`dune tools install ocamllsp` sometimes fails with `server error (error kind: Code_error)`

[shonfeder]

Expected Behavior

dune tools install ocamllsp succeeds or returns an informative user error

Actual Behavior

Non-deterministically, the attempt fails with

Error: Server returned error:
server error (error kind: Code_error)

Reproduction

I have not been able to repro this reliably. It sometimes occurs, and if I retry a couple times, the installation will just go thru.

Specifications

• Version of dune (output of dune --version): Nightly build 2026-08-10T03:08:22Z, git revision 4fd9626a1cc4c9019267e2c99fb02d9162a78213

Additional information

Looks like we are getting here somehow

https://github.com/shonfeder/dune/blob/5b109c3cb8bf4492415168f7597405aa13bfb569/src/rpc/server.ml#L526-L532

so maybe its a "server-side" issues? But in these cases no dune watch was running in the projects (AFAIK).

This will probably be fixed as a side effect of #12914

[Alizter]

dune build will now start a watch server every time, the old batch mode was removed. This server however does not accept RPC build requests.

It seems that dune tools install ocamllsp is doing something akin to dune build and then attempting to query the server and hitting this error under the hood.

I haven't looked into why exactly this is happening, but this would be my first thought in order to find the issue.

[Alizter]

Having a closer look I don't see how this would happen with a lone dune tools install ocamllsp. You said that there were no other watch instances, were there any other dune build instances during its running? That would explain things.

[shonfeder]

Possibly there are some other dune sessions running on my machine at those times. I'll look for them next time I hit this.