Timeout errors with Zig Language Server (zls)

[jedisct1]

Thanks for this awesome project. it’s a great idea and looks really promising!

I’ve been trying to use it with the Zig language server, but unfortunately I’m running into an issue. The tool seems to be correctly detected, but all requests consistently time out.

For example, calling diagnostics results in the following error:

Error executing MCP tool: MCP error -32001: Request timed out

This happens every time, without exception.

I also tried launching the tool with --enable-stderr-logs but it didn’t seem to change anything or provide additional output.

Is there anything specific I should configure for compatibility with zls, or is this a known issue?

Thanks in advance!

[isaacphi]

Thanks for the issue!

Which client are you using? Are there any other logs from mcp-language-server?

When I launch Claude Desktop with this configuration:

    "language-server": {
      "command": "mcp-language-server",
      "args": [
        "--workspace",
        "/Users/phil/dev/scratch/zig_sample/",
        "--lsp",
        "zls"
      ],
      "env": {
        "LOG_LEVEL": "DEBUG",
      }
    },

then it does launch, but hangs on any actual tool call. I'm not sure whether or not this is the same issue you're running into.

I see the following logs that point to what might be the problem:

2025/05/09 09:51:12.389547 [ERROR][lsp] Error reading message: failed to unmarshal message: json: cannot unmarshal string into Go struct field Message.id of type int32

or

2025/05/09 09:51:12.356114 [DEBUG][wire] <- Received: {"jsonrpc":"2.0","method":"window/logMessage","params":{"type":1,"message":"zig env failed with error_code: 1"}}

Where to find these logs depends on the client you're using. It seems like most likely we'd have to change the jsonrpc Message type to support strings. Might be an easy fix. zls might also need some additional setup.

I'm not a zig developer so I'm not likely to get to this right away but would certainly welcome any PRs!

[jedisct1]

Hi!

I'm using Roo, but it looks like the issue is exactly the same as the one you describe.

[adrichman]

@jedisct1
With Roo I got the MCP to connect with a configuration like this one:

# in {PROJECT_ROOT}/.roo/mcp.json
{
  "mcpServers": {
    "pyright": {
      "env": {},
      "command": "/FULL/PATH/TO/go/bin/mcp-language-server",
      "args": [
        "--workspace",
        "/FULL/PATH/TO/PYTHON/PROJECT/ROOT",
        "--lsp",
        "/FULL/PATH/TO/LANGUAGE/SERVER/BIN/node_modules/.bin/pyright-langserver",
        "--",
        "--stdio" # DON'T FORGET THIS
      ],
      "auto_start": false,
      "disabled": false,
      "alwaysAllow": []
    }
}

[isaacphi]

@jedisct1 Ok, I made some changes to properly support string IDs which means that zls can now properly connect (#31). The tools hover and diagnostics are working for me now with zls!

However, it looks like zls does not yet support the workspace/symbol method which I'm using for definition and references so those won't work: zigtools/zls#397