fix: support z.object() schemas in tool() method

Author: DaleSeo

src/server/mcp.ts

@@ -948,6 +948,13 @@ export class McpServer {
         cb: ToolCallback<Args>
     ): RegisteredTool;
 
+    /**
+     * Registers a tool with a ZodObject schema (e.g., z.object({ ... })).
+     * The schema's shape will be extracted and used for validation.
+     * @deprecated Use `registerTool` instead.
+     */
+    tool<Schema extends AnyObjectSchema>(name: string, paramsSchema: Schema, cb: ToolCallback<Schema>): RegisteredTool;
+
     /**
      * Registers a tool `name` (with a description) taking either parameter schema or annotations.
      * This unified overload handles both `tool(name, description, paramsSchema, cb)` and
@@ -964,6 +971,13 @@ export class McpServer {
         cb: ToolCallback<Args>
     ): RegisteredTool;
 
+    /**
+     * Registers a tool with a description and ZodObject schema (e.g., z.object({ ... })).
+     * The schema's shape will be extracted and used for validation.
+     * @deprecated Use `registerTool` instead.
+     */
+    tool<Schema extends AnyObjectSchema>(name: string, description: string, paramsSchema: Schema, cb: ToolCallback<Schema>): RegisteredTool;
+
     /**
      * Registers a tool with both parameter schema and annotations.
      * @deprecated Use `registerTool` instead.
@@ -975,6 +989,18 @@ export class McpServer {
         cb: ToolCallback<Args>
     ): RegisteredTool;
 
+    /**
+     * Registers a tool with a ZodObject schema and annotations.
+     * The schema's shape will be extracted and used for validation.
+     * @deprecated Use `registerTool` instead.
+     */
+    tool<Schema extends AnyObjectSchema>(
+        name: string,
+        paramsSchema: Schema,
+        annotations: ToolAnnotations,
+        cb: ToolCallback<Schema>
+    ): RegisteredTool;
+
     /**
      * Registers a tool with description, parameter schema, and annotations.
      * @deprecated Use `registerTool` instead.
@@ -987,6 +1013,19 @@ export class McpServer {
         cb: ToolCallback<Args>
     ): RegisteredTool;
 
+    /**
+     * Registers a tool with description, ZodObject schema, and annotations.
+     * The schema's shape will be extracted and used for validation.
+     * @deprecated Use `registerTool` instead.
+     */
+    tool<Schema extends AnyObjectSchema>(
+        name: string,
+        description: string,
+        paramsSchema: Schema,
+        annotations: ToolAnnotations,
+        cb: ToolCallback<Schema>
+    ): RegisteredTool;
+
     /**
      * tool() implementation. Parses arguments passed to overrides defined above.
      */
@@ -1023,8 +1062,31 @@ export class McpServer {
                     // Or: tool(name, description, paramsSchema, annotations, cb)
                     annotations = rest.shift() as ToolAnnotations;
                 }
+            } else if (typeof firstArg === 'object' && firstArg !== null && isZodSchemaInstance(firstArg)) {
+                // It's a Zod schema instance (like z.object()), extract its shape if it's an object schema
+                const shape = getObjectShape(firstArg as AnyObjectSchema);
+                if (shape) {
+                    // We found an object schema, use its shape
+                    inputSchema = shape;
+                    rest.shift();
+
+                    // Check if the next arg is potentially annotations
+                    if (
+                        rest.length > 1 &&
+                        typeof rest[0] === 'object' &&
+                        rest[0] !== null &&
+                        !isZodRawShapeCompat(rest[0]) &&
+                        !isZodSchemaInstance(rest[0])
+                    ) {
+                        annotations = rest.shift() as ToolAnnotations;
+                    }
+                } else {
+                    // It's a schema but not an object schema, treat as annotations
+                    // (This maintains backward compatibility for edge cases)
+                    annotations = rest.shift() as ToolAnnotations;
+                }
             } else if (typeof firstArg === 'object' && firstArg !== null) {
-                // Not a ZodRawShapeCompat, so must be annotations in this position
+                // Not a ZodRawShapeCompat or Zod schema, so must be annotations in this position
                 // Case: tool(name, annotations, cb)
                 // Or: tool(name, description, annotations, cb)
                 annotations = rest.shift() as ToolAnnotations;

test/server/mcp.test.ts

@@ -993,6 +993,107 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
             expect(result.tools[1].annotations).toEqual(result.tools[0].annotations);
         });
 
+        test('should accept z.object() schemas and extract arguments correctly', async () => {
+            const mcpServer = new McpServer({
+                name: 'test server',
+                version: '1.0'
+            });
+            const client = new Client({
+                name: 'test client',
+                version: '1.0'
+            });
+
+            // tool() with z.object() schema
+            mcpServer.tool(
+                'test-zobject',
+                'Test with ZodObject',
+                z.object({
+                    message: z.string()
+                }),
+                async ({ message }) => ({
+                    content: [{ type: 'text', text: `Echo: ${message}` }]
+                })
+            );
+
+            // tool() with z.object() and annotations
+            mcpServer.tool(
+                'test-zobject-annotations',
+                'Test with ZodObject and annotations',
+                z.object({
+                    name: z.string(),
+                    value: z.number()
+                }),
+                { title: 'ZodObject Tool', readOnlyHint: true },
+                async ({ name, value }) => ({
+                    content: [{ type: 'text', text: `${name}: ${value}` }]
+                })
+            );
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+
+            await Promise.all([client.connect(clientTransport), mcpServer.connect(serverTransport)]);
+
+            const listResult = await client.request({ method: 'tools/list' }, ListToolsResultSchema);
+
+            expect(listResult.tools).toHaveLength(2);
+            expect(listResult.tools[0].name).toBe('test-zobject');
+            expect(listResult.tools[0].inputSchema).toMatchObject({
+                type: 'object',
+                properties: {
+                    message: { type: 'string' }
+                }
+            });
+
+            expect(listResult.tools[1].name).toBe('test-zobject-annotations');
+            expect(listResult.tools[1].inputSchema).toMatchObject({
+                type: 'object',
+                properties: {
+                    name: { type: 'string' },
+                    value: { type: 'number' }
+                }
+            });
+            expect(listResult.tools[1].annotations).toEqual({
+                title: 'ZodObject Tool',
+                readOnlyHint: true
+            });
+
+            const result1 = await client.request(
+                {
+                    method: 'tools/call',
+                    params: {
+                        name: 'test-zobject',
+                        arguments: { message: 'Hello World' }
+                    }
+                },
+                CallToolResultSchema
+            );
+
+            expect(result1.content).toEqual([
+                {
+                    type: 'text',
+                    text: 'Echo: Hello World'
+                }
+            ]);
+
+            const result2 = await client.request(
+                {
+                    method: 'tools/call',
+                    params: {
+                        name: 'test-zobject-annotations',
+                        arguments: { name: 'test', value: 42 }
+                    }
+                },
+                CallToolResultSchema
+            );
+
+            expect(result2.content).toEqual([
+                {
+                    type: 'text',
+                    text: 'test: 42'
+                }
+            ]);
+        });
+
         /***
          * Test: Tool Argument Validation
          */