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Additional parameters are not passed through to tools #147
Description
Describe the bug
When you pass properties in the arguments of a tools/call message that are not defined in the schema, these are silently dropped from the args object.
To Reproduce
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { z } from "zod";
const echoServer = new McpServer({
name: "Echo",
version: "1.0.0",
});
const schema = {
message: z.string().describe("the message to echo"),
};
echoServer.tool("echo", "Echos the input message back", schema, async (args) => {
console.log({ args });
return { content: [{ type: "text", text: `Echo: ${props.message}` }] };
});Call the echo tool with e.g.
{
"method": "tools/call",
"params": {
"name": "echo",
"arguments": {
"message": "bar"
"other": "foo"
},
"_meta": {
"progressToken": 0
}
}
}
args will be { message: "bar" }
Expected behavior
It should be possible to pass additional properties to the tool.
This is the JSON Schema generated on the server, by default additionalProperties is set to true.
{
"tools": [
{
"name": "echo",
"description": "Echos the input message back",
"inputSchema": {
"type": "object",
"properties": {
"message": {
"type": "string",
"description": "the message to echo"
}
},
"required": [
"message"
],
"additionalProperties": true,
"$schema": "http://json-schema.org/draft-07/schema#"
}
}
]
}
Activity
uncomplexity commentedon Mar 11, 2025
Also how do we provide description to each tool?
felixweinberger commentedon Oct 24, 2025
Hi @au-re thanks for this report, apologies for the time it took to get back to this - is this still an issue for you?