diff --git a/libraries/dateformatter/api/src/main/kotlin/io/element/android/libraries/dateformatter/api/DateFormatter.kt b/libraries/dateformatter/api/src/main/kotlin/io/element/android/libraries/dateformatter/api/DateFormatter.kt
index 1f5c88f008..84cbf5efdd 100644
--- a/libraries/dateformatter/api/src/main/kotlin/io/element/android/libraries/dateformatter/api/DateFormatter.kt
+++ b/libraries/dateformatter/api/src/main/kotlin/io/element/android/libraries/dateformatter/api/DateFormatter.kt
@@ -8,7 +8,17 @@
 
 package io.element.android.libraries.dateformatter.api
 
+/**
+ * Formats timestamps for display, in the user's locale and time zone.
+ */
 interface DateFormatter {
+    /**
+     * Formats a timestamp according to [mode].
+     *
+     * @param timestamp milliseconds since the epoch; a `null` value yields an empty string, so callers need no null check of their own.
+     * @param mode how much of the date and time to include; see [DateFormatterMode] for examples of each.
+     * @param useRelative true to prefer wording relative to today, such as "Today" or "This month", where the mode allows it.
+     */
     fun format(
         timestamp: Long?,
         mode: DateFormatterMode = DateFormatterMode.Full,
diff --git a/libraries/dateformatter/api/src/main/kotlin/io/element/android/libraries/dateformatter/api/DurationFormatter.kt b/libraries/dateformatter/api/src/main/kotlin/io/element/android/libraries/dateformatter/api/DurationFormatter.kt
index ac297d10e9..712355c423 100644
--- a/libraries/dateformatter/api/src/main/kotlin/io/element/android/libraries/dateformatter/api/DurationFormatter.kt
+++ b/libraries/dateformatter/api/src/main/kotlin/io/element/android/libraries/dateformatter/api/DurationFormatter.kt
@@ -21,6 +21,11 @@ import kotlin.time.Duration
  * - 30 seconds → "30 seconds"
  */
 interface DurationFormatter {
+    /**
+     * Formats the duration using its largest appropriate unit, rounding rather than listing several units.
+     *
+     * @param duration the duration to describe.
+     */
     fun format(duration: Duration): String
 }
 
diff --git a/libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/Feature.kt b/libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/Feature.kt
index 395fd04c6c..4fd8706d89 100644
--- a/libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/Feature.kt
+++ b/libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/Feature.kt
@@ -10,6 +10,9 @@ package io.element.android.libraries.featureflag.api
 
 import io.element.android.libraries.core.meta.BuildMeta
 
+/**
+ * Describes one feature flag: how it is identified, how it is presented in the developer options, and what it defaults to.
+ */
 interface Feature {
     /**
      * Unique key to identify the feature.
diff --git a/libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlagService.kt b/libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlagService.kt
index 6a8cb2fd9d..3dc791317f 100644
--- a/libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlagService.kt
+++ b/libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlagService.kt
@@ -11,6 +11,9 @@ package io.element.android.libraries.featureflag.api
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.first
 
+/**
+ * Reads and, where a mutable provider is registered, changes the state of the feature flags.
+ */
 interface FeatureFlagService {
     /**
      * @param feature the feature to check for
diff --git a/libraries/matrixmedia/api/src/main/kotlin/io/element/android/libraries/matrix/ui/media/ImageLoaderHolder.kt b/libraries/matrixmedia/api/src/main/kotlin/io/element/android/libraries/matrix/ui/media/ImageLoaderHolder.kt
index 62b169744b..228fbd2cff 100644
--- a/libraries/matrixmedia/api/src/main/kotlin/io/element/android/libraries/matrix/ui/media/ImageLoaderHolder.kt
+++ b/libraries/matrixmedia/api/src/main/kotlin/io/element/android/libraries/matrix/ui/media/ImageLoaderHolder.kt
@@ -11,8 +11,24 @@ import coil3.ImageLoader
 import io.element.android.libraries.matrix.api.MatrixClient
 import io.element.android.libraries.matrix.api.core.SessionId
 
+/**
+ * Caches one Coil image loader per session, since loading Matrix media requires the session's own client to fetch and decrypt it.
+ */
 interface ImageLoaderHolder {
+    /** Returns a loader that can only fetch ordinary URLs, for images that do not come from a Matrix room. */
     fun get(): ImageLoader
+
+    /**
+     * Returns the loader of a session, creating it on first use.
+     *
+     * @param client the session whose media the loader will fetch.
+     */
     fun get(client: MatrixClient): ImageLoader
+
+    /**
+     * Drops the cached loader of a session, to be called when that session goes away.
+     *
+     * @param sessionId the session whose loader is dropped.
+     */
     fun remove(sessionId: SessionId)
 }
diff --git a/libraries/matrixmedia/api/src/main/kotlin/io/element/android/libraries/matrix/ui/media/InitialsAvatarBitmapGenerator.kt b/libraries/matrixmedia/api/src/main/kotlin/io/element/android/libraries/matrix/ui/media/InitialsAvatarBitmapGenerator.kt
index e6ff5d762d..f145383e87 100644
--- a/libraries/matrixmedia/api/src/main/kotlin/io/element/android/libraries/matrix/ui/media/InitialsAvatarBitmapGenerator.kt
+++ b/libraries/matrixmedia/api/src/main/kotlin/io/element/android/libraries/matrix/ui/media/InitialsAvatarBitmapGenerator.kt
@@ -14,6 +14,15 @@ import io.element.android.libraries.designsystem.components.avatar.AvatarData
  * Generates a bitmap for an initials avatar based on the provided [io.element.android.libraries.designsystem.components.avatar.AvatarData].
  */
 interface InitialsAvatarBitmapGenerator {
+    /**
+     * Draws the initials avatar, used where a bitmap is required rather than a Composable, such as in notifications and shortcuts.
+     *
+     * @param size the width and height of the bitmap in pixels.
+     * @param avatarData the name and id the initials and the background colour are derived from.
+     * @param useDarkTheme whether to draw the dark theme variant.
+     * @param fontSizePercentage the text height as a fraction of [size].
+     * @return the bitmap, or `null` when it could not be drawn.
+     */
     fun generateBitmap(
         size: Int,
         avatarData: AvatarData,
diff --git a/libraries/mediaviewer/api/src/main/kotlin/io/element/android/libraries/mediaviewer/api/local/LocalMediaFactory.kt b/libraries/mediaviewer/api/src/main/kotlin/io/element/android/libraries/mediaviewer/api/local/LocalMediaFactory.kt
index 383f985c4f..5a218f9e1e 100644
--- a/libraries/mediaviewer/api/src/main/kotlin/io/element/android/libraries/mediaviewer/api/local/LocalMediaFactory.kt
+++ b/libraries/mediaviewer/api/src/main/kotlin/io/element/android/libraries/mediaviewer/api/local/LocalMediaFactory.kt
@@ -12,9 +12,15 @@ import android.net.Uri
 import io.element.android.libraries.matrix.api.media.MediaFile
 import io.element.android.libraries.mediaviewer.api.MediaInfo
 
+/**
+ * Builds the [LocalMedia] the media viewer renders, from either a downloaded Matrix media file or an arbitrary content uri.
+ */
 interface LocalMediaFactory {
     /**
      * This method will create a [LocalMedia] with the given [MediaFile] and [MediaInfo].
+     *
+     * @param mediaFile the file already downloaded from the homeserver.
+     * @param mediaInfo what is known about the media from the event that carried it.
      */
     fun createFromMediaFile(
         mediaFile: MediaFile,
@@ -24,6 +30,11 @@ interface LocalMediaFactory {
     /**
      * This method will create a [LocalMedia] with the given mimeType, name and formattedFileSize
      * If any of those params are null, it'll try to read them from the content.
+     *
+     * @param uri the content to display, for instance one just picked by the user.
+     * @param mimeType the MIME type, or `null` to read it from the content.
+     * @param name the file name, or `null` to read it from the content.
+     * @param formattedFileSize the size to display, or `null` to read it from the content.
      */
     fun createFromUri(
         uri: Uri,
diff --git a/libraries/mediaviewer/api/src/main/kotlin/io/element/android/libraries/mediaviewer/api/local/LocalMediaRenderer.kt b/libraries/mediaviewer/api/src/main/kotlin/io/element/android/libraries/mediaviewer/api/local/LocalMediaRenderer.kt
index fb7f26101e..5bc84919be 100644
--- a/libraries/mediaviewer/api/src/main/kotlin/io/element/android/libraries/mediaviewer/api/local/LocalMediaRenderer.kt
+++ b/libraries/mediaviewer/api/src/main/kotlin/io/element/android/libraries/mediaviewer/api/local/LocalMediaRenderer.kt
@@ -10,7 +10,15 @@ package io.element.android.libraries.mediaviewer.api.local
 
 import androidx.compose.runtime.Composable
 
+/**
+ * Renders a media file held on the device, picking the right player or viewer for its type.
+ */
 interface LocalMediaRenderer {
+    /**
+     * Draws the media, filling the space it is given.
+     *
+     * @param localMedia the file to display, along with what is known about its type.
+     */
     @Composable
     fun Render(localMedia: LocalMedia)
 }
diff --git a/libraries/mediaviewer/api/src/main/kotlin/io/element/android/libraries/mediaviewer/api/util/FileExtensionExtractor.kt b/libraries/mediaviewer/api/src/main/kotlin/io/element/android/libraries/mediaviewer/api/util/FileExtensionExtractor.kt
index 094b4513d8..e29188e984 100644
--- a/libraries/mediaviewer/api/src/main/kotlin/io/element/android/libraries/mediaviewer/api/util/FileExtensionExtractor.kt
+++ b/libraries/mediaviewer/api/src/main/kotlin/io/element/android/libraries/mediaviewer/api/util/FileExtensionExtractor.kt
@@ -8,6 +8,13 @@
 
 package io.element.android.libraries.mediaviewer.api.util
 
+/**
+ * Extracts the extension of a file name, so it can be shown to the user and used to pick an icon.
+ */
 interface FileExtensionExtractor {
+    /**
+     * @param name the file name to inspect.
+     * @return the extension without its dot, or an empty string when the name has none.
+     */
     fun extractFromName(name: String): String
 }
diff --git a/libraries/troubleshoot/api/src/main/kotlin/io/element/android/libraries/troubleshoot/api/test/NotificationTroubleshootNavigator.kt b/libraries/troubleshoot/api/src/main/kotlin/io/element/android/libraries/troubleshoot/api/test/NotificationTroubleshootNavigator.kt
index a4be22e70b..69a0561d25 100644
--- a/libraries/troubleshoot/api/src/main/kotlin/io/element/android/libraries/troubleshoot/api/test/NotificationTroubleshootNavigator.kt
+++ b/libraries/troubleshoot/api/src/main/kotlin/io/element/android/libraries/troubleshoot/api/test/NotificationTroubleshootNavigator.kt
@@ -8,6 +8,10 @@
 
 package io.element.android.libraries.troubleshoot.api.test
 
+/**
+ * Lets a troubleshooting test's quick fix send the user to another screen, without the test itself knowing about navigation.
+ */
 interface NotificationTroubleshootNavigator {
+    /** Opens the blocked users screen, so the user can unblock someone whose messages were being filtered out. */
     fun navigateToBlockedUsers()
 }
diff --git a/libraries/troubleshoot/api/src/main/kotlin/io/element/android/libraries/troubleshoot/api/test/NotificationTroubleshootTest.kt b/libraries/troubleshoot/api/src/main/kotlin/io/element/android/libraries/troubleshoot/api/test/NotificationTroubleshootTest.kt
index 27f3bcf8ed..98898ad483 100644
--- a/libraries/troubleshoot/api/src/main/kotlin/io/element/android/libraries/troubleshoot/api/test/NotificationTroubleshootTest.kt
+++ b/libraries/troubleshoot/api/src/main/kotlin/io/element/android/libraries/troubleshoot/api/test/NotificationTroubleshootTest.kt
@@ -20,11 +20,36 @@ import kotlinx.coroutines.flow.StateFlow
  * the component they're injected into is bound to [SessionScope] and so should these (https://github.com/ZacSweers/metro/issues/1932).
  */
 interface NotificationTroubleshootTest {
+    /** Position of this test in the list shown to the user; tests run in ascending order. */
     val order: Int
+
+    /** The current state of this test, which the UI observes to show it as idle, running, successful or failed. */
     val state: StateFlow<NotificationTroubleshootTestState>
+
+    /**
+     * Whether this test applies to the current setup, so that irrelevant tests are hidden rather than shown as failing.
+     *
+     * @param data the current push provider and distributor, used to decide relevance.
+     */
     fun isRelevant(data: TestFilterData): Boolean = true
+
+    /**
+     * Runs the test, reporting its outcome through [state] rather than by returning or throwing.
+     *
+     * @param coroutineScope the scope to launch any work that must outlive the call itself.
+     */
     suspend fun run(coroutineScope: CoroutineScope)
+
+    /** Returns the test to its idle state so it can be run again. */
     suspend fun reset()
+
+    /**
+     * Attempts to fix what this test detected, for the tests that can offer it. Throws by default, so check the state's quick fix
+     * flag before calling.
+     *
+     * @param coroutineScope the scope to launch any work that must outlive the call itself.
+     * @param navigator used by the fixes that have to send the user to another screen.
+     */
     suspend fun quickFix(
         coroutineScope: CoroutineScope,
         navigator: NotificationTroubleshootNavigator,
diff --git a/libraries/workmanager/api/src/main/kotlin/io/element/android/libraries/workmanager/api/WorkManagerScheduler.kt b/libraries/workmanager/api/src/main/kotlin/io/element/android/libraries/workmanager/api/WorkManagerScheduler.kt
index a616e01573..b4c2950cd4 100644
--- a/libraries/workmanager/api/src/main/kotlin/io/element/android/libraries/workmanager/api/WorkManagerScheduler.kt
+++ b/libraries/workmanager/api/src/main/kotlin/io/element/android/libraries/workmanager/api/WorkManagerScheduler.kt
@@ -10,20 +10,31 @@ package io.element.android.libraries.workmanager.api
 
 import io.element.android.libraries.matrix.api.core.SessionId
 
+/**
+ * Schedules background work, wrapping `WorkManager` so the rest of the app does not depend on it directly.
+ */
 interface WorkManagerScheduler {
     /**
      * Submits a new work request built from [workManagerRequestBuilder] to run in `WorkManager`.
+     *
+     * @param workManagerRequestBuilder describes the work to schedule.
      */
     suspend fun submit(workManagerRequestBuilder: WorkManagerRequestBuilder)
 
     /**
      * Checks if there are any pending requests scheduled for the provided [sessionId] and [requestType].
+     *
+     * @param sessionId the session the work belongs to.
+     * @param requestType the kind of work to look for.
      */
     fun hasPendingWork(sessionId: SessionId, requestType: WorkManagerRequestType): Boolean
 
     /**
      * Cancel pending work requests for the session [SessionId].
      * If [requestType] is provided, it will only cancel requests for that type, otherwise it will cancel all requests.
+     *
+     * @param sessionId the session whose work is cancelled.
+     * @param requestType the kind of work to cancel, or `null` to cancel every kind.
      */
     fun cancel(sessionId: SessionId, requestType: WorkManagerRequestType? = null)
 }
diff --git a/libraries/workmanager/api/src/main/kotlin/io/element/android/libraries/workmanager/api/di/MetroWorkerFactory.kt b/libraries/workmanager/api/src/main/kotlin/io/element/android/libraries/workmanager/api/di/MetroWorkerFactory.kt
index 086d3baafa..d97f66f966 100644
--- a/libraries/workmanager/api/src/main/kotlin/io/element/android/libraries/workmanager/api/di/MetroWorkerFactory.kt
+++ b/libraries/workmanager/api/src/main/kotlin/io/element/android/libraries/workmanager/api/di/MetroWorkerFactory.kt
@@ -28,7 +28,13 @@ class MetroWorkerFactory(
       return workerProviders[Class.forName(workerClassName).kotlin]?.create(workerParameters)
     }
 
+    /**
+     * Creates one worker type with its dependencies injected, since `WorkManager` instantiates workers itself and cannot use the graph.
+     */
     interface WorkerInstanceFactory<T : ListenableWorker> {
+      /**
+       * @param params the parameters `WorkManager` built the worker with.
+       */
       fun create(params: WorkerParameters): T
     }
 }