diff --git a/HyperNerd.cabal b/HyperNerd.cabal
index f6bec78..8eadac3 100644
--- a/HyperNerd.cabal
+++ b/HyperNerd.cabal
@@ -151,6 +151,25 @@ executable HyperNerd
   -- Base language which the package is written in.
   default-language:    Haskell2010
 
+executable Fuzzy
+  ghc-options:         -threaded
+                       -Wall
+                       -fwarn-incomplete-patterns
+                       -fwarn-incomplete-uni-patterns
+
+  main-is:             FuzzyMain.hs
+
+  other-modules:       Bot.Expr
+
+  build-depends:       base
+                     , text
+                     , random
+                     , aeson
+
+  hs-source-dirs:      src
+
+  default-language:    Haskell2010
+
 executable Markov
   ghc-options:         -threaded
                        -Wall
diff --git a/src/Bot/Expr.hs b/src/Bot/Expr.hs
index 034bd1f..6f8f0bb 100644
--- a/src/Bot/Expr.hs
+++ b/src/Bot/Expr.hs
@@ -7,7 +7,6 @@ import Control.Applicative
 import Data.Char
 import qualified Data.Text as T
 import Data.Tuple
-import Effect
 
 data Expr
   = TextExpr T.Text
@@ -65,14 +64,16 @@ sepBy element sep = do
   args <- many (sep >> element)
   return (arg : args)
 
+funcallarg :: Parser Expr
+funcallarg = funcall <|> var <|> stringLiteral
+
 funcall :: Parser Expr
 funcall = do
   _ <- charP '%' >> whitespaces
   name <- symbol
   _ <- whitespaces >> charP '(' >> whitespaces
   args <-
-    sepBy (funcall <|> var <|> stringLiteral) (whitespaces >> charP ',') <|>
-    return []
+    sepBy funcallarg (whitespaces >> charP ',' >> whitespaces) <|> return []
   _ <- whitespaces >> charP ')'
   return $ FunCallExpr name args
 
@@ -117,7 +118,6 @@ expr = funcall <|> var <|> textBlock
 
 exprs :: Parser [Expr]
 exprs = many expr
-
 -- TODO(#600): interpretExprs is not implemented
-interpretExprs :: NameTable -> [Expr] -> Effect T.Text
-interpretExprs = undefined
+-- interpretExprs :: NameTable -> [Expr] -> Effect T.Text
+-- interpretExprs = undefined
diff --git a/src/FuzzyMain.hs b/src/FuzzyMain.hs
new file mode 100644
index 0000000..e12bf4a
--- /dev/null
+++ b/src/FuzzyMain.hs
@@ -0,0 +1,139 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Main where
+
+import Bot.Expr
+import Control.Monad
+import Data.Aeson
+import Data.Aeson.Types
+import Data.Char
+import Data.List
+import qualified Data.Text as T
+import System.Environment
+import System.Random
+import Text.Printf
+
+data FuzzParams = FuzzParams
+  { fpFuzzCount :: Int
+  , fpExprsRange :: (Int, Int)
+  , fpFunCallArgsRange :: (Int, Int)
+  , fpWordLenRange :: (Int, Int)
+  , fpTextWordCountRange :: (Int, Int)
+  } deriving (Show, Eq)
+
+instance ToJSON FuzzParams where
+  toJSON params =
+    object
+      [ "FuzzCount" .= fpFuzzCount params
+      , "ExprsRange" .= fpExprsRange params
+      , "FunCallArgsRange" .= fpFunCallArgsRange params
+      , "WordLenRange" .= fpWordLenRange params
+      , "TextWordCountRange" .= fpTextWordCountRange params
+      ]
+
+instance FromJSON FuzzParams where
+  parseJSON (Object params) =
+    FuzzParams <$> params .: "FuzzCount" <*> params .: "ExprsRange" <*>
+    params .: "FunCallArgsRange" <*>
+    params .: "WordLenRange" <*>
+    params .: "TextWordCountRange"
+  parseJSON invalid = typeMismatch "FuzzParams" invalid
+
+readFuzzParams :: FilePath -> IO FuzzParams
+readFuzzParams = fmap (either error id) . eitherDecodeFileStrict
+
+defaultFuzzParams :: FuzzParams
+defaultFuzzParams =
+  FuzzParams
+    { fpFuzzCount = 100
+    , fpExprsRange = (1, 100)
+    , fpFunCallArgsRange = (0, 2)
+    , fpWordLenRange = (2, 10)
+    , fpTextWordCountRange = (3, 5)
+    }
+
+unparseFunCallArg :: Expr -> T.Text
+unparseFunCallArg (TextExpr text) = "\"" <> text <> "\""
+unparseFunCallArg e = unparseExpr e
+
+unparseFunCallArgs :: [Expr] -> T.Text
+unparseFunCallArgs = T.concat . intersperse "," . map unparseFunCallArg
+
+unparseExpr :: Expr -> T.Text
+unparseExpr (TextExpr text) = text
+unparseExpr (VarExpr name) = "%" <> name
+unparseExpr (FunCallExpr name args) =
+  "%" <> name <> "(" <> unparseFunCallArgs args <> ")"
+
+unparseExprs :: [Expr] -> T.Text
+unparseExprs = T.concat . map unparseExpr
+
+randomChar :: IO Char
+randomChar = do
+  x <- randomRIO (0, ord 'z' - ord 'a')
+  return $ chr (x + ord 'a')
+
+randomText :: FuzzParams -> IO T.Text
+randomText params = do
+  n <- randomRIO $ fpTextWordCountRange params
+  T.concat . intersperse " " <$> replicateM n (randomWord params)
+
+randomWord :: FuzzParams -> IO T.Text
+randomWord params = do
+  n <- randomRIO $ fpWordLenRange params
+  T.pack <$> replicateM n randomChar
+
+randomTextExpr :: FuzzParams -> IO Expr
+randomTextExpr params = TextExpr <$> randomText params
+
+randomVarExpr :: FuzzParams -> IO Expr
+randomVarExpr params = VarExpr <$> randomWord params
+
+randomFunCallExpr :: FuzzParams -> IO Expr
+randomFunCallExpr params = do
+  name <- randomWord params
+  n <- randomRIO $ fpFunCallArgsRange params
+  args <- replicateM n (randomExpr params)
+  return $ FunCallExpr name args
+
+randomExpr :: FuzzParams -> IO Expr
+randomExpr params = do
+  n <- randomRIO (0, 2) :: IO Int
+  case n of
+    0 -> randomTextExpr params
+    1 -> randomVarExpr params
+    _ -> randomFunCallExpr params
+
+normalizeExprs :: [Expr] -> [Expr]
+normalizeExprs [] = []
+normalizeExprs (TextExpr t1:TextExpr t2:rest) =
+  normalizeExprs (TextExpr (t1 <> t2) : rest)
+normalizeExprs (_:rest) = normalizeExprs rest
+
+randomExprs :: FuzzParams -> IO [Expr]
+randomExprs params = do
+  n <- randomRIO $ fpExprsRange params
+  replicateM n (randomExpr params)
+
+fuzzIteration :: FuzzParams -> IO Bool
+fuzzIteration params = do
+  es <- normalizeExprs <$> randomExprs params
+  let es' = runParser exprs $ unparseExprs es
+  when (Right ("", es) /= es') $ do
+    print es
+    print es'
+    error "test"
+  return (Right ("", es) == es')
+
+fuzz :: FuzzParams -> IO ()
+fuzz params = do
+  report <- replicateM (fpFuzzCount params) (fuzzIteration params)
+  printf "Failures: %d\n" $ length $ filter not report
+  printf "Successes: %d\n" $ length $ filter id report
+
+mainWithArgs :: [String] -> IO ()
+mainWithArgs (fuzzParamsPath:_) = readFuzzParams fuzzParamsPath >>= fuzz
+mainWithArgs _ = error "Usage: Fuzz <fuzz.json>"
+
+main :: IO ()
+main = getArgs >>= mainWithArgs
diff --git a/test/Bot/ExprTest.hs b/test/Bot/ExprTest.hs
index 4ab75fc..9ba3a06 100644
--- a/test/Bot/ExprTest.hs
+++ b/test/Bot/ExprTest.hs
@@ -52,6 +52,7 @@ exprsTest =
             , TextExpr " baz ()"
             ]))
     , ("%f(%x)", Right ("", [FunCallExpr "f" [VarExpr "x"]]))
+    , ("%f(%x, %y)", Right ("", [FunCallExpr "f" [VarExpr "x", VarExpr "y"]]))
     , ( "\"hello %x world\""
       , Right ("", [TextExpr "\"hello ", VarExpr "x", TextExpr " world\""]))
     ]