Elixir
Detected from mix.exs. Builds a Mix release. Erlang and Elixir stay out of the run image.
See Buildpacksfor how detection and version policy work.
Detects
mix.exs in the base directory.
Versions
Default is Elixir 1.20 on Erlang/OTP 28. Published Elixir lines as of September 2026: 1.15-1.20. Published OTP lines: 26-29. You get the latest patch of the requested line.
Pin Elixir with, in this order:
- An
elixirentry in.tool-versions(asdf style1.18.2-otp-27uses the part before the hyphen) .elixir-version
The elixir: requirement in mix.exs is a range, not a pin. Nodion does not use it. An old ~> 1.17 still runs on a newer default.
Pin Erlang with an erlang entry in .tool-versions.
Install and build
mix local.hex, mix local.rebar, mix deps.get --only prod. If mix assets.deploy exists, that runs next. Then mix release --overwrite.
The app must be set up for Mix releases. The release bundles ERTS, so the run image is the release only.
Launch sets PHX_SERVER=true so a Phoenix endpoint starts. Harmless on non-Phoenix releases.
Start command
<release>/bin/<app> start, preferring the OTP app name from mix.exs. Override with a Procfile. Listen on PORT (Phoenix reads PORT when the release is generated that way).