diff --git a/mix.exs b/mix.exs
index e10732f..7261ce6 100644
--- a/mix.exs
+++ b/mix.exs
@@ -6,6 +6,7 @@ defmodule FeedReader.MixProject do
       app: :feed_reader,
       version: "0.1.0",
       elixir: "~> 1.20.4",
+      elixirc_paths: elixirc_paths(Mix.env()),
       start_permanent: Mix.env() == :prod,
       aliases: aliases(),
       deps: deps()
@@ -20,6 +21,9 @@ defmodule FeedReader.MixProject do
     [preferred_envs: [precommit: :test]]
   end
 
+  defp elixirc_paths(:test), do: ["lib", "test/support"]
+  defp elixirc_paths(_), do: ["lib"]
+
   defp deps do
     [
       {:credo, "~> 1.7.19", only: :dev},
diff --git a/test/feed_reader/format_contract_test.exs b/test/feed_reader/format_contract_test.exs
new file mode 100644
index 0000000..4816e4b
--- /dev/null
+++ b/test/feed_reader/format_contract_test.exs
@@ -0,0 +1,38 @@
+defmodule FeedReader.FormatContractTest do
+  use ExUnit.Case, async: true
+
+  alias FeedReader.{Feed, FormatRegistry, Item}
+  alias FeedReader.TestSupport.ExternalFormat
+
+  test "built-in registry has a fixed format priority" do
+    assert FormatRegistry.builtins() == [
+             FeedReader.Formats.RSS,
+             FeedReader.Formats.Atom,
+             FeedReader.Formats.RDF
+           ]
+  end
+
+  test "per-call formats append after built-ins without duplicate modules" do
+    formats = FormatRegistry.formats([ExternalFormat, FeedReader.Formats.RSS])
+
+    assert formats == FormatRegistry.builtins() ++ [ExternalFormat]
+  end
+
+  test "external formats implement detection, parser state, and normalization callbacks" do
+    sink = fn event -> send(self(), {:format_event, event}) end
+    state = ExternalFormat.new_parser_state(sink, mode: :test)
+
+    assert ExternalFormat.id() == :external_test
+    assert ExternalFormat.matches_prefix?("<external-feed>")
+    refute ExternalFormat.matches_prefix?("<other-feed>")
+    assert ExternalFormat.handler() == FeedReader.TestSupport.ExternalFormatHandler
+    assert state.event_sink == sink
+    assert state.opts == [mode: :test]
+
+    feed = ExternalFormat.normalize_feed(state, [])
+    assert %Feed{title: "External", type: :rss} = feed
+
+    assert ExternalFormat.normalize_item(%{title: "Entry"}, feed, []) ==
+             {:ok, %Item{title: "Entry"}}
+  end
+end
diff --git a/test/support/external_format.ex b/test/support/external_format.ex
new file mode 100644
index 0000000..f3f1651
--- /dev/null
+++ b/test/support/external_format.ex
@@ -0,0 +1,25 @@
+defmodule FeedReader.TestSupport.ExternalFormat do
+  @moduledoc false
+  @behaviour FeedReader.Format
+
+  alias FeedReader.{Feed, Item}
+
+  @impl true
+  def id, do: :external_test
+
+  @impl true
+  def matches_prefix?("<external-feed" <> _rest), do: true
+  def matches_prefix?(_prefix), do: false
+
+  @impl true
+  def handler, do: FeedReader.TestSupport.ExternalFormatHandler
+
+  @impl true
+  def new_parser_state(event_sink, opts), do: %{event_sink: event_sink, opts: opts}
+
+  @impl true
+  def normalize_feed(_state, _opts), do: %Feed{title: "External", type: :rss}
+
+  @impl true
+  def normalize_item(%{title: title}, _feed, _opts), do: {:ok, %Item{title: title}}
+end
diff --git a/test/support/external_format_handler.ex b/test/support/external_format_handler.ex
new file mode 100644
index 0000000..1d70649
--- /dev/null
+++ b/test/support/external_format_handler.ex
@@ -0,0 +1,7 @@
+defmodule FeedReader.TestSupport.ExternalFormatHandler do
+  @moduledoc false
+  @behaviour Saxy.Handler
+
+  @impl true
+  def handle_event(_event_type, _data, state), do: {:ok, state}
+end