diff --git a/web/workspace/src/lib/workspace/api/workspace-catalog.ts b/web/workspace/src/lib/workspace/api/workspace-catalog.ts
index 4287c9a31..90516e893 100644
--- a/web/workspace/src/lib/workspace/api/workspace-catalog.ts
+++ b/web/workspace/src/lib/workspace/api/workspace-catalog.ts
@@ -138,15 +138,18 @@ async function fetchJson(
   if (!response.ok) {
     let detail = `${response.status} ${response.statusText}`.trim();
     try {
-      const body = await response.json();
-      if (typeof body?.message === "string") detail = body.message;
-      else if (typeof body?.error === "string") detail = body.error;
+      const body: unknown = await response.json();
+      if (typeof body === "object" && body !== null && !Array.isArray(body)) {
+        const record = body as Record<string, unknown>;
+        if (typeof record.message === "string") detail = record.message.slice(0, 512);
+        else if (typeof record.error === "string") detail = record.error.slice(0, 512);
+      }
     } catch {
       // Preserve the bounded status text when the Backend did not return JSON.
     }
     throw new WorkspaceCatalogError(response.status, detail);
   }
-  return await response.json() as unknown;
+  return await response.json();
 }
 
 function errorMessage(error: unknown): string {
diff --git a/web/workspace/src/lib/workspace/sidebar/worker-actions.ts b/web/workspace/src/lib/workspace/sidebar/worker-actions.ts
index 2ac7adc89..3f3dcccf2 100644
--- a/web/workspace/src/lib/workspace/sidebar/worker-actions.ts
+++ b/web/workspace/src/lib/workspace/sidebar/worker-actions.ts
@@ -24,11 +24,18 @@ function workerPath(workspaceId: string, worker: WorkerActionTarget): string {
 async function responseError(response: Response): Promise<string> {
   const fallback = `${response.status} ${response.statusText}`.trim();
   try {
-    const payload = await response.json() as {
-      message?: string;
-      error?: { message?: string };
-    };
-    return payload.error?.message ?? payload.message ?? fallback;
+    const payload: unknown = await response.json();
+    if (typeof payload !== "object" || payload === null || Array.isArray(payload)) {
+      return fallback;
+    }
+    const record = payload as Record<string, unknown>;
+    if (typeof record.message === "string") return record.message.slice(0, 512);
+    const nested = record.error;
+    if (typeof nested === "object" && nested !== null && !Array.isArray(nested)) {
+      const message = (nested as Record<string, unknown>).message;
+      if (typeof message === "string") return message.slice(0, 512);
+    }
+    return fallback;
   } catch {
     return fallback;
   }