diff --git a/lib/livebook/evaluator/io_proxy.ex b/lib/livebook/evaluator/io_proxy.ex
index 10457a38a..ecb398169 100644
--- a/lib/livebook/evaluator/io_proxy.ex
+++ b/lib/livebook/evaluator/io_proxy.ex
@@ -132,11 +132,11 @@ defmodule Livebook.Evaluator.IOProxy do
   end
 
   defp io_request({:get_chars, prompt, count}, state) when count >= 0 do
-    get_chars(:latin1, prompt, state, count)
+    get_chars(:latin1, prompt, count, state)
   end
 
   defp io_request({:get_chars, encoding, prompt, count}, state) when count >= 0 do
-    get_chars(encoding, prompt, state, count)
+    get_chars(encoding, prompt, count, state)
   end
 
   defp io_request({:get_line, prompt}, state) do
@@ -147,12 +147,12 @@ defmodule Livebook.Evaluator.IOProxy do
     get_line(encoding, prompt, state)
   end
 
-  defp io_request({:get_until, _prompt, _mod, _fun, _args}, state) do
-    {{:error, :enotsup}, state}
+  defp io_request({:get_until, prompt, mod, fun, args}, state) do
+    get_until(:latin1, prompt, mod, fun, args, state)
   end
 
-  defp io_request({:get_until, _encoding, _prompt, _mod, _fun, _args}, state) do
-    {{:error, :enotsup}, state}
+  defp io_request({:get_until, encoding, prompt, mod, fun, args}, state) do
+    get_until(encoding, prompt, mod, fun, args, state)
   end
 
   defp io_request({:get_password, _encoding}, state) do
@@ -228,34 +228,29 @@ defmodule Livebook.Evaluator.IOProxy do
   end
 
   defp get_line(encoding, prompt, state) do
-    prompt = :unicode.characters_to_binary(prompt, encoding, state.encoding)
-
-    case get_input(prompt, state) do
-      input when is_binary(input) ->
-        {line, rest} = line_from_input(input)
-
-        line =
-          if is_binary(line) do
-            :unicode.characters_to_binary(line, state.encoding, encoding)
-          else
-            line
-          end
-
-        state = put_in(state.input_buffers[prompt], rest)
-        {line, state}
-
-      error ->
-        {error, state}
-    end
+    get_consume(encoding, prompt, state, fn input ->
+      line_from_input(input)
+    end)
   end
 
-  defp get_chars(encoding, prompt, state, count) do
+  defp get_chars(encoding, prompt, count, state) do
+    get_consume(encoding, prompt, state, fn input ->
+      chars_from_input(input, encoding, count)
+    end)
+  end
+
+  defp get_until(encoding, prompt, mod, fun, args, state) do
+    get_consume(encoding, prompt, state, fn input ->
+      get_until_from_input(input, encoding, mod, fun, args)
+    end)
+  end
+
+  defp get_consume(encoding, prompt, state, consume_fun) do
     prompt = :unicode.characters_to_binary(prompt, encoding, state.encoding)
 
     case get_input(prompt, state) do
       input when is_binary(input) ->
-        {chars, rest} = chars_from_input(input, encoding, count)
-
+        {chars, rest} = consume_fun.(input)
         state = put_in(state.input_buffers[prompt], rest)
         {chars, state}
 
@@ -297,61 +292,83 @@ defmodule Livebook.Evaluator.IOProxy do
         {input, ""}
 
       {pos, len} ->
-        size = byte_size(input)
-        line = binary_part(input, 0, pos + len)
-        rest = binary_part(input, pos + len, size - pos - len)
-        {line, rest}
+        :erlang.split_binary(input, pos + len)
     end
   end
 
-  defp chars_from_input("", _, _count), do: {:eof, ""}
+  defp chars_from_input("", _encoding, _count), do: {:eof, ""}
 
   defp chars_from_input(input, :unicode, count) do
-    if byte_size_utf8(input) >= count do
-      chars_part(input, :unicode, count)
-    else
-      {input, ""}
-    end
+    {:ok, count} = utf8_split_at(input, count)
+    :erlang.split_binary(input, count)
   end
 
   defp chars_from_input(input, :latin1, count) do
-    if byte_size(input) >= count do
-      chars_part(input, :latin1, count)
+    if byte_size(input) > count do
+      :erlang.split_binary(input, count)
     else
       {input, ""}
     end
   end
 
-  defp chars_part(chars, _, 0), do: {"", chars}
+  defp utf8_split_at(input, count), do: utf8_split_at(input, count, 0)
 
-  defp chars_part(input, :unicode, count) do
-    with {:ok, count} <- split_at(input, count, 0) do
-      <<chars::binary-size(count), rest::binary>> = input
-      {chars, rest}
+  defp utf8_split_at(_, 0, acc), do: {:ok, acc}
+
+  defp utf8_split_at(<<h::utf8, t::binary>>, count, acc),
+    do: utf8_split_at(t, count - 1, acc + byte_size(<<h::utf8>>))
+
+  defp utf8_split_at(<<_, _::binary>>, _count, _acc),
+    do: {:error, :invalid_unicode}
+
+  defp utf8_split_at(<<>>, _count, acc),
+    do: {:ok, acc}
+
+  defp get_until_from_input(input, encoding, mod, fun, args) do
+    {chars, rest} = get_until_from_input(input, encoding, mod, fun, args, [])
+    {get_until_result(chars, encoding), rest}
+  end
+
+  defp get_until_from_input("", encoding, mod, fun, args, continuation) do
+    case apply(mod, fun, [continuation, :eof | args]) do
+      {:done, result, :eof} ->
+        {result, ""}
+
+      {:done, result, rest} ->
+        {result, list_to_binary(rest, encoding)}
+
+      {:more, next_continuation} ->
+        get_until_from_input("", encoding, mod, fun, args, next_continuation)
     end
   end
 
-  defp chars_part(input, :latin1, count) do
-    <<chars::binary-size(count), rest::binary>> = input
-    {chars, rest}
+  defp get_until_from_input(input, encoding, mod, fun, args, continuation) do
+    {line, rest} = line_from_input(input)
+
+    case apply(mod, fun, [continuation, binary_to_list(line, encoding) | args]) do
+      {:done, result, :eof} ->
+        {result, rest}
+
+      {:done, result, extra} ->
+        {result, list_to_binary(extra, encoding) <> rest}
+
+      {:more, next_continuation} ->
+        get_until_from_input(rest, encoding, mod, fun, args, next_continuation)
+    end
   end
 
-  defp split_at(_, 0, acc), do: {:ok, acc}
+  defp binary_to_list(data, _) when is_list(data), do: data
+  defp binary_to_list(data, :unicode) when is_binary(data), do: String.to_charlist(data)
+  defp binary_to_list(data, :latin1) when is_binary(data), do: :erlang.binary_to_list(data)
 
-  defp split_at(<<h::utf8, t::binary>>, count, acc),
-    do: split_at(t, count - 1, acc + byte_size(<<h::utf8>>))
+  defp list_to_binary(data, _) when is_binary(data), do: data
+  defp list_to_binary(data, :unicode) when is_list(data), do: List.to_string(data)
+  defp list_to_binary(data, :latin1) when is_list(data), do: :erlang.list_to_binary(data)
 
-  defp split_at(<<_, _::binary>>, _count, _acc),
-    do: {:error, :invalid_unicode}
-
-  defp split_at(<<>>, _count, acc),
-    do: {:ok, acc}
-
-  defp byte_size_utf8(chars), do: byte_size_utf8(chars, 0)
-
-  defp byte_size_utf8(<<>>, size), do: size
-
-  defp byte_size_utf8(<<_h::utf8, t::binary>>, size), do: byte_size_utf8(t, size + 1)
+  # From https://erlang.org/doc/apps/stdlib/io_protocol.html - result can be any
+  # Erlang term, but if it is a list(), the I/O server can convert it to a binary().
+  defp get_until_result(data, encoding) when is_list(data), do: list_to_binary(data, encoding)
+  defp get_until_result(data, _), do: data
 
   defp io_reply(from, reply_as, reply) do
     send(from, {:io_reply, reply_as, reply})
diff --git a/test/livebook/evaluator/io_proxy_test.exs b/test/livebook/evaluator/io_proxy_test.exs
index 11e6db612..54abf00ca 100644
--- a/test/livebook/evaluator/io_proxy_test.exs
+++ b/test/livebook/evaluator/io_proxy_test.exs
@@ -37,12 +37,7 @@ defmodule Livebook.Evaluator.IOProxyTest do
     end
 
     test "IO.gets", %{io: io} do
-      pid =
-        spawn_link(fn ->
-          reply_to_input_request(:ref, "name: ", {:ok, "Jake Peralta"}, 1)
-        end)
-
-      IOProxy.configure(io, pid, :ref)
+      configure_owner_with_input(io, "name: ", "Jake Peralta")
 
       assert IO.gets(io, "name: ") == "Jake Peralta"
     end
@@ -57,15 +52,121 @@ defmodule Livebook.Evaluator.IOProxyTest do
 
       assert IO.gets(io, "name: ") == {:error, "no matching Livebook input found"}
     end
+
+    test "IO.getn with unicode input", %{io: io} do
+      configure_owner_with_input(io, "name: ", "🐈 test\n")
+
+      assert IO.getn(io, "name: ", 3) == "🐈 t"
+    end
+
+    test "IO.getn returns the given number of characters", %{io: io} do
+      configure_owner_with_input(io, "name: ", "Jake Peralta\nAmy Santiago\n")
+
+      assert IO.getn(io, "name: ", 13) == "Jake Peralta\n"
+      assert IO.getn(io, "name: ", 13) == "Amy Santiago\n"
+      assert IO.getn(io, "name: ", 13) == :eof
+    end
+
+    test "IO.getn returns all characters if requested more than available", %{io: io} do
+      configure_owner_with_input(io, "name: ", "Jake Peralta\nAmy Santiago\n")
+
+      assert IO.getn(io, "name: ", 10_000) == "Jake Peralta\nAmy Santiago\n"
+    end
+  end
+
+  # See https://github.com/elixir-lang/elixir/blob/v1.12.1/lib/elixir/test/elixir/string_io_test.exs
+  defmodule GetUntilCallbacks do
+    def until_eof(continuation, :eof) do
+      {:done, continuation, :eof}
+    end
+
+    def until_eof(continuation, content) do
+      {:more, continuation ++ content}
+    end
+
+    def until_eof_then_try_more('magic-stop-prefix' ++ continuation, :eof) do
+      {:done, continuation, :eof}
+    end
+
+    def until_eof_then_try_more(continuation, :eof) do
+      {:more, 'magic-stop-prefix' ++ continuation}
+    end
+
+    def until_eof_then_try_more(continuation, content) do
+      {:more, continuation ++ content}
+    end
+
+    def up_to_3_bytes(continuation, :eof) do
+      {:done, continuation, :eof}
+    end
+
+    def up_to_3_bytes(continuation, content) do
+      case continuation ++ content do
+        [a, b, c | tail] -> {:done, [a, b, c], tail}
+        str -> {:more, str}
+      end
+    end
+
+    def up_to_3_bytes_discard_rest(continuation, :eof) do
+      {:done, continuation, :eof}
+    end
+
+    def up_to_3_bytes_discard_rest(continuation, content) do
+      case continuation ++ content do
+        [a, b, c | _tail] -> {:done, [a, b, c], :eof}
+        str -> {:more, str}
+      end
+    end
+  end
+
+  describe ":get_until" do
+    test "with up_to_3_bytes", %{io: io} do
+      configure_owner_with_input(io, "name: ", "abcdefg")
+
+      result = get_until(io, :unicode, "name: ", GetUntilCallbacks, :up_to_3_bytes)
+      assert result == "abc"
+      assert IO.gets(io, "name: ") == "defg"
+    end
+
+    test "with up_to_3_bytes_discard_rest", %{io: io} do
+      configure_owner_with_input(io, "name: ", "abcdefg")
+
+      result = get_until(io, :unicode, "name: ", GetUntilCallbacks, :up_to_3_bytes_discard_rest)
+      assert result == "abc"
+      assert IO.gets(io, "name: ") == :eof
+    end
+
+    test "with until_eof", %{io: io} do
+      configure_owner_with_input(io, "name: ", "abc\nd")
+
+      result = get_until(io, :unicode, "name: ", GetUntilCallbacks, :until_eof)
+      assert result == "abc\nd"
+    end
+
+    test "with until_eof and \\r\\n", %{io: io} do
+      configure_owner_with_input(io, "name: ", "abc\r\nd")
+
+      result = get_until(io, :unicode, "name: ", GetUntilCallbacks, :until_eof)
+      assert result == "abc\r\nd"
+    end
+
+    test "with until_eof_then_try_more", %{io: io} do
+      configure_owner_with_input(io, "name: ", "abc\nd")
+
+      result = get_until(io, :unicode, "name: ", GetUntilCallbacks, :until_eof_then_try_more)
+      assert result == "abc\nd"
+    end
+
+    test "with raw bytes (latin1)", %{io: io} do
+      configure_owner_with_input(io, "name: ", <<181, 255, 194, ?\n>>)
+
+      result = get_until(io, :latin1, "name: ", GetUntilCallbacks, :until_eof)
+      assert result == <<181, 255, 194, ?\n>>
+    end
   end
 
   test "consumes the given input only once", %{io: io} do
-    pid =
-      spawn_link(fn ->
-        reply_to_input_request(:ref, "name: ", {:ok, "Jake Peralta\nAmy Santiago\n"}, 1)
-      end)
-
-    IOProxy.configure(io, pid, :ref)
+    configure_owner_with_input(io, "name: ", "Jake Peralta\nAmy Santiago\n")
 
     assert IO.gets(io, "name: ") == "Jake Peralta\n"
     assert IO.gets(io, "name: ") == "Amy Santiago\n"
@@ -121,43 +222,21 @@ defmodule Livebook.Evaluator.IOProxyTest do
     assert IOProxy.flush_widgets(io) == MapSet.new()
   end
 
-  test "getn/1 return first character", %{io: io} do
-    pid =
-      spawn_link(fn ->
-        reply_to_input_request(:ref, "name: ", {:ok, "🐈 test\n"}, 1)
-      end)
-
-    IOProxy.configure(io, pid, :ref)
-
-    assert IO.getn(io, "name: ") == "🐈"
-  end
-
-  test "getn/2 returns the number of defined characters ", %{io: io} do
-    pid =
-      spawn_link(fn ->
-        reply_to_input_request(:ref, "name: ", {:ok, "Jake Peralta\nAmy Santiago\n"}, 1)
-      end)
-
-    IOProxy.configure(io, pid, :ref)
-
-    assert IO.getn(io, "name: ", 13) == "Jake Peralta\n"
-    assert IO.getn(io, "name: ", 13) == "Amy Santiago\n"
-    assert IO.getn(io, "name: ", 13) == :eof
-  end
-
-  test "getn/2 all characters", %{io: io} do
-    pid =
-      spawn_link(fn ->
-        reply_to_input_request(:ref, "name: ", {:ok, "Jake Peralta\nAmy Santiago\n"}, 1)
-      end)
-
-    IOProxy.configure(io, pid, :ref)
-
-    assert IO.getn(io, "name: ", 10_000) == "Jake Peralta\nAmy Santiago\n"
-  end
-
   # Helpers
 
+  defp get_until(pid, encoding, prompt, module, function) do
+    :io.request(pid, {:get_until, encoding, prompt, module, function, []})
+  end
+
+  defp configure_owner_with_input(io, prompt, input) do
+    pid =
+      spawn_link(fn ->
+        reply_to_input_request(:ref, prompt, {:ok, input}, 1)
+      end)
+
+    IOProxy.configure(io, pid, :ref)
+  end
+
   defp reply_to_input_request(_ref, _prompt, _reply, 0), do: :ok
 
   defp reply_to_input_request(ref, prompt, reply, times) do