fix: reject plain JSON Schema objects passed as inputSchema (#1596)

Co-authored-by: Matt <77928207+mattzcarey@users.noreply.github.com>

src/server/mcp.ts

@@ -1024,9 +1024,10 @@ export class McpServer {
                     annotations = rest.shift() as ToolAnnotations;
                 }
             } else if (typeof firstArg === 'object' && firstArg !== null) {
-                // Not a ZodRawShapeCompat, so must be annotations in this position
-                // Case: tool(name, annotations, cb)
-                // Or: tool(name, description, annotations, cb)
+                // ToolAnnotations values are primitives. Nested objects indicate a misplaced schema
+                if (Object.values(firstArg).some(v => typeof v === 'object' && v !== null)) {
+                    throw new Error(`Tool ${name} expected a Zod schema or ToolAnnotations, but received an unrecognized object`);
+                }
                 annotations = rest.shift() as ToolAnnotations;
             }
         }
@@ -1386,7 +1387,7 @@ function isZodRawShapeCompat(obj: unknown): obj is ZodRawShapeCompat {
 
 /**
  * Converts a provided Zod schema to a Zod object if it is a ZodRawShapeCompat,
- * otherwise returns the schema as is.
+ * otherwise returns the schema as is. Throws if the value is not a valid Zod schema.
  */
 function getZodSchemaObject(schema: ZodRawShapeCompat | AnySchema | undefined): AnySchema | undefined {
     if (!schema) {
@@ -1397,6 +1398,10 @@ function getZodSchemaObject(schema: ZodRawShapeCompat | AnySchema | undefined):
         return objectFromShape(schema);
     }
 
+    if (!isZodSchemaInstance(schema as object)) {
+        throw new Error('inputSchema must be a Zod schema or raw shape, received an unrecognized object');
+    }
+
     return schema;
 }
 

test/server/mcp.test.ts

@@ -2050,6 +2050,37 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
             // Clean up spies
             warnSpy.mockRestore();
         });
+
+        test('should reject plain JSON Schema objects passed as inputSchema', () => {
+            const mcpServer = new McpServer({
+                name: 'test server',
+                version: '1.0'
+            });
+
+            const jsonSchema = {
+                type: 'object',
+                properties: {
+                    directory_id: {
+                        type: 'string',
+                        format: 'uuid',
+                        description: 'The UUID of the directory'
+                    }
+                },
+                required: ['directory_id']
+            } as any;
+
+            const cb = async ({ directory_id }: any) => ({
+                content: [{ type: 'text' as const, text: `Got: ${directory_id}` }]
+            });
+
+            expect(() => {
+                mcpServer.tool('test', 'A tool', jsonSchema, cb);
+            }).toThrow(/unrecognized object/);
+
+            expect(() => {
+                mcpServer.registerTool('test', { description: 'A tool', inputSchema: jsonSchema }, cb);
+            }).toThrow(/unrecognized object/);
+        });
     });
 
     describe('resource()', () => {