# Mix.env() doesn't work in releases

`Mix.env()` doesn't exist in production releases. Your conditional routes will silently fail, and you won't know until deployment. Use `Application.compile_env/2` instead.

```elixir
# ❌ WRONG: Mix.env() doesn't exist in releases
if Mix.env() == :dev do
  scope "/dev" do
    pipe_through(:browser)
    forward("/mailbox", Plug.Swoosh.MailboxPreview)
  end
end

# ✅ CORRECT: Use compile_env for conditional routes
if Application.compile_env(:my_app, :env) == :dev do
  scope "/dev" do
    pipe_through(:browser)
    forward("/mailbox", Plug.Swoosh.MailboxPreview)
  end
end
```

The `Mix` module is a build tool - it's not included in production releases. When you deploy with `mix release`, any code calling `Mix.env()` will crash with `UndefinedFunctionError`.

## The fix

Store your environment at compile time in `config/config.exs`:

```elixir
# config/config.exs
config :my_app, :env, config_env()
```

Then use `Application.compile_env/2` in your router:

```elixir
# router.ex
if Application.compile_env(:my_app, :env) == :dev do
  scope "/dev" do
    pipe_through(:browser)
    forward("/mailbox", Plug.Swoosh.MailboxPreview)
    live_dashboard("/dashboard", metrics: MyAppWeb.Telemetry)
  end
end
```

## When to use each function

| Function                    | When                   | Example                                   |
| --------------------------- | ---------------------- | ----------------------------------------- |
| `config_env()`              | Config files only      | `config/config.exs`, `config/runtime.exs` |
| `Application.compile_env/2` | Compile-time decisions | Router conditionals, module attributes    |
| `Application.get_env/2`     | Runtime values         | Reading config in functions               |

## Common patterns that break in releases

- `if Mix.env() == :test` for test-only routes
- `@some_attr if Mix.env() == :prod, do: X, else: Y`
- `Logger.level(if Mix.env() == :dev, do: :debug, else: :info)`

All of these need `Application.compile_env/2` instead.


---

Created by: almirsarajcic
Date: January 07, 2026
URL: https://elixirdrops.net/d/TRjMZGwf