Request for a "Hello, World!" RPC request

[ColinKennedy]

As a proof of concept I'd be looking to send bytes to stdin of an existing MCP server just to get something started but it's been a bit difficult to find a "this is an example of a raw message that you can send and you will get a response back".

For example with LSP a message could be echo -e "Content-Length: 12345\r\n\r\n...text here with stuff" | some_lsp_executable and as long as the message header is right + the server always posts a message back, you'll get a message back. I'm trying to find the MCP equivalent.

If echo is a but too raw it's fine if it's a blob of text. At this point I'm just trying to get 2 examples

1. A minimum message (even if it would just result in an error)
2. A "Hello, World!" InitializeRequest that can be sent from a client to some server, to start.

I've looked around at the spec, the python-sdk, and also Zed. The code in all of those places is really well abstracted which is great for getting stuff done but for learning "okay at the end of all this abstraction, X message gets generated" - it hasn't been good reference for that so far.

My context is I'm interested in writing a Lua client for Neovim and want to get an idea of what the on-ramping process would look like. Thank you!

[ezynda3]

Try checking the spec docs. All messages are JSON RPC and they depend of the transport being used. https://spec.modelcontextprotocol.io/specification/basic/utilities/ping

Most of the official servers all run on stdio right now meaning you can only send messages via stdin/stdout through a terminal session.

[ColinKennedy]

I read through the page and others but still am not much closer to knowing what the real input should be. The docs show the payload, yes, but not what would need to be sent via stdin.

Here's a real example. Using lua-language-server I can send

echo -e 'Content-Length: 433\r\n\r\n{"jsonrpc": "2.0", "id": 1, "method": "initialize", "params": {"processId": 12345, "rootUri": "file:///path/to/your/project", "capabilities": {"textDocument": {"completion": {"dynamicRegistration": true}, "hover": {"dynamicRegistration": true}, "signatureHelp": {"dynamicRegistration": true}}, "workspace": {"workspaceFolders": {"supported": true}}}, "workspaceFolders": [{"uri": "file:///path/to/your/project", "name": "project"}]}}' | lua-language-server

And it will return with

Content-Length: 239

{"jsonrpc":"2.0","method":"window/logMessage","params":{"message":"Log path: file:///home/selecaoone/repositories/personal/.config/nvim/mason_packages/Linux/packages/lua-language-server/libexec/log/file_path_to_your_project.log","type":4}}`

And I can send more messages to check the handshake between the client's capabilities and server's capabilities just from the terminal. How do I do this with MCP?

I was curious what MCP's first message from the client to a server might be so I downloaded the Python SDK and then fed it a fake mcp_server.sh file to try to peek at what the client sends, at least for the first message. But the output was not helpful.

mcp_server.sh

#!/usr/bin/env sh

# Read input line by line from stdin and print it
while IFS= read -r line; do
    echo "$line"
done

python -m mcp.client ~/temp/mcp_server.sh

Unfortunately the below is just an error message, nothing resembling a message that a client would send to a server.

Full Logs

INFO:client:Initializing session
INFO:client:Starting receive loop
  + Exception Group Traceback (most recent call last):
  |   File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
  |     return _run_code(code, main_globals, None,
  |   File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
  |     exec(code, run_globals)
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/mcp/client/__main__.py", line 76, in <module>
  |     cli()
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/click/core.py", line 1157, in __call__
  |     return self.main(*args, **kwargs)
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/click/core.py", line 1078, in main
  |     rv = self.invoke(ctx)
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/click/core.py", line 1434, in invoke
  |     return ctx.invoke(self.callback, **ctx.params)
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/click/core.py", line 783, in invoke
  |     return __callback(*args, **kwargs)
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/mcp/client/__main__.py", line 72, in cli
  |     anyio.run(partial(main, *args, **kwargs), backend="trio")
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/anyio/_core/_eventloop.py", line 74, in run
  |     return async_backend.run(func, args, {}, backend_options)
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/anyio/_backends/_trio.py", line 996, in run
  |     return trio.run(func, *args)
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/trio/_core/_run.py", line 2407, in run
  |     raise runner.main_task_outcome.error
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/mcp/client/__main__.py", line 56, in main
  |     async with stdio_client(server_parameters) as streams:
  |   File "/usr/lib/python3.10/contextlib.py", line 217, in __aexit__
  |     await self.gen.athrow(typ, value, traceback)
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/mcp/client/stdio.py", line 122, in stdio_client
  |     async with (
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/anyio/_backends/_trio.py", line 187, in __aexit__
  |     return await self._nursery_manager.__aexit__(exc_type, exc_val, exc_tb)
  |   File "/home/selecaoone/.local/lib/python3.10/site-packages/trio/_core/_run.py", line 1039, in __aexit__
  |     raise combined_error_from_nursery
  | exceptiongroup.ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  +-+---------------- 1 ----------------
    | Exception Group Traceback (most recent call last):
    |   File "/home/selecaoone/.local/lib/python3.10/site-packages/mcp/client/stdio.py", line 128, in stdio_client
    |     yield read_stream, write_stream
    |   File "/home/selecaoone/.local/lib/python3.10/site-packages/mcp/client/__main__.py", line 57, in main
    |     await run_session(*streams)
    |   File "/home/selecaoone/.local/lib/python3.10/site-packages/mcp/client/__main__.py", line 33, in run_session
    |     async with (
    |   File "/home/selecaoone/.local/lib/python3.10/site-packages/mcp/shared/session.py", line 122, in __aexit__
    |     return await self._task_group.__aexit__(exc_type, exc_val, exc_tb)
    |   File "/home/selecaoone/.local/lib/python3.10/site-packages/anyio/_backends/_trio.py", line 187, in __aexit__
    |     return await self._nursery_manager.__aexit__(exc_type, exc_val, exc_tb)
    |   File "/home/selecaoone/.local/lib/python3.10/site-packages/trio/_core/_run.py", line 1039, in __aexit__
    |     raise combined_error_from_nursery
    | exceptiongroup.ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
    +-+---------------- 1 ----------------
      | Traceback (most recent call last):
      |   File "/home/selecaoone/.local/lib/python3.10/site-packages/mcp/shared/session.py", line 217, in _receive_loop
      |     validated_request = self._receive_request_type.model_validate(
      |   File "/home/selecaoone/.local/lib/python3.10/site-packages/pydantic/main.py", line 568, in model_validate
      |     return cls.__pydantic_validator__.validate_python(
      | pydantic_core._pydantic_core.ValidationError: 5 validation errors for ServerRequest
      | PingRequest.method
      |   Input should be 'ping' [type=literal_error, input_value='initialize', input_type=str]
      |     For further information visit https://errors.pydantic.dev/2.8/v/literal_error
      | CreateMessageRequest.method
      |   Input should be 'sampling/createMessage' [type=literal_error, input_value='initialize', input_type=str]
      |     For further information visit https://errors.pydantic.dev/2.8/v/literal_error
      | CreateMessageRequest.params.messages
      |   Field required [type=missing, input_value={'protocolVersion': '2024...p', 'version': '0.1.0'}}, input_type=dict]
      |     For further information visit https://errors.pydantic.dev/2.8/v/missing
      | CreateMessageRequest.params.maxTokens
      |   Field required [type=missing, input_value={'protocolVersion': '2024...p', 'version': '0.1.0'}}, input_type=dict]
      |     For further information visit https://errors.pydantic.dev/2.8/v/missing
      | ListRootsRequest.method
      |   Input should be 'roots/list' [type=literal_error, input_value='initialize', input_type=str]
      |     For further information visit https://errors.pydantic.dev/2.8/v/literal_error
      +------------------------------------

What is MCP's equivalent of an initialization request?

[dsp-ant]

My go to way is

{"jsonrpc": "2.0", "id": 1, "method": "initialize", "params": {"protocolVersion": "2024-11-05", "capabilities": {}, "clientInfo": {"name": "test-client", "version": "0.1.0"}}}
{"jsonrpc": "2.0", "method": "notifications/initialized"}
{"jsonrpc": "2.0", "id": 2, "method": "prompts/list"}

Can just be provided line by line when running npx @modelcontextprotocol/server-everything

[raiki61]

echo '{"jsonrpc": "2.0", "id": 2, "method": "tools/call", "params": {"name": "list_allowed_directories"}}' | node src/filesystem/dist/index.js [your dir]

This works for me.