fix: ensure object schemas always include required field for OpenAI compatibility

.changeset/empty-zod-required.md

@@ -0,0 +1,6 @@
+---
+"@modelcontextprotocol/core": patch
+"@modelcontextprotocol/server": patch
+---
+
+Ensure object schemas always include required field for OpenAI strict JSON schema compatibility

packages/core/src/util/schema.ts

@@ -26,7 +26,13 @@ export type SchemaOutput<T extends AnySchema> = z.output<T>;
  * Converts a Zod schema to JSON Schema.
  */
 export function schemaToJson(schema: AnySchema, options?: { io?: 'input' | 'output' }): Record<string, unknown> {
-    return z.toJSONSchema(schema, options) as Record<string, unknown>;
+    const jsonSchema = z.toJSONSchema(schema, options) as Record<string, unknown>;
+    // Ensure object schemas always include 'required' (even if empty)
+    // for compatibility with OpenAI strict JSON schema mode
+    if (jsonSchema.type === 'object' && !('required' in jsonSchema)) {
+        jsonSchema.required = [];
+    }
+    return jsonSchema;
 }
 
 /**

packages/server/src/server/mcp.ts

@@ -1146,7 +1146,8 @@ function createToolExecutor(inputSchema: AnySchema | undefined, handler: AnyTool
 
 const EMPTY_OBJECT_JSON_SCHEMA = {
     type: 'object' as const,
-    properties: {}
+    properties: {},
+    required: [] as string[]
 };
 
 /**

test/integration/test/server/mcp.test.ts

@@ -432,7 +432,8 @@ describe('Zod v4', () => {
             expect(result.tools[0]!.name).toBe('test');
             expect(result.tools[0]!.inputSchema).toEqual({
                 type: 'object',
-                properties: {}
+                properties: {},
+                required: []
             });
 
             // Adding the tool before the connection was established means no notification was sent