If you run a Phoenix project on Ubuntu 24.04, you've seen these:
warning: the dependency :swoosh requires Elixir "~> 1.16" but you are running on v1.14.0
warning: the dependency :ecto_sqlite3 requires Elixir "~> 1.15" but you are running on v1.14.0
warning: the dependency :uri_query requires Elixir "~> 1.16" but you are running on v1.14.0
They compile. They pass tests. So you ignore them. I ignored them for months. Then I turned on --warnings-as-errors & my CI went red.
the problem
Ubuntu 24.04 ships Erlang/OTP 25 & Elixir 1.14. The Hex ecosystem moved to 1.15+ & 1.16+ minimums throughout 2025. Phoenix 1.7 still compiles on 1.14, but its dependency tree doesn't agree.
The warnings mean: "this works today by accident, not by contract." That gap closes without notice the next time you mix deps.update.
what actually fixes it
Ubuntu 26.04 LTS drops April 23, 2026. I checked the package archive:
- Erlang/OTP 27.3 (1:27.3.4.6+dfsg-1)
- Elixir 1.18.3 (1.18.3.dfsg-1build1)
Every version warning disappears. apt install erlang elixir on 26.04 gives you a supported toolchain with zero third-party repos.
recommendations
1. Don't chase the warnings today. Without --warnings-as-errors they stay cosmetic. Production releases compile with MIX_ENV=prod which skips the test deps that generate most of these anyway.
2. Don't add PPAs for Elixir. Third-party repos on CI runners create invisible dependencies. When they lag or drop your distro, your pipeline breaks & you don't know why.
3. If you need --warnings-as-errors now, pin your deps. Older versions of swoosh, ecto_sqlite3, & uri_query support Elixir 1.14. Stop running mix deps.update until you're ready to upgrade.
4. Plan for 26.04. Test against Elixir 1.18 & OTP 27 locally before the server upgrade. The jump from OTP 25 to 27 changes some behaviors (rebar3_lint needs OTP 26+, Process.set_label/1 exists now).
5. Separate build from deploy. The build server's Elixir version doesn't need to match production's Erlang — only the ERTS version baked into the release matters.
This happens every LTS cycle. Ubuntu 22.04 shipped Python 3.10 while the ecosystem ran on 3.11. The pattern repeats because LTS freezes & ecosystems don't. Five weeks. Then apt install elixir does what it should have done all along.