fix: support Zod runtimes that store literal values in _def.values[0]

packages/core/src/types/types.ts

@@ -2707,10 +2707,26 @@ export function getResultSchema<M extends RequestMethod>(method: M): z.ZodType<R
 type RequestSchemaType = (typeof ClientRequestSchema.options)[number] | (typeof ServerRequestSchema.options)[number];
 type NotificationSchemaType = (typeof ClientNotificationSchema.options)[number] | (typeof ServerNotificationSchema.options)[number];
 
-function buildSchemaMap<T extends { shape: { method: { value: string } } }>(schemas: readonly T[]): Record<string, T> {
+type MethodLiteralSchema = {
+    value?: unknown;
+    _def?: {
+        value?: unknown;
+        values?: readonly unknown[];
+    };
+};
+
+function getMethodLiteral(methodSchema: MethodLiteralSchema): string {
+    const method = methodSchema.value ?? methodSchema._def?.value ?? methodSchema._def?.values?.[0];
+    if (typeof method !== 'string') {
+        throw new TypeError('Schema method literal must be a string');
+    }
+    return method;
+}
+
+function buildSchemaMap<T extends { shape: { method: MethodLiteralSchema } }>(schemas: readonly T[]): Record<string, T> {
     const map: Record<string, T> = {};
     for (const schema of schemas) {
-        const method = schema.shape.method.value;
+        const method = getMethodLiteral(schema.shape.method);
         map[method] = schema;
     }
     return map;

test/integration/test/issues/test1380.zod.literal-values.test.ts

@@ -0,0 +1,62 @@
+/**
+ * Regression test for https://github.com/modelcontextprotocol/typescript-sdk/issues/1380
+ *
+ * Some Zod runtimes store literal values only in `._def.values[0]`, with both
+ * the top-level `.value` shortcut and `._def.value` absent. This test verifies
+ * McpServer initialization still succeeds in that scenario.
+ */
+
+import { afterEach, describe, expect, test, vi } from 'vitest';
+
+describe('Issue #1380: Zod literal method extraction', () => {
+    afterEach(() => {
+        vi.resetModules();
+        vi.doUnmock('zod/v4');
+    });
+
+    test('should construct McpServer when method literal is stored only in _def.values[0]', async () => {
+        vi.resetModules();
+        vi.doMock('zod/v4', async () => {
+            const actual = await vi.importActual<typeof import('zod/v4')>('zod/v4');
+
+            return {
+                ...actual,
+                literal: ((...args: Parameters<typeof actual.literal>) => {
+                    const schema = actual.literal(...args);
+
+                    // Simulate a Zod runtime that stores the literal value only in
+                    // _def.values[0], with neither the top-level .value shortcut
+                    // nor _def.value present.
+                    //
+                    // We wrap the schema in a plain object that shadows .value with
+                    // undefined and exposes a new _def that has only .values (no .value).
+                    const originalDef = (schema as Record<string, unknown>)._def as Record<string, unknown>;
+                    const value = originalDef.value ?? originalDef.values?.[0] ?? (schema as Record<string, unknown>).value;
+
+                    const strippedDef: Record<string, unknown> = {};
+                    for (const [k, v] of Object.entries(originalDef)) {
+                        if (k !== 'value') strippedDef[k] = v;
+                    }
+                    if (!strippedDef.values) {
+                        strippedDef.values = [value];
+                    }
+
+                    return Object.create(schema as object, {
+                        value: { get: () => void 0, enumerable: true, configurable: true },
+                        _def: { get: () => strippedDef, enumerable: true, configurable: true }
+                    }) as typeof schema;
+                }) as typeof actual.literal
+            };
+        });
+
+        const { McpServer } = await import('@modelcontextprotocol/server');
+
+        expect(
+            () =>
+                new McpServer({
+                    name: 'test server',
+                    version: '1.0'
+                })
+        ).not.toThrow();
+    });
+});