diff --git a/libraries/audio/api/src/main/kotlin/io/element/android/libraries/audio/api/AudioFocus.kt b/libraries/audio/api/src/main/kotlin/io/element/android/libraries/audio/api/AudioFocus.kt
index aeccaa9b8a..de2e5844d5 100644
--- a/libraries/audio/api/src/main/kotlin/io/element/android/libraries/audio/api/AudioFocus.kt
+++ b/libraries/audio/api/src/main/kotlin/io/element/android/libraries/audio/api/AudioFocus.kt
@@ -15,6 +15,9 @@ enum class AudioFocusRequester {
     MediaViewer,
 }
 
+/**
+ * Claims and releases system audio focus, so that the app's own players do not talk over each other or over other apps.
+ */
 interface AudioFocus {
     /**
      * Request audio focus for the given requester.
diff --git a/libraries/cachestore/api/src/main/kotlin/io/element/android/libraries/cachestore/api/CacheStore.kt b/libraries/cachestore/api/src/main/kotlin/io/element/android/libraries/cachestore/api/CacheStore.kt
index 5df446f688..bbfb7d3405 100644
--- a/libraries/cachestore/api/src/main/kotlin/io/element/android/libraries/cachestore/api/CacheStore.kt
+++ b/libraries/cachestore/api/src/main/kotlin/io/element/android/libraries/cachestore/api/CacheStore.kt
@@ -7,9 +7,33 @@
 
 package io.element.android.libraries.cachestore.api
 
+/**
+ * A generic on-disk key/value cache, where each entry records when it was written so callers can decide for themselves when it is stale.
+ */
 interface CacheStore {
+    /**
+     * Stores an entry, replacing any previous one under the same key.
+     *
+     * @param key the entry to write.
+     * @param data the value along with the time it was obtained.
+     */
     suspend fun storeData(key: String, data: CacheData)
+
+    /**
+     * Reads an entry without applying any expiry of its own; judging staleness is up to the caller.
+     *
+     * @param key the entry to read.
+     * @return the value and its timestamp, or `null` when nothing is stored under that key.
+     */
     suspend fun getData(key: String): CacheData?
+
+    /**
+     * Removes one entry.
+     *
+     * @param key the entry to delete.
+     */
     suspend fun deleteData(key: String)
+
+    /** Empties the whole cache. */
     suspend fun deleteAll()
 }
diff --git a/libraries/indicator/api/src/main/kotlin/io/element/android/libraries/indicator/api/IndicatorService.kt b/libraries/indicator/api/src/main/kotlin/io/element/android/libraries/indicator/api/IndicatorService.kt
index 0409a0f846..d2fde82d8a 100644
--- a/libraries/indicator/api/src/main/kotlin/io/element/android/libraries/indicator/api/IndicatorService.kt
+++ b/libraries/indicator/api/src/main/kotlin/io/element/android/libraries/indicator/api/IndicatorService.kt
@@ -15,9 +15,11 @@ import androidx.compose.runtime.State
  * A set of State<Boolean> to observe to display or not the indicators in the UI.
  */
 interface IndicatorService {
+    /** Whether to show the red dot on the room list top bar, which aggregates the reasons the user should open the settings. */
     @Composable
     fun showRoomListTopBarIndicator(): State<Boolean>
 
+    /** Whether to show the red dot on the chat backup setting, i.e. when recovery needs the user's attention. */
     @Composable
     fun showSettingChatBackupIndicator(): State<Boolean>
 }
diff --git a/libraries/slashcommands/api/src/main/kotlin/io/element/android/libraries/slashcommands/api/SlashCommandService.kt b/libraries/slashcommands/api/src/main/kotlin/io/element/android/libraries/slashcommands/api/SlashCommandService.kt
index 9dfca26078..7ae056e38b 100644
--- a/libraries/slashcommands/api/src/main/kotlin/io/element/android/libraries/slashcommands/api/SlashCommandService.kt
+++ b/libraries/slashcommands/api/src/main/kotlin/io/element/android/libraries/slashcommands/api/SlashCommandService.kt
@@ -9,7 +9,19 @@ package io.element.android.libraries.slashcommands.api
 
 import io.element.android.libraries.matrix.api.timeline.Timeline
 
+/**
+ * Recognises and runs the slash commands the user can type in the composer, such as `/me` or `/join`.
+ *
+ * The composer first [parse]s what was typed, then hands the result to whichever proceed method matches its kind; navigation
+ * commands and the error cases are handled by the composer itself.
+ */
 interface SlashCommandService {
+    /**
+     * Returns the commands whose name starts with what has been typed, so the composer can offer an autocomplete list.
+     *
+     * @param text the current composer content.
+     * @param isInThread whether the composer is in a thread, since not every command is available there.
+     */
     suspend fun getSuggestions(
         text: String,
         isInThread: Boolean,
@@ -17,6 +29,11 @@ interface SlashCommandService {
 
     /**
      * Parse the message and return a SlashCommand.
+     * Ordinary text comes back as `NotACommand`, and a malformed or unsupported command as one of the error cases, so this never throws.
+     *
+     * @param textMessage the composer content as plain text.
+     * @param formattedMessage the composer content as HTML, or `null` when unformatted.
+     * @param isInThreadTimeline whether the composer is in a thread.
      */
     suspend fun parse(
         textMessage: CharSequence,
@@ -26,6 +43,9 @@ interface SlashCommandService {
 
     /**
      * Proceed a SlashCommandSendMessage.
+     *
+     * @param slashCommand the parsed command, carrying the content to send.
+     * @param timeline the timeline to send it to.
      */
     suspend fun proceedSendMessage(
         slashCommand: SlashCommand.SlashCommandSendMessage,
@@ -33,7 +53,9 @@ interface SlashCommandService {
     ): Result<Unit>
 
     /**
-     * Proceed a SlashCommandAdmin.
+     * Proceed a SlashCommandAdmin, i.e. one that acts on the room or its members rather than sending a message.
+     *
+     * @param slashCommand the parsed command to run.
      */
     suspend fun proceedAdmin(
         slashCommand: SlashCommand.SlashCommandAdmin,
diff --git a/libraries/voiceplayer/api/src/main/kotlin/io/element/android/libraries/voiceplayer/api/VoiceMessagePresenterFactory.kt b/libraries/voiceplayer/api/src/main/kotlin/io/element/android/libraries/voiceplayer/api/VoiceMessagePresenterFactory.kt
index 0229ed991f..1b576a6d72 100644
--- a/libraries/voiceplayer/api/src/main/kotlin/io/element/android/libraries/voiceplayer/api/VoiceMessagePresenterFactory.kt
+++ b/libraries/voiceplayer/api/src/main/kotlin/io/element/android/libraries/voiceplayer/api/VoiceMessagePresenterFactory.kt
@@ -13,7 +13,17 @@ import io.element.android.libraries.matrix.api.core.EventId
 import io.element.android.libraries.matrix.api.media.MediaSource
 import kotlin.time.Duration
 
+/**
+ * Creates the presenter of a single voice message player, one per voice message shown in a timeline.
+ */
 interface VoiceMessagePresenterFactory {
+    /**
+     * @param eventId the event carrying the voice message, or `null` while it is still a local echo.
+     * @param mediaSource where the audio is fetched from.
+     * @param mimeType the MIME type of the audio, or `null` when the event did not state it.
+     * @param filename the file name to display, or `null` when the event did not state it.
+     * @param duration the length of the recording, used to render the waveform before the audio is downloaded.
+     */
     fun createVoiceMessagePresenter(
         eventId: EventId?,
         mediaSource: MediaSource,
diff --git a/services/apperror/api/src/main/kotlin/io/element/android/services/apperror/api/AppErrorStateService.kt b/services/apperror/api/src/main/kotlin/io/element/android/services/apperror/api/AppErrorStateService.kt
index 7c592e2102..d9ae37c097 100644
--- a/services/apperror/api/src/main/kotlin/io/element/android/services/apperror/api/AppErrorStateService.kt
+++ b/services/apperror/api/src/main/kotlin/io/element/android/services/apperror/api/AppErrorStateService.kt
@@ -11,10 +11,26 @@ package io.element.android.services.apperror.api
 import androidx.annotation.StringRes
 import kotlinx.coroutines.flow.StateFlow
 
+/**
+ * Surfaces app-wide error dialogs from anywhere, for the failures that are not tied to the screen the user is on.
+ */
 interface AppErrorStateService {
+    /** The error to display, if any; observed once at the top of the app rather than per screen. */
     val appErrorStateFlow: StateFlow<AppErrorState>
 
+    /**
+     * Shows an error dialog with already resolved text.
+     *
+     * @param title the dialog title.
+     * @param body the dialog message; never put user content or secrets in it.
+     */
     fun showError(title: String, body: String)
 
+    /**
+     * Shows an error dialog from string resources, which is the preferred form since it stays localised.
+     *
+     * @param titleRes resource id of the dialog title.
+     * @param bodyRes resource id of the dialog message.
+     */
     fun showError(@StringRes titleRes: Int, @StringRes bodyRes: Int)
 }