diff --git a/lib/protox/json_decode.ex b/lib/protox/json_decode.ex
index c19a6ae..5008030 100644
--- a/lib/protox/json_decode.ex
+++ b/lib/protox/json_decode.ex
@@ -7,7 +7,7 @@ defmodule Protox.JsonDecode do
 
   use Protox.{Float, Integer}
 
-  @spec decode!(iodata(), atom(), fun()) :: any() | no_return()
+  @spec decode!(iodata(), atom(), fun()) :: struct() | no_return()
   def decode!(input, mod, json_decode) do
     case json_decode.(input) do
       nil -> raise JsonDecodingError.new("invalid top-level null")
diff --git a/lib/protox/json_message_decoder.ex b/lib/protox/json_message_decoder.ex
index 33c0d98..05f5017 100644
--- a/lib/protox/json_message_decoder.ex
+++ b/lib/protox/json_message_decoder.ex
@@ -5,7 +5,7 @@ defprotocol Protox.JsonMessageDecoder do
 
   @doc since: "1.6.0"
   @fallback_to_any true
-  # @spec decode_message(atom(), struct()) :: struct()
+  @spec decode_message(struct(), any()) :: struct()
   def decode_message(initial_message, json)
 end