diff --git a/src/server/mcp.ts b/src/server/mcp.ts
index dccfa04e..a8e7e5f8 100644
--- a/src/server/mcp.ts
+++ b/src/server/mcp.ts
@@ -660,6 +660,20 @@ export class McpServer {
       throw new Error(`Tool ${name} is already registered`);
     }
     
+    // Runtime type guard for Zod types that works across bundles/realms
+    const isZodType = (value: unknown): value is ZodTypeAny => {
+      if (typeof value !== "object" || value === null) return false;
+
+      if (!("_def" in value)) return false;
+
+      const def = (value as { _def: unknown })._def;
+      if (typeof def !== "object" || def === null) return false;
+
+      if (!("typeName" in def)) return false;
+
+      return typeof (def as { typeName: unknown }).typeName === "string";
+    };
+
     // Helper to check if an object is a Zod schema (ZodRawShape)
     const isZodRawShape = (obj: unknown): obj is ZodRawShape => {
       if (typeof obj !== "object" || obj === null) return false;
@@ -667,7 +681,9 @@ export class McpServer {
       const isEmptyObject = z.object({}).strict().safeParse(obj).success;
 
       // Check if object is empty or at least one property is a ZodType instance
-      return isEmptyObject || Object.values(obj as object).some(v => v instanceof ZodType);
+      return (
+        isEmptyObject || Object.values(obj as object).some((v) => isZodType(v))
+      );
     };
 
     let description: string | undefined;