diff --git a/src/lib/collaboration/realtime.ts b/src/lib/collaboration/realtime.ts
index e5b0cb8..800e273 100644
--- a/src/lib/collaboration/realtime.ts
+++ b/src/lib/collaboration/realtime.ts
@@ -77,6 +77,21 @@ export class RealtimeConnection {
     this.resetInactivityTimer()
     this.clearConnectionTimer()
 
+    // Ensure the Supabase client has a valid auth session before connecting.
+    // On initial page load, the session may still be loading from cookies.
+    try {
+      const { data: { session } } = await this.supabase.auth.getSession()
+      if (!session && !this.isDestroyed) {
+        // No session yet - wait briefly and retry
+        this.scheduleReconnect()
+        return
+      }
+    } catch {
+      // Session check failed - proceed anyway, channel will handle auth errors
+    }
+
+    if (this.isDestroyed || this.isPaused) return
+
     // Set a timeout: if we don't connect within CONNECTION_TIMEOUT_MS, retry
     this.connectionTimer = setTimeout(() => {
       if (this.isDestroyed || this.isPaused) return