modified it.

3. Default resolution — DefaultsProvider#all applies active overrides when resolving defaults, so code reading SiteSetting.defaults[:setting_name] gets the overridden value.

4. Frontend display — DefaultsProvider#upcoming_change_override_metadata returns { old_default:, new_default:, change_setting_name: } for active overrides. The site settings UI (admin/components/site-setting.gjs) shows a warning linking to the upcoming changes page.

Key Behaviors

• Non-destructive: If an admin has manually set a custom value for the target setting, the override does not apply — it only affects the default.
• Reversible: Disabling the upcoming change deactivates the override and restores the original default.
• Default-locale only: Overrides currently only apply on the default locale.

Conditional Display

Some upcoming changes only make sense to show admins under certain conditions — for example, a Horizon-themed change is irrelevant if Horizon isn't installed, or a change might only apply when another setting is enabled. UpcomingChanges::ConditionalDisplay (in lib/upcoming_changes.rb) lets the framework hide a change from the admin UI without removing it from site_settings.yml.

How It Works

1. Filtering — UpcomingChanges::List#fetch_upcoming_changes calls UpcomingChanges::ConditionalDisplay.should_display?(setting_name) on every change after the status filter, before group/image enrichment. Changes that return false are dropped from the result entirely.
2. Resolution — should_display? first hides the change outright if its owning plugin is not configurable (owning_plugin_configurable?), or if it is plugin-owned and the owning plugin is disabled (owning_plugin_enabled?) — unless the change opts out with requires_plugin_enabled: false. Then it checks for a class method named should_display_<upcoming_change_name>? on ConditionalDisplay. If defined, its return value is used. Otherwise it evaluates enabled plugin callbacks registered for that setting. If no method or callback is defined, the change is always displayed (returns true).
3. Definition site — Core gates can live directly on ConditionalDisplay, typically next to the relevant subsystem's code. Plugin gates should be registered from the plugin initializer via register_upcoming_change_conditional_display(:setting_name) { ... } so disabled plugins are filtered by DiscoursePluginRegistry.

Key Behaviors

• Display-only, with one exception: A gate normally affects whether the change appears in the admin UI, not whether it's enabled — enabled? / enabled_for_user? still resolve normally, so code paths gated on the change continue to work. The exception is the owning-plugin checks, which enabled? consults too (see below).
• Don't express the owning-plugin gate with a conditional display callback: it's already the default for plugin-owned changes, and a callback couldn't express it anyway — DiscoursePluginRegistry filters callbacks from disabled plugins, so it would never run in the only case that matters. See Requiring the Owning Plugin.
• N+1 by design: should_display? is called once per change in the loop. If a gating method does expensive work (DB queries, plugin lookups), memoize inside the method to avoid repeated cost.
• Notifications are gated too: Beyond the List service, NotifyAdminsOfAvailableUpcomingChanges and NotifyPromotion also consult should_display?, so a hidden change is not notified about. TrackAddedChanges / TrackRemovedChanges do not — the audit trail records every change regardless. A gate should still reflect a long-lived condition (plugin missing, theme not installed) rather than transient state.

Requiring the Owning Plugin

A plugin-owned change that gates a feature inside its plugin is useless while that plugin is disabled — the plugin's code isn't running, so the admin is being pitched (and notified about) a toggle that does nothing. This is the default for every plugin-owned change: no metadata is needed to get it.

enable_your_plugin_feature:
  default: false
  client: true
  hidden: true
  upcoming_change:
    status: experimental
    impact: feature,all_members
    # requires_plugin_enabled defaults to true for plugin-owned changes -- nothing to add.

While the plugin is off, UpcomingChanges.owning_plugin_enabled? returns false, which both hides the change (should_display?) and stops it resolving (enabled?), so its hide_settings and default overrides don't apply either. The admin's opt-in stays in the database and resumes when they re-enable the plugin.

Opting out

Plenty of plugin-owned changes are the opposite: they exist to get the plugin adopted, and only make sense while it is disabled. Leaving one gated makes it unreachable — it would be hidden from exactly the sites it targets. These must opt out with requires_plugin_enabled: false:

enable_your_plugin_feature:
  default: false
  client: true
  hidden: true
  upcoming_change:
    status: experimental
    impact: feature,all_members
    requires_plugin_enabled: false

Current examples, all of which set requires_plugin_enabled: false:

Rule of thumb: if the change's purpose is "try this feature we added to the plugin you already run", leave it gated (the default). If its purpose is "start using this plugin", opt out with false.

Guardrails

The integrity spec (spec/integrity/upcoming_change_metadata_spec.rb) enforces that requires_plugin_enabled, when present, is a boolean and is only set on plugin-owned changes. It also requires a change that is its plugin's own enabled_site_setting to set requires_plugin_enabled: false — leaving the default gate on it would be self-gating and would recurse (Plugin::Instance#enabled? reads the setting, which resolves back through enabled?). owning_plugin_enabled? also guards against the recursion at runtime, so a mistake surfaces as a hidden change rather than a stack overflow. The spec derives ownership and the plugin's enabled_site_setting from the file path and plugin.rb, so it holds regardless of which plugins are loaded in the run.

CSS Opt-In

A change can opt into having a uc-{dasherized-key} class added to <body> when it is enabled for the current user, so stylesheets can gate visuals on the change. This is opt-in via the body_class: true metadata key — body classes are not emitted for every enabled change, only those that ask for them. This keeps the body class list small and intentional, and avoids leaking the names of unrelated (non-visual) changes into the DOM.

enable_your_feature_name:
  default: false
  client: true
  hidden: true
  upcoming_change:
    status: experimental
    impact: feature,all_members
    body_class: true

How It Works

1. Parsing — lib/site_setting_extension.rb reads body_class from the upcoming_change: metadata and stores it (defaulting to nil/falsey) in upcoming_change_metadata.
2. Serialization — SiteSerializer#upcoming_changes_with_css filters SiteSetting.upcoming_change_site_settings down to the change names whose metadata has body_class truthy, and exposes them to the frontend as site.upcoming_changes_with_css.
3. Body class generation — app/controllers/application.js iterates siteSettings.currentUserUpcomingChanges and pushes a uc-{dasherize(key)} class only when both the setting is truthy for the user and site.upcoming_changes_with_css.includes(key). A change must be enabled and opted-in to get a class.

Key Behaviors

• Opt-in only: Omitting body_class (or setting it false) means no body class — the default. Add it only when you actually have CSS keyed on uc-{name}.
• Always wrap in :where(): Style the class as :where(.uc-{name}), never a bare .uc-{name}, so the transitional class adds zero specificity and stays safe to unwrap and delete later — the discourse/uc-classes-in-where stylelint rule enforces this.
• Put the CSS in dedicated files imported from app/assets/stylesheets/common/upcoming-changes/_index.scss.
• Enabled-for-user gated: The class only appears for users the change is enabled for (via currentUserUpcomingChanges), not globally. Anonymous/ineligible users won't get it.
• Integrity-checked: body_class is in the integrity spec's allowed_keys and must be a boolean — see Mocking Metadata for how to set it in tests.

Permanent Soon Warning

Once a change reaches stable status, the admin page shows a warning on its row: "This change will become permanent soon. You will no longer be able to opt-out." This is opt-out via the permanent_warning: metadata key — every stable change shows the warning unless it explicitly sets permanent_warning: false.

enable_your_feature_name:
  default: false
  client: true
  hidden: true
  upcoming_change:
    status: "stable"
    impact: "site_setting_default,all_members"
    permanent_warning: false

How It Works

1. Parsing — lib/site_setting_extension.rb reads permanent_warning from the upcoming_change: metadata and normalizes it to a boolean (!= false, so an omitted key becomes true) in upcoming_change_metadata.
2. Rendering — UpcomingChangeItem#showPermanentSoonNotice (admin/components/admin-config-areas/upcoming-change-item.gjs) renders the notice when status === "stable" and permanent_warning !== false. The !== false comparison (rather than a truthy check) means metadata that omits the key — including hashes built by mock_upcoming_change_metadata — still shows the notice.

Key Behaviors

• Opt-out, not opt-in: The default is to warn. Suppress it only when the warning would be misleading — the usual case is a site_setting_default change, where becoming permanent just changes another setting's default and the admin can still set that setting to whatever they want.
• Stable-only: The key has no effect below stable, and permanent changes don't show the notice either (they already are permanent).
• Independent of impact_type: Before this key existed, the notice was implicitly suppressed for every site_setting_default change. That coupling is gone — impact type no longer affects the notice.
• Integrity-checked: permanent_warning is in the integrity spec's allowed_keys and must be a boolean.

Hiding Settings While Enabled

A change can declare other site settings that should be hidden from admins while it is enabled, via the optional hide_settings: metadata key. This is for legacy settings that stop making sense once the change replaces them — they disappear from the admin UI rather than being deleted, and reappear if the change is disabled.

enable_your_feature_name:
  default: false
  client: true
  hidden: true
  upcoming_change:
    status: experimental
    impact: feature,all_members
    hide_settings:
      - legacy_setting_one
      - legacy_setting_two

How It Works

1. Parsing — lib/site_setting_extension.rb reads hide_settings from the upcoming_change: metadata, normalizes it to an array of symbols, and stores it in upcoming_change_metadata.
2. Live computation — UpcomingChanges.settings_hidden_while_enabled scans the metadata and, for each change that declares hide_settings, calls enabled?(change_name) and concatenates the declared settings when the change is on. enabled? is only called for the (usually zero) changes that declare hide_settings, so the common case is a cheap metadata scan with no DB hit; it returns [] immediately when no metadata exists.
3. Application — SiteSettings::HiddenProvider#all unions the result into the hidden set (before the :hidden_site_settings modifier) on every SiteSetting.hidden_settings read.

Key Behaviors

• Computed live, not toggled: The hidden set is recomputed per request rather than flipped imperatively at opt-in time. This means it automatically tracks both opt-in paths — manual admin enable and auto-promotion — and stays correct if the change is later disabled.
• Multisite-safe: The metadata (and thus the declared hide_settings) is process-global, but enabled? resolves per-site. A setting is only hidden for sites that have actually opted into the change.
• Plugins can still un-hide: The union happens before the :hidden_site_settings modifier, so a plugin can explicitly remove a setting from the hidden set via that modifier.
• Integrity-checked: hide_settings is in the integrity spec's allowed_keys. When present it must be an array, and every referenced name must be a real site setting (SiteSetting.respond_to?) — typos fail the spec. See Mocking Metadata for how to set it in tests.

Key Design Decisions

Caching Strategy

The current_statuses and permanent_upcoming_changes caches are keyed by git version (Discourse.git_version). This means they're naturally invalidated on every deploy — no TTL needed. Within a deploy, TrackStatusChanges calls clear_caches! when it detects metadata changes. Always call clear_caches! in tests after modifying metadata.

Auto-Promotion

The resolved_value method is the single source of truth for whether a setting is "on." Auto-promotion happens implicitly: when a setting's status meets the threshold, resolved_value returns true regardless of the DB value. The DB value only changes when an admin explicitly toggles. This separation means promotion is reversible by the admin without losing the original opt-in/opt-out state.

Notification Merging

When multiple changes need notifications, NotificationDataMerger consolidates them into a single notification per admin. It finds existing unread notifications and merges the change names array. The frontend notification types handle singular ("Feature X"), dual ("Feature X and Feature Y"), and many ("Feature X and 2 others") display.

For "change available" notifications specifically, merging also happens across the batch processed by the weekly NotifyAdminsOfAvailableUpcomingChanges job — for admins without an existing unread notification, the job builds a single new notification per admin that lists every newly-available change in that run, rather than emitting one notification per change.

Back-Catalogue Notification Suppression

Admins should never be notified about changes that pre-date their ability to act on them. Two mechanisms cover this.

The one-hour window. UpcomingChanges.should_notify_admins? is false while Migration::Helpers.new_site? is true (a site is "new" if its first schema migration was less than 1 hour ago). Both the weekly NotifyAdminsOfAvailableUpcomingChanges job and the NotifyPromotion service guard on it. This only covers the first hour, so it is a backstop, not the main mechanism — the weekly job runs weekly, and NotifyPromotion fires as soon as the hour elapses.

Backfilled events. UpcomingChanges::Action::BackfillNotifiedEvents writes added plus the appropriate notified event (admins_notified_available_change at promote_upcoming_changes_on_status - 1, admins_notified_automatic_promotion at/above the promote status) so both notification paths permanently treat those changes as handled. It is called from:

• db/fixtures/995_upcoming_changes.rb, on new sites (skipped in development so notifications remain testable locally). This covers every installed plugin's changes too, enabled or not — plugin settings are registered regardless of enabled state, so they are already in SiteSetting.upcoming_change_site_settings.
• The :site_setting_changed handler in config/initializers/015-track-upcoming-change-toggle.rb, when a plugin's enabled_site_setting is turned on. Until that moment ConditionalDisplay is the only thing holding the plugin's notifications back, so enabling it would otherwise release the whole back-catalogue at once.

Changes below the notification threshold at backfill time are deliberately left alone, so they notify normally when they later reach it.

Promotion still happens. The backfill only suppresses notifications. NotifyPromotion keys its "already handled?" policy (promotion_not_already_handled) on the automatically_promoted event, and separately skips only the notification steps when an admins_notified_automatic_promotion event exists. So a backfilled change still gets its automatically_promoted event and still fires DiscourseEvent(:upcoming_change_enabled). That matters because the event is what applies a change's real side effects, and because the automatically_promoted record is read elsewhere as the "when did this turn on" anchor — see BrowserPageviewEvent#beacon_cutover_date and SiteSetting::Action::RemoveAndReplaceUncategorizedToggled::SNAPSHOT_EVENT_TYPES.

When adding a policy to NotifyPromotion, be clear about which of the two questions it answers. A policy aborts the whole service, so anything about notification belongs in the notify_admins? guard instead — putting it in a policy would silently stop the change from promoting.

Group-Based Access

Group restrictions use a separate SiteSettingGroup model rather than storing groups on the setting itself. This allows the caching layer (site_setting_group_ids) to work independently. Group IDs are pipe-separated in the DB for efficient single-row storage.

The allow_enabled_for metadata key on an upcoming change restricts which "Enabled for" dropdown options the admin sees. It accepts an array of any subset of [everyone, staff, specific_groups]; the No one option is always present and cannot be removed. When the key is omitted, all four options are shown (the permissive default). Rule: if everyone is present it must be the only value — everyone cannot combine with staff or specific_groups. The integrity spec enforces these rules. Server-side enforcement lives in UpcomingChanges::Toggle (validates the target when no groups are configured) and SiteSetting::UpsertGroups (validates group selection: a [staff]-only selection needs staff allowed; any other selection needs specific_groups allowed). When neither staff nor specific_groups is in the allow list, Toggle also clears any stale SiteSettingGroup records.

Auto-promoted display: When allow_enabled_for excludes :everyone and a change is enabled without an explicit admin selection (typically because it reached the promotion threshold), enabled_for_with_groups returns the broadest allowed display target — the staff group name if :staff is permitted, otherwise "groups". This is display-only; enabled_for_user? is unchanged and still treats the change as on for all users until the admin scopes it via the dropdown.

Event Idempotency

UpcomingChangeEvent has unique indexes on specific event type + change name combinations. This prevents duplicate added, removed, or notification events even if the job runs multiple times. Always check for existing events before creating new ones in service code.

Common Modification Scenarios

Adding a New Status

1. Add the status and its numeric value to UpcomingChanges.statuses in lib/upcoming_changes.rb
2. The numeric ordering determines hierarchy — meets_or_exceeds_status? uses these values
3. Update previous_status mapping if the new status fits in the progression
4. Add status badge styling in app/assets/stylesheets/admin/upcoming-changes.scss (.upcoming-change__badge.--status-{name})
5. Add translations for the status label

Adding a New Event Type

1. Add the enum value to UpcomingChangeEvent (app/models/upcoming_change_event.rb)
2. If the event should be unique per change, add a unique index in a migration
3. Create or update the relevant service to emit the event

Modifying the Admin UI

The three main components to know:

• Container (upcoming-changes.gjs) — Filtering logic and data management
• Item (upcoming-change-item.gjs) — Individual change row rendering and interactions
• User view (admin-user-upcoming-changes.gjs) — Read-only per-user view

State is managed via trackedObject for reactivity. API calls go through ajax() directly in the item component.

Restricting "Enabled for" options

To constrain which dropdown options an admin can pick for a change, add allow_enabled_for to its upcoming_change: metadata:

my_upcoming_change_setting:
  default: false
  client: true
  hidden: true
  upcoming_change:
    status: experimental
    impact: feature,all_members
    allow_enabled_for:
      - staff
      - specific_groups

Valid value sets:

everyone cannot be combined with staff or specific_groups — when present, it must be the only value. No one is always available. The integrity spec rejects invalid combinations.

Adding a Conditional Display Rule

To hide an upcoming change from the admin UI under certain conditions:

1. For plugin-owned changes, register a callback from plugin.rb:

   register_upcoming_change_conditional_display(:enable_plugin_feature) do
     SiteSetting.some_dependency_enabled
   end
   

   The condition must be something other than the plugin's own enabled_site_setting — core already hides changes owned by a disabled plugin, and the registry would filter the callback out anyway.
2. For core changes, reopen UpcomingChanges::ConditionalDisplay and define a class method named should_display_<upcoming_change_name>? that returns a boolean:

   module UpcomingChanges
     class ConditionalDisplay
       def self.should_display_enable_horizon_blah?
         Discourse.plugins_by_name["horizon"].present?
       end
     end
   end
   

3. Place core gates near the related subsystem so the gate lives with the code that owns the condition.
4. If the check is expensive, memoize inside the method or callback — List calls should_display? once per change.
5. Multiple plugin callbacks for the same setting are combined with all?, so any enabled plugin can hide the change.
6. Test by stubbing the predicate or registering a callback — see Testing Conditional Display.

Adding a Default Override

To make an upcoming change control the default of another setting:

1. Add upcoming_change_default_override metadata to the target setting in config/site_settings.yml:

   target_setting:
     default: original_value
     upcoming_change_default_override:
       upcoming_change: trigger_setting_name
       new_default: new_value
   

2. Ensure the trigger setting has impact: "site_setting_default,..." in its upcoming_change: metadata
3. The override activates automatically when the trigger is enabled — no additional code needed
4. Test with mock_upcoming_change_default_overrides — see Mocking Default Overrides

Hiding Legacy Settings While a Change Is Enabled

To hide other settings from admins while an upcoming change is enabled (e.g. legacy settings the change replaces):

1. Add a hide_settings: array to the change's upcoming_change: metadata in config/site_settings.yml, listing the setting names to hide:

   enable_your_feature_name:
     default: false
     client: true
     hidden: true
     upcoming_change:
       status: experimental
       impact: feature,all_members
       hide_settings:
         - legacy_setting_one
   

2. The settings are hidden automatically whenever the change is enabled (manual opt-in or auto-promotion) and reappear when it is disabled — no additional code needed.
3. Every name must be a real site setting or the integrity spec fails. See Hiding Settings While Enabled.

Changing Resolution Logic

All value resolution goes through resolved_value in lib/upcoming_changes.rb. If you need to change how settings are evaluated (e.g., adding a new override condition), this is the single place to modify. The method checks in order: permanent status, admin manual override, auto-promotion threshold.

Testing Patterns

Mocking Metadata

Use the test helper to mock upcoming change metadata — never modify site_settings.yml in tests:

mock_upcoming_change_metadata(
  {
    enable_some_feature: {
      impact: "feature,all_members",
      status: :experimental,
      impact_type: "feature",
      impact_role: "all_members",
      body_class: true, # optional — opts into the uc-{name} body class
      permanent_warning: false, # optional — suppresses the "becomes permanent soon" notice at stable
      hide_settings: %i[legacy_setting_one], # optional — settings hidden while enabled
    },
  },
)

To test hide_settings, mock a change with a hide_settings: array, then assert the named settings are absent from SiteSetting.hidden_settings (or HiddenProvider#all) until the change is enabled and present once it is — remember to enable the change (SiteSetting.<name> = true) and clean up with remove_override! + UpcomingChanges.clear_caches!. See spec/lib/upcoming_changes_spec.rb (.settings_hidden_while_enabled) and spec/lib/site_settings/hidden_provider_spec.rb.

To test the CSS opt-in serializer (SiteSerializer#upcoming_changes_with_css), mock two changes — one with body_class: true and one with body_class: false — and assert the serialized array includes the former and excludes the latter. See spec/serializers/site_serializer_spec.rb. System coverage for the resulting uc-{name} body class lives in spec/system/member_upcoming_changes_spec.rb.

Mocking Default Overrides

Use mock_upcoming_change_default_overrides to set up override metadata in tests — never modify site_settings.yml:

mock_upcoming_change_default_overrides(
  {
    suggested_topics_max_days_old: {
      upcoming_change: :increase_suggested_topics_max_days_old_default,
      new_default: 1000,
    },
  },
)

# Enable the trigger setting and refresh to activate the override
SiteSetting.increase_suggested_topics_max_days_old_default = true
SiteSetting.refresh!

# Now SiteSetting.suggested_topics_max_days_old returns 1000 (the overridden default)

To test that the override does NOT apply when the admin has customized the target setting:

# Admin sets a custom value before the override activates
SiteSetting.suggested_topics_max_days_old = 730
SiteSetting.increase_suggested_topics_max_days_old_default = true
SiteSetting.refresh!

# Override is not applied — admin's explicit choice is preserved
expect(SiteSetting.suggested_topics_max_days_old).to eq(730)

Testing Conditional Display

For plugin-owned changes, test the registry path:

Plugin::Instance
  .new
  .register_upcoming_change_conditional_display(:enable_plugin_feature) { false }

For core changes, stub the predicate method directly on UpcomingChanges::ConditionalDisplay rather than redefining it — this avoids leaking method definitions across examples:

UpcomingChanges::ConditionalDis

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