diff --git a/apps/docs-mcp/src/__tests__/mcp.test.ts b/apps/docs-mcp/src/__tests__/mcp.test.ts
index c9fde62d2..abd98867a 100644
--- a/apps/docs-mcp/src/__tests__/mcp.test.ts
+++ b/apps/docs-mcp/src/__tests__/mcp.test.ts
@@ -1,6 +1,6 @@
 import { Client } from "@modelcontextprotocol/sdk/client/index.js";
 import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js";
-import { describe, expect, test } from "vitest";
+import { describe, expect, test, vi } from "vitest";
 import { app } from "../index";
 
 describe("private MCP transport", () => {
@@ -34,6 +34,61 @@ describe("private MCP transport", () => {
     }
   });
 
+  test.each([
+    {
+      args: { libs: ["nextjs"] },
+      error:
+        "Failed to list features: Set OPENROUTER_API_KEY on the docs-mcp worker",
+      name: "get-available-features",
+    },
+    {
+      args: { query: "tracing" },
+      error:
+        "Failed to search docs: Failed to fetch https://docs.sentry.io/doctree.json: 503 Service Unavailable",
+      name: "search-docs",
+    },
+  ])("marks $name failures as MCP errors", async ({ args, error, name }) => {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(
+        async () =>
+          new Response("Unavailable", {
+            status: 503,
+            statusText: "Service Unavailable",
+          }),
+      ),
+    );
+
+    const client = new Client({ name: "docs-mcp-test", version: "1.0.0" });
+    const transport = new StreamableHTTPClientTransport(
+      new URL("https://docs-mcp.internal/mcp"),
+      {
+        fetch: async (input, init) =>
+          await app.request(new Request(input, init), undefined, {
+            CF_VERSION_METADATA: { id: "test", tag: "test", timestamp: "test" },
+          }),
+      },
+    );
+
+    try {
+      await client.connect(transport);
+      const result = await client.callTool({ arguments: args, name });
+
+      expect(result).toEqual({
+        content: [
+          {
+            text: JSON.stringify({ error }),
+            type: "text",
+          },
+        ],
+        isError: true,
+      });
+    } finally {
+      vi.unstubAllGlobals();
+      await client.close();
+    }
+  });
+
   test("rejects MCP requests outside the service binding", async () => {
     const response = await app.request(
       "https://docs-mcp.getsentry.workers.dev/mcp",
diff --git a/apps/docs-mcp/src/mcp.ts b/apps/docs-mcp/src/mcp.ts
index 39d4ad744..f3d46ee38 100644
--- a/apps/docs-mcp/src/mcp.ts
+++ b/apps/docs-mcp/src/mcp.ts
@@ -24,6 +24,7 @@ function errorEnvelope(message: string) {
         type: "text" as const,
       },
     ],
+    isError: true,
   };
 }