diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -12,9 +12,9 @@
 
 Callbacks are configured through the `$XDG_CONFIG_HOME/imm/callbacks.dhall` file. A commented template file is bundled with the project.
 
-`imm` will call each callback once per feed element, and will fill its standard input (`stdin`) with a JSON object structured as follows:
+`imm` will call each callback once per feed element, and will fill its standard input (`stdin`) with a [MessagePack][4]-encoded object structured as follows:
 
-```json
+```
 {
   "feed": "RSS document or Atom feed (XML)",
   "element": "RSS item or Atom entry (XML)"
@@ -107,3 +107,4 @@
 [1]: http://hackage.haskell.org/package/imm
 [2]: https://www.haskell.org
 [3]: https://dhall-lang.org/
+[4]: https://msgpack.org/
diff --git a/imm.cabal b/imm.cabal
--- a/imm.cabal
+++ b/imm.cabal
@@ -1,6 +1,6 @@
 cabal-version:       2.2
 name:                imm
-version:             1.6.1.0
+version:             1.7.0.0
 synopsis:            Execute arbitrary callbacks for each element of RSS/Atom feeds
 description:         Cf README file
 homepage:            https://github.com/k0ral/imm
@@ -48,6 +48,7 @@
     filepath,
     hashable,
     http-types,
+    msgpack,
     microlens,
     monad-time,
     prettyprinter,
@@ -67,7 +68,7 @@
 
 executable imm
   import: common
-  build-depends: imm, aeson, async, atom-conduit >=0.7, bytestring, case-insensitive, conduit, connection, containers, dhall, directory, fast-logger, filepath, http-client >=0.4.30, http-client-tls, opml-conduit >=0.7, optparse-applicative, prettyprinter-ansi-terminal, refined >=0.4.1, rss-conduit >=0.4.1, safe-exceptions, stm, stm-chans, streaming-bytestring, streaming-with, text, typed-process, uri-bytestring, xml-conduit >=1.5, xml-types
+  build-depends: imm, aeson, async, atom-conduit >=0.7, bytestring, case-insensitive, conduit, connection, containers, dhall, directory, fast-logger, filepath, http-client >=0.4.30, http-client-tls, msgpack, opml-conduit >=0.7, optparse-applicative, prettyprinter-ansi-terminal, refined >=0.4.1, rss-conduit >=0.4.1, safe-exceptions, stm, stm-chans, streaming-bytestring, streaming-with, text, typed-process, uri-bytestring, xml-conduit >=1.5, xml-types
   main-is: Main.hs
   other-modules: Core, Database, HTTP, Logger, Options, Paths_imm, XML
   autogen-modules: Paths_imm
@@ -76,14 +77,14 @@
 
 executable imm-writefile
   import: common
-  build-depends: imm, aeson, atom-conduit, blaze-html, blaze-markup, bytestring, directory, filepath, optparse-applicative, prettyprinter, rss-conduit, streaming-bytestring, streaming-with, text, time, uri-bytestring
+  build-depends: imm, atom-conduit, blaze-html, blaze-markup, bytestring, directory, filepath, msgpack, optparse-applicative, prettyprinter, rss-conduit, streaming-bytestring, streaming-with, text, time, uri-bytestring
   main-is: Main.hs
   hs-source-dirs: src/write-file
   ghc-options: -Wall -fno-warn-unused-do-bind -threaded
 
 executable imm-sendmail
   import: common
-  build-depends: imm, aeson, atom-conduit, blaze-html, blaze-markup, bytestring, directory, filepath, HaskellNet >=0.5, HaskellNet-SSL >= 0.3.3.0, mime-mail, network <3, optparse-applicative, prettyprinter, refined, rss-conduit, text, time, uri-bytestring
+  build-depends: imm, atom-conduit, blaze-html, blaze-markup, bytestring, directory, filepath, HaskellNet >=0.5, HaskellNet-SSL >= 0.3.3.0, msgpack, mime-mail, network <3, optparse-applicative, prettyprinter, refined, rss-conduit, text, time, uri-bytestring
   main-is: Main.hs
   hs-source-dirs: src/send-mail
   ghc-options: -Wall -fno-warn-unused-do-bind -threaded
diff --git a/src/lib/Imm/Callback.hs b/src/lib/Imm/Callback.hs
--- a/src/lib/Imm/Callback.hs
+++ b/src/lib/Imm/Callback.hs
@@ -1,14 +1,15 @@
 {-# LANGUAGE DeriveGeneric     #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE TypeApplications  #-}
 module Imm.Callback where
 
 -- {{{ Imports
 import           Imm.Feed
 
-import           Data.Aeson
-import           Data.Aeson.Types
+import qualified Data.Map                  as Map
+import           Data.MessagePack.Object
 import           Data.Text.Prettyprint.Doc
-import           Dhall
+import           Dhall                     hiding (maybe)
 -- }}}
 
 -- | External program run for each feed element.
@@ -28,12 +29,13 @@
 -- | All information passed to external programs about a new feed item, are stored in this structure.
 data Message = Message Feed FeedElement deriving(Eq, Generic, Ord, Show)
 
-instance ToJSON Message where
-  toJSON (Message feed element) = object ["feed" .= renderFeed feed, "element" .= renderFeedElement element]
+instance MessagePack Message where
+  toObject (Message feed element) = toObject @(Map Text Text)
+    $ Map.insert "feed" (renderFeed feed)
+    $ Map.insert "element" (renderFeedElement element) mempty
 
-instance FromJSON Message where
-  parseJSON = withObject "Message" $ \v -> Message
-    <$> (v .: "feed" >>= (liftEither . parseFeed))
-    <*> (v .: "element" >>= (liftEither . parseFeedElement))
-    where liftEither :: Either e a -> Parser a
-          liftEither = either (const mempty) return
+  fromObject object = fromObject object >>= \m -> Message
+    <$> (lookup @(Map Text Text) "feed" m >>= eitherToMaybe . parseFeed)
+    <*> (lookup @(Map Text Text) "element" m >>= eitherToMaybe . parseFeedElement)
+    where eitherToMaybe (Right a) = Just a
+          eitherToMaybe _         = Nothing
diff --git a/src/main/Main.hs b/src/main/Main.hs
--- a/src/main/Main.hs
+++ b/src/main/Main.hs
@@ -22,9 +22,9 @@
 import           Control.Concurrent.Async
 import           Control.Concurrent.STM.TMChan
 import           Control.Exception.Safe
-import           Data.Aeson                    hiding (Error)
 import           Data.Conduit.Combinators      as Conduit (stdin)
 import qualified Data.Map                      as Map
+import qualified Data.MessagePack              as MsgPack
 import qualified Data.Text                     as Text
 import qualified Data.Text.IO                  as Text
 import           Dhall
@@ -123,7 +123,7 @@
     newItem <- atomically $ readTMChan newItemsChan
     forM_ newItem $ \(feedID, feed, element) -> do
       results <- forM callbacks $ \callback@(Callback executable arguments) -> do
-        let processInput = byteStringInput $ encode $ Message feed element
+        let processInput = byteStringInput $ MsgPack.pack $ Message feed element
             processConfig = proc executable (map toString arguments) & setStdin processInput
 
         log logger Debug $ "Running" <+> cyan (pretty executable) <+> "on" <+> magenta (pretty feedID) <+> "/" <+> yellow (pretty $ getTitle element)
diff --git a/src/send-mail/Main.hs b/src/send-mail/Main.hs
--- a/src/send-mail/Main.hs
+++ b/src/send-mail/Main.hs
@@ -7,8 +7,8 @@
 import           Imm.Feed
 import           Imm.Pretty
 
-import           Data.Aeson
 import           Data.ByteString             (getContents)
+import qualified Data.MessagePack            as MsgPack
 import           Data.Text                   as Text (intercalate)
 import           Data.Time
 import           Network.HaskellNet.SMTP
@@ -103,15 +103,15 @@
 main = do
   CliOptions smtpServer recipients dryRun <- parseOptions
 
-  message <- getContents <&> fromStrict <&> eitherDecode
+  message <- getContents <&> fromStrict <&> MsgPack.unpack
   case message of
-    Left e -> putStrLn e
-    Right (Message feed element) -> do
+    Just (Message feed element) -> do
       timezone <- io getCurrentTimeZone
       currentTime <- io getCurrentTime
       let formatMail = FormatMail defaultFormatFrom defaultFormatSubject defaultFormatBody (const $ const recipients)
           mail = buildMail formatMail currentTime timezone feed element
       unless dryRun $ io $ withSMTPConnection smtpServer $ sendMimeMail2 mail
+    _ -> putStrLn "Invalid input" >> exitFailure
 
   return ()
 
diff --git a/src/write-file/Main.hs b/src/write-file/Main.hs
--- a/src/write-file/Main.hs
+++ b/src/write-file/Main.hs
@@ -8,10 +8,10 @@
 import           Imm.Feed
 import           Imm.Pretty
 
-import           Data.Aeson
-import           Data.ByteString (getContents)
+import           Data.ByteString               (getContents)
 import           Data.ByteString.Builder
 import           Data.ByteString.Streaming     (toStreamingByteString)
+import qualified Data.MessagePack              as MsgPack
 import qualified Data.Text                     as Text (null, replace)
 import           Data.Time
 import           Options.Applicative
@@ -46,16 +46,16 @@
 main :: IO ()
 main = do
   CliOptions directory dryRun <- parseOptions
-  message <- getContents <&> fromStrict <&> eitherDecode
+  message <- getContents <&> fromStrict <&> MsgPack.unpack
   case message of
-    Left e -> putStrLn e
-    Right (Message feed element) -> do
+    Just (Message feed element) -> do
       let content = defaultFileContent feed element
           filePath = defaultFilePath directory feed element
       putStrLn filePath
       unless dryRun $ do
         createDirectoryIfMissing True $ takeDirectory filePath
         writeBinaryFile filePath $ toStreamingByteString content
+    _ -> putStrLn "Invalid input" >> exitFailure
   return ()
 
 -- * Default behavior