Mock definition

This document lists the fields of the definition file that mcpeak-mock reads. The matching rules and the design first workflow are covered in mock server.

{
  "tools": [
    {
      "name": "get_weather",
      "description": "도시의 현재 날씨",
      "inputSchema": {
        "type": "object",
        "properties": { "city": { "type": "string" } },
        "required": ["city"]
      }
    }
  ],
  "responses": [
    { "tool": "get_weather", "args": { "city": "서울" }, "result": { "tempC": 21 } },
    { "tool": "get_weather", "args": { "city": "없는도시" },
      "result": "→ '없는도시' 는 모르는 도시입니다", "isError": true },
    { "tool": "get_weather", "result": { "tempC": 0 } }
  ]
}

tools[]

The tools advertised through tools/list. A tool not listed here cannot be used in responses either.

Field	Type	Required	Meaning
name	string	yes	The tool name
description	string	no	The description a client reads when choosing the tool
inputSchema	JSON Schema	yes	The argument schema. Advertised as is and also used to check arguments

inputSchema is emitted exactly as given, as JSON Schema. There is no Zod conversion.

responses[]

Field	Type	Required	Meaning
tool	string	yes	A name present in tools. If not, the definition is rejected when it is read
args	JSON object	no	The response to give when called with these arguments. If omitted, any arguments that pass the schema are accepted
result	JSON value	yes	The response payload. The mock wraps it as content: [{ type: "text", … }]
isError	boolean	no	If true, this response is a rejection. Omitting it is the same as false

responses itself can be omitted. Every call then falls through as a match miss, and an isError: true response is returned stating what is registered.

When the definition is malformed

→ 올바르지 않은 목 정의입니다 — weather.mock.json: responses[0] 의 툴 '없는툴' 이 tools 에 없습니다. 있는 툴: get_weather, add
→ 형식: { "tools": [ { "name": ..., "inputSchema": ... } ], "responses": [ { "tool": ..., "result": ... } ] }

What is checked:

• tools is an array, and each entry has a string name and an inputSchema.
• If responses is present, it is an array, each entry has a string tool and a result, and isError, if present, is a boolean.
• responses[].tool is present in tools.

Arguments that cannot be turned into a key

The following args values are rejected when the definition is read. Since an MCP call arrives as JSON, none of these values can ever be reached by any call, and leaving them in would make the injection look like it succeeded while it never matches.

Value	Example
Circular reference	o.self = o
Sparse array	[1, , 3]
NaN / Infinity	{ n: NaN }
Non JSON value	Date, function, symbol, BigInt, Map

The nesting depth limit is 512. If a call's arguments exceed it, the server reports isError: true instead of being killed.

Validating in code

import { assertMockDefinition } from "@mcpeak/mock";

assertMockDefinition(JSON.parse(text), "weather.mock.json");

The second argument is the source name used in the error message.