fix: handle empty path array in getParseErrorMessage

Zod sets path: [] for root-level errors; ![] is false so the early-return
never fired. Match main's standardSchema.ts:167 approach with ?.length.
Also add changeset.

Author: felixweinberger

.changeset/fix-zod-error-message-priority.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/sdk': patch
+---
+
+Prioritize `error.issues[].message` over `error.message` in `getParseErrorMessage` so custom Zod error messages surface correctly. In Zod v4, `error.message` is a JSON blob of all issues, not a readable string.

src/server/zod-compat.ts

@@ -214,7 +214,7 @@ export function getParseErrorMessage(error: unknown): string {
         if ('issues' in error && Array.isArray(error.issues) && error.issues.length > 0) {
             return error.issues
                 .map((i: { message: string; path?: (string | number)[] }) => {
-                    if (!i.path) {
+                    if (!i.path?.length) {
                         return i.message;
                     }
                     return `${i.message} at ${getDotPath(i.path)}`;