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OCaml on Miren

OCaml isn't auto-detected, so you deploy it with a Dockerfile.miren that builds a native binary with opam and dune and runs it on a minimal image. This guide uses the Dream web framework as the example.

Let your agent do this

Ask your AI coding agent to "set up this OCaml app on Miren" after installing the Miren agent skills. It adds the Dockerfile.miren, confirms the server binds 0.0.0.0:$PORT, wires up environment variables, and deploys — using this page as its reference.

Do you need a Dockerfile?​

Yes. Miren doesn't auto-detect OCaml, so add a Dockerfile.miren to your project root. Miren builds from it instead of guessing the stack — see Using Dockerfile.miren.

Want native support?

Miren auto-detects and builds common stacks (Python, Node, Bun, Go, Ruby, Rust) without a Dockerfile. This language isn't one of them yet — if you'd like first-class support, request it.

Bind to the injected port​

Miren injects PORT and routes traffic to it, so your server must read PORT and listen on 0.0.0.0. With Dream:

let () =
  let port =
    match Sys.getenv_opt "PORT" with
    | Some p -> int_of_string p
    | None -> 8080
  in
  Dream.run ~interface:"0.0.0.0" ~port
  @@ Dream.logger
  @@ Dream.router [
       Dream.get "/" (fun _ -> Dream.respond "Hello from OCaml on Miren!\n");
     ]

~interface:"0.0.0.0" is required — Dream binds to localhost by default, which Miren can't route to.

The Dockerfile​

Create Dockerfile.miren in your project root:

# ----- Build stage -----
FROM ocaml/opam:debian-12-ocaml-5.2 AS builder

# Install system libraries Dream needs. Miren's build sandbox runs without
# new privileges, so `sudo` fails here — switch to root instead.
USER root
RUN apt-get update -y \
    && apt-get install -y libev-dev libgmp-dev libssl-dev pkg-config m4
USER opam

WORKDIR /app
COPY --chown=opam:opam . .
# System deps are already installed above, so skip opam's sudo-based depext step.
RUN opam install -y --no-depexts dream dune
RUN opam exec -- dune build --profile release ./main.exe

# ----- Runtime stage -----
FROM debian:12-slim

RUN apt-get update -y \
    && apt-get install -y libev4 libgmp10 libssl3 ca-certificates \
    && apt-get clean && rm -rf /var/lib/apt/lists/*

COPY --from=builder /app/_build/default/main.exe /usr/local/bin/app

EXPOSE 8080
CMD ["app"]
Use USER root, not sudo, for system packages

The ocaml/opam image runs as the opam user and expects sudo for apt-get. Miren's build sandbox runs with no new privileges, so sudo fails with an exit code. Switch to USER root to install packages, switch back to USER opam, and pass --no-depexts to opam install so it doesn't try to sudo apt-get the same libraries again.

A minimal dune-project and dune:

; dune-project
(lang dune 3.0)
; dune
(executable
 (name main)
 (libraries dream))

.dockerignore​

.git
_build

Set up the app​

Even with a Dockerfile.miren, Miren needs at least one service defined — it doesn't use the image's CMD as the start command. Add a Procfile:

web: /usr/local/bin/app

Then create .miren/app.toml naming your app and deploy from your project root:

name = "ocaml-bench"
miren deploy
Deploying without a service fails

If no service is defined, the build succeeds but the deploy stops with no services defined: please define at least one service in a Procfile or .miren/app.toml.

Environment variables​

Set variables with miren env set — -e for plain values, -s for secrets (masked in output and logs). Read them with Sys.getenv_opt "KEY":

miren env set -e LOG_LEVEL=info
miren env set -s DATABASE_URL

You can also declare variables in .miren/app.toml:

[[env]]
key = "DATABASE_URL"
value = ""
required = true
sensitive = true

See App Configuration — Environment Variables.

Agent quick reference​

  • Detection: none — requires Dockerfile.miren (native binary)
  • Build: opam install --no-depexts dream dune, dune build --profile release ./main.exe
  • System deps: use USER root for apt-get (sudo fails in the build sandbox); runtime needs libev4 libgmp10 libssl3
  • Service is required: define a Procfile (web: /usr/local/bin/app) — the image CMD is not used
  • Port: read Sys.getenv_opt "PORT"; Dream needs ~interface:"0.0.0.0"
  • Env vars: miren env set -e/-s; read with Sys.getenv_opt

Next steps​