feat(compat): widen completable() constraint to StandardSchemaV1

v1 accepted any zod schema (AnySchema). v2 narrowed to StandardSchemaWithJSON,
which requires ~standard.jsonSchema. completable() itself only needs validate
(to type the callback's value param); JSON-Schema generation happens at the
outer registerPrompt argsSchema level, not per-field. Widening to
StandardSchemaV1 restores the v1 surface (any Standard Schema lib, zod >=3.24).

Adds raw-shape registerPrompt test with a completable() field.

Author: felixweinberger

packages/server/src/server/completable.ts

@@ -1,19 +1,19 @@
-import type { StandardSchemaWithJSON } from '@modelcontextprotocol/core';
+import type { StandardSchemaV1 } from '@modelcontextprotocol/core';
 
 export const COMPLETABLE_SYMBOL: unique symbol = Symbol.for('mcp.completable');
 
-export type CompleteCallback<T extends StandardSchemaWithJSON = StandardSchemaWithJSON> = (
-    value: StandardSchemaWithJSON.InferInput<T>,
+export type CompleteCallback<T extends StandardSchemaV1 = StandardSchemaV1> = (
+    value: StandardSchemaV1.InferInput<T>,
     context?: {
         arguments?: Record<string, string>;
     }
-) => StandardSchemaWithJSON.InferInput<T>[] | Promise<StandardSchemaWithJSON.InferInput<T>[]>;
+) => StandardSchemaV1.InferInput<T>[] | Promise<StandardSchemaV1.InferInput<T>[]>;
 
-export type CompletableMeta<T extends StandardSchemaWithJSON = StandardSchemaWithJSON> = {
+export type CompletableMeta<T extends StandardSchemaV1 = StandardSchemaV1> = {
     complete: CompleteCallback<T>;
 };
 
-export type CompletableSchema<T extends StandardSchemaWithJSON> = T & {
+export type CompletableSchema<T extends StandardSchemaV1> = T & {
     [COMPLETABLE_SYMBOL]: CompletableMeta<T>;
 };
 
@@ -48,7 +48,7 @@ export type CompletableSchema<T extends StandardSchemaWithJSON> = T & {
  *
  * @see {@linkcode server/mcp.McpServer.registerPrompt | McpServer.registerPrompt} for using completable schemas in prompt argument definitions
  */
-export function completable<T extends StandardSchemaWithJSON>(schema: T, complete: CompleteCallback<T>): CompletableSchema<T> {
+export function completable<T extends StandardSchemaV1>(schema: T, complete: CompleteCallback<T>): CompletableSchema<T> {
     Object.defineProperty(schema as object, COMPLETABLE_SYMBOL, {
         value: { complete } as CompletableMeta<T>,
         enumerable: false,
@@ -61,14 +61,14 @@ export function completable<T extends StandardSchemaWithJSON>(schema: T, complet
 /**
  * Checks if a schema is completable (has completion metadata).
  */
-export function isCompletable(schema: unknown): schema is CompletableSchema<StandardSchemaWithJSON> {
+export function isCompletable(schema: unknown): schema is CompletableSchema<StandardSchemaV1> {
     return !!schema && typeof schema === 'object' && COMPLETABLE_SYMBOL in (schema as object);
 }
 
 /**
  * Gets the completer callback from a completable schema, if it exists.
  */
-export function getCompleter<T extends StandardSchemaWithJSON>(schema: T): CompleteCallback<T> | undefined {
+export function getCompleter<T extends StandardSchemaV1>(schema: T): CompleteCallback<T> | undefined {
     const meta = (schema as unknown as { [COMPLETABLE_SYMBOL]?: CompletableMeta<T> })[COMPLETABLE_SYMBOL];
     return meta?.complete as CompleteCallback<T> | undefined;
 }

packages/server/test/server/mcp.compat.test.ts

@@ -3,6 +3,7 @@ import { InMemoryTransport, isStandardSchema, LATEST_PROTOCOL_VERSION } from '@m
 import { describe, expect, it, vi } from 'vitest';
 import * as z from 'zod/v4';
 import { McpServer } from '../../src/index.js';
+import { completable } from '../../src/server/completable.js';
 
 describe('registerTool/registerPrompt accept raw Zod shape (auto-wrapped)', () => {
     it('registerTool accepts a raw shape for inputSchema and auto-wraps it', () => {
@@ -56,6 +57,25 @@ describe('registerTool/registerPrompt accept raw Zod shape (auto-wrapped)', () =
         expect(isStandardSchema(prompts['p']?.argsSchema)).toBe(true);
     });
 
+    it('registerPrompt raw shape accepts completable() fields (v1 pattern)', () => {
+        const server = new McpServer({ name: 't', version: '1.0.0' });
+
+        server.registerPrompt(
+            'p',
+            {
+                argsSchema: {
+                    language: completable(z.string(), v => ['typescript', 'python'].filter(l => l.startsWith(v)))
+                }
+            },
+            async ({ language }) => ({
+                messages: [{ role: 'user' as const, content: { type: 'text' as const, text: language } }]
+            })
+        );
+
+        const prompts = (server as unknown as { _registeredPrompts: Record<string, { argsSchema?: unknown }> })._registeredPrompts;
+        expect(isStandardSchema(prompts['p']?.argsSchema)).toBe(true);
+    });
+
     it('callback receives validated, typed args end-to-end via tools/call', async () => {
         const server = new McpServer({ name: 't', version: '1.0.0' });