feat(compat): registerTool/registerPrompt accept raw Zod shape (auto-wrap with z.object)

Author: felixweinberger

.changeset/register-rawshape-compat.md

@@ -0,0 +1,6 @@
+---
+'@modelcontextprotocol/core': patch
+'@modelcontextprotocol/server': patch
+---
+
+`registerTool`/`registerPrompt` accept a raw Zod shape (`{ field: z.string() }`) for `inputSchema`/`outputSchema`/`argsSchema` in addition to a wrapped Standard Schema. Raw shapes are auto-wrapped with `z.object()`. Both forms are first-class.

packages/core/src/util/standardSchema.ts

@@ -6,6 +6,8 @@
 
 /* eslint-disable @typescript-eslint/no-namespace */
 
+import * as z from 'zod/v4';
+
 // Standard Schema interfaces — vendored from https://standardschema.dev (spec v1, Jan 2025)
 
 export interface StandardTypedV1<Input = unknown, Output = Input> {
@@ -136,6 +138,38 @@ export function isStandardSchemaWithJSON(schema: unknown): schema is StandardSch
     return isStandardJSONSchema(schema) && isStandardSchema(schema);
 }
 
+/**
+ * Detects a "raw shape" — a plain object whose values are Zod (or other
+ * Standard Schema) field schemas, e.g. `{ name: z.string() }`. Powers the
+ * auto-wrap in {@linkcode normalizeRawShapeSchema}.
+ *
+ * @internal
+ */
+export function isZodRawShape(obj: unknown): obj is Record<string, StandardSchemaV1> {
+    if (typeof obj !== 'object' || obj === null) return false;
+    if (isStandardSchema(obj)) return false;
+    const values = Object.values(obj);
+    return values.length > 0 && values.every(v => isStandardSchema(v) || (typeof v === 'object' && v !== null && '_def' in v));
+}
+
+/**
+ * Accepts either a {@linkcode StandardSchemaWithJSON} or a raw Zod shape
+ * `{ field: z.string() }` and returns a {@linkcode StandardSchemaWithJSON}.
+ * Raw shapes are wrapped with `z.object()` so the rest of the pipeline sees a
+ * uniform schema type; already-wrapped schemas pass through unchanged.
+ *
+ * @internal
+ */
+export function normalizeRawShapeSchema(
+    schema: StandardSchemaWithJSON | Record<string, StandardSchemaV1> | undefined
+): StandardSchemaWithJSON | undefined {
+    if (schema === undefined) return undefined;
+    if (isZodRawShape(schema)) {
+        return z.object(schema as z.ZodRawShape) as StandardSchemaWithJSON;
+    }
+    return schema;
+}
+
 // JSON Schema conversion
 
 /**

packages/server/src/server/mcp.ts

@@ -30,6 +30,7 @@ import type {
 import {
     assertCompleteRequestPrompt,
     assertCompleteRequestResourceTemplate,
+    normalizeRawShapeSchema,
     promptArgumentsFromStandardSchema,
     ProtocolError,
     ProtocolErrorCode,
@@ -873,6 +874,31 @@ export class McpServer {
             _meta?: Record<string, unknown>;
         },
         cb: ToolCallback<InputArgs>
+    ): RegisteredTool;
+    /** Raw-shape form: `inputSchema`/`outputSchema` may be a plain `{ field: z.string() }` record; it is auto-wrapped with `z.object()`. */
+    registerTool<InputArgs extends ZodRawShape, OutputArgs extends ZodRawShape | StandardSchemaWithJSON | undefined = undefined>(
+        name: string,
+        config: {
+            title?: string;
+            description?: string;
+            inputSchema?: InputArgs;
+            outputSchema?: OutputArgs;
+            annotations?: ToolAnnotations;
+            _meta?: Record<string, unknown>;
+        },
+        cb: LegacyToolCallback<InputArgs>
+    ): RegisteredTool;
+    registerTool(
+        name: string,
+        config: {
+            title?: string;
+            description?: string;
+            inputSchema?: StandardSchemaWithJSON | ZodRawShape;
+            outputSchema?: StandardSchemaWithJSON | ZodRawShape;
+            annotations?: ToolAnnotations;
+            _meta?: Record<string, unknown>;
+        },
+        cb: ToolCallback<StandardSchemaWithJSON | undefined> | LegacyToolCallback<ZodRawShape>
     ): RegisteredTool {
         if (this._registeredTools[name]) {
             throw new Error(`Tool ${name} is already registered`);
@@ -884,8 +910,8 @@ export class McpServer {
             name,
             title,
             description,
-            inputSchema,
-            outputSchema,
+            normalizeRawShapeSchema(inputSchema),
+            normalizeRawShapeSchema(outputSchema),
             annotations,
             { taskSupport: 'forbidden' },
             _meta,
@@ -928,6 +954,27 @@ export class McpServer {
             _meta?: Record<string, unknown>;
         },
         cb: PromptCallback<Args>
+    ): RegisteredPrompt;
+    /** Raw-shape form: `argsSchema` may be a plain `{ field: z.string() }` record; it is auto-wrapped with `z.object()`. */
+    registerPrompt<Args extends ZodRawShape>(
+        name: string,
+        config: {
+            title?: string;
+            description?: string;
+            argsSchema?: Args;
+            _meta?: Record<string, unknown>;
+        },
+        cb: LegacyPromptCallback<Args>
+    ): RegisteredPrompt;
+    registerPrompt(
+        name: string,
+        config: {
+            title?: string;
+            description?: string;
+            argsSchema?: StandardSchemaWithJSON | ZodRawShape;
+            _meta?: Record<string, unknown>;
+        },
+        cb: PromptCallback<StandardSchemaWithJSON> | LegacyPromptCallback<ZodRawShape>
     ): RegisteredPrompt {
         if (this._registeredPrompts[name]) {
             throw new Error(`Prompt ${name} is already registered`);
@@ -939,7 +986,7 @@ export class McpServer {
             name,
             title,
             description,
-            argsSchema,
+            normalizeRawShapeSchema(argsSchema),
             cb as PromptCallback<StandardSchemaWithJSON | undefined>,
             _meta
         );
@@ -1062,6 +1109,25 @@ export class ResourceTemplate {
     }
 }
 
+/**
+ * A plain record of field schemas, e.g. `{ name: z.string() }`. Accepted by
+ * `registerTool`/`registerPrompt` as a shorthand; auto-wrapped with `z.object()`.
+ */
+export type ZodRawShape = Record<string, StandardSchemaWithJSON>;
+
+/** Infers the parsed-output type of a {@linkcode ZodRawShape}. */
+export type InferRawShape<S extends ZodRawShape> = { [K in keyof S]: StandardSchemaWithJSON.InferOutput<S[K]> };
+
+/** {@linkcode ToolCallback} variant used when `inputSchema` is a {@linkcode ZodRawShape}. */
+export type LegacyToolCallback<Args extends ZodRawShape | undefined> = Args extends ZodRawShape
+    ? (args: InferRawShape<Args>, ctx: ServerContext) => CallToolResult | Promise<CallToolResult>
+    : (ctx: ServerContext) => CallToolResult | Promise<CallToolResult>;
+
+/** {@linkcode PromptCallback} variant used when `argsSchema` is a {@linkcode ZodRawShape}. */
+export type LegacyPromptCallback<Args extends ZodRawShape | undefined> = Args extends ZodRawShape
+    ? (args: InferRawShape<Args>, ctx: ServerContext) => GetPromptResult | Promise<GetPromptResult>
+    : (ctx: ServerContext) => GetPromptResult | Promise<GetPromptResult>;
+
 export type BaseToolCallback<
     SendResultT extends Result,
     Ctx extends ServerContext,

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

@@ -0,0 +1,54 @@
+import { isStandardSchema } from '@modelcontextprotocol/core';
+import { describe, expect, it, vi } from 'vitest';
+import * as z from 'zod/v4';
+import { McpServer } from '../../src/index.js';
+
+describe('registerTool/registerPrompt accept raw Zod shape (auto-wrapped)', () => {
+    it('registerTool accepts a raw shape for inputSchema, auto-wraps, and does not warn', () => {
+        const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+        const server = new McpServer({ name: 't', version: '1.0.0' });
+
+        server.registerTool('a', { inputSchema: { x: z.number() } }, async ({ x }) => ({
+            content: [{ type: 'text' as const, text: String(x) }]
+        }));
+        server.registerTool('b', { inputSchema: { y: z.number() } }, async ({ y }) => ({
+            content: [{ type: 'text' as const, text: String(y) }]
+        }));
+
+        const tools = (server as unknown as { _registeredTools: Record<string, { inputSchema?: unknown }> })._registeredTools;
+        expect(Object.keys(tools)).toEqual(['a', 'b']);
+        // raw shape was wrapped into a Standard Schema (z.object)
+        expect(isStandardSchema(tools['a']?.inputSchema)).toBe(true);
+
+        expect(warn).not.toHaveBeenCalled();
+        warn.mockRestore();
+    });
+
+    it('registerTool with z.object() inputSchema also works without warning', () => {
+        const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+        const server = new McpServer({ name: 't', version: '1.0.0' });
+
+        server.registerTool('c', { inputSchema: z.object({ x: z.number() }) }, async ({ x }) => ({
+            content: [{ type: 'text' as const, text: String(x) }]
+        }));
+
+        expect(warn).not.toHaveBeenCalled();
+        warn.mockRestore();
+    });
+
+    it('registerPrompt accepts a raw shape for argsSchema and does not warn', () => {
+        const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+        const server = new McpServer({ name: 't', version: '1.0.0' });
+
+        server.registerPrompt('p', { argsSchema: { topic: z.string() } }, async ({ topic }) => ({
+            messages: [{ role: 'user' as const, content: { type: 'text' as const, text: topic } }]
+        }));
+
+        const prompts = (server as unknown as { _registeredPrompts: Record<string, { argsSchema?: unknown }> })._registeredPrompts;
+        expect(Object.keys(prompts)).toContain('p');
+        expect(isStandardSchema(prompts['p']?.argsSchema)).toBe(true);
+
+        expect(warn).not.toHaveBeenCalled();
+        warn.mockRestore();
+    });
+});