diff --git a/rest/nodejs/src/utils/signature.ts b/rest/nodejs/src/utils/signature.ts
index 6868afe..6c99a55 100644
--- a/rest/nodejs/src/utils/signature.ts
+++ b/rest/nodejs/src/utils/signature.ts
@@ -19,6 +19,7 @@ import { type Context, type MiddlewareHandler } from "hono";
 import { type ContentfulStatusCode } from "hono/utils/http-status";
 
 import { signatureConfig } from "./config";
+import { UcpError, ucpErrorResponse } from "./ucp_error";
 
 // RFC 9421 HTTP Message Signatures for UCP, using the default UCP profile.
 // This is the Node twin of the Python server's ucp_signing.py: RFC 9421
@@ -708,18 +709,20 @@ export async function fetchSigningKeys(
 }
 
 function signatureErrorResponse(c: Context, exc: SignatureError): Response {
-  // The same wire shape as the Python server's error envelope, inside the
-  // { detail } wrapper this server already uses for its 4xx responses.
-  return c.json(
-    {
-      detail: {
-        status: "error",
-        errors: [
-          { code: exc.code, message: exc.message, severity: "critical" },
-        ],
-      },
-    },
-    exc.statusCode as ContentfulStatusCode
+  // ucpErrorResponse is what this server answers its other protocol-level
+  // rejections with, so the signature path uses it too. error_response.json
+  // requires a ucp object plus messages[], and message_error.json constrains
+  // severity to the four values in its enum. Unrecoverable is the right one
+  // here because no resource exists to act on when a request is turned away
+  // at the signature layer, which is how message_error.json defines it.
+  return ucpErrorResponse(
+    c,
+    new UcpError(
+      exc.message,
+      exc.code,
+      exc.statusCode as ContentfulStatusCode,
+      "unrecoverable"
+    )
   );
 }
 
diff --git a/rest/nodejs/test/signature.test.ts b/rest/nodejs/test/signature.test.ts
index a5e10b0..7b1a37c 100644
--- a/rest/nodejs/test/signature.test.ts
+++ b/rest/nodejs/test/signature.test.ts
@@ -46,9 +46,23 @@ const AUTHORITY = "merchant.test";
 const ORIGIN = `http://${AUTHORITY}`;
 
 type ErrorEnvelope = {
-  detail: { status: string; errors: Array<{ code: string; message: string }> };
+  ucp?: { version?: string; status?: string };
+  messages?: Array<{
+    type?: string;
+    code?: string;
+    content?: string;
+    severity?: string;
+  }>;
+  detail?: unknown;
 };
 
+const ERROR_SEVERITIES = new Set([
+  "recoverable",
+  "requires_buyer_input",
+  "requires_buyer_review",
+  "unrecoverable",
+]);
+
 // A minimal app wired like src/index.ts (verifySignature ahead of validation)
 // but without the pino middleware, following the lifecycle.test.ts convention.
 function buildApp() {
@@ -222,10 +236,48 @@ async function assertError(
 ): Promise<void> {
   assert.equal(response.status, status, await response.clone().text());
   const envelope = (await response.json()) as ErrorEnvelope;
-  assert.equal(envelope.detail.status, "error");
-  assert.equal(envelope.detail.errors[0]?.code, code);
+  assert.equal(envelope.ucp?.status, "error");
+  assert.equal(envelope.messages?.[0]?.code, code);
 }
 
+test("enforced: a rejection body is the UCP error envelope", async () => {
+  // error_response.json requires ucp and messages[]; each message is a
+  // message_error.json with type, code, content and an in-enum severity. The
+  // rest of this server already answers in that shape (see
+  // error_envelope.test.ts), so the signature path should not differ.
+  enforce();
+  const body = checkoutBody();
+  const response = await postCheckout(
+    {
+      "UCP-Agent": `profile="${profileUrl}"`,
+      "Idempotency-Key": "envelope-1",
+      "Request-Id": "envelope-1",
+      "Content-Type": "application/json",
+    },
+    body
+  );
+
+  assert.equal(response.status, 401, await response.clone().text());
+  const envelope = (await response.json()) as ErrorEnvelope;
+
+  assert.equal(envelope.detail, undefined, "flat detail shape must be gone");
+  assert.equal(envelope.ucp?.status, "error");
+  assert.ok(envelope.ucp?.version, "ucp.version must be present");
+  assert.ok(
+    Array.isArray(envelope.messages) && envelope.messages.length === 1,
+    "messages[] must carry exactly the failure"
+  );
+
+  const message = envelope.messages![0];
+  assert.equal(message.type, "error");
+  assert.equal(message.code, "signature_missing");
+  assert.ok(message.content, "content must explain the failure");
+  assert.ok(
+    ERROR_SEVERITIES.has(message.severity ?? ""),
+    `severity ${message.severity} is outside the message_error.json enum`
+  );
+});
+
 /* Permissive (default) mode: existing clients keep working. */
 
 test("permissive: an unsigned request skips verification entirely", async () => {