diff --git a/src/fcm.erl b/src/fcm.erl
index 01a199b..f7c044b 100644
--- a/src/fcm.erl
+++ b/src/fcm.erl
@@ -17,11 +17,13 @@
 
 -export([push/4]).
 
--spec start_pool_with_api_key(atom(), string()) -> {ok, pid()}.
+-spec start_pool_with_api_key(atom(), string()) ->
+       {ok, pid()} | {error, {already_started, pid()}} | {error, _}.
 start_pool_with_api_key(Name, ApiKey) ->
     fcm_sup:start_child(Name, #{fcm_key => ApiKey}).
 
--spec start_pool_with_json_service_file(atom(), string()) -> {ok, pid()}.
+-spec start_pool_with_json_service_file(atom(), string()) ->
+       {ok, pid()} | {error, {already_started, pid()}} | {error, _}.
 start_pool_with_json_service_file(Name, FilePath) ->
     fcm_sup:start_child(Name, #{service_file => FilePath}).
 
@@ -33,7 +35,7 @@ stop(Name) ->
             atom(),
             binary() | list(binary()),
             list(tuple()) | map(),
-            integer()) -> list(tuple()).
+            integer()) -> list(tuple()) | {error, term()}.
 push(Name, RegIds, Message, Retry) when is_list(Message) ->
     push(Name, RegIds, maps:from_list(Message), Retry);
 push(Name, RegId, Message, Retry) when is_binary(RegId) ->
diff --git a/src/fcm_api_v1.erl b/src/fcm_api_v1.erl
index be02a7a..a7f60d7 100644
--- a/src/fcm_api_v1.erl
+++ b/src/fcm_api_v1.erl
@@ -12,7 +12,7 @@
 -define(HTTP_OPTS, [{timeout, 5000}]).
 -define(REQ_OPTS, [{full_result, false}, {body_format, binary}]).
 
--spec push(list(binary()), map(), string(), string()) -> list(tuple()).
+-spec push(list(binary()), map(), map()) -> {ok, list(tuple()), map()}.
 push(RegIds, Message, State) ->
     push(RegIds, Message, State, []).
 
diff --git a/src/fcm_sup.erl b/src/fcm_sup.erl
index f8161f7..474f61b 100644
--- a/src/fcm_sup.erl
+++ b/src/fcm_sup.erl
@@ -25,6 +25,6 @@ init([]) ->
     {ok, {{simple_one_for_one, 5, 10}, [?CHILD(fcm, worker)]}}.
 
 -spec start_child(atom(), map()) ->
-       {'error',_} | {'ok','undefined' | pid()} | {'ok','undefined' | pid(),_}.
+       {'error',{'already_started',pid()}} | {'error',_} | {'ok',pid()}.
 start_child(Name, Opts) ->
     supervisor:start_child(?MODULE, [Name, Opts]).