fix(core): strip _meta before custom-handler schema validation; route example notification to request stream; add changeset

- setCustomRequestHandler/setCustomNotificationHandler now strip _meta from
  params before validating against the user schema, so .strict() schemas do
  not reject SDK-injected fields like progressToken. _meta remains available
  via ctx.mcpReq._meta. Adds regression test.
- examples/server/src/customMethodExample.ts: pass relatedRequestId so the
  acme/statusUpdate notification routes to the request response stream as
  the comment claims (was going to the standalone SSE stream).
- Add .changeset/custom-method-handlers.md (minor bump for client+server).

Author: felixweinberger

.changeset/custom-method-handlers.md

@@ -0,0 +1,6 @@
+---
+'@modelcontextprotocol/client': minor
+'@modelcontextprotocol/server': minor
+---
+
+Add `setCustomRequestHandler` / `setCustomNotificationHandler` / `sendCustomRequest` / `sendCustomNotification` (plus `remove*` variants) on `Protocol` for non-standard JSON-RPC methods. Restores typed registration for vendor-specific methods (e.g. `mcp-ui/*`) that #1446/#1451 closed off, without reintroducing class-level generics. Handlers share the standard dispatch path (context, cancellation, tasks); a collision guard rejects standard MCP methods.

examples/server/src/customMethodExample.ts

@@ -34,8 +34,13 @@ const getServer = () => {
     server.setCustomRequestHandler('acme/search', SearchParamsSchema, async (params, ctx) => {
         console.log(`[server] acme/search query="${params.query}" limit=${params.limit ?? 'unset'} (req ${ctx.mcpReq.id})`);
 
-        // Send a custom server→client notification on the same SSE stream as this response.
-        await server.sendCustomNotification('acme/statusUpdate', { status: 'busy', detail: `searching "${params.query}"` });
+        // Send a custom server→client notification on the same SSE stream as this response
+        // (relatedRequestId routes it to the request's stream rather than the standalone SSE stream).
+        await server.sendCustomNotification(
+            'acme/statusUpdate',
+            { status: 'busy', detail: `searching "${params.query}"` },
+            { relatedRequestId: ctx.mcpReq.id }
+        );
 
         return {
             results: [

packages/core/src/shared/protocol.ts

@@ -1077,7 +1077,9 @@ export abstract class Protocol<ContextT extends BaseContext> {
             throw new Error(`"${method}" is a standard MCP request method. Use setRequestHandler() instead.`);
         }
         this._requestHandlers.set(method, (request, ctx) => {
-            const parsed = parseSchema(paramsSchema, request.params);
+            const { _meta, ...userParams } = (request.params ?? {}) as Record<string, unknown>;
+            void _meta;
+            const parsed = parseSchema(paramsSchema, userParams);
             if (!parsed.success) {
                 throw new ProtocolError(ProtocolErrorCode.InvalidParams, `Invalid params for ${method}: ${parsed.error.message}`);
             }
@@ -1112,7 +1114,9 @@ export abstract class Protocol<ContextT extends BaseContext> {
             throw new Error(`"${method}" is a standard MCP notification method. Use setNotificationHandler() instead.`);
         }
         this._notificationHandlers.set(method, notification => {
-            const parsed = parseSchema(paramsSchema, notification.params);
+            const { _meta, ...userParams } = (notification.params ?? {}) as Record<string, unknown>;
+            void _meta;
+            const parsed = parseSchema(paramsSchema, userParams);
             if (!parsed.success) {
                 throw new ProtocolError(ProtocolErrorCode.InvalidParams, `Invalid params for ${method}: ${parsed.error.message}`);
             }

packages/core/test/shared/customMethods.test.ts

@@ -62,6 +62,19 @@ describe('custom request handlers', () => {
         expect(received?.mcpReq.method).toBe('acme/ctx');
     });
 
+    test('strict schema: SDK-injected _meta is stripped before validation', async () => {
+        let receivedQ: string | undefined;
+        let receivedMeta: unknown;
+        server.setCustomRequestHandler('acme/strict', z.object({ q: z.string() }).strict(), (params, ctx) => {
+            receivedQ = params.q;
+            receivedMeta = ctx.mcpReq._meta;
+            return {};
+        });
+        await expect(client.sendCustomRequest('acme/strict', { q: 'hi' }, z.object({}), { onprogress: () => {} })).resolves.toEqual({});
+        expect(receivedQ).toBe('hi');
+        expect(receivedMeta).toMatchObject({ progressToken: expect.anything() });
+    });
+
     test('invalid params -> InvalidParams ProtocolError', async () => {
         server.setCustomRequestHandler('acme/search', SearchParams, () => ({ hits: [], total: 0 }));