fix: use getLiteralValue for method literal extraction in setRequestHandler

Replace inline Zod v3/v4 literal extraction in Server.setRequestHandler
and Client.setRequestHandler with the existing getLiteralValue() helper
from zod-compat.ts, which already handles the _def.values[0] fallback
for late Zod v3 releases (e.g., zod@3.25.1) where the literal value is
stored under _def.values instead of _def.value.

This fixes "Schema method literal must be a string" errors when
constructing McpServer with zod@3.25.x.

Fixes #1380

Author: JosephDoUrden

src/client/index.ts

@@ -56,11 +56,9 @@ import type { JsonSchemaType, JsonSchemaValidator, jsonSchemaValidator } from '.
 import {
     AnyObjectSchema,
     SchemaOutput,
+    getLiteralValue,
     getObjectShape,
-    isZ4Schema,
     safeParse,
-    type ZodV3Internal,
-    type ZodV4Internal
 } from '../server/zod-compat.js';
 import type { RequestHandlerExtra } from '../shared/protocol.js';
 import { ExperimentalClientTasks } from '../experimental/tasks/client.js';
@@ -339,18 +337,7 @@ export class Client<
             throw new Error('Schema is missing a method literal');
         }
 
-        // Extract literal value using type-safe property access
-        let methodValue: unknown;
-        if (isZ4Schema(methodSchema)) {
-            const v4Schema = methodSchema as unknown as ZodV4Internal;
-            const v4Def = v4Schema._zod?.def;
-            methodValue = v4Def?.value ?? v4Schema.value;
-        } else {
-            const v3Schema = methodSchema as unknown as ZodV3Internal;
-            const legacyDef = v3Schema._def;
-            methodValue = legacyDef?.value ?? v3Schema.value;
-        }
-
+        const methodValue = getLiteralValue(methodSchema);
         if (typeof methodValue !== 'string') {
             throw new Error('Schema method literal must be a string');
         }

src/server/index.ts

@@ -46,12 +46,10 @@ import { AjvJsonSchemaValidator } from '../validation/ajv-provider.js';
 import type { JsonSchemaType, jsonSchemaValidator } from '../validation/types.js';
 import {
     AnyObjectSchema,
+    getLiteralValue,
     getObjectShape,
-    isZ4Schema,
     safeParse,
     SchemaOutput,
-    type ZodV3Internal,
-    type ZodV4Internal
 } from './zod-compat.js';
 import { RequestHandlerExtra } from '../shared/protocol.js';
 import { ExperimentalServerTasks } from '../experimental/tasks/server.js';
@@ -228,18 +226,7 @@ export class Server<
             throw new Error('Schema is missing a method literal');
         }
 
-        // Extract literal value using type-safe property access
-        let methodValue: unknown;
-        if (isZ4Schema(methodSchema)) {
-            const v4Schema = methodSchema as unknown as ZodV4Internal;
-            const v4Def = v4Schema._zod?.def;
-            methodValue = v4Def?.value ?? v4Schema.value;
-        } else {
-            const v3Schema = methodSchema as unknown as ZodV3Internal;
-            const legacyDef = v3Schema._def;
-            methodValue = legacyDef?.value ?? v3Schema.value;
-        }
-
+        const methodValue = getLiteralValue(methodSchema);
         if (typeof methodValue !== 'string') {
             throw new Error('Schema method literal must be a string');
         }