fix: allow zod 4 transformations (#1213)

Author: mattzcarey

src/server/mcp.test.ts

@@ -4765,6 +4765,201 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
         });
     });
 
+    describe('Tools with transformation schemas', () => {
+        test('should support z.preprocess() schemas', async () => {
+            const server = new McpServer({
+                name: 'test',
+                version: '1.0.0'
+            });
+
+            const client = new Client({
+                name: 'test-client',
+                version: '1.0.0'
+            });
+
+            // z.preprocess() allows transforming input before validation
+            const preprocessSchema = z.preprocess(
+                input => {
+                    // Normalize input by trimming strings
+                    if (typeof input === 'object' && input !== null) {
+                        const obj = input as Record<string, unknown>;
+                        if (typeof obj.name === 'string') {
+                            return { ...obj, name: obj.name.trim() };
+                        }
+                    }
+                    return input;
+                },
+                z.object({ name: z.string() })
+            );
+
+            server.registerTool('preprocess-test', { inputSchema: preprocessSchema }, async args => {
+                return {
+                    content: [{ type: 'text' as const, text: `Hello, ${args.name}!` }]
+                };
+            });
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+            await server.connect(serverTransport);
+            await client.connect(clientTransport);
+
+            // Test with input that has leading/trailing whitespace
+            const result = await client.callTool({
+                name: 'preprocess-test',
+                arguments: { name: '  World  ' }
+            });
+
+            expect(result.content).toEqual([
+                {
+                    type: 'text',
+                    text: 'Hello, World!'
+                }
+            ]);
+        });
+
+        test('should support z.transform() schemas', async () => {
+            const server = new McpServer({
+                name: 'test',
+                version: '1.0.0'
+            });
+
+            const client = new Client({
+                name: 'test-client',
+                version: '1.0.0'
+            });
+
+            // z.transform() allows transforming validated output
+            const transformSchema = z
+                .object({
+                    firstName: z.string(),
+                    lastName: z.string()
+                })
+                .transform(data => ({
+                    ...data,
+                    fullName: `${data.firstName} ${data.lastName}`
+                }));
+
+            server.registerTool('transform-test', { inputSchema: transformSchema }, async args => {
+                return {
+                    content: [{ type: 'text' as const, text: `Full name: ${args.fullName}` }]
+                };
+            });
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+            await server.connect(serverTransport);
+            await client.connect(clientTransport);
+
+            const result = await client.callTool({
+                name: 'transform-test',
+                arguments: { firstName: 'John', lastName: 'Doe' }
+            });
+
+            expect(result.content).toEqual([
+                {
+                    type: 'text',
+                    text: 'Full name: John Doe'
+                }
+            ]);
+        });
+
+        test('should support z.pipe() schemas', async () => {
+            const server = new McpServer({
+                name: 'test',
+                version: '1.0.0'
+            });
+
+            const client = new Client({
+                name: 'test-client',
+                version: '1.0.0'
+            });
+
+            // z.pipe() chains multiple schemas together
+            const pipeSchema = z
+                .object({ value: z.string() })
+                .transform(data => ({ ...data, processed: true }))
+                .pipe(z.object({ value: z.string(), processed: z.boolean() }));
+
+            server.registerTool('pipe-test', { inputSchema: pipeSchema }, async args => {
+                return {
+                    content: [{ type: 'text' as const, text: `Value: ${args.value}, Processed: ${args.processed}` }]
+                };
+            });
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+            await server.connect(serverTransport);
+            await client.connect(clientTransport);
+
+            const result = await client.callTool({
+                name: 'pipe-test',
+                arguments: { value: 'test' }
+            });
+
+            expect(result.content).toEqual([
+                {
+                    type: 'text',
+                    text: 'Value: test, Processed: true'
+                }
+            ]);
+        });
+
+        test('should support nested transformation schemas', async () => {
+            const server = new McpServer({
+                name: 'test',
+                version: '1.0.0'
+            });
+
+            const client = new Client({
+                name: 'test-client',
+                version: '1.0.0'
+            });
+
+            // Complex schema with both preprocess and transform
+            const complexSchema = z.preprocess(
+                input => {
+                    if (typeof input === 'object' && input !== null) {
+                        const obj = input as Record<string, unknown>;
+                        // Convert string numbers to actual numbers
+                        if (typeof obj.count === 'string') {
+                            return { ...obj, count: parseInt(obj.count, 10) };
+                        }
+                    }
+                    return input;
+                },
+                z
+                    .object({
+                        name: z.string(),
+                        count: z.number()
+                    })
+                    .transform(data => ({
+                        ...data,
+                        doubled: data.count * 2
+                    }))
+            );
+
+            server.registerTool('complex-transform', { inputSchema: complexSchema }, async args => {
+                return {
+                    content: [{ type: 'text' as const, text: `${args.name}: ${args.count} -> ${args.doubled}` }]
+                };
+            });
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+            await server.connect(serverTransport);
+            await client.connect(clientTransport);
+
+            // Pass count as string, preprocess will convert it
+            const result = await client.callTool({
+                name: 'complex-transform',
+                arguments: { name: 'items', count: '5' }
+            });
+
+            expect(result.content).toEqual([
+                {
+                    type: 'text',
+                    text: 'items: 5 -> 10'
+                }
+            ]);
+        });
+    });
+
     describe('resource()', () => {
         /***
          * Test: Resource Registration with URI and Read Callback

src/server/mcp.ts

@@ -1312,17 +1312,9 @@ const EMPTY_OBJECT_JSON_SCHEMA = {
     properties: {}
 };
 
-// Helper to check if an object is a Zod schema (ZodRawShapeCompat)
-function isZodRawShapeCompat(obj: unknown): obj is ZodRawShapeCompat {
-    if (typeof obj !== 'object' || obj === null) return false;
-
-    const isEmptyObject = Object.keys(obj).length === 0;
-
-    // Check if object is empty or at least one property is a ZodType instance
-    // Note: use heuristic check to avoid instanceof failure across different Zod versions
-    return isEmptyObject || Object.values(obj as object).some(isZodTypeLike);
-}
-
+/**
+ * Checks if a value looks like a Zod schema by checking for parse/safeParse methods.
+ */
 function isZodTypeLike(value: unknown): value is AnySchema {
     return (
         value !== null &&
@@ -1334,6 +1326,46 @@ function isZodTypeLike(value: unknown): value is AnySchema {
     );
 }
 
+/**
+ * Checks if an object is a Zod schema instance (v3 or v4).
+ *
+ * Zod schemas have internal markers:
+ * - v3: `_def` property
+ * - v4: `_zod` property
+ *
+ * This includes transformed schemas like z.preprocess(), z.transform(), z.pipe().
+ */
+function isZodSchemaInstance(obj: object): boolean {
+    return '_def' in obj || '_zod' in obj || isZodTypeLike(obj);
+}
+
+/**
+ * Checks if an object is a "raw shape" - a plain object where values are Zod schemas.
+ *
+ * Raw shapes are used as shorthand: `{ name: z.string() }` instead of `z.object({ name: z.string() })`.
+ *
+ * IMPORTANT: This must NOT match actual Zod schema instances (like z.preprocess, z.pipe),
+ * which have internal properties that could be mistaken for schema values.
+ */
+function isZodRawShapeCompat(obj: unknown): obj is ZodRawShapeCompat {
+    if (typeof obj !== 'object' || obj === null) {
+        return false;
+    }
+
+    // If it's already a Zod schema instance, it's NOT a raw shape
+    if (isZodSchemaInstance(obj)) {
+        return false;
+    }
+
+    // Empty objects are valid raw shapes (tools with no parameters)
+    if (Object.keys(obj).length === 0) {
+        return true;
+    }
+
+    // A raw shape has at least one property that is a Zod schema
+    return Object.values(obj).some(isZodTypeLike);
+}
+
 /**
  * Converts a provided Zod schema to a Zod object if it is a ZodRawShapeCompat,
  * otherwise returns the schema as is.