diff --git a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/analytics/GetDatabaseSizesUseCase.kt b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/analytics/GetDatabaseSizesUseCase.kt
index fde02fb2c6..1ca9adfaca 100644
--- a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/analytics/GetDatabaseSizesUseCase.kt
+++ b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/analytics/GetDatabaseSizesUseCase.kt
@@ -9,6 +9,14 @@ package io.element.android.libraries.matrix.api.analytics
 
 import io.element.android.libraries.matrix.api.core.SessionId
 
+/**
+ * Reads the on-disk size of a session's SDK stores, so they can be reported as analytics.
+ */
 fun interface GetDatabaseSizesUseCase {
+    /**
+     * Returns the size of each store of the given session.
+     *
+     * @param sessionId the session whose stores are measured.
+     */
     operator fun invoke(sessionId: SessionId): Result<SdkStoreSizes>
 }
diff --git a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/core/ProgressCallback.kt b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/core/ProgressCallback.kt
index a7abc0f797..5c8a75e384 100644
--- a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/core/ProgressCallback.kt
+++ b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/core/ProgressCallback.kt
@@ -8,6 +8,15 @@
 
 package io.element.android.libraries.matrix.api.core
 
+/**
+ * Reports the progress of a long running transfer, such as a media upload or download.
+ */
 interface ProgressCallback {
+    /**
+     * Called as the transfer advances, possibly many times and from a background thread.
+     *
+     * @param current the number of bytes transferred so far.
+     * @param total the total number of bytes to transfer.
+     */
     fun onProgress(current: Long, total: Long)
 }
diff --git a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/core/SendHandle.kt b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/core/SendHandle.kt
index f3b8904fff..02c8a85d58 100644
--- a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/core/SendHandle.kt
+++ b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/core/SendHandle.kt
@@ -8,6 +8,10 @@
 
 package io.element.android.libraries.matrix.api.core
 
+/**
+ * A handle on an event sitting in a room's send queue, obtained from a timeline item that failed to send.
+ */
 fun interface SendHandle {
+    /** Puts the event back in the send queue for another attempt. */
     suspend fun retry(): Result<Unit>
 }
diff --git a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/permalink/MatrixToConverter.kt b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/permalink/MatrixToConverter.kt
index cc3e2b5cf6..d6a2e71b8f 100644
--- a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/permalink/MatrixToConverter.kt
+++ b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/permalink/MatrixToConverter.kt
@@ -20,6 +20,9 @@ interface MatrixToConverter {
      * - https://riot.im/develop/#/room/#element-android:matrix.org  ->  https://matrix.to/#/#element-android:matrix.org
      * - https://app.element.io/#/room/#element-android:matrix.org   ->  https://matrix.to/#/#element-android:matrix.org
      * - https://www.example.org/#/room/#element-android:matrix.org  ->  https://matrix.to/#/#element-android:matrix.org
+     *
+     * @param uri the uri to convert.
+     * @return the equivalent matrix.to uri, or `null` when the input is not a permalink this converter recognises.
      */
     fun convert(uri: Uri): Uri?
 }
diff --git a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/permalink/PermalinkBuilder.kt b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/permalink/PermalinkBuilder.kt
index 878b0e7a69..9140c312c7 100644
--- a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/permalink/PermalinkBuilder.kt
+++ b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/permalink/PermalinkBuilder.kt
@@ -11,8 +11,22 @@ package io.element.android.libraries.matrix.api.permalink
 import io.element.android.libraries.matrix.api.core.RoomAlias
 import io.element.android.libraries.matrix.api.core.UserId
 
+/**
+ * Builds `matrix.to` permalinks, which are the shareable links to a user or a room; see [PermalinkParser] for the other direction.
+ */
 interface PermalinkBuilder {
+    /**
+     * Builds a permalink to a user, failing with [PermalinkBuilderError.InvalidData] when the id is malformed.
+     *
+     * @param userId the user to link to.
+     */
     fun permalinkForUser(userId: UserId): Result<String>
+
+    /**
+     * Builds a permalink to a room from its alias, failing with [PermalinkBuilderError.InvalidData] when the alias is malformed.
+     *
+     * @param roomAlias the room alias to link to.
+     */
     fun permalinkForRoomAlias(roomAlias: RoomAlias): Result<String>
 }
 
diff --git a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/permalink/PermalinkParser.kt b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/permalink/PermalinkParser.kt
index 5365b8776a..467b147d98 100644
--- a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/permalink/PermalinkParser.kt
+++ b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/permalink/PermalinkParser.kt
@@ -18,7 +18,10 @@ package io.element.android.libraries.matrix.api.permalink
 interface PermalinkParser {
     /**
      * Turns a uri string to a [PermalinkData].
+     * A uri that is not a Matrix permalink is reported as a fallback link rather than as an error, so this never throws.
      * https://github.com/matrix-org/matrix-doc/blob/master/proposals/1704-matrix.to-permalinks.md
+     *
+     * @param uriString the uri to parse.
      */
     fun parse(uriString: String): PermalinkData
 }
diff --git a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/server/UserServerResolver.kt b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/server/UserServerResolver.kt
index 3529b17d85..6c3c16b35e 100644
--- a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/server/UserServerResolver.kt
+++ b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/server/UserServerResolver.kt
@@ -8,6 +8,10 @@
 
 package io.element.android.libraries.matrix.api.server
 
+/**
+ * Resolves the server name of the logged in user, which is the part of their user id after the colon.
+ */
 interface UserServerResolver {
+    /** Returns the server name, for instance `matrix.org` for `@alice:matrix.org`. */
     fun resolve(): String
 }
diff --git a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/tracing/TracingService.kt b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/tracing/TracingService.kt
index d4ef27fd0d..f5de427ce0 100644
--- a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/tracing/TracingService.kt
+++ b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/tracing/TracingService.kt
@@ -10,8 +10,21 @@ package io.element.android.libraries.matrix.api.tracing
 
 import timber.log.Timber
 
+/**
+ * Routes the app's own logs into the SDK's tracing subsystem, so that Kotlin and Rust logs end up interleaved in the same files.
+ */
 interface TracingService {
+    /**
+     * Creates a Timber tree that forwards what the app logs to the SDK tracing subsystem.
+     *
+     * @param target the tracing target name the logs are attributed to, used to tell subsystems apart in the output.
+     */
     fun createTimberTree(target: String): Timber.Tree
 
+    /**
+     * Turns writing logs to files on or off at runtime, which is how the user-facing logging toggle is applied.
+     *
+     * @param config where to write the log files, or the disabled configuration to stop writing them.
+     */
     fun updateWriteToFilesConfiguration(config: WriteToFilesConfiguration)
 }
diff --git a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/widget/CallAnalyticCredentialsProvider.kt b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/widget/CallAnalyticCredentialsProvider.kt
index 5b2a691afc..1b45c4f1a0 100644
--- a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/widget/CallAnalyticCredentialsProvider.kt
+++ b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/widget/CallAnalyticCredentialsProvider.kt
@@ -8,10 +8,24 @@
 
 package io.element.android.libraries.matrix.api.widget
 
+/**
+ * Supplies the analytics and crash reporting credentials passed to the embedded Element Call widget.
+ *
+ * Every member is `null` when the corresponding service is not configured for this build, or when the user has not consented to analytics.
+ */
 interface CallAnalyticCredentialsProvider {
+    /** The PostHog identifier of the current user, so call analytics can be tied to the same person as app analytics. */
     val posthogUserId: String?
+
+    /** The PostHog host the widget should send its events to. */
     val posthogApiHost: String?
+
+    /** The PostHog API key the widget should use. */
     val posthogApiKey: String?
+
+    /** The URL the widget should submit its rageshakes to. */
     val rageshakeSubmitUrl: String?
+
+    /** The Sentry DSN the widget should report its crashes to. */
     val sentryDsn: String?
 }
diff --git a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/widget/CallWidgetSettingsProvider.kt b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/widget/CallWidgetSettingsProvider.kt
index d0686e8e5f..92510c4b8f 100644
--- a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/widget/CallWidgetSettingsProvider.kt
+++ b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/widget/CallWidgetSettingsProvider.kt
@@ -10,7 +10,18 @@ package io.element.android.libraries.matrix.api.widget
 
 import java.util.UUID
 
+/**
+ * Builds the widget settings used to embed Element Call in a room, applying the app's own preferences on top of the SDK defaults.
+ */
 interface CallWidgetSettingsProvider {
+    /**
+     * @param baseUrl the URL Element Call is served from.
+     * @param widgetId a unique identifier for this widget instance.
+     * @param encrypted whether the room the call takes place in is encrypted.
+     * @param direct whether the call happens in a direct message room.
+     * @param isAudioCall whether the call should start without video.
+     * @param hasActiveCall whether a call is already ongoing in the room, which affects whether the user joins straight away.
+     */
     suspend fun provide(
         baseUrl: String,
         widgetId: String = UUID.randomUUID().toString(),
diff --git a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/widget/MatrixWidgetDriver.kt b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/widget/MatrixWidgetDriver.kt
index d8cca95842..3b863279cf 100644
--- a/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/widget/MatrixWidgetDriver.kt
+++ b/libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/widget/MatrixWidgetDriver.kt
@@ -10,10 +10,25 @@ package io.element.android.libraries.matrix.api.widget
 
 import kotlinx.coroutines.flow.Flow
 
+/**
+ * Bridges a widget running in a WebView with the Matrix room it is embedded in, relaying the widget API messages in both directions.
+ *
+ * The driver owns SDK resources and must be closed once the widget goes away; Element Call is the main user of this.
+ */
 interface MatrixWidgetDriver : AutoCloseable {
+    /** The id of the widget this driver serves, matching the one in its settings. */
     val id: String
+
+    /** The messages coming from the room that should be forwarded to the widget. */
     val incomingMessages: Flow<String>
 
+    /** Starts relaying messages and suspends for as long as the driver runs; calling it again while it is running does nothing. */
     suspend fun run()
+
+    /**
+     * Forwards a message from the widget to the room. Does nothing once the driver has been closed.
+     *
+     * @param message the raw widget API message emitted by the WebView.
+     */
     suspend fun send(message: String)
 }