diff --git a/apps/emqx_gateway/src/lwm2m/emqx_lwm2m_cmd.erl b/apps/emqx_gateway/src/lwm2m/emqx_lwm2m_cmd.erl
index d0b362dda..090af3e87 100644
--- a/apps/emqx_gateway/src/lwm2m/emqx_lwm2m_cmd.erl
+++ b/apps/emqx_gateway/src/lwm2m/emqx_lwm2m_cmd.erl
@@ -138,7 +138,7 @@ mqtt_to_coap(AlternatePath, InputCmd = #{<<"msgType">> := <<"discover">>, <<"dat
             [
                 {uri_path, FullPathList},
                 {uri_query, QueryList},
-                {'accept', ?LWM2M_FORMAT_LINK}
+                {accept, ?LWM2M_FORMAT_LINK}
             ]
         ),
         InputCmd
@@ -241,6 +241,7 @@ empty_ack_to_mqtt(Ref) ->
 coap_failure_to_mqtt(Ref, MsgType) ->
     make_base_response(maps:put(<<"msgType">>, MsgType, Ref)).
 
+%% TODO: application/link-format
 content_to_mqtt(CoapPayload, <<"text/plain">>, Ref) ->
     emqx_lwm2m_message:text_to_json(extract_path(Ref), CoapPayload);
 content_to_mqtt(CoapPayload, <<"application/octet-stream">>, Ref) ->
diff --git a/apps/emqx_gateway/src/lwm2m/emqx_lwm2m_session.erl b/apps/emqx_gateway/src/lwm2m/emqx_lwm2m_session.erl
index 19cd5c25d..595543046 100644
--- a/apps/emqx_gateway/src/lwm2m/emqx_lwm2m_session.erl
+++ b/apps/emqx_gateway/src/lwm2m/emqx_lwm2m_session.erl
@@ -513,12 +513,16 @@ observe_object_list(AlternatePath, ObjectList, Session) ->
             true ->
                 Acc;
             false ->
-                try
-                    emqx_lwm2m_xml_object_db:find_objectid(binary_to_integer(ObjId)),
-                    observe_object(AlternatePath, ObjectPath, Acc)
-                catch
-                    error:no_xml_definition ->
-                        Acc
+                case emqx_lwm2m_xml_object_db:find_objectid(binary_to_integer(ObjId)) of
+                    {error, no_xml_definition} ->
+                        ?SLOG(warning, #{
+                            msg => "ignore_observer_resource",
+                            reason => no_xml_definition,
+                            object_id => ObjId
+                        }),
+                        Acc;
+                    _ ->
+                        observe_object(AlternatePath, ObjectPath, Acc)
                 end
         end
     end,
@@ -538,15 +542,20 @@ deliver_auto_observe_to_coap(AlternatePath, TermData, Session) ->
         path => AlternatePath,
         data => TermData
     }),
-    {Req, Ctx} = emqx_lwm2m_cmd:mqtt_to_coap(AlternatePath, TermData),
+    {Req0, Ctx} = emqx_lwm2m_cmd:mqtt_to_coap(AlternatePath, TermData),
+    Req = alloc_token(Req0),
     maybe_do_deliver_to_coap(Ctx, Req, 0, false, Session).
 
 is_auto_observe() ->
     emqx:get_config([gateway, lwm2m, auto_observe]).
 
+alloc_token(Req = #coap_message{}) ->
+    Req#coap_message{token = crypto:strong_rand_bytes(4)}.
+
 %%--------------------------------------------------------------------
 %% Response
 %%--------------------------------------------------------------------
+
 handle_coap_response(
     {Ctx = #{<<"msgType">> := EventType}, #coap_message{
         method = CoapMsgMethod,
diff --git a/apps/emqx_gateway/src/lwm2m/emqx_lwm2m_xml_object_db.erl b/apps/emqx_gateway/src/lwm2m/emqx_lwm2m_xml_object_db.erl
index 19335768f..58373e114 100644
--- a/apps/emqx_gateway/src/lwm2m/emqx_lwm2m_xml_object_db.erl
+++ b/apps/emqx_gateway/src/lwm2m/emqx_lwm2m_xml_object_db.erl
@@ -57,6 +57,7 @@
 start_link(XmlDir) ->
     gen_server:start_link({local, ?MODULE}, ?MODULE, [XmlDir], []).
 
+-spec find_objectid(integer()) -> {error, no_xml_definition} | xmerl:xmlElement().
 find_objectid(ObjectId) ->
     ObjectIdInt =
         case is_list(ObjectId) of
@@ -65,9 +66,10 @@ find_objectid(ObjectId) ->
         end,
     case ets:lookup(?LWM2M_OBJECT_DEF_TAB, ObjectIdInt) of
         [] -> {error, no_xml_definition};
-        [{ObjectId, Xml}] -> Xml
+        [{_ObjectId, Xml}] -> Xml
     end.
 
+-spec find_name(string()) -> {error, no_xml_definition} | xmerl:xmlElement().
 find_name(Name) ->
     NameBinary =
         case is_list(Name) of
@@ -77,10 +79,11 @@ find_name(Name) ->
     case ets:lookup(?LWM2M_OBJECT_NAME_TO_ID_TAB, NameBinary) of
         [] ->
             {error, no_xml_definition};
-        [{NameBinary, ObjectId}] ->
+        [{_NameBinary, ObjectId}] ->
             find_objectid(ObjectId)
     end.
 
+-spec stop() -> ok.
 stop() ->
     gen_server:stop(?MODULE).
 
diff --git a/apps/emqx_gateway/src/lwm2m/include/emqx_lwm2m.hrl b/apps/emqx_gateway/src/lwm2m/include/emqx_lwm2m.hrl
index 1f02a1637..e1a6ec0d6 100644
--- a/apps/emqx_gateway/src/lwm2m/include/emqx_lwm2m.hrl
+++ b/apps/emqx_gateway/src/lwm2m/include/emqx_lwm2m.hrl
@@ -36,8 +36,39 @@
 -define(ERR_BAD_REQUEST, <<"Bad Request">>).
 -define(REG_PREFIX, <<"rd">>).
 
+%%--------------------------------------------------------------------
+%% Data formats for transferring resource information, defined in
+%% OMA-TS-LightweightM2M-V1_0_1-20170704-A
+
+%% 0: Plain text. 0 is numeric value used in CoAP Content-Format option.
+%% The plain text format is used for "Read" and "Write" operations on singular
+%% Resources. i.e: /3/0/0
+%%
+%% This data format has a Media Type of "text/plain".
 -define(LWM2M_FORMAT_PLAIN_TEXT, 0).
+
+%% 40: Link format. 40 is numeric value used in CoAP Content-Format option.
+%%
 -define(LWM2M_FORMAT_LINK, 40).
+
+%% 42: Opaque. 41 is numeric value used in CoAP Content-Format option.
+%% The opaque format is used for "Read" and "Write" operations on singular
+%% Resources where the value of the Resource is an opaque binary value.
+%% i.e: firmware images or opaque value from top layer.
+%%
+%% This data format has a Media Type of "application/octet-stream".
 -define(LWM2M_FORMAT_OPAQUE, 42).
+
+%% 11542: TLV. 11542 is numeric value used in CoAP Content-Format option.
+%% For "Read" and "Write" operation, the binary TLV format is used to represent
+%% an array of values or a single value using a compact binary representation.
+%%
+%% This data format has a Media Type of "application/vnd.oma.lwm2m+tlv".
 -define(LWM2M_FORMAT_TLV, 11542).
--define(LWMWM_FORMAT_JSON, 11543).
+
+%% 11543: JSON. 11543 is numeric value used in CoAP Content-Format option.
+%% The client may support the JSON format for "Read" and "Write" operations to
+%% represent multiple resource or single resource values.
+%%
+%% This data format has a Media Type of "application/vnd.oma.lwm2m+json".
+-define(LWM2M_FORMAT_OMA_JSON, 11543).