Local path dependencies are described here: https://hexdocs.pm/mix/Mix.Tasks.Deps.html

For cases like that, I’d sometimes do the following:

defp deps() do
  [
    ecto: ecto_dep()
  ]
end

defp ecto_dep() do
  if path = System.get_env("ECTO_PATH") do
    {:ecto, path: path}
  else
    {:ecto, "~> 3.0"}
  end
end

Figured it out. If you’re using path dep and dont want to have to start stop your phoenix app to have changes reflected you just need to add the path to elixirc_paths in mix.exs e.g.

  # Specifies which paths to compile per environment.
  defp elixirc_paths(:test), do: ["lib", "test/support"]
  defp elixirc_paths(_), do: ["lib", "/Path/to/dependency"]

You can modify files in deps/ and then run mix deps.compile APP to recompile it.

In the case where I need to actually make changes to a dependency I like that I need to explicitly declare it as a path dependency. It’s very little work compared to making a PR and with an explicit dep I cannot accidentally commit changes to the project without updating the dep or reverting changes that depend on my local checkout changes.