diff --git a/clients/web/src/test/core/mcp/inspectorClient-skills.test.ts b/clients/web/src/test/core/mcp/inspectorClient-skills.test.ts
index c8215362..602d501b 100644
--- a/clients/web/src/test/core/mcp/inspectorClient-skills.test.ts
+++ b/clients/web/src/test/core/mcp/inspectorClient-skills.test.ts
@@ -150,8 +150,9 @@ describe("InspectorClient skills methods (#2234)", () => {
 
   it("requires the modern list envelope on a modern connection", async () => {
     // SEP-2640: "In protocol versions 2026-07-28 and later, the result also
-    // carries … `ttlMs` and `cacheScope`." Nothing else validates it —
-    // `skills/*` is consumer-owned, so the SDK codec never sees it.
+    // carries … `ttlMs` and `cacheScope`." The SDK codec checks and removes
+    // `resultType` on every modern result, but for a consumer-owned method it
+    // checks neither caching attribute — so this schema is what does.
     const client = makeClient();
     internals(client).protocolEra = "modern";
     stubRequest(client, { skills: [] });
diff --git a/clients/web/src/test/integration/mcp/inspectorClient-advertised-extensions.test.ts b/clients/web/src/test/integration/mcp/inspectorClient-advertised-extensions.test.ts
index 97ca51c4..fc7f2c44 100644
--- a/clients/web/src/test/integration/mcp/inspectorClient-advertised-extensions.test.ts
+++ b/clients/web/src/test/integration/mcp/inspectorClient-advertised-extensions.test.ts
@@ -117,8 +117,9 @@ describe("advertised-extension gating (#1739)", () => {
 
   it("omits the gated tool when the extension is not advertised", async () => {
     const started = await startGatedServer();
-    // Disable the only registry extension → the client sends no extensions,
-    // so the server never enables the gated tool.
+    // Disable the Tasks extension → the client still sends the other registry
+    // extensions (MCP Apps UI, Skills), but not the one this tool is gated on,
+    // so the server never enables it.
     const connected = await connect(started.url, {
       [TASKS_EXTENSION_KEY]: false,
     });
diff --git a/clients/web/src/test/integration/mcp/inspectorClient-coverage-backfill.test.ts b/clients/web/src/test/integration/mcp/inspectorClient-coverage-backfill.test.ts
index 18ff9258..afbdff41 100644
--- a/clients/web/src/test/integration/mcp/inspectorClient-coverage-backfill.test.ts
+++ b/clients/web/src/test/integration/mcp/inspectorClient-coverage-backfill.test.ts
@@ -880,10 +880,11 @@ describe("InspectorClient coverage backfill", () => {
   });
 
   describe("constructor capability branches and createReceiverTask options", () => {
-    it("advertises only the tasks extension when no other capabilities are set", async () => {
+    it("still advertises the registry extensions when no other capabilities are set", async () => {
       // sample:false + elicit:false + no roots + no receiverTasks → the only
-      // advertised capability is the always-on Tasks extension (#1631), so
-      // `capabilities` is non-empty and is always attached.
+      // advertised capabilities are the default-on registry extensions (Tasks
+      // since #1631, Skills since #2373), so `capabilities` is non-empty and
+      // is always attached.
       client = new InspectorClient(
         {
           type: "stdio",
@@ -901,6 +902,7 @@ describe("InspectorClient coverage backfill", () => {
       expect(caps.elicitation).toBeUndefined();
       expect(caps.roots).toBeUndefined();
       expect(caps.extensions?.["io.modelcontextprotocol/tasks"]).toBeDefined();
+      expect(caps.extensions?.["io.modelcontextprotocol/skills"]).toBeDefined();
       await client.connect();
       expect(client.getStatus()).toBe("connected");
     });
diff --git a/core/mcp/inspectorClient.ts b/core/mcp/inspectorClient.ts
index d3c6aacb..04293238 100644
--- a/core/mcp/inspectorClient.ts
+++ b/core/mcp/inspectorClient.ts
@@ -5739,11 +5739,13 @@ export class InspectorClient extends InspectorClientEventTarget {
       ...(cursor !== undefined ? { cursor } : {}),
     };
     // Era-aware: a modern (2026-07-28+) `skills/list` result also carries the
-    // base list envelope (`resultType` / `ttlMs` / `cacheScope`). `skills/*` is
-    // consumer-owned, so the SDK codec validates none of it — without picking
-    // the schema here a modern server could answer `{ skills: [] }` and the
-    // conformance UI would show a clean list. Legacy stays permissive: those
-    // are 2026-era attributes.
+    // base list envelope (`resultType` / `ttlMs` / `cacheScope`). The two halves
+    // are checked in different places: the SDK codec checks `resultType` on
+    // every modern result and removes it before this schema runs (#2373), but
+    // `skills/*` is consumer-owned, so it checks neither caching attribute —
+    // without picking the schema here a modern server could answer
+    // `{ skills: [] }` and the conformance UI would show a clean list. Legacy
+    // stays permissive: those are 2026-era attributes.
     const resultSchema = this.isModernEra()
       ? ModernListSkillsResultSchema
       : ListSkillsResultSchema;
@@ -5773,8 +5775,10 @@ export class InspectorClient extends InspectorClientEventTarget {
   }
 
   /**
-   * `skills/get` as the server sent it — the `{ skill }` envelope **and any
-   * other members it carried**.
+   * `skills/get` as the SDK decoded it — the `{ skill }` envelope **and any
+   * other members the server sent**, such as the caching attributes. On a
+   * modern connection `resultType` is not among them: the codec checks it and
+   * removes it before the result gets here (#2373).
    *
    * Separate from {@link getSkill} because the callers differ: the UIs want the
    * entry, while the CLI prints the result and must not reshape it. SEP-2640
diff --git a/core/mcp/skills.ts b/core/mcp/skills.ts
index fb7b9113..cacb8f9f 100644
--- a/core/mcp/skills.ts
+++ b/core/mcp/skills.ts
@@ -11,12 +11,15 @@
  * shared by every client, and produce a structured finding list rather than a
  * boolean — a report is useful, "invalid" is not.
  *
- * ⚠️ Skills is a **server-declared** extension, read off the connecting server's
- * `capabilities.extensions`. It deliberately does NOT belong in
- * `ADVERTISABLE_EXTENSIONS` (`core/mcp/extensions.ts`), which is the catalog of
- * extensions the *Inspector* advertises and the user toggles in Server Settings.
- * The precedent is `appElicitation.ts`, which reads the server side the same
- * way; getting it backwards would put a meaningless toggle in Server Settings.
+ * ⚠️ Skills is negotiated from **both** sides (SEP-2133), and the two halves
+ * live in different places. This module reads the *server's* declaration off
+ * the connecting server's `capabilities.extensions` — that is what gates the
+ * Skills screen. The *Inspector's* own declaration is a separate entry in
+ * `ADVERTISABLE_EXTENSIONS` (`core/mcp/extensions.ts`): a strict server refuses
+ * `skills/*` to a client that did not declare it, and turning that toggle off
+ * in Server Settings is how to check a server's refusal path (#2373). An
+ * earlier revision of this comment said Skills must never be in that registry;
+ * that was the missing client declaration #2373 fixed.
  *
  * The Inspector is an inspector, not a host: a `resources/read` of a `SKILL.md`
  * is explicitly not a load and confers no standing, so none of the SEP's host
diff --git a/core/mcp/skillsSchemas.ts b/core/mcp/skillsSchemas.ts
index ed21db36..a91cb2ed 100644
--- a/core/mcp/skillsSchemas.ts
+++ b/core/mcp/skillsSchemas.ts
@@ -181,7 +181,10 @@ export const GetSkillEnvelopeSchema = z.looseObject({
 });
 
 /**
- * The `skills/get` result as the server sent it, envelope and all.
+ * The `skills/get` result envelope as the SDK decoded it: the `skill` wrapper
+ * plus any caching attributes the server sent. On a modern connection
+ * `resultType` is not in it — the codec checks it and removes it before this
+ * schema runs (#2373).
  *
  * Exported alongside the unwrapping schema below because the two callers want
  * different things: the UIs want the entry, while the CLI's job is to print
diff --git a/docs/test-servers.md b/docs/test-servers.md
index 87718044..e40057bd 100644
--- a/docs/test-servers.md
+++ b/docs/test-servers.md
@@ -105,7 +105,9 @@ than one for both:
    `io.modelcontextprotocol/clientCapabilities` `_meta` entry (Modern).
 2. Open **Server Settings → Advertised Extensions**, uncheck
    **Skills (io.modelcontextprotocol/skills)**, and reconnect.
-3. `skills/list` now fails with `-32601` — the refusal a strict server sends.
+3. `skills/list` now fails — with `-32021` MissingRequiredClientCapability on
+   the modern file and `-32601` on the legacy one, the refusal a strict server
+   sends in each era.
 
 **Both `skills/*` results carry the full modern base envelope** (`resultType` /
 `ttlMs` / `cacheScope`). They are consumer-owned methods, so the SDK stamps
@@ -528,7 +530,7 @@ Requiredness is a property of the **expression**, not the variable: RFC 6570 dro
 
 1. Connect — the Inspector advertises the Tasks extension by default, so the Tools list shows both `echo` and `get_weather`.
 2. Open **Server Settings → Advertised Extensions**, uncheck **Tasks (io.modelcontextprotocol/tasks)**, and reconnect.
-3. The client now advertises no extensions, the server never enables `get_weather`, and the Tools list shows only `echo`.
+3. The client no longer advertises the Tasks extension (it still advertises the others, such as MCP Apps UI and Skills), the server never enables `get_weather`, and the Tools list shows only `echo`.
 
 This is the debugging knob for a server legitimately changing tool registration based on what the client advertises. Legacy stateful leg only — the modern per-request leg has no persistent `oninitialized`.
 
diff --git a/test-servers/src/composable-test-server.ts b/test-servers/src/composable-test-server.ts
index fef18353..013e9624 100644
--- a/test-servers/src/composable-test-server.ts
+++ b/test-servers/src/composable-test-server.ts
@@ -615,11 +615,14 @@ export interface ServerConfig {
    */
   skills?: boolean;
   /**
-   * With {@link ServerConfig.skills}, answer `skills/list`, `skills/get` and
-   * `resources/directory/read` with `-32601` unless the client declared
+   * With {@link ServerConfig.skills}, refuse `skills/list`, `skills/get` and
+   * `resources/directory/read` unless the client declared
    * `io.modelcontextprotocol/skills` in its own capabilities — a strict
-   * SEP-2133 server (#2373). Off by default so the existing skills fixtures
-   * keep serving any client. Ignored without `skills`.
+   * SEP-2133 server (#2373). A modern request is refused with `-32021`
+   * MissingRequiredClientCapability carrying `data.requiredCapabilities`; a
+   * legacy one, whose era has no such code, with `-32601`. Off by default so
+   * the existing skills fixtures keep serving any client. Ignored without
+   * `skills`.
    */
   skillsRequireClientExtension?: boolean;
   /**
diff --git a/test-servers/src/skills.ts b/test-servers/src/skills.ts
index b65a3b8a..eddc7c98 100644
--- a/test-servers/src/skills.ts
+++ b/test-servers/src/skills.ts
@@ -729,7 +729,9 @@ function hasSkillsExtension(capabilities: unknown): boolean {
 }
 
 /**
- * Throw `-32601` when the client did not declare the Skills extension.
+ * Refuse the request when the client did not declare the Skills extension:
+ * `-32021` MissingRequiredClientCapability for a modern request, `-32601` for
+ * a legacy one.
  *
  * The two eras keep the declaration in different places. A modern
  * (2026-07-28) request carries it in its own `_meta` envelope, which the SDK