feat: support Standard Schema for tool/prompt schemas (#1689)

Co-authored-by: Matt Carey <mcarey@cloudflare.com>

docs/migration-SKILL.md

@@ -209,7 +209,7 @@ if (error instanceof OAuthError && error.code === OAuthErrorCode.InvalidClient)
 
 The variadic `.tool()`, `.prompt()`, `.resource()` methods are removed. Use the `register*` methods with a config object.
 
-**IMPORTANT**: v2 requires full Zod schemas — raw shapes like `{ name: z.string() }` are no longer supported. You must wrap with `z.object()`. This applies to `inputSchema`, `outputSchema`, and `argsSchema`.
+**IMPORTANT**: v2 requires schema objects implementing [Standard Schema](https://standardschema.dev/) — raw shapes like `{ name: z.string() }` are no longer supported. Wrap with `z.object()` (Zod v4), or use ArkType's `type({...})`, or Valibot. For raw JSON Schema, wrap with `fromJsonSchema(schema, validator)` from `@modelcontextprotocol/core`. Applies to `inputSchema`, `outputSchema`, and `argsSchema`.
 
 ### Tools
 
@@ -279,13 +279,22 @@ Note: the third argument (`metadata`) is required — pass `{}` if no metadata.
 
 ### Schema Migration Quick Reference
 
-| v1 (raw shape) | v2 (Zod schema) |
+| v1 (raw shape) | v2 (Standard Schema object) |
 |----------------|-----------------|
 | `{ name: z.string() }` | `z.object({ name: z.string() })` |
 | `{ count: z.number().optional() }` | `z.object({ count: z.number().optional() })` |
 | `{}` (empty) | `z.object({})` |
 | `undefined` (no schema) | `undefined` or omit the field |
 
+### Removed core exports
+
+| Removed from `@modelcontextprotocol/core` | Replacement |
+|---|---|
+| `schemaToJson(schema)` | `standardSchemaToJsonSchema(schema)` |
+| `parseSchemaAsync(schema, data)` | `validateStandardSchema(schema, data)` |
+| `SchemaInput<T>` | `StandardSchemaWithJSON.InferInput<T>` |
+| `getSchemaShape`, `getSchemaDescription`, `isOptionalSchema`, `unwrapOptionalSchema` | none (internal Zod introspection helpers) |
+
 ## 7. Headers API
 
 Transport constructors and `RequestInfo.headers` now use the Web Standard `Headers` object instead of plain objects.

docs/migration.md

@@ -200,17 +200,17 @@ import * as z from 'zod/v4';
 const server = new McpServer({ name: 'demo', version: '1.0.0' });
 
 // Tool with schema
-server.registerTool('greet', { inputSchema: { name: z.string() } }, async ({ name }) => {
+server.registerTool('greet', { inputSchema: z.object({ name: z.string() }) }, async ({ name }) => {
     return { content: [{ type: 'text', text: `Hello, ${name}!` }] };
 });
 
 // Tool with description
-server.registerTool('greet', { description: 'Greet a user', inputSchema: { name: z.string() } }, async ({ name }) => {
+server.registerTool('greet', { description: 'Greet a user', inputSchema: z.object({ name: z.string() }) }, async ({ name }) => {
     return { content: [{ type: 'text', text: `Hello, ${name}!` }] };
 });
 
 // Prompt
-server.registerPrompt('summarize', { argsSchema: { text: z.string() } }, async ({ text }) => {
+server.registerPrompt('summarize', { argsSchema: z.object({ text: z.string() }) }, async ({ text }) => {
     return { messages: [{ role: 'user', content: { type: 'text', text: `Summarize: ${text}` } }] };
 });
 
@@ -220,9 +220,9 @@ server.registerResource('config', 'config://app', {}, async uri => {
 });
 ```
 
-### Zod schemas required (raw shapes no longer supported)
+### Standard Schema objects required (raw shapes no longer supported)
 
-v2 requires full Zod schemas for `inputSchema` and `argsSchema`. Raw object shapes are no longer accepted.
+v2 requires schema objects implementing the [Standard Schema spec](https://standardschema.dev/) for `inputSchema`, `outputSchema`, and `argsSchema`. Raw object shapes are no longer accepted. Zod v4, ArkType, and Valibot all implement the spec.
 
 **Before (v1):**
 
@@ -240,11 +240,23 @@ server.registerTool('greet', {
 ```typescript
 import * as z from 'zod/v4';
 
-// Must wrap with z.object()
+// Wrap with z.object() (or use any Standard Schema library)
 server.registerTool('greet', {
-  inputSchema: z.object({ name: z.string() })  // full Zod schema
+  inputSchema: z.object({ name: z.string() })
 }, async ({ name }) => { ... });
 
+// ArkType works too
+import { type } from 'arktype';
+server.registerTool('greet', {
+  inputSchema: type({ name: 'string' })
+}, async ({ name }) => { ... });
+
+// Raw JSON Schema via fromJsonSchema
+import { fromJsonSchema, AjvJsonSchemaValidator } from '@modelcontextprotocol/core';
+server.registerTool('greet', {
+  inputSchema: fromJsonSchema({ type: 'object', properties: { name: { type: 'string' } } }, new AjvJsonSchemaValidator())
+}, handler);
+
 // For tools with no parameters, use z.object({})
 server.registerTool('ping', {
   inputSchema: z.object({})
@@ -256,6 +268,15 @@ This applies to:
 - `outputSchema` in `registerTool()`
 - `argsSchema` in `registerPrompt()`
 
+**Removed Zod-specific helpers** from `@modelcontextprotocol/core` (use Standard Schema equivalents):
+
+| Removed | Replacement |
+|---|---|
+| `schemaToJson(schema)` | `standardSchemaToJsonSchema(schema)` |
+| `parseSchemaAsync(schema, data)` | `validateStandardSchema(schema, data)` |
+| `SchemaInput<T>` | `StandardSchemaWithJSON.InferInput<T>` |
+| `getSchemaShape`, `getSchemaDescription`, `isOptionalSchema`, `unwrapOptionalSchema` | No replacement — these are now internal Zod introspection helpers |
+
 ### Host header validation moved
 
 Express-specific middleware (`hostHeaderValidation()`, `localhostHostValidation()`) moved from the server package to `@modelcontextprotocol/express`. The server package now exports framework-agnostic functions instead: `validateHostHeader()`, `localhostAllowedHostnames()`,