docs(core): clarify schema-bundle overload validates only; add test

The {params, result} schema bundle in sendCustomRequest/sendCustomNotification
is a type guard, not a transformer — the caller-provided value is sent as-is,
matching request()/v1 behavior. Transforms/defaults on the params schema are
not applied outbound (parsed.data is intentionally unused on the send path).
Adds JSDoc and a test asserting params are sent verbatim.

Author: felixweinberger

packages/core/src/shared/protocol.ts

@@ -1143,6 +1143,10 @@ export abstract class Protocol<ContextT extends BaseContext> {
      *
      * Pass a `{ params, result }` schema bundle as the third argument to get typed `params` and
      * pre-send validation; pass a bare result schema for loose, unvalidated params.
+     *
+     * The `params` schema is used only for validation — the value you pass is sent as-is.
+     * Transforms (e.g. `.trim()`) and defaults (e.g. `.default(n)`) on the schema are not
+     * applied to outbound data, matching the behavior of {@linkcode Protocol.request | request}.
      */
     sendCustomRequest<P extends AnySchema, R extends AnySchema>(
         method: string,
@@ -1185,7 +1189,8 @@ export abstract class Protocol<ContextT extends BaseContext> {
      * standard MCP notifications.
      *
      * Pass a `{ params }` schema bundle as the third argument to get typed `params` and pre-send
-     * validation.
+     * validation. The schema validates only — transforms and defaults are not applied to
+     * outbound data; the value you pass is sent as-is.
      */
     sendCustomNotification<P extends AnySchema>(
         method: string,

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

@@ -191,6 +191,18 @@ describe('sendCustomRequest', () => {
             })
         ).rejects.toSatisfy((e: unknown) => e instanceof ProtocolError && e.code === ProtocolErrorCode.InvalidParams);
     });
+
+    test('schema bundle overload: params sent as-is (validate-only, no outbound transforms)', async () => {
+        const [client, server] = await linkedPair();
+        const P = z.object({ query: z.string().transform(s => s.trim()), page: z.number() });
+        let received: unknown;
+        server.setCustomRequestHandler('acme/q', z.unknown(), p => {
+            received = p;
+            return {};
+        });
+        await client.sendCustomRequest('acme/q', { query: '  hi  ', page: 1 }, { params: P, result: z.object({}) });
+        expect(received).toEqual({ query: '  hi  ', page: 1 });
+    });
 });
 
 describe('sendCustomNotification', () => {