diff --git a/avers-server.cabal b/avers-server.cabal
--- a/avers-server.cabal
+++ b/avers-server.cabal
@@ -1,5 +1,5 @@
 name:                avers-server
-version:             0.0.3
+version:             0.0.4
 synopsis:            Server implementation of the Avers API
 description:         See README.md
 homepage:            http://github.com/wereHamster/avers-server
diff --git a/src/Avers/Server.hs b/src/Avers/Server.hs
--- a/src/Avers/Server.hs
+++ b/src/Avers/Server.hs
@@ -6,6 +6,7 @@
 {-# LANGUAGE TypeFamilies        #-}
 {-# LANGUAGE FlexibleInstances   #-}
 {-# LANGUAGE TypeOperators       #-}
+{-# LANGUAGE FlexibleContexts    #-}
 
 module Avers.Server
     ( serveAversCoreAPI, serveAversSessionAPI
@@ -18,8 +19,6 @@
 
 import           Control.Monad
 import           Control.Monad.Except
--- import Control.Monad.IO.Class
-import           Control.Monad.Trans.Either
 
 import           Control.Concurrent
 import           Control.Concurrent.STM
@@ -77,23 +76,23 @@
 
 -- | Convert the 'Credentials' into an 'ObjId' to which the ceredentials refer.
 -- That's the object the client is authenticated as.
-credentialsObjId :: Handle -> Credentials -> EitherT ServantErr IO ObjId
+credentialsObjId :: Handle -> Credentials -> ExceptT ServantErr IO ObjId
 credentialsObjId aversH cred = do
     errOrObjId <- case cred of
         SessionIdCredential sId -> liftIO $ evalAvers aversH $
             sessionObjId <$> lookupSession sId
 
     case errOrObjId of
-        Left _  -> left err401
+        Left _  -> throwError err401
         Right s -> pure s
 
 
-failWith :: Text -> EitherT ServantErr IO b
-failWith e = left $ err500
+failWith :: Text -> ExceptT ServantErr IO b
+failWith e = throwError $ err500
     { errBody = LBS.fromChunks [T.encodeUtf8 e] }
 
 
-aversResult :: Either AversError a -> EitherT ServantErr IO a
+aversResult :: Either AversError a -> ExceptT ServantErr IO a
 aversResult res = case res of
     Left e -> case e of
         DatabaseError detail                  -> failWith $ "database " <> detail
@@ -108,15 +107,15 @@
     Right r -> pure r
 
 
-reqAvers :: Handle -> Avers a -> EitherT ServantErr IO a
+reqAvers :: Handle -> Avers a -> ExceptT ServantErr IO a
 reqAvers aversH m = liftIO (evalAvers aversH m) >>= aversResult
 
 
-cacheableResponse :: (ToJSON a) => Maybe Text -> a -> EitherT ServantErr IO (Cacheable a)
+cacheableResponse :: (ToJSON a) => Maybe Text -> a -> ExceptT ServantErr IO (Cacheable a)
 cacheableResponse mbValidationToken a = do
     let etag = T.decodeUtf8 $ BS64.encode $ hashlazy $ encode a
     if mbValidationToken == Just (etagVersion <> ":" <> etag)
-        then left $ err304
+        then throwError err304
         else pure $ addHeader "no-cache, public, max-age=63072000"
                   $ addHeader (etagVersion <> ":" <> etag) a
 
@@ -202,7 +201,7 @@
             , porResultingPatches = resultingPatches
             }
 
-    serveDeleteObject _ _ = left err500
+    serveDeleteObject _ _ = throwError err500
 
 
     ----------------------------------------------------------------------------
@@ -239,9 +238,9 @@
 
 
 
-    serveCreateRelease _ _ _ = left err500
-    serveLookupRelease _ _ _ _ = left err500
-    serveLookupLatestRelease _ _ _ = left err500
+    serveCreateRelease _ _ _ = throwError err500
+    serveLookupRelease _ _ _ _ = throwError err500
+    serveLookupLatestRelease _ _ _ = throwError err500
 
 
     ----------------------------------------------------------------------------
@@ -277,7 +276,7 @@
             loop connection subscriptions chan
 
 
-    serveChangeSecret _ _ = left err500
+    serveChangeSecret _ _ = throwError err500
 
 
 
@@ -293,7 +292,7 @@
     sessionCookieName     = "session"
     sessionExpirationTime = 2 * 365 * 24 * 60 * 60
 
-    mkSetCookie :: SessionId -> EitherT ServantErr IO SetCookie
+    mkSetCookie :: SessionId -> ExceptT ServantErr IO SetCookie
     mkSetCookie sId = do
         now <- liftIO $ getCurrentTime
         pure $ def
diff --git a/src/Avers/Server/Authorization.hs b/src/Avers/Server/Authorization.hs
--- a/src/Avers/Server/Authorization.hs
+++ b/src/Avers/Server/Authorization.hs
@@ -11,7 +11,7 @@
 
 
 import Control.Monad.IO.Class
-import Control.Monad.Trans.Either
+import Control.Monad.Except
 
 import Data.Text (Text)
 
@@ -64,16 +64,16 @@
 --------------------------------------------------------------------------------
 -- | Run the authorization logic inside of the Servant monad.
 
-runAuthorization :: Handle -> Authz -> EitherT ServantErr IO ()
+runAuthorization :: Handle -> Authz -> ExceptT ServantErr IO ()
 runAuthorization _      []     = pure ()
 runAuthorization aversH (x:xs) = do
     res <- liftIO $ evalAvers aversH x
     case res of
-        Left _  -> left $ err500
+        Left _  -> throwError err500
         Right r -> case r of
             ContinueR -> runAuthorization aversH xs
             AllowR    -> pure ()
-            RejectR   -> left $ err401
+            RejectR   -> throwError err401
 
 
 
diff --git a/src/Avers/Server/Instances.hs b/src/Avers/Server/Instances.hs
--- a/src/Avers/Server/Instances.hs
+++ b/src/Avers/Server/Instances.hs
@@ -1,13 +1,14 @@
-{-# LANGUAGE OverloadedStrings   #-}
+{-# LANGUAGE OverloadedStrings     #-}
 
-{-# LANGUAGE DataKinds           #-}
-{-# LANGUAGE TypeFamilies        #-}
-{-# LANGUAGE FlexibleInstances   #-}
-{-# LANGUAGE TypeOperators       #-}
+{-# LANGUAGE DataKinds             #-}
+{-# LANGUAGE TypeFamilies          #-}
+{-# LANGUAGE FlexibleInstances     #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE TypeOperators         #-}
 
-{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE ScopedTypeVariables   #-}
 
-{-# OPTIONS_GHC -fno-warn-orphans #-}
+{-# OPTIONS_GHC -fno-warn-orphans  #-}
 
 module Avers.Server.Instances where
 
@@ -22,43 +23,47 @@
 import Servant.Server
 import Servant.Server.Internal
 
-import Network.HTTP.Types.Status
-
 import Network.Wai
 
 import Web.Cookie
 
 
 
-instance (HasServer sublayout) => HasServer (Credentials :> sublayout) where
+instance (HasServer sublayout context) => HasServer (Credentials :> sublayout) context where
 
     type ServerT (Credentials :> sublayout) m =
         Credentials -> ServerT sublayout m
 
-    route Proxy subserver request respond = do
-        let mbCookieHeaders = lookup "cookie" (requestHeaders request)
-            mbSessionIdText = fromText =<< lookup "session" =<< fmap parseCookiesText mbCookieHeaders
-            mbCredentials   = fmap (SessionIdCredential . SessionId) mbSessionIdText
+    route Proxy context subserver = WithRequest $ \request ->
+        route (Proxy :: Proxy sublayout) context (addCapture subserver $ do
+            let mbCookieHeaders = lookup "cookie" (requestHeaders request)
+                mbSessionIdText = lookup "session" =<< fmap parseCookiesText mbCookieHeaders
 
-        case mbCredentials of
-            Nothing -> respond $ RR $ Right $ responseLBS status401 [] ""
-            Just cred -> route (Proxy :: Proxy sublayout) (subserver cred) request respond
+            pure $ case mbSessionIdText of
+                Nothing -> FailFatal err401
+                Just sessionIdText -> case parseQueryParam sessionIdText of
+                    Left _ -> FailFatal err401
+                    Right sId -> Route $ (SessionIdCredential $ SessionId sId)
+        )
 
 
 
-instance (HasServer sublayout) => HasServer (SessionId :> sublayout) where
+instance (HasServer sublayout context) => HasServer (SessionId :> sublayout) context where
 
     type ServerT (SessionId :> sublayout) m =
         SessionId -> ServerT sublayout m
 
-    route Proxy subserver request respond = do
-        let mbCookieHeaders = lookup "cookie" (requestHeaders request)
-            mbSessionIdText = fromText =<< lookup "session" =<< fmap parseCookiesText mbCookieHeaders
-            mbSessionId     = fmap SessionId mbSessionIdText
+    route Proxy context subserver = WithRequest $ \request ->
+        route (Proxy :: Proxy sublayout) context (addCapture subserver $ do
+            let mbCookieHeaders = lookup "cookie" (requestHeaders request)
+                mbSessionIdText = lookup "session" =<< fmap parseCookiesText mbCookieHeaders
 
-        case mbSessionId of
-            Nothing -> respond $ RR $ Right $ responseLBS status401 [] ""
-            Just sId -> route (Proxy :: Proxy sublayout) (subserver sId) request respond
+            pure $ case mbSessionIdText of
+                Nothing -> FailFatal err401
+                Just sessionIdText -> case parseQueryParam sessionIdText of
+                    Left _ -> FailFatal err401
+                    Right sId -> Route $ SessionId sId
+        )
 
 
 instance ToByteString SetCookie where