diff --git a/lib/bundler/compact_index_client/cache_file.rb b/lib/bundler/compact_index_client/cache_file.rb
index 299d683438b8..03659aeac19e 100644
--- a/lib/bundler/compact_index_client/cache_file.rb
+++ b/lib/bundler/compact_index_client/cache_file.rb
@@ -103,7 +103,7 @@ def open(write_mode = "wb", perm = @perm, &block)
       # Returns false without appending when no digests since appending is too error prone to do without digests.
       def append(data)
         return false unless digests?
-        open("a") {|f| f.write data }
+        open("ab") {|f| f.write data }
         verify && commit
       end
 
diff --git a/lib/rubygems/compact_index_client/cache_file.rb b/lib/rubygems/compact_index_client/cache_file.rb
index 3656645d8220..00e5f541ddf6 100644
--- a/lib/rubygems/compact_index_client/cache_file.rb
+++ b/lib/rubygems/compact_index_client/cache_file.rb
@@ -99,7 +99,7 @@ def open(write_mode = "wb", perm = @perm, &block)
     # Returns false without appending when no digests since appending is too error prone to do without digests.
     def append(data)
       return false unless digests?
-      open("a") {|f| f.write data }
+      open("ab") {|f| f.write data }
       verify && commit
     end
 
diff --git a/spec/bundler/compact_index_client/cache_file_spec.rb b/spec/bundler/compact_index_client/cache_file_spec.rb
new file mode 100644
index 000000000000..c9a87c3c2d44
--- /dev/null
+++ b/spec/bundler/compact_index_client/cache_file_spec.rb
@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require "bundler/compact_index_client"
+require "bundler/compact_index_client/cache_file"
+require "tmpdir"
+
+RSpec.describe Bundler::CompactIndexClient::CacheFile do
+  let(:path) { Pathname.new(Dir.mktmpdir("localpath")).join("versions") }
+
+  def sha256(data)
+    { "sha-256" => Digest::SHA256.base64digest(data) }
+  end
+
+  it "appends in binary mode so line endings are preserved" do
+    path.binwrite "created_at: 2026-06-10\n---\n"
+
+    appended = nil
+    described_class.copy(path) do |file|
+      file.digests = sha256("created_at: 2026-06-10\n---\nrake 13.0.0\n")
+      appended = file.append("rake 13.0.0\n")
+    end
+
+    expect(appended).to be_truthy
+    # On Windows a text-mode append rewrites the appended LF as CRLF while the
+    # digest is computed over the pre-write bytes, so verify passes but the file
+    # on disk is corrupted. Read raw bytes to catch any stray carriage return.
+    expect(path.binread).not_to include("\r")
+    expect(path.binread).to eq("created_at: 2026-06-10\n---\nrake 13.0.0\n")
+  end
+end
diff --git a/spec/support/shards.rb b/spec/support/shards.rb
index 5718e775b23a..8416190d000b 100644
--- a/spec/support/shards.rb
+++ b/spec/support/shards.rb
@@ -143,6 +143,7 @@ module Shards
         "spec/bundler/ci_detector_spec.rb",
       ],
       shard_d: [
+        "spec/bundler/compact_index_client/cache_file_spec.rb",
         "spec/bundler/rubygems_ext_spec.rb",
         "spec/bundler/resolver/cooldown_spec.rb",
         "spec/install/cooldown_spec.rb",
diff --git a/test/rubygems/test_gem_compact_index_client_cache_file.rb b/test/rubygems/test_gem_compact_index_client_cache_file.rb
index 193ae327e9c2..f2e2633646b4 100644
--- a/test/rubygems/test_gem_compact_index_client_cache_file.rb
+++ b/test/rubygems/test_gem_compact_index_client_cache_file.rb
@@ -82,6 +82,23 @@ def test_append_with_matching_digests
     assert_equal "abcdef", @path.read
   end
 
+  def test_append_writes_in_binary_mode
+    @path.binwrite "created_at: 2026-06-10\n---\n"
+
+    appended = nil
+    CacheFile.copy(@path) do |file|
+      file.digests = sha256("created_at: 2026-06-10\n---\nrake 13.0.0\n")
+      appended = file.append("rake 13.0.0\n")
+    end
+
+    assert appended
+    # On Windows a text-mode append rewrites the appended LF as CRLF while the
+    # digest is computed over the pre-write bytes, so verify passes but the file
+    # on disk is corrupted. Read raw bytes to catch any stray carriage return.
+    refute_includes @path.binread, "\r"
+    assert_equal "created_at: 2026-06-10\n---\nrake 13.0.0\n", @path.binread
+  end
+
   def test_append_with_mismatched_digests_keeps_original
     @path.binwrite "abc"