Additional parameters are not passed through to tools

[au-re]

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#"
      }
    }
  ]
}

[uncomplexity]

Also how do we provide description to each tool?

server.tool("add",
  { a: z.number(), b: z.number() },
  async ({ a, b }) => ({
    content: [{ type: "text", text: String(a + b) }]
  })
);

[felixweinberger]

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?

[mcp-claude]

bug is present on main (v2 alpha), not on v1.x. zod v4's ~standard.jsonSchema omits additionalProperties for default strip-mode objects, so the advertised inputSchema implicitly allows extras while ~standard.validate silently drops them.

workaround: use z.object({...}).strict() if you want validation to reject unknowns (consistent), or .passthrough() if you want them preserved (also consistent — passthrough emits "additionalProperties": {} and the validate result keeps the extra keys).

repro script (test/integration/repro.ts)

import { Client } from "@modelcontextprotocol/client";
import { InMemoryTransport } from "@modelcontextprotocol/core";
import { McpServer } from "@modelcontextprotocol/server";
import * as z from "zod/v4";

const schema = z.object({
  message: z.string().describe("the message to echo"),
});

const server = new McpServer({ name: "Echo", version: "1.0.0" });
let receivedArgs: unknown = null;

server.registerTool("echo", { description: "echo", inputSchema: schema }, async (args) => {
  receivedArgs = args;
  return { content: [{ type: "text" as const, text: `Echo: ${(args as any).message}` }] };
});

const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
const client = new Client({ name: "test-client", version: "1.0.0" });
await Promise.all([server.connect(serverTransport), client.connect(clientTransport)]);

const toolsList = await client.listTools();
console.log("inputSchema:", JSON.stringify(toolsList.tools[0]?.inputSchema));
// → no additionalProperties key (defaults to true = allowed)

await client.callTool({ name: "echo", arguments: { message: "bar", other: "foo" } });
console.log("received:", JSON.stringify(receivedArgs));
// → {"message":"bar"}  — "other" is gone

command + output

$ cd test/integration && npx tsx repro.ts
Advertised inputSchema:
{
  "type": "object",
  "properties": { "message": { "type": "string", "description": "the message to echo" } },
  "required": ["message"],
  "$schema": "https://json-schema.org/draft/2020-12/schema"
}

additionalProperties in schema: undefined
Tool received args: {"message":"bar"}
...
FAIL: extra property 'other' was dropped (got: undefined)
Bug confirmed: schema advertises additionalProperties: undefined but Zod strips unknowns

code path

1. McpServer.validateToolInput → validateStandardSchema(tool.inputSchema, args) — mcp.ts:252
2. validateStandardSchema calls schema['~standard'].validate(data) — standardSchema.ts:176
3. Zod v4's default z.object strips unknown keys; result is {message:"bar"} with other gone
4. standardSchemaToJsonSchema calls schema['~standard'].jsonSchema.input({target:'draft-2020-12'}) — standardSchema.ts:152; Zod returns schema without additionalProperties, leaving the default (true = allowed) in place

on v1.x: zodToJsonSchema already emits "additionalProperties": false so the behavior there is consistent (schema and runtime both say no extras); the mismatch is main-only.

suggested fix

// packages/core/src/util/standardSchema.ts
-    return { type: 'object', ...result };
+    const merged = { type: 'object', ...result };
+    if (!('additionalProperties' in merged) && !('unevaluatedProperties' in merged)) {
+        merged.additionalProperties = false;
+    }
+    return merged;

test to verify: assert that standardSchemaToJsonSchema(z.object({ message: z.string() }), 'input').additionalProperties === false (previously undefined), while .passthrough() variant does not set it to false.