Dune RPC

Starting from dune 3.0, dune’s watch mode also runs an RPC server. This is a general mechanism introduced to integrate with various dune functionality. Some use cases we have in mind are:

  • Text editors that would like to receive dune’s error reporting

  • In the future, custom actions that need to communicate with dune

There’s no fixed scope for RPC, and we encourage users to submit requests to cover more functionality.

The purpose of this documentation is to explain how RPC works, and how to connect to it as a client. More concrete information such as what requests are available is in Dune_rpc

dune-rpc library

We provide a client library dune-rpc to make it easy to write clients. The library has a versioned interface and we guarantee to maintain its stability. The library documentation describes the guarantees in more detail.

RPC clients written with this library are guaranteed to work for all versions of the RPC server greater than it. In other words, a client using version X will work with all servers >= X. However, it will not work with any servers < X.

The library contains a client implementation parameterised over a concurrency monad. The dune-rpc-lwt package provides an instantiation of the RPC client defined in dune-rpc using lwt to implement the concurrency monad. It is of course possible to instantiate the dune RPC client using alternative concurrency implementations.

The dune-rpc library provides an API to do the following:

  • Initialise an RPC session over a channel

  • Send RPC requests and notifications

  • Handle notifications received from the server

  • Definitions of available requests, notifications, and their associated types.

  • Subscribe to streams of build progress events

Here is an example of a simple Dune RPC client that uses the dune-rpc and dune-rpc-lwt packages.


To connect to Dune’s RPC server, it needs to be started in watch mode. It is possible to use dune build --passive-watch-mode to start an RPC server which will listen for requests without starting a build by itself. Then dune rpc build . will connect to it, trigger a build, and report status.