diff --git a/app/api/mcp/route.ts b/app/api/mcp/route.ts
index e341f14..52a45a3 100644
--- a/app/api/mcp/route.ts
+++ b/app/api/mcp/route.ts
@@ -1273,7 +1273,13 @@ async function ensureServiceUp(uuid: string): Promise<{
   let composeDiag = '';
   try {
     const r = await composeUp('service', uuid, { timeoutMs: 600_000 });
-    composeDiag = (r.stderr || r.stdout).trim().slice(-400);
+    // Strip ANSI / control chars (compose progress output uses \r and
+    // ANSI escapes) so the diag survives JSON serialization cleanly.
+    composeDiag = (r.stderr || r.stdout)
+      .replace(/\x1b\[[0-9;]*[a-zA-Z]/g, '')
+      .replace(/[\x00-\x08\x0B-\x1F]/g, '')
+      .trim()
+      .slice(-400);
     if (r.code === 0) {
       return { started: true, startMethod: 'compose-up', diag: '' };
     }