Fix Zod v4 schema description extraction (#1296)

test/issues/test_1277_zod_v4_description.test.ts

@@ -0,0 +1,65 @@
+/**
+ * Regression test for https://github.com/modelcontextprotocol/typescript-sdk/issues/1277
+ *
+ * Zod v4 stores `.describe()` descriptions directly on the schema object,
+ * not in `._zod.def.description`. This test verifies that descriptions are
+ * correctly extracted for prompt arguments.
+ */
+
+import { Client } from '../../src/client/index.js';
+import { InMemoryTransport } from '../../src/inMemory.js';
+import { ListPromptsResultSchema } from '../../src/types.js';
+import { McpServer } from '../../src/server/mcp.js';
+import { zodTestMatrix, type ZodMatrixEntry } from '../../src/__fixtures__/zodTestMatrix.js';
+
+describe.each(zodTestMatrix)('Issue #1277: $zodVersionLabel', (entry: ZodMatrixEntry) => {
+    const { z } = entry;
+
+    test('should preserve argument descriptions from .describe()', async () => {
+        const mcpServer = new McpServer({
+            name: 'test server',
+            version: '1.0'
+        });
+        const client = new Client({
+            name: 'test client',
+            version: '1.0'
+        });
+
+        mcpServer.prompt(
+            'test',
+            {
+                name: z.string().describe('The user name'),
+                value: z.string().describe('The value to set')
+            },
+            async ({ name, value }) => ({
+                messages: [
+                    {
+                        role: 'assistant',
+                        content: {
+                            type: 'text',
+                            text: `${name}: ${value}`
+                        }
+                    }
+                ]
+            })
+        );
+
+        const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+
+        await Promise.all([client.connect(clientTransport), mcpServer.server.connect(serverTransport)]);
+
+        const result = await client.request(
+            {
+                method: 'prompts/list'
+            },
+            ListPromptsResultSchema
+        );
+
+        expect(result.prompts).toHaveLength(1);
+        expect(result.prompts[0].name).toBe('test');
+        expect(result.prompts[0].arguments).toEqual([
+            { name: 'name', required: true, description: 'The user name' },
+            { name: 'value', required: true, description: 'The value to set' }
+        ]);
+    });
+});