diff --git a/cabal-gild.cabal b/cabal-gild.cabal
--- a/cabal-gild.cabal
+++ b/cabal-gild.cabal
@@ -11,7 +11,7 @@
 maintainer: Taylor Fausak
 name: cabal-gild
 synopsis: Formats package descriptions.
-version: 1.5.0.3
+version: 1.6.0.0
 
 source-repository head
   type: git
@@ -75,6 +75,7 @@
     CabalGild.Unstable.Action.ReflowText
     CabalGild.Unstable.Action.Render
     CabalGild.Unstable.Action.StripBlanks
+    CabalGild.Unstable.Class.MonadHandle
     CabalGild.Unstable.Class.MonadLog
     CabalGild.Unstable.Class.MonadRead
     CabalGild.Unstable.Class.MonadWalk
@@ -84,6 +85,8 @@
     CabalGild.Unstable.Exception.InvalidLeniency
     CabalGild.Unstable.Exception.InvalidMode
     CabalGild.Unstable.Exception.InvalidOption
+    CabalGild.Unstable.Exception.MoreThanOneCabalFileFound
+    CabalGild.Unstable.Exception.NoCabalFileFound
     CabalGild.Unstable.Exception.ParseError
     CabalGild.Unstable.Exception.SpecifiedOutputWithCheckMode
     CabalGild.Unstable.Exception.SpecifiedStdinWithFileInput
diff --git a/source/library/CabalGild/Unstable/Class/MonadHandle.hs b/source/library/CabalGild/Unstable/Class/MonadHandle.hs
new file mode 100644
--- /dev/null
+++ b/source/library/CabalGild/Unstable/Class/MonadHandle.hs
@@ -0,0 +1,9 @@
+module CabalGild.Unstable.Class.MonadHandle where
+
+import qualified System.IO as IO
+
+class MonadHandle m where
+  isTerminalDevice :: IO.Handle -> m Bool
+
+instance MonadHandle IO where
+  isTerminalDevice = IO.hIsTerminalDevice
diff --git a/source/library/CabalGild/Unstable/Exception/MoreThanOneCabalFileFound.hs b/source/library/CabalGild/Unstable/Exception/MoreThanOneCabalFileFound.hs
new file mode 100644
--- /dev/null
+++ b/source/library/CabalGild/Unstable/Exception/MoreThanOneCabalFileFound.hs
@@ -0,0 +1,18 @@
+module CabalGild.Unstable.Exception.MoreThanOneCabalFileFound where
+
+import qualified CabalGild.Unstable.Type.Flag as Flag
+import qualified Control.Monad.Catch as Exception
+
+-- | This exception is thrown when no input or output file has been specified,
+-- stdin is a terminal device, and there are multiple Cabal files in the
+-- current directory.
+data MoreThanOneCabalFileFound
+  = MoreThanOneCabalFileFound
+  deriving (Eq, Show)
+
+instance Exception.Exception MoreThanOneCabalFileFound where
+  displayException =
+    const $
+      "More than one package description found in the current directory. Use the --"
+        <> Flag.ioOption
+        <> " option to specify a *.cabal file."
diff --git a/source/library/CabalGild/Unstable/Exception/NoCabalFileFound.hs b/source/library/CabalGild/Unstable/Exception/NoCabalFileFound.hs
new file mode 100644
--- /dev/null
+++ b/source/library/CabalGild/Unstable/Exception/NoCabalFileFound.hs
@@ -0,0 +1,18 @@
+module CabalGild.Unstable.Exception.NoCabalFileFound where
+
+import qualified CabalGild.Unstable.Type.Flag as Flag
+import qualified Control.Monad.Catch as Exception
+
+-- | This exception is thrown when no input or output file has been specified,
+-- stdin is a terminal device, and there are no Cabal files in the current
+-- directory.
+data NoCabalFileFound
+  = NoCabalFileFound
+  deriving (Eq, Show)
+
+instance Exception.Exception NoCabalFileFound where
+  displayException =
+    const $
+      "No package descriptions found in the current directory. Use the --"
+        <> Flag.ioOption
+        <> " option to specify a *.cabal file."
diff --git a/source/library/CabalGild/Unstable/Main.hs b/source/library/CabalGild/Unstable/Main.hs
--- a/source/library/CabalGild/Unstable/Main.hs
+++ b/source/library/CabalGild/Unstable/Main.hs
@@ -9,6 +9,7 @@
 import qualified CabalGild.Unstable.Action.ReflowText as ReflowText
 import qualified CabalGild.Unstable.Action.Render as Render
 import qualified CabalGild.Unstable.Action.StripBlanks as StripBlanks
+import qualified CabalGild.Unstable.Class.MonadHandle as MonadHandle
 import qualified CabalGild.Unstable.Class.MonadLog as MonadLog
 import qualified CabalGild.Unstable.Class.MonadRead as MonadRead
 import qualified CabalGild.Unstable.Class.MonadWalk as MonadWalk
@@ -54,7 +55,8 @@
 -- constraints so that it can be run in pure code if so desired. But most often
 -- this will be run in 'IO'.
 mainWith ::
-  ( MonadLog.MonadLog m,
+  ( MonadHandle.MonadHandle m,
+    MonadLog.MonadLog m,
     MonadRead.MonadRead m,
     Exception.MonadThrow m,
     MonadWalk.MonadWalk m,
diff --git a/source/library/CabalGild/Unstable/Type/Context.hs b/source/library/CabalGild/Unstable/Type/Context.hs
--- a/source/library/CabalGild/Unstable/Type/Context.hs
+++ b/source/library/CabalGild/Unstable/Type/Context.hs
@@ -1,6 +1,10 @@
 module CabalGild.Unstable.Type.Context where
 
+import qualified CabalGild.Unstable.Class.MonadHandle as MonadHandle
 import qualified CabalGild.Unstable.Class.MonadLog as MonadLog
+import qualified CabalGild.Unstable.Class.MonadWalk as MonadWalk
+import qualified CabalGild.Unstable.Exception.MoreThanOneCabalFileFound as MoreThanOneCabalFileFound
+import qualified CabalGild.Unstable.Exception.NoCabalFileFound as NoCabalFileFound
 import qualified CabalGild.Unstable.Exception.SpecifiedOutputWithCheckMode as SpecifiedOutputWithCheckMode
 import qualified CabalGild.Unstable.Exception.SpecifiedStdinWithFileInput as SpecifiedStdinWithFileInput
 import qualified CabalGild.Unstable.Type.Config as Config
@@ -19,6 +23,7 @@
 import qualified Paths_cabal_gild as This
 import qualified System.Console.GetOpt as GetOpt
 import qualified System.Exit as Exit
+import qualified System.IO as IO
 
 -- | Represents the context necessary to run the program. This is essentially a
 -- simplified 'Config.Config'.
@@ -35,7 +40,11 @@
 -- requested, then this will throw an 'Exit.ExitSuccess'. Otherwise this makes
 -- sure the config is valid before returning the context.
 fromConfig ::
-  (MonadLog.MonadLog m, Exception.MonadThrow m) =>
+  ( MonadHandle.MonadHandle m,
+    MonadLog.MonadLog m,
+    Exception.MonadThrow m,
+    MonadWalk.MonadWalk m
+  ) =>
   Config.Config ->
   m Context
 fromConfig config = do
@@ -67,15 +76,26 @@
       Exception.throwM SpecifiedOutputWithCheckMode.SpecifiedOutputWithCheckMode
     _ -> pure ()
 
-  let theInput = Maybe.fromMaybe Input.Stdin . Optional.toMaybe $ Config.input config
-      filePath = case theInput of
+  let preInput = Maybe.fromMaybe Input.Stdin . Optional.toMaybe $ Config.input config
+      filePath = case preInput of
         Input.Stdin -> "."
         Input.File f -> f
+      preOutput = Maybe.fromMaybe Output.Stdout . Optional.toMaybe $ Config.output config
+  (theInput, theOutput) <- do
+    isTerm <- MonadHandle.isTerminalDevice IO.stdin
+    if preInput == Input.Stdin && preOutput == Output.Stdout && isTerm
+      then do
+        cabalFiles <- MonadWalk.walk "." ["*.cabal"] []
+        case cabalFiles of
+          [fp] -> pure (Input.File fp, Output.File fp)
+          [] -> Exception.throwM NoCabalFileFound.NoCabalFileFound
+          _ -> Exception.throwM MoreThanOneCabalFileFound.MoreThanOneCabalFileFound
+      else pure (preInput, preOutput)
   pure
     Context
       { crlf = Maybe.fromMaybe Leniency.Lenient . Optional.toMaybe $ Config.crlf config,
         input = theInput,
         mode = Maybe.fromMaybe Mode.Format . Optional.toMaybe $ Config.mode config,
-        output = Maybe.fromMaybe Output.Stdout . Optional.toMaybe $ Config.output config,
+        output = theOutput,
         stdin = Maybe.fromMaybe filePath . Optional.toMaybe $ Config.stdin config
       }
diff --git a/source/test-suite/Main.hs b/source/test-suite/Main.hs
--- a/source/test-suite/Main.hs
+++ b/source/test-suite/Main.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 
+import qualified CabalGild.Unstable.Class.MonadHandle as MonadHandle
 import qualified CabalGild.Unstable.Class.MonadLog as MonadLog
 import qualified CabalGild.Unstable.Class.MonadRead as MonadRead
 import qualified CabalGild.Unstable.Class.MonadWalk as MonadWalk
@@ -7,6 +8,8 @@
 import qualified CabalGild.Unstable.Exception.CheckFailure as CheckFailure
 import qualified CabalGild.Unstable.Exception.DuplicateOption as DuplicateOption
 import qualified CabalGild.Unstable.Exception.InvalidOption as InvalidOption
+import qualified CabalGild.Unstable.Exception.MoreThanOneCabalFileFound as MoreThanOneCabalFileFound
+import qualified CabalGild.Unstable.Exception.NoCabalFileFound as NoCabalFileFound
 import qualified CabalGild.Unstable.Exception.SpecifiedOutputWithCheckMode as SpecifiedOutputWithCheckMode
 import qualified CabalGild.Unstable.Exception.SpecifiedStdinWithFileInput as SpecifiedStdinWithFileInput
 import qualified CabalGild.Unstable.Exception.UnexpectedArgument as UnexpectedArgument
@@ -35,13 +38,13 @@
 main :: IO ()
 main = Hspec.hspec . Hspec.parallel . Hspec.describe "cabal-gild" $ do
   Hspec.it "shows the help" $ do
-    let (a, s, w) = runGild ["--help"] [] (".", [])
+    let (a, s, w) = runGild ["--help"] [] (".", []) False
     a `shouldBeFailure` Exit.ExitSuccess
     w `Hspec.shouldNotBe` []
     s `Hspec.shouldBe` Map.empty
 
   Hspec.it "shows the version" $ do
-    let (a, s, w) = runGild ["--version"] [] (".", [])
+    let (a, s, w) = runGild ["--version"] [] (".", []) False
     a `shouldBeFailure` Exit.ExitSuccess
     w `Hspec.shouldNotBe` []
     s `Hspec.shouldBe` Map.empty
@@ -68,6 +71,7 @@
             ["--crlf=strict", "--crlf=strict"]
             [(Input.Stdin, String.toUtf8 "")]
             (".", [])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.singleton Output.Stdout (String.toUtf8 "")
@@ -102,6 +106,7 @@
             ["--input", "input.cabal"]
             [(Input.File "input.cabal", String.toUtf8 "")]
             (".", [])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.singleton Output.Stdout (String.toUtf8 "")
@@ -112,6 +117,7 @@
             ["--output", "output.cabal"]
             [(Input.Stdin, String.toUtf8 "")]
             (".", [])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.singleton (Output.File "output.cabal") (String.toUtf8 "")
@@ -122,6 +128,7 @@
             ["--mode", "check"]
             [(Input.Stdin, String.toUtf8 "pass: yes\n")]
             (".", [])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.empty
@@ -132,6 +139,7 @@
             ["--mode", "check"]
             [(Input.Stdin, String.toUtf8 "pass: no")]
             (".", [])
+            False
     a `shouldBeFailure` CheckFailure.CheckFailure
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.empty
@@ -142,6 +150,7 @@
             ["--mode", "check"]
             [(Input.Stdin, String.toUtf8 "pass: yes\r\n")]
             (".", [])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.empty
@@ -152,6 +161,7 @@
             ["--crlf", "strict", "--mode", "check"]
             [(Input.Stdin, String.toUtf8 "pass: no\r\n")]
             (".", [])
+            False
     a `shouldBeFailure` CheckFailure.CheckFailure
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.empty
@@ -162,6 +172,7 @@
             ["--input", "f", "--stdin", "g"]
             []
             (".", [])
+            False
     a `shouldBeFailure` SpecifiedStdinWithFileInput.SpecifiedStdinWithFileInput
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.empty
@@ -172,6 +183,7 @@
             ["--mode", "check", "--output", "-"]
             []
             (".", [])
+            False
     a `shouldBeFailure` SpecifiedOutputWithCheckMode.SpecifiedOutputWithCheckMode
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.empty
@@ -182,6 +194,7 @@
             ["--io", "io.cabal"]
             [(Input.File "io.cabal", String.toUtf8 "")]
             (".", [])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.empty
@@ -192,6 +205,7 @@
             ["--input", "p.cabal"]
             [(Input.File "p.cabal", String.toUtf8 "")]
             (".", [])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.singleton Output.Stdout (String.toUtf8 "")
@@ -202,6 +216,7 @@
             ["--input", "p.cabal", "--output", "q.cabal"]
             [(Input.File "p.cabal", String.toUtf8 "")]
             (".", [])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.singleton (Output.File "q.cabal") (String.toUtf8 "")
@@ -212,6 +227,7 @@
             ["--output", "q.cabal"]
             [(Input.Stdin, String.toUtf8 "")]
             (".", [])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.singleton (Output.File "q.cabal") (String.toUtf8 "")
@@ -222,6 +238,7 @@
             ["--io", "io.cabal"]
             [(Input.File "io.cabal", String.toUtf8 "f:a")]
             (".", [])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.singleton (Output.File "io.cabal") (String.toUtf8 "f: a\n")
@@ -232,6 +249,7 @@
             ["--io", "p.cabal"]
             [(Input.File "p.cabal", String.toUtf8 "s\r\n")]
             (".", [])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.empty
@@ -242,6 +260,7 @@
             ["--crlf", "strict", "--io", "p.cabal"]
             [(Input.File "p.cabal", String.toUtf8 "s\r\n")]
             (".", [])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.singleton (Output.File "p.cabal") (String.toUtf8 "s\n")
@@ -252,6 +271,7 @@
             ["--stdin", "d/p.cabal"]
             [(Input.Stdin, String.toUtf8 "library\n -- cabal-gild: discover\n exposed-modules:")]
             ("d", [["M.hs"]])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.singleton Output.Stdout (String.toUtf8 "library\n  -- cabal-gild: discover\n  exposed-modules: M\n")
@@ -1300,6 +1320,7 @@
             ["--input", FilePath.combine d "io.cabal"]
             [(Input.File $ FilePath.combine d "io.cabal", String.toUtf8 "library\n -- cabal-gild: discover src --exclude src/N.hs\n exposed-modules:")]
             (d, [["src", "M.hs"], ["src", "N.hs"]])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.singleton Output.Stdout (String.toUtf8 "library\n  -- cabal-gild: discover src --exclude src/N.hs\n  exposed-modules: M\n")
@@ -1322,6 +1343,7 @@
             []
             [(Input.Stdin, String.toUtf8 "-- cabal-gild: discover --unknown\nsignatures:")]
             (".", [])
+            False
     a `shouldBeFailure` UnknownOption.UnknownOption "--unknown"
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.empty
@@ -1332,6 +1354,7 @@
             []
             [(Input.Stdin, String.toUtf8 "-- cabal-gild: discover --exclude\nsignatures:")]
             (".", [])
+            False
     a `shouldBeFailure` InvalidOption.InvalidOption "option `--exclude' requires an argument PATTERN"
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.empty
@@ -1495,6 +1518,7 @@
             ["--input", FilePath.combine d "io.cabal"]
             [(Input.File $ FilePath.combine d "io.cabal", String.toUtf8 "library\n -- cabal-gild: discover src --include src/M.hs\n exposed-modules:")]
             (d, [["src", "M.hs"], ["src", "N.hs"]])
+            False
     a `Hspec.shouldSatisfy` Either.isRight
     w `Hspec.shouldBe` []
     s `Hspec.shouldBe` Map.singleton Output.Stdout (String.toUtf8 "library\n  -- cabal-gild: discover src --include src/M.hs\n  exposed-modules: M\n")
@@ -1698,6 +1722,31 @@
             "    N.M2"
           ]
 
+  Hspec.it "successfully determine that stdin is terminal and find/format the cabal file" $ do
+    let filePath = "stdin-is-terminal.cabal"
+        fileData = "FOO :  bar"
+        (a, s, w) =
+          runGild
+            []
+            [(Input.File "stdin-is-terminal.cabal", String.toUtf8 fileData)]
+            (".", [[filePath]])
+            True
+    a `Hspec.shouldSatisfy` Either.isRight
+    w `Hspec.shouldBe` []
+    s `Hspec.shouldBe` Map.singleton (Output.File filePath) (String.toUtf8 "foo: bar\n")
+
+  Hspec.it "fails if no cabal file is found" $ do
+    let (a, s, w) = runGild [] [] (".", []) True
+    a `shouldBeFailure` NoCabalFileFound.NoCabalFileFound
+    w `Hspec.shouldBe` []
+    s `Hspec.shouldBe` Map.empty
+
+  Hspec.it "fails if more than one cabal file is found" $ do
+    let (a, s, w) = runGild [] [] (".", [["0.cabal"], ["1.cabal"]]) True
+    a `shouldBeFailure` MoreThanOneCabalFileFound.MoreThanOneCabalFileFound
+    w `Hspec.shouldBe` []
+    s `Hspec.shouldBe` Map.empty
+
 withTemporaryDirectory :: IO () -> IO ()
 withTemporaryDirectory =
   Temp.withSystemTempDirectory "cabal-gild"
@@ -1723,7 +1772,7 @@
   ByteString.ByteString ->
   Hspec.Expectation
 expectStable files input = do
-  let (a, s, w) = runGild [] [(Input.Stdin, input)] files
+  let (a, s, w) = runGild [] [(Input.Stdin, input)] files False
   a `Hspec.shouldSatisfy` Either.isRight
   w `Hspec.shouldBe` []
   output <- case Map.toList s of
@@ -1738,7 +1787,7 @@
   String ->
   Hspec.Expectation
 expectDiscover files input expected = do
-  let (a, s, w) = runGild [] [(Input.Stdin, String.toUtf8 input)] files
+  let (a, s, w) = runGild [] [(Input.Stdin, String.toUtf8 input)] files False
   a `Hspec.shouldSatisfy` Either.isRight
   w `Hspec.shouldBe` []
   actual <- case Map.toList s of
@@ -1751,20 +1800,25 @@
   [String] ->
   [(Input.Input, ByteString.ByteString)] ->
   (String, [[String]]) ->
+  Bool ->
   (Either E (), S, W)
-runGild arguments inputs files =
+runGild arguments inputs files tty =
   runTest
     (Gild.mainWith arguments)
-    ( Map.fromList inputs,
-      fmap FilePath.joinPath <$> uncurry Map.singleton files
+    ( ( Map.fromList inputs,
+        fmap FilePath.joinPath <$> uncurry Map.singleton files
+      ),
+      tty
     )
     Map.empty
 
 expectException ::
   (Stack.HasCallStack, Eq e, Exception.Exception e) =>
-  [String] -> e -> Hspec.Expectation
+  [String] ->
+  e ->
+  Hspec.Expectation
 expectException flags exception = do
-  let (a, s, w) = runGild flags [] (".", [])
+  let (a, s, w) = runGild flags [] (".", []) False
   a `shouldBeFailure` exception
   w `Hspec.shouldBe` []
   s `Hspec.shouldBe` Map.empty
@@ -1776,7 +1830,12 @@
 
 type E = Exception.SomeException
 
-type R = (Map.Map Input.Input ByteString.ByteString, Map.Map FilePath [FilePath])
+type R =
+  ( ( Map.Map Input.Input ByteString.ByteString, -- For 'MonadRead'.
+      Map.Map FilePath [FilePath] -- For 'MonadWalk'.
+    ),
+    Bool -- For 'MonadHandle'.
+  )
 
 type S = Map.Map Output.Output ByteString.ByteString
 
@@ -1792,7 +1851,7 @@
 
 instance (Monad m) => MonadRead.MonadRead (TestT m) where
   read k = do
-    m <- TestT . Trans.lift . RWST.asks $ Map.lookup k . fst
+    m <- TestT . Trans.lift . RWST.asks $ Map.lookup k . fst . fst
     case m of
       Nothing -> Exception.throwM . userError $ "read " <> show k
       Just x -> pure x
@@ -1802,7 +1861,7 @@
 
 instance (Monad m) => MonadWalk.MonadWalk (TestT m) where
   walk d i x = do
-    result <- TestT . Trans.lift . RWST.asks $ Map.lookup d . snd
+    result <- TestT . Trans.lift . RWST.asks $ Map.lookup d . snd . fst
     case result of
       Nothing -> Exception.throwM . userError $ "walk " <> show d
       Just fs ->
@@ -1813,3 +1872,6 @@
 
 instance (Monad m) => MonadWrite.MonadWrite (TestT m) where
   write k = TestT . Trans.lift . RWST.modify . Map.insert k
+
+instance (Monad m) => MonadHandle.MonadHandle (TestT m) where
+  isTerminalDevice = const . TestT . Trans.lift $ RWST.asks snd