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ListTools request handler fails to generate inputSchema (jsonSchema) #1028
Description
Describe the bug
When a tool - either with or without an input schema - is registered onto the mcp server, it gets registered correctly with its input schema being a ZodObject ({type: 'object', shape: <accessor>}), the server starts and the MCP inspector is able to connect to it.
When a list tools request is sent from the mcp inspector, all toolDefinitions are generated, converting the existing tool inputSchema (Zod) into a jsonSchema. However, despite the shape and type of the zod input schema, the zodToJsonSchema function always returns {type: "string", $schema: "http://json-schema.org/draft-07/schema#"}. This turns into an error on the MCP inspector, being unable to list tools, thus unable to execute any of them.
To Reproduce
Steps to reproduce the behavior:
- Create an instance of McpServer:
server = new McpServer({...serverInfo}, {
capabilities,
instructions,
});
- Register a tool with an inputSchema:
server.registerTool(
name,
{
title: name,
description: description,
inputSchema: z.object({...schema}).shape,
},
handler,
);
- Start MCP inspector
- Connect to MCP server from inspector
- Try to list tools
- Get the error:
Uncaught (in promise) ZodError: [
{
"received": "string",
"code": "invalid_literal",
"expected": "object",
"path": [
"tools",
0,
"inputSchema",
"type"
],
"message": "Invalid literal value, expected \"object\""
}
]
Expected behavior
The input Schema of the tool should be converted into a valid, matching jsonSchema.
Additional Context
Debugging the application, I could see that the zodToJsonSchema function calls the parseDef function with the zod definition. When the jsonSchemaOrGetterconstant gets calculated through the selectParser function, it passes def, def.typeName and refs. The second argument is undefined. Then teh selectParser function switches typeName on all possible ZodFirstPartyTypeKind.<ZodType>. However, ZodFirstPartyTypeKind is {}, so the first one matches, which is the string matcher.
I have @modelcontextprotocol/sdk@1.20.0 dependency installed.
Activity
juanlluvallord commentedon Oct 16, 2025
This seemed to be an incompatibility with the installed Zod version (zod4). Sorry for the inconvenience 😅
timurguvenkaya commentedon Oct 16, 2025
Same issue here. Also doesn't work with import { z } from 'zod/v3';
felixweinberger commentedon Oct 17, 2025
Related: #869
Currently trying to figure out the path to supporting zod v4
max-rousseau commentedon Nov 24, 2025
Anyone seen any movement on this issue or is it tied to the 'v2' work now? As far as I can tell, this makes the tools, like filesystem, unusable.
domdomegg commentedon Nov 25, 2025
I think underlying this is #555. I think the solution might be to use https://github.com/modelcontextprotocol/typescript-sdk/releases/tag/1.23.0-beta.0 which has backwards-compatible support for Zod 4.
RahulLanjewar93 commentedon Dec 2, 2025
Same issue, in mcp insepctor i get the same error as mentioned in the issue description
However i am also getting a typescript error when i use the code mentioned in example.
Attaching image below, using zod@4.1.13 and @modelcontextprotocol/sdk@1.23.0
The error mentioned in the issue is fixed by using the below code. But typescript error is still there.
If anyone has insights on how to fix this, please let me know. Thanks in advance :)
Fix Zod version: downgrade to v3 for zod-to-json-schema compatibility
felixweinberger commentedon Oct 5, 2026
Fixed by #1040, released in 1.23.0. v2 takes schemas through Standard Schema and needs zod 4.2 or newer. Closing.