diff --git a/apps/api/src/collectives/provisioner.config.ts b/apps/api/src/collectives/provisioner.config.ts
index 5f5148b..d62483f 100644
--- a/apps/api/src/collectives/provisioner.config.ts
+++ b/apps/api/src/collectives/provisioner.config.ts
@@ -115,7 +115,13 @@ export class MatrixHttpClient implements MatrixAdminPort {
     if (typeof body.room_id !== "string" || body.room_id === "") {
       throw new ProvisionerUnavailableError("createRoom response missing room_id");
     }
-    return { roomId: body.room_id };
+    // Some homeservers return the bare localpart; the full room ID carries the
+    // server name, and every downstream consumer (DB checks, invites, joins)
+    // requires the complete form.
+    const roomId = body.room_id.includes(":")
+      ? body.room_id
+      : `${body.room_id}:${this.config.homeserverUrl.host}`;
+    return { roomId };
   }
 
   async sendStateEvent(