fix(core): make fromJsonSchema() use runtime-aware default validator … (#1825)

Co-authored-by: Felix Weinberger <3823880+felixweinberger@users.noreply.github.com>

Author: KKonstantinov

docs/migration-SKILL.md

@@ -210,7 +210,7 @@ Zod schemas, all callback return types. Note: `callTool()` and `request()` signa
 
 The variadic `.tool()`, `.prompt()`, `.resource()` methods are removed. Use the `register*` methods with a config object.
 
-**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/server`. 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)` from `@modelcontextprotocol/server` (validator defaults automatically; pass an explicit validator for custom configurations). Applies to `inputSchema`, `outputSchema`, and `argsSchema`.
 
 ### Tools
 

docs/migration.md

@@ -274,10 +274,10 @@ server.registerTool('greet', {
   inputSchema: type({ name: 'string' })
 }, async ({ name }) => { ... });
 
-// Raw JSON Schema via fromJsonSchema
-import { fromJsonSchema, AjvJsonSchemaValidator } from '@modelcontextprotocol/server';
+// Raw JSON Schema via fromJsonSchema (validator defaults to runtime-appropriate choice)
+import { fromJsonSchema } from '@modelcontextprotocol/server';
 server.registerTool('greet', {
-  inputSchema: fromJsonSchema({ type: 'object', properties: { name: { type: 'string' } } }, new AjvJsonSchemaValidator())
+  inputSchema: fromJsonSchema({ type: 'object', properties: { name: { type: 'string' } } })
 }, handler);
 
 // For tools with no parameters, use z.object({})

packages/client/src/fromJsonSchema.ts

@@ -0,0 +1,9 @@
+import { DefaultJsonSchemaValidator } from '@modelcontextprotocol/client/_shims';
+import type { JsonSchemaType, jsonSchemaValidator, StandardSchemaWithJSON } from '@modelcontextprotocol/core';
+import { fromJsonSchema as coreFromJsonSchema } from '@modelcontextprotocol/core';
+
+let _defaultValidator: jsonSchemaValidator | undefined;
+
+export function fromJsonSchema<T = unknown>(schema: JsonSchemaType, validator?: jsonSchemaValidator): StandardSchemaWithJSON<T, T> {
+    return coreFromJsonSchema<T>(schema, validator ?? (_defaultValidator ??= new DefaultJsonSchemaValidator()));
+}

packages/client/src/index.ts

@@ -73,5 +73,8 @@ export { StreamableHTTPClientTransport } from './client/streamableHttp.js';
 // experimental exports
 export { ExperimentalClientTasks } from './experimental/tasks/client.js';
 
+// runtime-aware wrapper (shadows core/public's fromJsonSchema with optional validator)
+export { fromJsonSchema } from './fromJsonSchema.js';
+
 // re-export curated public API from core
 export * from '@modelcontextprotocol/core/public';

packages/core/src/exports/public/index.ts

@@ -138,5 +138,6 @@ export type { StandardSchemaWithJSON } from '../../util/standardSchema.js';
 export { AjvJsonSchemaValidator } from '../../validators/ajvProvider.js';
 export type { CfWorkerSchemaDraft } from '../../validators/cfWorkerProvider.js';
 export { CfWorkerJsonSchemaValidator } from '../../validators/cfWorkerProvider.js';
-export { fromJsonSchema } from '../../validators/fromJsonSchema.js';
+// fromJsonSchema is intentionally NOT exported here — the server and client packages
+// provide runtime-aware wrappers that default to the appropriate validator via _shims.
 export type { JsonSchemaType, JsonSchemaValidator, jsonSchemaValidator, JsonSchemaValidatorResult } from '../../validators/types.js';

packages/core/src/validators/fromJsonSchema.ts

@@ -10,6 +10,11 @@ import type { JsonSchemaType, jsonSchemaValidator } from './types.js';
  * The callback arguments will be typed `unknown` (raw JSON Schema has no TypeScript
  * types attached). Cast at the call site, or use the generic `fromJsonSchema<MyType>(...)`.
  *
+ * @param schema - A JSON Schema object describing the expected shape
+ * @param validator - A validator provider. When importing `fromJsonSchema` from
+ *   `@modelcontextprotocol/server` or `@modelcontextprotocol/client`, a runtime-appropriate
+ *   default is provided automatically (AJV on Node.js, CfWorker on edge runtimes).
+ *
  * @example
  * ```ts source="./fromJsonSchema.examples.ts#fromJsonSchema_basicUsage"
  * const inputSchema = fromJsonSchema<{ name: string }>(

packages/server/src/fromJsonSchema.ts

@@ -0,0 +1,9 @@
+import type { JsonSchemaType, jsonSchemaValidator, StandardSchemaWithJSON } from '@modelcontextprotocol/core';
+import { fromJsonSchema as coreFromJsonSchema } from '@modelcontextprotocol/core';
+import { DefaultJsonSchemaValidator } from '@modelcontextprotocol/server/_shims';
+
+let _defaultValidator: jsonSchemaValidator | undefined;
+
+export function fromJsonSchema<T = unknown>(schema: JsonSchemaType, validator?: jsonSchemaValidator): StandardSchemaWithJSON<T, T> {
+    return coreFromJsonSchema<T>(schema, validator ?? (_defaultValidator ??= new DefaultJsonSchemaValidator()));
+}

packages/server/src/index.ts

@@ -43,5 +43,8 @@ export type { CreateTaskRequestHandler, TaskRequestHandler, ToolTaskHandler } fr
 export { ExperimentalMcpServerTasks } from './experimental/tasks/mcpServer.js';
 export { ExperimentalServerTasks } from './experimental/tasks/server.js';
 
+// runtime-aware wrapper (shadows core/public's fromJsonSchema with optional validator)
+export { fromJsonSchema } from './fromJsonSchema.js';
+
 // re-export curated public API from core
 export * from '@modelcontextprotocol/core/public';

test/integration/test/standardSchema.test.ts

@@ -6,7 +6,7 @@
 import { Client } from '@modelcontextprotocol/client';
 import type { TextContent } from '@modelcontextprotocol/core';
 import { AjvJsonSchemaValidator, fromJsonSchema, InMemoryTransport } from '@modelcontextprotocol/core';
-import { completable, McpServer } from '@modelcontextprotocol/server';
+import { completable, fromJsonSchema as serverFromJsonSchema, McpServer } from '@modelcontextprotocol/server';
 import { toStandardJsonSchema } from '@valibot/to-json-schema';
 import { type } from 'arktype';
 import * as v from 'valibot';
@@ -428,6 +428,44 @@ describe('Standard Schema Support', () => {
         });
     });
 
+    describe('fromJsonSchema with default validator (server wrapper)', () => {
+        test('should use runtime-appropriate default validator when none is provided', async () => {
+            const inputSchema = serverFromJsonSchema<{ name: string }>({
+                type: 'object',
+                properties: { name: { type: 'string' } },
+                required: ['name']
+            });
+
+            mcpServer.registerTool('greet-default', { inputSchema }, async ({ name }) => ({
+                content: [{ type: 'text', text: `Hello, ${name}!` }]
+            }));
+
+            await connectClientAndServer();
+
+            const result = await client.request({ method: 'tools/call', params: { name: 'greet-default', arguments: { name: 'World' } } });
+            expect((result.content[0] as TextContent).text).toBe('Hello, World!');
+        });
+
+        test('should reject invalid input with default validator', async () => {
+            const inputSchema = serverFromJsonSchema({ type: 'object', properties: { count: { type: 'number' } }, required: ['count'] });
+
+            mcpServer.registerTool('double-default', { inputSchema }, async args => {
+                const { count } = args as { count: number };
+                return { content: [{ type: 'text', text: `${count * 2}` }] };
+            });
+
+            await connectClientAndServer();
+
+            const result = await client.request({
+                method: 'tools/call',
+                params: { name: 'double-default', arguments: { count: 'not a number' } }
+            });
+            expect(result.isError).toBe(true);
+            const errorText = (result.content[0] as TextContent).text;
+            expect(errorText).toContain('Input validation error');
+        });
+    });
+
     describe('Prompt completions with Zod completable', () => {
         // Note: completable() is currently Zod-specific
         // These tests verify that Zod schemas with completable still work