z.preprocess() causes "Mixed Zod versions detected" error #1190
Description
When using Zod v4's z.preprocess() with McpServer.registerTool(), the SDK throws:
Error: Mixed Zod versions detected in object shape.
This happens even when only a single Zod version is installed.
To Reproduce
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { z } from "zod";
const server = new McpServer({
name: "test-server",
version: "1.0.0",
});
// This works fine - regular z.object schema
const WorkingSchema = z.object({
name: z.string(),
});
// This fails - z.preprocess schema
const BrokenSchema = z.preprocess(
(input) => input,
z.object({
name: z.string(),
})
);
console.log("Registering tool with z.object schema...");
server.registerTool(
"working-tool",
{
description: "A tool with a regular z.object schema",
inputSchema: WorkingSchema,
},
async (args) => ({ content: [{ type: "text", text: "OK" }] })
);
console.log("Success: z.object schema works\n");
// Register a tool with the broken schema - this throws
console.log("Registering tool with z.preprocess schema...");
try {
server.registerTool(
"broken-tool",
{
description: "A tool with a z.preprocess schema",
inputSchema: BrokenSchema,
},
async (args) => ({ content: [{ type: "text", text: "OK" }] })
);
console.log("Success: z.preprocess schema works");
} catch (error) {
console.log("Error:", error.message);
}
Output from the above example:
Registering tool with z.object schema...
Success: z.object schema works
Registering tool with z.preprocess schema...
Error: Mixed Zod versions detected in object shape.
Expected behavior
Tools with z.preprocess() schemas should register successfully, just like tools with z.object() schemas.
Logs
If applicable, add logs to help explain your problem.
Additional context
- @modelcontextprotocol/sdk: 1.23.0
- zod: 4.1.x
- Node.js: 24.5