diff --git a/server/services/DownloadBuilder.js b/server/services/DownloadBuilder.js
index 64fc235..fce6509 100644
--- a/server/services/DownloadBuilder.js
+++ b/server/services/DownloadBuilder.js
@@ -1,9 +1,21 @@
 // Copyright (c) 2026 Gordon Bolton. MIT License.
+
+/**
+ * DownloadBuilder - Aggregates and matches download data from multiple sources.
+ * This service processes cached data from SABnzbd, qBittorrent, Sonarr, and Radarr to build
+ * a unified view of downloads for each user, matching downloads to media metadata via tags.
+ */
+
 const { mapTorrentToDownload } = require('../utils/qbittorrent');
 const TagMatcher = require('./TagMatcher');
 const DownloadAssembler = require('./DownloadAssembler');
 
-// Build series map from queue and history records
+/**
+ * Builds a Map of series metadata from Sonarr queue and history records.
+ * @param {Array} queueRecords - Sonarr queue records
+ * @param {Array} historyRecords - Sonarr history records
+ * @returns {Map} Map of seriesId to series object
+ */
 function buildSeriesMapFromRecords(queueRecords, historyRecords) {
   const seriesMap = new Map();
   for (const r of queueRecords) {
@@ -15,7 +27,12 @@ function buildSeriesMapFromRecords(queueRecords, historyRecords) {
   return seriesMap;
 }
 
-// Build movies map from queue and history records
+/**
+ * Builds a Map of movie metadata from Radarr queue and history records.
+ * @param {Array} queueRecords - Radarr queue records
+ * @param {Array} historyRecords - Radarr history records
+ * @returns {Map} Map of movieId to movie object
+ */
 function buildMoviesMapFromRecords(queueRecords, historyRecords) {
   const moviesMap = new Map();
   for (const r of queueRecords) {
@@ -27,7 +44,14 @@ function buildMoviesMapFromRecords(queueRecords, historyRecords) {
   return moviesMap;
 }
 
-// Get slot status and speed based on queue status
+/**
+ * Determines the status and speed for a SABnzbd slot based on queue state.
+ * @param {Object} slot - SABnzbd queue slot
+ * @param {string} queueStatus - Overall queue status (e.g., 'Paused')
+ * @param {string} queueSpeed - Queue speed string
+ * @param {string} queueKbpersec - Queue speed in KB/s
+ * @returns {Object} Object with status and speed properties
+ */
 function getSlotStatusAndSpeed(slot, queueStatus, queueSpeed, queueKbpersec) {
   if (queueStatus === 'Paused') {
     return { status: 'Paused', speed: '0' };
@@ -38,7 +62,12 @@ function getSlotStatusAndSpeed(slot, queueStatus, queueSpeed, queueKbpersec) {
   };
 }
 
-// Match SABnzbd queue slots to Sonarr/Radarr activity
+/**
+ * Matches SABnzbd queue slots to Sonarr/Radarr activity using download IDs and title matching.
+ * @param {Array} slots - SABnzbd queue slots
+ * @param {Object} context - Matching context with records, maps, and user info
+ * @returns {Array} Array of matched download objects
+ */
 function matchSabSlots(slots, context) {
   const {
     sonarrQueueRecords,
@@ -215,7 +244,12 @@ function matchSabSlots(slots, context) {
   return matched;
 }
 
-// Match SABnzbd history slots to Sonarr/Radarr activity
+/**
+ * Matches SABnzbd history slots to Sonarr/Radarr activity using title matching.
+ * @param {Array} slots - SABnzbd history slots
+ * @param {Object} context - Matching context with records, maps, and user info
+ * @returns {Array} Array of matched download objects
+ */
 function matchSabHistory(slots, context) {
   const {
     sonarrHistoryRecords,
@@ -313,7 +347,12 @@ function matchSabHistory(slots, context) {
   return matched;
 }
 
-// Match qBittorrent torrents to Sonarr/Radarr activity
+/**
+ * Matches qBittorrent torrents to Sonarr/Radarr activity using title matching.
+ * @param {Array} torrents - qBittorrent torrent list
+ * @param {Object} context - Matching context with records, maps, and user info
+ * @returns {Array} Array of matched download objects
+ */
 function matchTorrents(torrents, context) {
   const {
     sonarrQueueRecords,
@@ -529,9 +568,31 @@ function matchTorrents(torrents, context) {
   return matched;
 }
 
-// Build user downloads from cache snapshot
+/**
+ * Builds a unified list of downloads for a user from multiple download clients.
+ * Matches SABnzbd and qBittorrent downloads to Sonarr/Radarr activity using tags.
+ * @param {Object} cacheSnapshot - Cached data from all services
+ * @param {Object} options - User context and metadata maps
+ * @param {string} options.username - Lowercase username for tag matching
+ * @param {string} options.usernameSanitized - Original username
+ * @param {boolean} options.isAdmin - Whether user is admin
+ * @param {boolean} options.showAll - Whether to show all users' downloads
+ * @param {Map} options.seriesMap - Map of seriesId to series object
+ * @param {Map} options.moviesMap - Map of movieId to movie object
+ * @param {Map} options.sonarrTagMap - Map of Sonarr tag IDs to labels
+ * @param {Map} options.radarrTagMap - Map of Radarr tag IDs to labels
+ * @param {Map} options.embyUserMap - Map of Emby users for admin view
+ * @returns {Array} Array of download objects for the user
+ */
 function buildUserDownloads(cacheSnapshot, { username, usernameSanitized, isAdmin, showAll, seriesMap, moviesMap, sonarrTagMap, radarrTagMap, embyUserMap }) {
-  // Handle null/undefined cache data
+  // Input validation
+  if (!cacheSnapshot || typeof cacheSnapshot !== 'object') {
+    console.error('[DownloadBuilder] Invalid cacheSnapshot provided');
+    return [];
+  }
+
+  try {
+    // Handle null/undefined cache data
   const sabnzbdQueue = cacheSnapshot.sabnzbdQueue || { data: { queue: { slots: [] } } };
   const sabnzbdHistory = cacheSnapshot.sabnzbdHistory || { data: { history: { slots: [] } } };
   const sonarrQueue = cacheSnapshot.sonarrQueue || { data: { records: [] } };
@@ -599,7 +660,11 @@ function buildUserDownloads(cacheSnapshot, { username, usernameSanitized, isAdmi
     }
   }
 
-  return userDownloads;
+    return userDownloads;
+  } catch (error) {
+    console.error('[DownloadBuilder] Error building user downloads:', error.message);
+    return [];
+  }
 }
 
 module.exports = {