diff --git a/extension/background.js b/extension/background.js
index 9f44a08..44dfe5a 100644
--- a/extension/background.js
+++ b/extension/background.js
@@ -1,6 +1,7 @@
 import { EVENTS, PROTOCOL_VERSION, OFFICIAL_SERVER_URL, OFFICIAL_SERVER_TOKEN, EPISODE_LOBBY_TIMEOUT, FORCE_SYNC_TIMEOUT } from './shared/constants.js';
 import { generateUsername } from './shared/names.js';
 import { loadLocale, getMessage, getSystemLanguage } from './i18n.js';
+import { sameEpisode } from './episode-utils.js';
 
 // --- Uninstall URL Initialization ---
 chrome.runtime.onInstalled.addListener((details) => {
@@ -191,25 +192,7 @@ let forceSyncTimeout = null;
 let episodeLobby = null; // { expectedTitle, initiatorPeerId, readyPeers: [], createdAt }
 let episodeLobbyTimeout = null;
 
-// --- Episode Title Extraction (synced with content.js) ---
-function extractEpisodeId(title) {
-    if (!title || typeof title !== 'string') return null;
-    const se = title.match(/S(?:eason\s*)?(\d+)[\s\-\.]*E(?:pisode\s*)?(\d+)/i);
-    if (se) return `S${String(se[1]).padStart(2, '0')}E${String(se[2]).padStart(2, '0')}`;
-    const ep = title.match(/(?:Episode|Folge|Ep\.?|#)\s*(\d+)/i);
-    if (ep) return `EP${String(ep[1]).padStart(3, '0')}`;
-    return null;
-}
-
-function sameEpisode(titleA, titleB) {
-    if (!titleA && !titleB) return true; // Both unknown → assume same (backward compat)
-    if (!titleA || !titleB) return false; // One unknown, one known → different
-    const idA = extractEpisodeId(titleA);
-    const idB = extractEpisodeId(titleB);
-    if (idA && idB) return idA === idB; // Both have parseable IDs → compare IDs
-    if (idA || idB) return false;       // One has ID, other doesn't → different
-    return titleA === titleB;            // Neither has ID → exact string match
-}
+// --- Episode Title Extraction (synced with content.js via episode-utils.js) ---
 
 // --- Storage Utils ---
 
diff --git a/extension/content.js b/extension/content.js
index b91c6c2..ce1118b 100644
--- a/extension/content.js
+++ b/extension/content.js
@@ -290,28 +290,27 @@
     // Extract a canonical episode identifier from a title string.
     // Handles: S01E01, S1E1, S01 - E01, Season 1 Episode 1, "Folge 5", "Episode 5", "Ep. 5", "#5"
     // Returns null if no episode pattern found.
+    // --- SHARED_EPISODE_UTILS_INJECT_START ---
+    // This block is automatically replaced by /scripts/build-extension.js
     function extractEpisodeId(title) {
         if (!title || typeof title !== 'string') return null;
-        // S01E01 patterns (with any non-alphanumeric separator between season and E)
         const se = title.match(/S(?:eason\s*)?(\d+)[^a-zA-Z0-9]*E(?:pisode\s*)?(\d+)/i);
         if (se) return `S${String(se[1]).padStart(2, '0')}E${String(se[2]).padStart(2, '0')}`;
-        // "Episode X", "Folge X", "Ep. X", "#X"
         const ep = title.match(/(?:Episode|Folge|Ep\.?|#)\s*(\d+)/i);
         if (ep) return `EP${String(ep[1]).padStart(3, '0')}`;
         return null;
     }
 
-    // Returns true if two titles likely refer to the same episode.
-    // Strict: both must have IDs and match, OR neither has IDs and exact match.
     function sameEpisode(titleA, titleB) {
-        if (!titleA && !titleB) return true; // Both unknown → assume same (backward compat)
-        if (!titleA || !titleB) return false; // One unknown, one known → different
+        if (!titleA && !titleB) return true;
+        if (!titleA || !titleB) return false;
         const idA = extractEpisodeId(titleA);
         const idB = extractEpisodeId(titleB);
-        if (idA && idB) return idA === idB; // Both have parseable IDs → compare IDs
-        if (idA || idB) return false;       // One has ID, other doesn't → different
-        return titleA === titleB;            // Neither has ID → exact string match
+        if (idA && idB) return idA === idB;
+        if (idA || idB) return false;
+        return titleA === titleB;
     }
+    // --- SHARED_EPISODE_UTILS_INJECT_END ---
 
     // Returns true only when we are CERTAIN the episodes differ.
     // Permissive: only blocks if BOTH titles have parseable IDs AND they differ.
diff --git a/extension/episode-utils.js b/extension/episode-utils.js
new file mode 100644
index 0000000..ab68b07
--- /dev/null
+++ b/extension/episode-utils.js
@@ -0,0 +1,24 @@
+/**
+ * KoalaSync Episode Title Utilities
+ * Single source of truth — synced to content.js by build-extension.js.
+ * Keep in sync with the injection block in content.js!
+ */
+
+export function extractEpisodeId(title) {
+    if (!title || typeof title !== 'string') return null;
+    const se = title.match(/S(?:eason\s*)?(\d+)[^a-zA-Z0-9]*E(?:pisode\s*)?(\d+)/i);
+    if (se) return `S${String(se[1]).padStart(2, '0')}E${String(se[2]).padStart(2, '0')}`;
+    const ep = title.match(/(?:Episode|Folge|Ep\.?|#)\s*(\d+)/i);
+    if (ep) return `EP${String(ep[1]).padStart(3, '0')}`;
+    return null;
+}
+
+export function sameEpisode(titleA, titleB) {
+    if (!titleA && !titleB) return true;
+    if (!titleA || !titleB) return false;
+    const idA = extractEpisodeId(titleA);
+    const idB = extractEpisodeId(titleB);
+    if (idA && idB) return idA === idB;
+    if (idA || idB) return false;
+    return titleA === titleB;
+}
diff --git a/scripts/build-extension.js b/scripts/build-extension.js
index ffcb971..4f69299 100644
--- a/scripts/build-extension.js
+++ b/scripts/build-extension.js
@@ -97,6 +97,25 @@ function copyExtensionFiles(targetDir, browserName) {
           console.warn('⚠️ WARNING: Heartbeat markers not found in content.js');
         }
 
+        // 3. Inject Episode Utils
+        const euStart = '// --- SHARED_EPISODE_UTILS_INJECT_START ---';
+        const euEnd = '// --- SHARED_EPISODE_UTILS_INJECT_END ---';
+        const euPattern = new RegExp(euStart.replace(/[.*+?^${}()|[\]\\]/g, '\\$&') + '[\\s\\S]+?' + euEnd.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'));
+        const euPath = path.join(rootDir, 'extension', 'episode-utils.js');
+        if (fs.existsSync(euPath)) {
+          const euContent = fs.readFileSync(euPath, 'utf8');
+          const stripped = euContent
+            .replace(/^\/\*\*[\s\S]*?\*\/\s*/m, '')
+            .replace(/export function /g, 'function ')
+            .trim();
+          const euRep = `${euStart}\n    // This block is automatically updated by /scripts/build-extension.js\n${stripped.split('\n').map(l => '    ' + l).join('\n')}\n    ${euEnd}`;
+          if (euPattern.test(content)) {
+            content = content.replace(euPattern, euRep);
+          } else {
+            console.warn('⚠ WARNING: Episode utils markers not found in content.js');
+          }
+        }
+
         fs.writeFileSync(destPath, content);
         console.log('✓ Injected shared constants into content.js');
       } else if (item === 'background.js') {