diff --git a/.github/scripts/check_newline_preservation.sh b/.github/scripts/check_newline_preservation.sh
new file mode 100644
index 00000000..620b9da8
--- /dev/null
+++ b/.github/scripts/check_newline_preservation.sh
@@ -0,0 +1,67 @@
+#!/usr/bin/env bash
+#
+# Checks that formatting a file keeps the line endings it came with. The
+# detection is exercised through the binary rather than the library, because
+# reading the file, detecting the ending and writing it back out are three
+# separate steps and any one of them can drop it.
+#
+# Usage: bash ./.github/scripts/check_newline_preservation.sh <jbeam-edit-binary>
+
+set -euo pipefail
+
+BINARY=$(realpath "${1:?usage: check_newline_preservation.sh <jbeam-edit-binary>}")
+SOURCE_FILE="examples/jbeam/frame.jbeam"
+
+WORK_DIR=$(mktemp -d)
+trap 'rm -rf "$WORK_DIR"' EXIT
+
+# The binary runs from the work directory, so point it back at the repo for the
+# default ruleset it ships as a data file.
+export jbeam_edit_datadir="$PWD"
+
+failed=0
+
+carriage_returns() {
+  grep -c $'\r' "$1" || true
+}
+
+check() {
+  local label="$1" expected="$2" target="$WORK_DIR/$1.jbeam"
+
+  if [[ "$label" == crlf ]]; then
+    sed 's/\r$//; s/$/\r/' "$SOURCE_FILE" >"$target"
+  else
+    sed 's/\r$//' "$SOURCE_FILE" >"$target"
+  fi
+
+  local before after
+  before=$(carriage_returns "$target")
+  if [[ "$before" -ne "$expected" ]]; then
+    echo "setup error: $label input has $before carriage returns, expected $expected"
+    failed=1
+    return
+  fi
+
+  (cd "$WORK_DIR" && "$BINARY" "$label.jbeam" >/dev/null)
+
+  after=$(carriage_returns "$target")
+  if [[ "$expected" -eq 0 && "$after" -ne 0 ]]; then
+    echo "$label: line endings changed, LF input came out with $after carriage returns"
+    failed=1
+  elif [[ "$expected" -ne 0 && "$after" -eq 0 ]]; then
+    echo "$label: line endings changed, CRLF input came out with no carriage returns"
+    failed=1
+  fi
+}
+
+lf_lines=$(sed 's/\r$//' "$SOURCE_FILE" | wc -l)
+
+check lf 0
+check crlf "$lf_lines"
+
+if [[ "$failed" -ne 0 ]]; then
+  echo "formatting does not preserve line endings"
+  exit 1
+fi
+
+echo "line endings survive a format for both LF and CRLF input"
diff --git a/.github/scripts/prepare_installer.sh b/.github/scripts/prepare_installer.sh
index fce5227f..c9cf0fd1 100644
--- a/.github/scripts/prepare_installer.sh
+++ b/.github/scripts/prepare_installer.sh
@@ -49,8 +49,11 @@ cp ./examples/jbeam-edit.yaml ./.jbeam-edit.yaml
 
 JBEAM_DIR="./examples/jbeam"
 
+# Line endings are checked separately by check_newline_preservation.sh. Here we
+# only care about content, and the two sides can disagree on endings because the
+# input comes from a blob while the expected file comes from the working tree.
 custom_diff() {
-  diff --color=always --suppress-common-lines "$1" "$2"
+  diff --color=always --suppress-common-lines --strip-trailing-cr "$1" "$2"
 }
 
 mapfile -t JBEAM_FILES < <(find "$JBEAM_DIR" -maxdepth 1 -name "*.jbeam" -printf "%f\n")
diff --git a/.github/workflows/build-and-test.yaml b/.github/workflows/build-and-test.yaml
index 9bfff6b2..42196181 100644
--- a/.github/workflows/build-and-test.yaml
+++ b/.github/workflows/build-and-test.yaml
@@ -198,3 +198,8 @@ jobs:
         run: cabal build --project-file cabal.project.ci all
       - name: Run tests (GHC ${{ steps.setup-ghc.outputs.ghc-version }})
         run: cabal test --project-file cabal.project.ci
+      - name: Check that formatting preserves line endings
+        shell: bash
+        run: |
+          bash ./.github/scripts/check_newline_preservation.sh \
+            "$(cabal list-bin --project-file cabal.project.ci jbeam-edit)"
diff --git a/exe/jbeam-edit/Main.hs b/exe/jbeam-edit/Main.hs
index c65c9e25..f037284d 100644
--- a/exe/jbeam-edit/Main.hs
+++ b/exe/jbeam-edit/Main.hs
@@ -5,6 +5,7 @@ module Main (
 import CommandLineOptions
 import Control.Monad (when)
 import Data.Text (Text)
+import JbeamEdit.Core.Newline
 import JbeamEdit.Core.Node (Node)
 import JbeamEdit.Formatting (RuleSet, formatNodeAndWrite)
 import JbeamEdit.Formatting.Config
@@ -12,12 +13,9 @@ import JbeamEdit.IOUtils
 import JbeamEdit.Parsing.Jbeam (parseNodes)
 import System.Directory.OsPath
 import System.Environment (getArgs)
+import System.IO (Newline)
 import System.OsPath
 
-#ifdef ENABLE_WINDOWS_NEWLINES
-import Data.Text qualified as T
-#endif
-
 #ifdef ENABLE_TRANSFORMATION
 import JbeamEdit.Transformation
 import JbeamEdit.Transformation.Config
@@ -48,16 +46,19 @@ editFile opts = do
     Just filename -> do
       createBackupFile filename opts
       contents <- tryReadFile [] filename
-      case contents >>= parseNodes of
-        Right ns -> processNodes opts filename ns formattingConfig
+      case contents of
+        Right contents' ->
+          case parseNodes contents' of
+            Right ns -> processNodes (detectNewline contents') opts filename ns formattingConfig
+            Left err -> putErrorLine err
         Left err -> putErrorLine err
     Nothing -> putErrorLine "missing arg filename"
 
-processNodes :: Options -> OsPath -> Node -> RuleSet -> IO ()
-processNodes opts outFile nodes formattingConfig = do
+processNodes :: Newline -> Options -> OsPath -> Node -> RuleSet -> IO ()
+processNodes newline opts outFile nodes formattingConfig = do
   transformedNode <- applyTransform formattingConfig opts nodes
   case transformedNode of
-    Right transformedNode' -> formatNodeAndWrite formattingConfig outFile transformedNode'
+    Right transformedNode' -> formatNodeAndWrite newline formattingConfig outFile transformedNode'
     Left err -> putErrorLine err
 
 applyTransform :: RuleSet -> Options -> Node -> IO (Either Text Node)
diff --git a/jbeam-edit.cabal b/jbeam-edit.cabal
index a60dfd62..c101eea4 100644
--- a/jbeam-edit.cabal
+++ b/jbeam-edit.cabal
@@ -11,7 +11,7 @@ license-file:       LICENSE
 copyright:          2025 webdevred
 maintainer:         example@example.com
 author:             webdevred
-tested-with:        ghc ==9.10.3 ghc ==9.14.1
+tested-with:        ghc ==9.8.4 ghc ==9.10.3 ghc ==9.14.1
 homepage:           https://github.com/webdevred/jbeam-edit#readme
 bug-reports:        https://github.com/webdevred/jbeam-edit/issues
 synopsis:
@@ -63,6 +63,7 @@ flag windows-example-paths
 
 library
     exposed-modules:
+        JbeamEdit.Core.Newline
         JbeamEdit.Core.Node
         JbeamEdit.Core.NodeCursor
         JbeamEdit.Core.NodePath
@@ -118,9 +119,6 @@ library
     if (os(windows) && flag(windows-example-paths))
         cpp-options: -DWINDOWS_EXAMPLE_PATHS
 
-    if os(windows)
-        cpp-options: -DENABLE_WINDOWS_NEWLINES
-
 library jbeam-edit-transformation
     exposed-modules:
         JbeamEdit.Transformation
@@ -262,6 +260,7 @@ test-suite jbeam-edit-test
     build-tool-depends: hspec-discover:hspec-discover
     hs-source-dirs:     test
     other-modules:
+        Core.NewlineSpec
         Core.NodeCursorSpec
         Core.NodePathSpec
         Core.NodeSpec
diff --git a/package.yaml b/package.yaml
index e07ccea6..04f0a267 100644
--- a/package.yaml
+++ b/package.yaml
@@ -25,7 +25,7 @@ description: >-
   and can automatically rename nodes and update references. Custom formatting rules
   are supported via JBFL (JBeam Formatting Language). See the README for usage instructions
   and examples: https://github.com/webdevred/jbeam-edit#readme
-tested-with: [GHC == 9.10.3, GHC == 9.14.1]
+tested-with: [GHC == 9.8.4, GHC == 9.10.3, GHC == 9.14.1]
 
 dependencies:
   - base >= 4.19
@@ -101,8 +101,6 @@ library:
       cpp-options: -DENABLE_TRANSFORMATION
     - condition: os(windows) && flag(windows-example-paths)
       cpp-options: -DWINDOWS_EXAMPLE_PATHS
-    - condition: os(windows)
-      cpp-options: -DENABLE_WINDOWS_NEWLINES
 
 _jbeam-options: &jbeam-options
   main: Main.hs
diff --git a/src-extra/transformation/JbeamEdit/Transformation.hs b/src-extra/transformation/JbeamEdit/Transformation.hs
index 8e043502..081e5142 100644
--- a/src-extra/transformation/JbeamEdit/Transformation.hs
+++ b/src-extra/transformation/JbeamEdit/Transformation.hs
@@ -5,7 +5,7 @@ import Data.Bifunctor (first)
 import Data.Bool (bool)
 import Data.Foldable.Extra (notNull)
 import Data.Function (on)
-import Data.List (partition)
+import Data.List (foldl', partition)
 import Data.List.NonEmpty (NonEmpty)
 import Data.List.NonEmpty qualified as NE
 import Data.Map (Map)
@@ -24,6 +24,7 @@ import Data.Traversable (mapAccumL)
 import Data.Vector (Vector, (!), (!?), (//))
 import Data.Vector qualified as V
 import GHC.IsList
+import JbeamEdit.Core.Newline
 import JbeamEdit.Core.Node
 import JbeamEdit.Core.NodeCursor (newCursor)
 import JbeamEdit.Core.NodeCursor qualified as NC
@@ -492,12 +493,15 @@ filterJbeamFiles excludedFilenames = filter go
 updateOtherFiles :: RuleSet -> UpdateNamesMap -> OsPath -> IO ()
 updateOtherFiles formattingConfig updatedNames filepath = do
   contents <- tryReadFile [] filepath
-  case contents >>= parseNodes of
-    Right node ->
-      let node' = findAndUpdateTextInNode updatedNames newCursor node
-       in when
-            (node /= node')
-            (formatNodeAndWrite formattingConfig filepath node')
+  case contents of
+    Right contents' ->
+      case parseNodes contents' of
+        Right node ->
+          let node' = findAndUpdateTextInNode updatedNames newCursor node
+           in when
+                (node /= node')
+                (formatNodeAndWrite (detectNewline contents') formattingConfig filepath node')
+        Left err -> putErrorLine err
     Left err -> putErrorLine err
 
 transform
diff --git a/src-extra/transformation/JbeamEdit/Transformation/BeamValidation.hs b/src-extra/transformation/JbeamEdit/Transformation/BeamValidation.hs
index f0c3aee3..e4e34df3 100644
--- a/src-extra/transformation/JbeamEdit/Transformation/BeamValidation.hs
+++ b/src-extra/transformation/JbeamEdit/Transformation/BeamValidation.hs
@@ -7,6 +7,7 @@ module JbeamEdit.Transformation.BeamValidation (
 ) where
 
 import Control.Monad (forM_, unless)
+import Data.List (foldl')
 import Data.List.NonEmpty (toList)
 import Data.Map qualified as M
 import Data.Set (Set)
diff --git a/src/JbeamEdit/Core/Newline.hs b/src/JbeamEdit/Core/Newline.hs
new file mode 100644
index 00000000..b1a43020
--- /dev/null
+++ b/src/JbeamEdit/Core/Newline.hs
@@ -0,0 +1,15 @@
+module JbeamEdit.Core.Newline (detectNewline) where
+
+import Data.ByteString.Lazy (LazyByteString)
+import Data.ByteString.Lazy qualified as LBS
+import Data.ByteString.Lazy.Char8 qualified as LBS8 (elemIndex)
+import System.IO (Newline (..))
+
+detectNewline :: LazyByteString -> Newline
+detectNewline content =
+  case LBS8.elemIndex '\r' content of
+    Nothing -> LF
+    Just idx ->
+      case LBS.uncons (LBS.drop (idx + 1) content) of
+        Just (10, _) -> CRLF -- '\n'
+        _ -> detectNewline (LBS.drop (idx + 1) content)
diff --git a/src/JbeamEdit/Formatting.hs b/src/JbeamEdit/Formatting.hs
index 320f0a06..34ebabb2 100644
--- a/src/JbeamEdit/Formatting.hs
+++ b/src/JbeamEdit/Formatting.hs
@@ -52,6 +52,7 @@ import JbeamEdit.Formatting.Rules (
 import JbeamEdit.Formatting.Rules.ComplexNewLine qualified as CNL
 import JbeamEdit.Formatting.Rules.TrailingComma qualified as TC
 import System.File.OsPath qualified as OS (writeFile)
+import System.IO (Newline (..))
 import System.OsPath (OsPath)
 
 data FormattingState = FormattingState
@@ -459,22 +460,23 @@ formatWithCursor rs _ cursor n =
 formatNode :: RuleSet -> Node -> Text
 formatNode rs node = formatWithCursor rs emptyState newCursor node <> T.singleton '\n'
 
-#ifdef ENABLE_WINDOWS_NEWLINES
-replaceNewlines :: Text -> Text
-replaceNewlines = T.replace "\n" "\r\n"
-#else
-replaceNewlines :: Text -> Text
-replaceNewlines = id
-#endif
+{- | 'formatNode' always emits LF, so rewrite the line endings when the file
+came with CRLF. The handle's newline mode cannot do this, it only applies to
+text-mode writes and the output goes out as bytes.
+-}
+applyNewline :: Newline -> Text -> Text
+applyNewline CRLF = T.replace "\n" "\r\n"
+applyNewline LF = id
 
 formatNodeAndWrite
-  :: RuleSet
+  :: Newline
+  -> RuleSet
   -> OsPath
   -> Node
   -> IO ()
-formatNodeAndWrite rs outFile =
+formatNodeAndWrite newline rs outFile =
   OS.writeFile outFile
     . LBS.fromStrict
     . encodeUtf8
-    . replaceNewlines
+    . applyNewline newline
     . formatNode rs
diff --git a/src/JbeamEdit/Parsing/Jbeam.hs b/src/JbeamEdit/Parsing/Jbeam.hs
index a4fa95b9..be8784e9 100644
--- a/src/JbeamEdit/Parsing/Jbeam.hs
+++ b/src/JbeamEdit/Parsing/Jbeam.hs
@@ -98,7 +98,8 @@ associationDirection st = bool PreviousNode NextNode (lastNodeEndedWithNewline s
 
 commentStripSpace :: Text -> Text
 commentStripSpace initialText =
-  let initialNewline = mwhen (T.isPrefixOf "\n" initialText) "\n"
+  let startsWithNewline text = T.isPrefixOf "\n" text || T.isPrefixOf "\r\n" text
+      initialNewline = mwhen (startsWithNewline initialText) "\n"
       trimTrailingSpaces = T.dropWhileEnd (charBoth (/= '\n') isSpace)
       endingNewline = mwhen (T.isSuffixOf "\n" $ trimTrailingSpaces initialText) "\n"
       go = T.intercalate "\n" . filter (not . T.all isSpace) . map T.strip . T.lines
diff --git a/test/Core/NewlineSpec.hs b/test/Core/NewlineSpec.hs
new file mode 100644
index 00000000..59bb68fd
--- /dev/null
+++ b/test/Core/NewlineSpec.hs
@@ -0,0 +1,17 @@
+module Core.NewlineSpec (spec) where
+
+import JbeamEdit.Core.Newline (detectNewline)
+import System.IO (Newline (..))
+import Test.Hspec
+
+spec :: Spec
+spec =
+  describe "detectNewline" $ do
+    it "detects LF for unix line endings" $
+      detectNewline "a\nb\n" `shouldBe` LF
+    it "detects CRLF for windows line endings" $
+      detectNewline "a\r\nb\r\n" `shouldBe` CRLF
+    it "detects CRLF when line endings are mixed" $
+      detectNewline "a\r\nb\n" `shouldBe` CRLF
+    it "defaults to LF when there is no line ending" $
+      detectNewline "abc" `shouldBe` LF