diff --git a/rest/nodejs/src/api/discovery.ts b/rest/nodejs/src/api/discovery.ts
index 1518fdd..c6a442a 100644
--- a/rest/nodejs/src/api/discovery.ts
+++ b/rest/nodejs/src/api/discovery.ts
@@ -14,7 +14,6 @@
 
 import { type Context } from "hono";
 import { UCP_VERSION } from "../utils/config";
-import { type Jwk } from "../utils/signature";
 import { publicJwk } from "../utils/webhook_signer";
 
 // overview.md (Discovery) requires the profile response to carry a
@@ -23,11 +22,18 @@ import { publicJwk } from "../utils/webhook_signer";
 // reference (samples#153), which serves `public, max-age=3600`.
 const PROFILE_CACHE_CONTROL = "public, max-age=3600";
 
-type DiscoveryCapability = {
+export type DiscoveryCapability = {
   version: string;
   spec: string;
   schema: string;
-  extends?: string;
+  // capability.json $defs/base: extends is oneOf [reverse_domain_name,
+  // array<reverse_domain_name> (minItems 1)] at both the 2026-04-08 pin
+  // this server declares and at 2026-08-25 -- "Use array for multi-parent
+  // extensions." Node's own `discount` entry only has one parent today, so
+  // this was a type-safety gap, not a live bug; it becomes load-bearing the
+  // moment a second-parent extension (e.g. a future cart capability) is
+  // added.
+  extends?: string | string[];
 };
 
 type DiscoveryServiceBinding = {
@@ -53,7 +59,6 @@ type UcpDiscoveryMetadata = {
   services: Record<string, DiscoveryServiceBinding[]>;
   capabilities: Record<string, DiscoveryCapability[]>;
   payment_handlers: Record<string, DiscoveryPaymentHandler[]>;
-  keys: Jwk[];
 };
 
 /**
@@ -126,16 +131,18 @@ export class DiscoveryService {
 
     // Publish the webhook-signing public key so platforms can verify our
     // order-event deliveries (order.md, Webhook Signature Verification /
-    // signatures.md, Key Discovery). The discovery profile schema places
-    // signing_keys[] at the top level of the served document (a sibling of
-    // `ucp`); it is mirrored into ucp.keys[], the RFC 7517 JWK Set this
-    // server's own verifier (signature.ts extractKeys) resolves, so both
-    // discovery conventions find the key.
+    // signatures.md, Key Discovery). source/discovery/profile_schema.json
+    // $defs/base at the 2026-04-08 pin this server declares (config.ts
+    // UCP_VERSION) requires `ucp` and separately declares `signing_keys` as
+    // a top-level sibling of `ucp` -- that schema defines no `keys` field
+    // anywhere, nested or otherwise, so publish signing_keys[] only. ucp#566
+    // renames this field to a top-level keys[] for 2026-08-25 and later;
+    // when UCP_VERSION moves to that pin, this field name must move with it
+    // in the same change, together with signature.ts's extractKeys().
     const webhookJwk = publicJwk();
 
     const ucp = {
       version: this.ucpVersion,
-      keys: [webhookJwk],
       services: {
         "dev.ucp.shopping": [
           {
@@ -162,30 +169,6 @@ export class DiscoveryService {
             schema: `https://ucp.dev/${this.ucpVersion}/schemas/shopping/order.json`,
           },
         ],
-        "dev.ucp.shopping.refund": [
-          {
-            version: this.ucpVersion,
-            spec: `https://ucp.dev/${this.ucpVersion}/specification/shopping/refund`,
-            schema: `https://ucp.dev/${this.ucpVersion}/schemas/shopping/refund.json`,
-            extends: "dev.ucp.shopping.order",
-          },
-        ],
-        "dev.ucp.shopping.return": [
-          {
-            version: this.ucpVersion,
-            spec: `https://ucp.dev/${this.ucpVersion}/specification/shopping/return`,
-            schema: `https://ucp.dev/${this.ucpVersion}/schemas/shopping/return.json`,
-            extends: "dev.ucp.shopping.order",
-          },
-        ],
-        "dev.ucp.shopping.dispute": [
-          {
-            version: this.ucpVersion,
-            spec: `https://ucp.dev/${this.ucpVersion}/specification/shopping/dispute`,
-            schema: `https://ucp.dev/${this.ucpVersion}/schemas/shopping/dispute.json`,
-            extends: "dev.ucp.shopping.order",
-          },
-        ],
         "dev.ucp.shopping.discount": [
           {
             version: this.ucpVersion,
diff --git a/rest/nodejs/src/utils/signature.ts b/rest/nodejs/src/utils/signature.ts
index 6868afe..3dff2ff 100644
--- a/rest/nodejs/src/utils/signature.ts
+++ b/rest/nodejs/src/utils/signature.ts
@@ -643,20 +643,29 @@ export async function assertProfileUrlAllowed(
   }
 }
 
-// Pulls the signing keys out of a profile document. keys[] is the canonical
-// RFC 7517 JWK Set field per ucp#566, which removed the earlier
-// signing_keys[]; this reference verifier reads only keys[].
+// Pulls the signing keys out of a profile document. At the UCP version this
+// server declares (2026-04-08, config.ts UCP_VERSION),
+// source/discovery/profile_schema.json $defs/base requires `ucp` and
+// separately declares `signing_keys` as a top-level sibling of `ucp` -- not
+// a field nested inside it. `ucp` is required on every real profile
+// document, so a reader that looks inside `ucp` whenever it is present can
+// never see a top-level sibling field on any real document; this reads the
+// top level directly instead.
+//
+// ucp#566 (merged upstream) renames this field to a top-level keys[] for
+// 2026-08-25 and later. When this server's UCP_VERSION moves to that pin,
+// this field name must move with it, in the same change, together with
+// discovery.ts's publication side.
 export function extractKeys(document: unknown): Jwk[] {
   if (typeof document !== "object" || document === null) return [];
   const doc = document as Record<string, unknown>;
-  const ucp = "ucp" in doc ? doc["ucp"] : doc;
-  if (typeof ucp !== "object" || ucp === null || Array.isArray(ucp)) return [];
-  const value = (ucp as Record<string, unknown>)["keys"];
+  const value = doc["signing_keys"];
   return Array.isArray(value) && value.length ? (value as Jwk[]) : [];
 }
 
 // Fetches and caches a signer's published signing keys from its UCP profile
-// (the keys[] of the document behind the UCP-Agent profile URL).
+// (the top-level signing_keys[] of the document behind the UCP-Agent
+// profile URL).
 export async function fetchSigningKeys(
   profileUrl: string,
   options: { allowInsecure?: boolean } = {}
diff --git a/rest/nodejs/test/discovery.test.ts b/rest/nodejs/test/discovery.test.ts
index 4a4fae1..99aade1 100644
--- a/rest/nodejs/test/discovery.test.ts
+++ b/rest/nodejs/test/discovery.test.ts
@@ -17,13 +17,18 @@ import { test } from "node:test";
 
 import { Hono } from "hono";
 
-import { DiscoveryService } from "../src/api/discovery";
+import {
+  DiscoveryService,
+  type DiscoveryCapability,
+} from "../src/api/discovery";
 
 type DiscoveryResponse = {
   ucp: {
     services: Record<string, Array<{ endpoint: string; transport: string }>>;
     capabilities: Record<string, Array<{ version: string }>>;
+    keys?: unknown;
   };
+  signing_keys?: unknown;
 };
 
 test("merchant profile uses schema-compliant discovery registries", async () => {
@@ -47,11 +52,8 @@ test("merchant profile uses schema-compliant discovery registries", async () =>
     "dev.ucp.shopping.buyer_consent",
     "dev.ucp.shopping.checkout",
     "dev.ucp.shopping.discount",
-    "dev.ucp.shopping.dispute",
     "dev.ucp.shopping.fulfillment",
     "dev.ucp.shopping.order",
-    "dev.ucp.shopping.refund",
-    "dev.ucp.shopping.return",
   ]);
 
   for (const [name, declarations] of Object.entries(body.ucp.capabilities)) {
@@ -61,6 +63,75 @@ test("merchant profile uses schema-compliant discovery registries", async () =>
   }
 });
 
+test("merchant profile declares no capability without a schema at its own version", async () => {
+  // dev.ucp.shopping.refund/.return/.dispute have no schema file under
+  // source/schemas/shopping/ at 2026-04-08 (this server's declared version)
+  // or at 2026-08-25, no route implements them, and neither reference
+  // python samples server (upstream or our 08-25 golden reference) declares
+  // them. A capability catalog entry with nothing behind it is not a real
+  // capability.
+  const app = new Hono();
+  const discoveryService = new DiscoveryService();
+  app.get("/.well-known/ucp", discoveryService.getMerchantProfile);
+
+  const response = await app.request("/.well-known/ucp");
+  const body = (await response.json()) as DiscoveryResponse;
+
+  for (const nonSpecCapability of [
+    "dev.ucp.shopping.refund",
+    "dev.ucp.shopping.return",
+    "dev.ucp.shopping.dispute",
+  ]) {
+    assert.equal(
+      body.ucp.capabilities[nonSpecCapability],
+      undefined,
+      `${nonSpecCapability} has no schema at any pin and must not be declared`
+    );
+  }
+});
+
+test("merchant profile publishes signing_keys[] at the top level, not ucp.keys[]", async () => {
+  // source/discovery/profile_schema.json $defs/base at the 2026-04-08 pin
+  // this server declares (config.ts UCP_VERSION) requires `ucp` and
+  // separately declares `signing_keys` as a top-level sibling of `ucp`.
+  // That schema defines no `keys` field, nested or otherwise.
+  const app = new Hono();
+  const discoveryService = new DiscoveryService();
+  app.get("/.well-known/ucp", discoveryService.getMerchantProfile);
+
+  const response = await app.request("/.well-known/ucp");
+  const body = (await response.json()) as DiscoveryResponse;
+
+  assert.ok(
+    Array.isArray(body.signing_keys) && body.signing_keys.length > 0,
+    "signing_keys[] must be published at the top level"
+  );
+  assert.equal(
+    body.ucp.keys,
+    undefined,
+    "ucp.keys[] has no basis in the 2026-04-08 schema and must not be published"
+  );
+});
+
+test("DiscoveryCapability.extends accepts a multi-parent array", () => {
+  // capability.json $defs/base at both the 2026-04-08 pin (this server's
+  // declared version) and 2026-08-25: extends is oneOf [reverse_domain_name,
+  // array<reverse_domain_name> minItems 1] -- "Use array for multi-parent
+  // extensions." This is a compile-time check: if extends is typed as a
+  // bare string, this literal fails tsc; the assertion below is the runtime
+  // half so the test still reports as a test, not just a build step.
+  const multiParent: DiscoveryCapability = {
+    version: "2026-04-08",
+    spec: "https://ucp.dev/2026-04-08/specification/shopping/discount",
+    schema: "https://ucp.dev/2026-04-08/schemas/shopping/discount.json",
+    extends: ["dev.ucp.shopping.checkout", "dev.ucp.shopping.cart"],
+  };
+  assert.deepEqual(multiParent.extends, [
+    "dev.ucp.shopping.checkout",
+    "dev.ucp.shopping.cart",
+  ]);
+});
+
 test("merchant profile sends a public, cacheable Cache-Control header", async () => {
   // overview.md (Discovery) MUST: "Profile responses MUST include a
   // Cache-Control header with `public` and `max-age` of at least 60 seconds.
diff --git a/rest/nodejs/test/signature.test.ts b/rest/nodejs/test/signature.test.ts
index a5e10b0..ada0249 100644
--- a/rest/nodejs/test/signature.test.ts
+++ b/rest/nodejs/test/signature.test.ts
@@ -116,8 +116,12 @@ before(async () => {
   const edJwk = jwkFromPublicKey(edKeys.publicKey, ED_KID);
   // A deliberately unsupported (RSA) JWK to exercise algorithm_unsupported.
   const rsaJwk = { kid: "rsa-key", kty: "RSA", n: "abc", e: "AQAB" };
+  // source/discovery/profile_schema.json $defs/base at the 2026-04-08 pin
+  // this server declares (config.ts UCP_VERSION): signing_keys is a
+  // top-level sibling of `ucp`, not a field nested inside it.
   const good = JSON.stringify({
-    ucp: { keys: [agentJwk, edJwk, rsaJwk] },
+    ucp: {},
+    signing_keys: [agentJwk, edJwk, rsaJwk],
   });
   const keyless = JSON.stringify({ ucp: {} });
 
diff --git a/rest/nodejs/test/signing.test.ts b/rest/nodejs/test/signing.test.ts
index 80e17f8..dd102c7 100644
--- a/rest/nodejs/test/signing.test.ts
+++ b/rest/nodejs/test/signing.test.ts
@@ -922,11 +922,19 @@ function profileUrl(path: string): string {
   return `http://127.0.0.1:${profilePort}${path}`;
 }
 
-test("keys[] is read from the ucp envelope", async () => {
+test("signing_keys[] is read from the top level, a sibling of ucp", async () => {
+  // source/discovery/profile_schema.json $defs/base at the 2026-04-08 pin
+  // (this server's own declared UCP_VERSION, config.ts) requires `ucp` and
+  // separately declares `signing_keys` as a sibling top-level property --
+  // not a field nested inside `ucp`. A real profile document always carries
+  // `ucp` (schema-required), so this is the realistic shape.
   clearKeyCache();
   profileResponses["/envelope.json"] = {
     status: 200,
-    body: JSON.stringify({ ucp: { keys: [{ kid: "a" }] } }),
+    body: JSON.stringify({
+      ucp: { version: "2026-04-08" },
+      signing_keys: [{ kid: "a" }],
+    }),
   };
   const keys = await fetchSigningKeys(profileUrl("/envelope.json"), {
     allowInsecure: true,
@@ -934,11 +942,11 @@ test("keys[] is read from the ucp envelope", async () => {
   assert.equal(keys[0]?.kid, "a");
 });
 
-test("a top-level keys[] array (no ucp wrapper) is read", async () => {
+test("a top-level signing_keys[] array (no ucp wrapper) is read", async () => {
   clearKeyCache();
   profileResponses["/top.json"] = {
     status: 200,
-    body: JSON.stringify({ keys: [{ kid: "b" }] }),
+    body: JSON.stringify({ signing_keys: [{ kid: "b" }] }),
   };
   const keys = await fetchSigningKeys(profileUrl("/top.json"), {
     allowInsecure: true,
@@ -946,7 +954,10 @@ test("a top-level keys[] array (no ucp wrapper) is read", async () => {
   assert.equal(keys[0]?.kid, "b");
 });
 
-test("a profile with only the removed signing_keys[] is profile_malformed", async () => {
+test("signing_keys[] nested under ucp has no basis in the schema and is not read", async () => {
+  // The 2026-04-08 profile schema places signing_keys as a sibling of ucp,
+  // never inside it. Nesting it under ucp is not a legacy shape to tolerate
+  // -- it was never a valid location at any pin.
   clearKeyCache();
   profileResponses["/legacy.json"] = {
     status: 200,
@@ -958,6 +969,22 @@ test("a profile with only the removed signing_keys[] is profile_malformed", asyn
   );
 });
 
+test("a top-level keys[] (the 2026-08-25 field name) is not yet read at the declared 2026-04-08 version", async () => {
+  // ucp#566 renames signing_keys[] to a top-level keys[] for 2026-08-25 and
+  // later. This server declares 2026-04-08 (config.ts UCP_VERSION); reading
+  // keys[] now would verify against a field name this server has not earned
+  // by bumping its own declared version yet.
+  clearKeyCache();
+  profileResponses["/future.json"] = {
+    status: 200,
+    body: JSON.stringify({ ucp: {}, keys: [{ kid: "future" }] }),
+  };
+  await assertSignatureErrorAsync(
+    () => fetchSigningKeys(profileUrl("/future.json"), { allowInsecure: true }),
+    "profile_malformed"
+  );
+});
+
 test("a 3xx response is treated as unreachable (no redirects allowed)", async () => {
   clearKeyCache();
   profileResponses["/redirect.json"] = {
@@ -999,7 +1026,7 @@ test("a second fetch within the TTL is served from the cache", async () => {
   clearKeyCache();
   profileResponses["/cached.json"] = {
     status: 200,
-    body: JSON.stringify({ ucp: { keys: [{ kid: "c" }] } }),
+    body: JSON.stringify({ ucp: {}, signing_keys: [{ kid: "c" }] }),
   };
   await fetchSigningKeys(profileUrl("/cached.json"), { allowInsecure: true });
   const hitsAfterFirst = profileHits.filter((p) => p === "/cached.json").length;
@@ -1011,21 +1038,38 @@ test("a second fetch within the TTL is served from the cache", async () => {
   assert.equal(hitsAfterSecond, 1);
 });
 
-/* extractKeys reads keys[] (canonical per ucp#566) and tolerates junk. */
+/* extractKeys reads the top-level signing_keys[] this server's declared
+ * 2026-04-08 version defines (a sibling of ucp, never nested inside it),
+ * and tolerates junk. */
 
 test("a non-object profile yields no keys, not an error", () => {
   assert.deepEqual(extractKeys(["not", "a", "dict"]), []);
 });
 
-test("keys[] under the ucp envelope is the canonical source", () => {
-  assert.deepEqual(extractKeys({ ucp: { keys: [{ kid: "k" }] } }), [
-    { kid: "k" },
-  ]);
+test("top-level signing_keys[], a sibling of ucp, is the canonical source", () => {
+  assert.deepEqual(
+    extractKeys({
+      ucp: { version: "2026-04-08" },
+      signing_keys: [{ kid: "k" }],
+    }),
+    [{ kid: "k" }]
+  );
 });
 
-test("the removed signing_keys[] field is not read (ucp#566)", () => {
+test("signing_keys[] nested under ucp is not read -- it is a top-level field only", () => {
   assert.deepEqual(
     extractKeys({ ucp: { signing_keys: [{ kid: "old" }] } }),
     []
   );
 });
+
+test("keys[] nested under ucp is not read -- no schema at any pin nests keys under ucp", () => {
+  assert.deepEqual(extractKeys({ ucp: { keys: [{ kid: "k" }] } }), []);
+});
+
+test("a top-level keys[] (2026-08-25 name) is not read while this server declares 2026-04-08", () => {
+  assert.deepEqual(
+    extractKeys({ ucp: { version: "2026-04-08" }, keys: [{ kid: "future" }] }),
+    []
+  );
+});
diff --git a/rest/nodejs/test/webhook_signing.test.ts b/rest/nodejs/test/webhook_signing.test.ts
index 2148fff..4cee61d 100644
--- a/rest/nodejs/test/webhook_signing.test.ts
+++ b/rest/nodejs/test/webhook_signing.test.ts
@@ -530,9 +530,12 @@ test("an unsupported key type is rejected with a clear message", async () => {
 
 test("the profile publishes the webhook signing key", async () => {
   // signatures.md, Key Discovery: public keys live in the profile's
-  // signing_keys[] (a top-level sibling of `ucp` per the discovery profile
-  // schema). It is also mirrored into ucp.keys[], the JWK Set this server's
-  // own verifier resolves.
+  // signing_keys[], a top-level sibling of `ucp` per the discovery profile
+  // schema at this server's declared version (source/discovery/profile_schema.json
+  // $defs/base at the 2026-04-08 pin -- config.ts UCP_VERSION). That schema
+  // defines no `keys` field anywhere, nested or otherwise; this server must
+  // not publish one, so a peer's strict 2026-04-08 verifier sees exactly the
+  // fields the schema promises and nothing it does not.
   const profile = await fetchProfile();
   const jwk = publicJwk();
   const signingKeys = profile["signing_keys"] as Jwk[];
@@ -542,8 +545,9 @@ test("the profile publishes the webhook signing key", async () => {
     "signing_keys[] must carry the webhook public JWK"
   );
   const ucp = profile["ucp"] as { keys?: Jwk[] };
-  assert.ok(
-    Array.isArray(ucp.keys) && ucp.keys.some((k) => isDeepStrictEqual(k, jwk)),
-    "ucp.keys[] must mirror the webhook public JWK"
+  assert.equal(
+    ucp.keys,
+    undefined,
+    "ucp.keys[] has no basis in the 2026-04-08 schema and must not be published"
   );
 });