Mock STDIO responses for MCP servers

Modern AI development increasingly relies on local tools that communicate via STDIO, particularly Model Context Protocol (MCP) servers. These servers receive JSON-RPC requests via stdin and send JSON-RPC responses via stdout. Using Dev Proxy, you can intercept and mock STDIO communication to test your AI client applications without running actual server logic.

Prerequisites

• Dev Proxy installed and running

Mock MCP server responses

To mock MCP server responses, you use the stdio command with the MockStdioResponsePlugin. The plugin intercepts stdin and returns mock responses via stdout or stderr.

1. Create a Dev Proxy configuration file

Create a devproxyrc.json file with the following content:

{
  "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v2.1.0/rc.schema.json",
  "plugins": [
    {
      "name": "MockSTDIOResponsePlugin",
      "enabled": true,
      "pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll",
      "configSection": "mockSTDIOResponsePlugin"
    },
    {
      "name": "DevToolsPlugin",
      "enabled": true,
      "pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll",
      "configSection": "devTools"
    }
  ],
  "devTools": {
    "preferredBrowser": "Edge"
  },
  "mockStdioResponsePlugin": {
    "mocksFile": "stdio-mocks.json"
  }
}

2. Create a mocks file

Create a stdio-mocks.json file with mock responses for the MCP server:

{
  "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v2.1.0/mockstdioresponseplugin.schema.json",
  "mocks": [
    {
      "request": {
        "bodyFragment": "initialize"
      },
      "response": {
        "stdout": "{\"jsonrpc\":\"2.0\",\"id\":@stdin.body.id,\"result\":{\"protocolVersion\":\"2024-11-05\",\"capabilities\":{\"tools\":{}},\"serverInfo\":{\"name\":\"Mock MCP Server\",\"version\":\"1.0.0\"}}}\n"
      }
    },
    {
      "request": {
        "bodyFragment": "tools/list"
      },
      "response": {
        "stdout": "{\"jsonrpc\":\"2.0\",\"id\":@stdin.body.id,\"result\":{\"tools\":[{\"name\":\"get_weather\",\"description\":\"Get current weather for a location\",\"inputSchema\":{\"type\":\"object\",\"properties\":{\"location\":{\"type\":\"string\",\"description\":\"City name\"}},\"required\":[\"location\"]}}]}}\n"
      }
    },
    {
      "request": {
        "bodyFragment": "tools/call"
      },
      "response": {
        "stdout": "{\"jsonrpc\":\"2.0\",\"id\":@stdin.body.id,\"result\":{\"content\":[{\"type\":\"text\",\"text\":\"Mock response from the tool\"}]}}\n"
      }
    }
  ]
}

3. Start Dev Proxy

Run Dev Proxy with the STDIO command, specifying the command to run:

devproxy stdio npx -y @modelcontextprotocol/server-filesystem

Dev Proxy starts the MCP server as a child process and intercepts all stdin/stdout communication. When stdin contains text matching a mock's bodyFragment, Dev Proxy returns the mock response instead of forwarding the request to the actual server.

Use placeholders for dynamic responses

To create dynamic responses that include values from the request, use @stdin.body.* placeholders:

{
  "mocks": [
    {
      "request": {
        "bodyFragment": "echo"
      },
      "response": {
        "stdout": "{\"jsonrpc\":\"2.0\",\"id\":@stdin.body.id,\"result\":{\"message\":\"You said: @stdin.body.params.text\"}}\n"
      }
    }
  ]
}

Available placeholders:

Placeholder	Description
@stdin.body.id	JSON-RPC request ID
@stdin.body.method	JSON-RPC method name
@stdin.body.params.*	Access to request parameters

Block unmocked requests

To prevent unmocked requests from reaching the child process, set blockUnmockedRequests to true:

{
  "mockStdioResponsePlugin": {
    "mocksFile": "stdio-mocks.json",
    "blockUnmockedRequests": true
  }
}

Blocking unmocked requests is useful when you want to fully mock the MCP server without running its actual logic.

Load mock responses from files

For complex responses, load the content from external files:

{
  "mocks": [
    {
      "request": {
        "bodyFragment": "initialize"
      },
      "response": {
        "stdout": "@initialize-response.json"
      }
    }
  ]
}

Create the initialize-response.json file:

{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2024-11-05","capabilities":{"tools":{}},"serverInfo":{"name":"Mock MCP Server","version":"1.0.0"}}}

Inspect STDIO traffic in DevTools

When you enable the DevToolsPlugin, Dev Proxy opens Chrome DevTools where you can inspect all STDIO communication:

• Network tab: View all stdin/stdout/stderr messages
• URLs: Messages appear as stdio://command-name
• Methods: Requests show as stdin
• Status codes: stdout appears as 200, stderr as 500
• Timing: See how long each request/response took

Using the DevToolsPlugin is invaluable for debugging communication issues between your AI client and MCP servers.

Simulate latency

To test how your application handles slow MCP server responses, add the LatencyPlugin:

{
  "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v2.1.0/rc.schema.json",
  "plugins": [
    {
      "name": "LatencyPlugin",
      "enabled": true,
      "pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll",
      "configSection": "latencyPlugin"
    },
    {
      "name": "MockStdioResponsePlugin",
      "enabled": true,
      "pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll",
      "configSection": "mockStdioResponsePlugin"
    }
  ],
  "latencyPlugin": {
    "minMs": 100,
    "maxMs": 500
  },
  "mockStdioResponsePlugin": {
    "mocksFile": "stdio-mocks.json"
  }
}

Next step

Learn more about the STDIO proxy feature:

stdio command reference