diff --git a/lib/feed_reader/format_detector.ex b/lib/feed_reader/format_detector.ex
new file mode 100644
index 0000000..3ab301a
--- /dev/null
+++ b/lib/feed_reader/format_detector.ex
@@ -0,0 +1,152 @@
+defmodule FeedReader.FormatDetector do
+  @moduledoc "Internal bounded detection and replay for `FeedReader`."
+
+  alias FeedReader.FormatRegistry
+
+  @default_max_detection_bytes 65_536
+
+  @doc "Returns the detected format id, or `:unknown`, from a bounded prefix."
+  @spec detect(binary(), keyword()) :: FeedReader.format_id()
+  def detect(content, opts) when is_binary(content) do
+    {format, _prefix, _replayable} = detect_and_replay(content, opts)
+    format
+  end
+
+  def detect(_content, _opts), do: :unknown
+
+  @doc "Returns the detected id, inspected prefix, and replayable input stream."
+  @spec detect_and_replay(binary() | Enumerable.t(), keyword()) ::
+          {FeedReader.format_id(), binary(), Enumerable.t()}
+  def detect_and_replay(content, opts) when is_binary(content) do
+    max_bytes = max_detection_bytes(opts)
+    prefix_size = min(byte_size(content), max_bytes)
+    prefix = binary_part(content, 0, prefix_size)
+    remainder = binary_part(content, prefix_size, byte_size(content) - prefix_size)
+
+    {detect_prefix(prefix, opts), prefix, Stream.concat([prefix, remainder])}
+  end
+
+  def detect_and_replay(content, opts) do
+    max_bytes = max_detection_bytes(opts)
+
+    if max_bytes == 0 do
+      {:unknown, "", content}
+    else
+      state = %{mode: :detect, chunks: [], size: 0, remainder: ""}
+      reduce_and_replay(content, state, max_bytes, opts)
+    end
+  end
+
+  defp reduce_and_replay(content, state, max_bytes, opts) do
+    case Enumerable.reduce(content, {:cont, state}, fn chunk, state ->
+           consume_chunk(chunk, state, max_bytes)
+         end) do
+      {:suspended, state, continuation} ->
+        prefix = prefix(state)
+        rest = Stream.concat([state.remainder], continuation_stream(continuation, state))
+        {detect_prefix(prefix, opts), prefix, Stream.concat([prefix], rest)}
+
+      {:done, state} ->
+        prefix = prefix(state)
+        {detect_prefix(prefix, opts), prefix, [prefix]}
+
+      {:halted, state} ->
+        prefix = prefix(state)
+        {:unknown, prefix, [prefix]}
+    end
+  end
+
+  @doc "Checks whether the bounded prefix matches the explicitly selected format."
+  @spec validate(binary(), atom() | module(), keyword()) :: boolean()
+  def validate(content, format, opts) when is_binary(content) do
+    {_detected, prefix, _replayable} = detect_and_replay(content, opts)
+
+    case resolve_format(format, formats(opts)) do
+      {:ok, formatter} -> matches?(formatter, prefix)
+      :error -> false
+    end
+  end
+
+  def validate(_content, _format, _opts), do: false
+
+  defp max_detection_bytes(opts) do
+    case Keyword.get(opts, :max_detection_bytes, @default_max_detection_bytes) do
+      max_bytes when is_integer(max_bytes) and max_bytes > 0 -> max_bytes
+      _ -> 0
+    end
+  end
+
+  defp formats(opts), do: FormatRegistry.formats(Keyword.get(opts, :formats, []))
+
+  defp detect_prefix(prefix, opts) do
+    opts
+    |> formats()
+    |> Enum.find(&matches?(&1, prefix))
+    |> format_id()
+  end
+
+  defp consume_chunk(chunk, %{mode: :detect} = state, max_bytes) when is_binary(chunk) do
+    remaining = max_bytes - state.size
+    length = min(byte_size(chunk), remaining)
+    prefix_chunk = chunk |> binary_part(0, length) |> :binary.copy()
+    state = %{state | chunks: [prefix_chunk | state.chunks], size: state.size + length}
+    remainder = binary_part(chunk, length, byte_size(chunk) - length)
+    state = %{state | remainder: remainder}
+
+    if state.size == max_bytes, do: {:suspend, state}, else: {:cont, state}
+  end
+
+  defp consume_chunk(chunk, %{mode: :replay} = state, _max_bytes) do
+    {:suspend, Map.put(state, :replay_chunk, chunk)}
+  end
+
+  defp consume_chunk(_chunk, state, _max_bytes), do: {:halt, state}
+
+  defp continuation_stream(continuation, state) do
+    Stream.resource(
+      fn -> {continuation, %{state | mode: :replay}} end,
+      fn {continuation, state} ->
+        case continuation.({:cont, state}) do
+          {:suspended, next_state, next_continuation} ->
+            chunk = Map.fetch!(next_state, :replay_chunk)
+            {[chunk], {next_continuation, Map.delete(next_state, :replay_chunk)}}
+
+          {:done, _state} ->
+            {:halt, nil}
+
+          {:halted, _state} ->
+            {:halt, nil}
+        end
+      end,
+      fn _state -> :ok end
+    )
+  end
+
+  defp prefix(state), do: state.chunks |> Enum.reverse() |> IO.iodata_to_binary()
+
+  defp matches?(format, prefix) do
+    Code.ensure_loaded?(format) and function_exported?(format, :matches_prefix?, 1) and
+      format.matches_prefix?(prefix)
+  end
+
+  defp format_id(nil), do: :unknown
+
+  defp format_id(format) do
+    if function_exported?(format, :id, 0), do: format.id(), else: :unknown
+  end
+
+  defp resolve_format(format, formats) when is_atom(format) do
+    if Code.ensure_loaded?(format) and function_exported?(format, :matches_prefix?, 1) do
+      {:ok, format}
+    else
+      Enum.find_value(formats, :error, &resolve_id(&1, format))
+    end
+  end
+
+  defp resolve_format(_format, _formats), do: :error
+
+  defp resolve_id(module, id) do
+    if Code.ensure_loaded?(module) and function_exported?(module, :id, 0) and module.id() == id,
+      do: {:ok, module}
+  end
+end