x/tools/gopls: Improve MCP Port Management for `gopls` in Multi-Workspace Environments

[tekert]

Description:

The current implementation of the gopls MCP server (https://tip.golang.org/gopls/features/mcp) presents challenges when working with multiple workspaces in environments like VS Code. Specifically, managing the MCP port (-mcp.listen) is cumbersome, and there is no seamless way to configure it for both shared and standalone gopls instances.

---

Problem:

1. Random MCP Port:

   • When using -mcp.listen=localhost:0, the MCP server assigns a random port. This makes it difficult to pin down the port for use in the mcp.json file or other tools that rely on a static MCP endpoint.
   • Dynamically updating the mcp.json file to reflect the random port is non-trivial.

2. Fixed MCP Port:

   • Using a fixed MCP port allows for per-workspace configurations, but this requires manual setup for each workspace. This is not ideal for users who want a more automated solution.

3. Shared gopls Daemon:

   • A shared gopls daemon with -mcp.listen and -remote flags is a viable solution. However, the -remote=auto flag does not currently support forwarding the -mcp.listen flag to the shared daemon. This makes it impossible to configure the MCP port for the shared daemon in an automated way.

---

Proposed Solutions:

1. Enhance -remote=auto:
   • Allow the -remote=auto flag to forward the -mcp.listen flag (or other arbitrary flags) to the shared gopls daemon. This would enable users to configure the MCP port for the shared daemon seamlessly.

[madelinekalil]

Thanks for your suggestion! This sounds like something we should support. If you wouldn't mind sharing more, I'm curious about your setup, and how you chose to use the MCP server in attached mode with -mcp.listen instead of the detached mode?

[tekert]

Ah, yes, the detached mode had some errors, for some reason all the request to the tool in detached mode had empty responses, I don't know the internals maybe the paths, workspace in vscode, or the way it works, just dind't work, i also asked random IAs to use the tool and diagnose problems with the mcp prompt instructions (just to debug it) and 90% of the calls failed, when switching to atached mode, all worked, so i defaulted to that and tought detached was not compatible with vscode.
I usually work with 3 go projects at the same time, in 3 windows, but recently had to abandon the use of the daemon since for some reason used 3GB of ram while the non daemon version each project uses 350MB, but that is another problem.

[madelinekalil]

That's strange, which MCP client / AI agent are you using with VSCode? Would be great to figure out why detached mode isn't working in this case.

[tekert]

Tested it with gtp 4o mostly, the latest golang.org/x/tools/gopls v0.20.0
when passing the npc intructions and a root workspace and asking, Do you see the files?

Response:
It seems that the gopls tool is not detecting your project as a valid Go workspace. The errors indicate issues such as "no package metadata for file" and "This is not a Go workspace." This suggests that the directory you have open (goetw) might not be properly set up as a Go module or workspace.

This is a valid workspace,

ERROR while calling tool: MPC 0: no package metadata for file file:///E:/Sources/go/projects/goetw
Please check your input and try again.

and that is a valid directory, this works with attached, but not with detachhed. same prompt, same version.

Every other tool call that doesn't error returns:
This is not a Go workspace. To work on Go code, open a directory inside a Go module.

[madelinekalil]

Thank you so much for explaining the error in more detail. In what directory are you running the "gopls mcp" command from to start the mcp server, is it from a directory with a go.mod? I've seen this error when running the command while not within a Go workspace.

[tekert]

uhmm I don't know, good question, vscode runs this from the go extension. We'll have to search the go extension itself.

[madelinekalil]

Are you using GitHub Copilot, or which agent are you running? What does your mcp config file look like? (.vscode/mcp.json)

[tekert]

Yes, Copilot, mcp looks like this:

{
	"servers": {
		"microsoft-docs": {
			"url": "https://learn.microsoft.com/api/mcp",
			"type": "http"
		},
		"goplsDetached": {
			"type": "stdio",
			"command": "gopls",
			"args": [
				"mcp"
			]
		}
		// "goplsAttached": {
		// 	"type": "http",
		// 	"url": "http://localhost:8092/sessions/1"
		// }
	},
	"inputs": []
}