diff --git a/definitions/src/Simplex/Messaging/Broker.hs b/definitions/src/Simplex/Messaging/Broker.hs
index eb430b90fa..16eadaf5a8 100644
--- a/definitions/src/Simplex/Messaging/Broker.hs
+++ b/definitions/src/Simplex/Messaging/Broker.hs
@@ -1,7 +1,6 @@
 {-# OPTIONS_GHC -fno-warn-unticked-promoted-constructors #-}
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 {-# LANGUAGE DataKinds             #-}
-{-# LANGUAGE FlexibleInstances     #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 
 module Simplex.Messaging.Broker where
@@ -10,6 +9,12 @@ import ClassyPrelude
 import Simplex.Messaging.Protocol
 import Simplex.Messaging.Types
 
+-- For some reason PrfCmd type-class requirement is not enforced here:
+-- if I change one of the connection states to one for which
+-- instance of PrfCmd does not exist (i.e. Command cannot be constructed),
+-- it compiles anyway.
+-- Creating Command instance here would prevent compilation
+-- in case the types are incorrect, as it is done in Client.hs
 instance ProtocolFunc Broker CreateConnCmd
          CreateConnRequest CreateConnResponse -- request response
          None New Idle Idle                   -- connection states
@@ -22,3 +27,17 @@ bCreateConn :: Connection Broker None Idle
                       (CreateConnResponse, Connection Broker New Idle)
 bCreateConn (Connection str) _ = Right (CreateConnResponse "1" "2", Connection str)
 -- TODO stub
+
+
+-- below code should NOT compile, but it does
+instance ProtocolFunc Broker CreateConnCmd
+         CreateConnRequest CreateConnResponse -- request response
+         None Secured Idle Idle -- "Secured" should not be allowed here,
+         where                  -- such command does not exist, so no instance of
+  protoFunc _ = bCreateConn'    -- PrfCmd exist...? But it compiles.
+
+bCreateConn' :: Connection Broker None Idle
+            -> CreateConnRequest
+            -> Either String
+                      (CreateConnResponse, Connection Broker Secured Idle)
+bCreateConn' (Connection str) _ = Right (CreateConnResponse "1" "2", Connection str)
diff --git a/definitions/src/Simplex/Messaging/Client.hs b/definitions/src/Simplex/Messaging/Client.hs
index c71e1ccd03..96d4bbbf47 100644
--- a/definitions/src/Simplex/Messaging/Client.hs
+++ b/definitions/src/Simplex/Messaging/Client.hs
@@ -1,8 +1,8 @@
 {-# OPTIONS_GHC -fno-warn-unticked-promoted-constructors #-}
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 {-# LANGUAGE DataKinds             #-}
-{-# LANGUAGE FlexibleInstances     #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE TypeApplications      #-}
 
 module Simplex.Messaging.Client where
 
@@ -15,11 +15,11 @@ instance ProtocolAction Recipient CreateConnCmd
          CreateConnRequest CreateConnResponse  -- request responce
          None New Idle Idle                    -- connection states
          where 
-  protoAction _ = rCreateConn
+  protoAction _ = \(Connection str) _ _ -> Connection str --  TODO stub
 
 
 rCreateConn :: Connection Recipient None Idle
             -> CreateConnRequest
             -> Either String CreateConnResponse
             -> Connection Recipient New Idle
-rCreateConn (Connection str) _ _ = Connection str --  TODO stub
+rCreateConn = protoAction $ CreateConn @None
diff --git a/definitions/src/Simplex/Messaging/Protocol.hs b/definitions/src/Simplex/Messaging/Protocol.hs
index 3ee78f0b2d..b3883a8c0f 100644
--- a/definitions/src/Simplex/Messaging/Protocol.hs
+++ b/definitions/src/Simplex/Messaging/Protocol.hs
@@ -297,20 +297,26 @@ type family PConnSt (p :: Participant) state where
   PConnSt Sender    (AllConnState r b s) = s
 
 
--- type class to ensure consistency of types of implementations
+-- Type classes to ensure consistency of types of implementations
 -- of participants actions/functions and connection state transitions (types)
--- with types of protocol commands defined here
+-- with types of protocol commands defined above.
 
--- TODO it still allows to construct invalid implementations
--- there should be added proof that such command can be constructed
--- (or it should be constructed by implementations, but it looks ugly)
+-- TODO for some reason this type class is not enforced -
+-- it still allows to construct invalid implementations.
+-- See comment in Broker.hs
+-- As done in Client.hs it type-checks, but it looks ugly
 class PrfCmd cmd arg res from to s s' ss ss' n n' where
   command :: Command cmd arg res from to s s' ss ss' n n'
+instance Prf HasState Sender s
+         => PrfCmd CreateConnCmd
+              CreateConnRequest CreateConnResponse
+              Recipient Broker
+              (AllConnState None None s)
+              (AllConnState New New s)
+              Idle Idle 0 0
+              where
+  command = CreateConn
 
--- TODO have to be specific commands, as is it allows to construct any command
--- not matching any real command in the protocol
-instance PrfCmd cmd arg res from to s s' ss ss' n n' where
-  command = AnyCmd @cmd @arg @res @from @to @s @s' @ss @ss' @n @n'
 
 class ProtocolFunc (p :: Participant) (cmd :: ProtocolCmd) arg res ps ps' ss ss' where
   protoFunc    :: ( PrfCmd cmd arg res from p s s' ss ss' n n'