diff --git a/simplexmq.cabal b/simplexmq.cabal
index b7160fb05..b4923024c 100644
--- a/simplexmq.cabal
+++ b/simplexmq.cabal
@@ -103,6 +103,7 @@ library
       Simplex.Messaging.Agent.Store.SQLite.Migrations.M20231225_failed_work_items
       Simplex.Messaging.Agent.TAsyncs
       Simplex.Messaging.Agent.TRcvQueues
+      Simplex.Messaging.Builder
       Simplex.Messaging.Client
       Simplex.Messaging.Client.Agent
       Simplex.Messaging.Crypto
diff --git a/src/Simplex/FileTransfer/Client.hs b/src/Simplex/FileTransfer/Client.hs
index 1e5aa5bb7..f991798e6 100644
--- a/src/Simplex/FileTransfer/Client.hs
+++ b/src/Simplex/FileTransfer/Client.hs
@@ -13,7 +13,7 @@ import Control.Monad
 import Control.Monad.Except
 import Crypto.Random (ChaChaDRG)
 import Data.Bifunctor (first)
-import Data.ByteString.Builder (Builder)
+import qualified Data.ByteString.Builder as BB
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
 import Data.Int (Int64)
@@ -35,6 +35,7 @@ import Simplex.Messaging.Client
     transportClientConfig,
   )
 import Simplex.Messaging.Client.Agent ()
+import Simplex.Messaging.Builder (Builder, builder)
 import qualified Simplex.Messaging.Crypto as C
 import qualified Simplex.Messaging.Crypto.Lazy as LC
 import Simplex.Messaging.Encoding.String
@@ -152,9 +153,9 @@ sendXFTPTransmission XFTPClient {config, http2Client = http2@HTTP2Client {sessio
       _ -> pure (r, body)
     Left e -> throwError $ PCEResponseError e
   where
-    streamBody :: (Builder -> IO ()) -> IO () -> IO ()
+    streamBody :: (BB.Builder -> IO ()) -> IO () -> IO ()
     streamBody send done = do
-      send t
+      send $ builder t
       forM_ chunkSpec_ $ \XFTPChunkSpec {filePath, chunkOffset, chunkSize} ->
         withFile filePath ReadMode $ \h -> do
           hSeek h AbsoluteSeek $ fromIntegral chunkOffset
diff --git a/src/Simplex/FileTransfer/Protocol.hs b/src/Simplex/FileTransfer/Protocol.hs
index 659db2201..dad1b059f 100644
--- a/src/Simplex/FileTransfer/Protocol.hs
+++ b/src/Simplex/FileTransfer/Protocol.hs
@@ -17,7 +17,7 @@ import Control.Applicative ((<|>))
 import qualified Data.Aeson.TH as J
 import qualified Data.Attoparsec.ByteString.Char8 as A
 import Data.Bifunctor (first)
-import Data.ByteString.Builder (Builder)
+import Simplex.Messaging.Builder (Builder)
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
 import Data.Kind (Type)
diff --git a/src/Simplex/FileTransfer/Server.hs b/src/Simplex/FileTransfer/Server.hs
index 661b4dc98..c1fc4be41 100644
--- a/src/Simplex/FileTransfer/Server.hs
+++ b/src/Simplex/FileTransfer/Server.hs
@@ -43,6 +43,7 @@ import Simplex.FileTransfer.Server.Stats
 import Simplex.FileTransfer.Server.Store
 import Simplex.FileTransfer.Server.StoreLog
 import Simplex.FileTransfer.Transport
+import Simplex.Messaging.Builder (builder)
 import qualified Simplex.Messaging.Crypto as C
 import qualified Simplex.Messaging.Crypto.Lazy as LC
 import Simplex.Messaging.Encoding.String
@@ -241,7 +242,7 @@ processRequest HTTP2Request {sessionId, reqBody = body@HTTP2Body {bodyHead}, sen
               send "padding error" -- TODO respond with BLOCK error?
               done
             Right t -> do
-              send t
+              send $ builder t
               -- timeout sending file in the same way as receiving
               forM_ serverFile_ $ \ServerFile {filePath, fileSize, sbState} -> do
                 withFile filePath ReadMode $ \h -> sendEncFile h send sbState (fromIntegral fileSize)
diff --git a/src/Simplex/Messaging/Client.hs b/src/Simplex/Messaging/Client.hs
index aac4b69da..7d69de1c7 100644
--- a/src/Simplex/Messaging/Client.hs
+++ b/src/Simplex/Messaging/Client.hs
@@ -85,10 +85,10 @@ import Control.Monad
 import Control.Monad.IO.Class (liftIO)
 import Control.Monad.Trans.Except
 import qualified Data.Aeson.TH as J
-import Data.ByteString.Builder (Builder, lazyByteString)
+import Simplex.Messaging.Builder (Builder)
+import qualified Simplex.Messaging.Builder as BB
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
-import qualified Data.ByteString.Lazy.Char8 as LB
 import Data.Functor (($>))
 import Data.Int (Int64)
 import Data.List (find)
@@ -675,14 +675,12 @@ sendProtocolCommand c@ProtocolClient {client_ = PClient {sndQ}, batch, blockSize
     -- two separate "atomically" needed to avoid blocking
     sendRecv :: SentRawTransmission -> Request err msg -> IO (Either (ProtocolClientError err) msg)
     sendRecv t r
-      | sLen > blockSize - 2 = pure $ Left $ PCETransportError TELargeMsg
+      | BB.length s > blockSize - 2 = pure $ Left $ PCETransportError TELargeMsg
       | otherwise = atomically (writeTBQueue sndQ s) >> response <$> getResponse c r
       where
-        (sLen, s)
-          | batch = (tLen + 3, tEncodeBatch 1 . encodeLarge $ tEncode t)
-          | otherwise = (tLen, lazyByteString $ tEncode t)
-        t' = tEncode t
-        tLen = fromIntegral $ LB.length t'
+        s
+          | batch = tEncodeBatch 1 . encodeLarge $ tEncode t
+          | otherwise = tEncode t
 
 -- TODO switch to timeout or TimeManager that supports Int64
 getResponse :: ProtocolClient err msg -> Request err msg -> IO (Response err msg)
diff --git a/src/Simplex/Messaging/Crypto.hs b/src/Simplex/Messaging/Crypto.hs
index 174956751..64e20543a 100644
--- a/src/Simplex/Messaging/Crypto.hs
+++ b/src/Simplex/Messaging/Crypto.hs
@@ -191,10 +191,10 @@ import Data.ByteArray (ByteArrayAccess)
 import qualified Data.ByteArray as BA
 import Data.ByteString.Base64 (decode, encode)
 import qualified Data.ByteString.Base64.URL as U
-import Data.ByteString.Builder (Builder, byteString, toLazyByteString, word16BE)
+import Simplex.Messaging.Builder (Builder, byteString, word16BE)
+import qualified Simplex.Messaging.Builder as BB
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
-import qualified Data.ByteString.Lazy.Char8 as LB
 import Data.ByteString.Lazy (fromStrict, toStrict)
 import Data.Constraint (Dict (..))
 import Data.Kind (Constraint, Type)
@@ -924,10 +924,10 @@ pad msg paddedLen
 
 pad' :: Builder -> Int -> Either CryptoError Builder
 pad' msg paddedLen
-  | len <= maxMsgLen && padLen >= 0 = Right $ byteString (encodeWord16 $ fromIntegral len) <> msg <> byteString (B.replicate padLen '#')
+  | len <= maxMsgLen && padLen >= 0 = Right $ word16BE (fromIntegral len) <> msg <> byteString (B.replicate padLen '#')
   | otherwise = Left CryptoLargeMsgError
   where
-    len = fromIntegral $ LB.length $ toLazyByteString msg
+    len = BB.length msg
     padLen = paddedLen - len - 2
 
 unPad :: ByteString -> Either CryptoError ByteString
diff --git a/src/Simplex/Messaging/Encoding.hs b/src/Simplex/Messaging/Encoding.hs
index ac9c8b7d8..9c5f331e2 100644
--- a/src/Simplex/Messaging/Encoding.hs
+++ b/src/Simplex/Messaging/Encoding.hs
@@ -22,10 +22,10 @@ where
 import Data.Attoparsec.ByteString.Char8 (Parser)
 import qualified Data.Attoparsec.ByteString.Char8 as A
 import Data.Bits (shiftL, shiftR, (.|.))
-import Data.ByteString.Builder (Builder, byteString, lazyByteString, word16BE)
+import Simplex.Messaging.Builder (Builder, word16BE)
+import qualified Simplex.Messaging.Builder as BB
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
-import qualified Data.ByteString.Lazy.Char8 as LB
 import Data.ByteString.Internal (c2w, w2c)
 import Data.Int (Int64)
 import qualified Data.List.NonEmpty as L
@@ -141,8 +141,8 @@ instance Encoding Large where
     Large <$> A.take len
   {-# INLINE smpP #-}
 
-encodeLarge :: LB.ByteString -> Builder
-encodeLarge s = byteString (encodeWord16 $ fromIntegral $ LB.length s) <> lazyByteString s
+encodeLarge :: Builder -> Builder
+encodeLarge s = word16BE (fromIntegral $ BB.length s) <> s
 {-# INLINE encodeLarge #-}
 
 instance Encoding SystemTime where
diff --git a/src/Simplex/Messaging/Protocol.hs b/src/Simplex/Messaging/Protocol.hs
index 98f282ca5..d4e20bc96 100644
--- a/src/Simplex/Messaging/Protocol.hs
+++ b/src/Simplex/Messaging/Protocol.hs
@@ -161,7 +161,8 @@ import Data.Aeson (FromJSON (..), ToJSON (..))
 import qualified Data.Aeson.TH as J
 import Data.Attoparsec.ByteString.Char8 (Parser, (<?>))
 import qualified Data.Attoparsec.ByteString.Char8 as A
-import Data.ByteString.Builder (Builder, char8, lazyByteString)
+import Simplex.Messaging.Builder (Builder, char8, lazyByteString)
+import qualified Simplex.Messaging.Builder as BB
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
 import qualified Data.ByteString.Lazy.Char8 as LB
@@ -1313,31 +1314,29 @@ batchTransmissions batch bSize = batchTransmissions' batch bSize . L.map (,())
 -- | encodes and batches transmissions into blocks,
 batchTransmissions' :: forall r. Bool -> Int -> NonEmpty (SentRawTransmission, r) -> [TransportBatch r]
 batchTransmissions' batch bSize
-  | batch = addBatch . foldr addTransmission ([], mempty, 0, 0, [])
+  | batch = addBatch . foldr addTransmission ([], mempty, 0, [])
   | otherwise = map mkBatch1 . L.toList
   where
     mkBatch1 :: (SentRawTransmission, r) -> TransportBatch r
     mkBatch1 (t, r)
       -- 2 bytes are reserved for pad size
-      | LB.length s <= fromIntegral (bSize - 2) = TBTransmission (lazyByteString s) r
+      | BB.length s <= fromIntegral (bSize - 2) = TBTransmission s r
       | otherwise = TBLargeTransmission r
       where
         s = tEncode t
-    addTransmission :: (SentRawTransmission, r) -> ([TransportBatch r], Builder, Int, Int, [r]) -> ([TransportBatch r], Builder, Int, Int, [r])
-    addTransmission (t, r) acc@(bs, b, len, n, rs)
-      | len' <= bSize - 3 && n < 255 = (bs, s <> b, len', 1 + n, r : rs)
-      | sLen <= bSize - 3 = (addBatch acc, s, sLen, 1, [r])
-      | otherwise = (TBLargeTransmission r : addBatch acc, mempty, 0, 0, [])
+    addTransmission :: (SentRawTransmission, r) -> ([TransportBatch r], Builder, Int, [r]) -> ([TransportBatch r], Builder, Int, [r])
+    addTransmission (t, r) acc@(bs, b, n, rs)
+      | len + BB.length b <= bSize - 3 && n < 255 = (bs, s <> b, 1 + n, r : rs)
+      | len <= bSize - 3 = (addBatch acc, s, 1, [r])
+      | otherwise = (TBLargeTransmission r : addBatch acc, mempty, 0, [])
       where
-        s = encodeLarge t'
-        sLen = 2 + fromIntegral (LB.length t') -- 2-bytes length is added by encodeLarge
-        t' = tEncode t
-        len' = sLen + len
-    addBatch :: ([TransportBatch r], Builder, Int, Int, [r]) -> [TransportBatch r]
-    addBatch (bs, b, _, n, rs) = if n == 0 then bs else TBTransmissions b n rs : bs
+        s = encodeLarge $ tEncode t
+        len = BB.length s
+    addBatch :: ([TransportBatch r], Builder, Int, [r]) -> [TransportBatch r]
+    addBatch (bs, b, n, rs) = if n == 0 then bs else TBTransmissions b n rs : bs
 
-tEncode :: SentRawTransmission -> LB.ByteString
-tEncode (sig, t) = LB.chunk (smpEncode $ C.signatureBytes sig) (LB.fromStrict t)
+tEncode :: SentRawTransmission -> Builder
+tEncode (sig, t) = lazyByteString $ LB.chunk (smpEncode $ C.signatureBytes sig) (LB.fromStrict t)
 {-# INLINE tEncode #-}
 
 tEncodeBatch :: Int -> Builder -> Builder
diff --git a/src/Simplex/Messaging/Transport.hs b/src/Simplex/Messaging/Transport.hs
index a1ff259ad..e0c6e088f 100644
--- a/src/Simplex/Messaging/Transport.hs
+++ b/src/Simplex/Messaging/Transport.hs
@@ -67,7 +67,7 @@ import qualified Data.Aeson.TH as J
 import Data.Attoparsec.ByteString.Char8 (Parser)
 import Data.Bifunctor (first)
 import Data.Bitraversable (bimapM)
-import Data.ByteString.Builder (Builder, byteString, toLazyByteString)
+import Simplex.Messaging.Builder (Builder, byteString, toLazyByteString)
 import Data.ByteString.Char8 (ByteString)
 import qualified Data.ByteString.Char8 as B
 import qualified Data.ByteString.Lazy.Char8 as LB
diff --git a/tests/AgentTests/NotificationTests.hs b/tests/AgentTests/NotificationTests.hs
index 9e88c7e60..8d1ef241f 100644
--- a/tests/AgentTests/NotificationTests.hs
+++ b/tests/AgentTests/NotificationTests.hs
@@ -508,7 +508,7 @@ testNotificationsSMPRestartBatch n t APNSMockServer {apnsQ} = do
     conns <- replicateM (n :: Int) $ makeConnection a b
     _ <- registerTestToken a "abcd" NMInstant apnsQ
     liftIO $ threadDelay 5000000
-    forM_ (zip [0..] conns) $ \(i, (aliceId, bobId)) -> do
+    forM_ conns $ \(aliceId, bobId) -> do
       msgId <- sendMessage b aliceId (SMP.MsgFlags True) "hello"
       get b ##> ("", aliceId, SENT msgId)
       void $ messageNotification apnsQ
diff --git a/tests/CoreTests/BatchingTests.hs b/tests/CoreTests/BatchingTests.hs
index 28f9f0435..c21ec8c7a 100644
--- a/tests/CoreTests/BatchingTests.hs
+++ b/tests/CoreTests/BatchingTests.hs
@@ -4,10 +4,10 @@ module CoreTests.BatchingTests (batchingTests) where
 
 import Control.Concurrent.STM
 import Control.Monad
-import Data.ByteString.Builder (Builder, toLazyByteString)
 import Data.ByteString.Char8 (ByteString)
-import qualified Data.ByteString.Lazy.Char8 as LB
 import qualified Data.List.NonEmpty as L
+import Simplex.Messaging.Builder (Builder)
+import qualified Simplex.Messaging.Builder as BB
 import Simplex.Messaging.Client
 import qualified Simplex.Messaging.Crypto as C
 import Simplex.Messaging.Protocol
@@ -171,9 +171,7 @@ randomSENDCmd c len = do
   mkTransmission c (Just rpKey, sId, Cmd SSender $ SEND noMsgFlags msg)
 
 lenOk :: Builder -> Bool
-lenOk s = 0 < len && len <= smpBlockSize - 2
-  where
-    len = fromIntegral . LB.length $ toLazyByteString s
+lenOk s = 0 < BB.length s && BB.length s <= smpBlockSize - 2
 
 lenOk1 :: TransportBatch r -> Bool
 lenOk1 = \case