diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 50c29da..e7069ba 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -7,11 +7,12 @@ permissions:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     env:
       RUST_BACKTRACE: full
     strategy:
       matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
         python-version: ["3.10", "3.11", "3.12", "3.13", "3.14"]
     steps:
     - uses: actions/checkout@v5
diff --git a/tests/test_replay_session.py b/tests/test_replay_session.py
index 380bc88..226fb02 100644
--- a/tests/test_replay_session.py
+++ b/tests/test_replay_session.py
@@ -70,7 +70,7 @@ def test_session_does_not_save_failed_snippet():
 
 
 def test_session_supports_readonly_directory(tmp_path):
-    (tmp_path / "message.txt").write_text("hello\n")
+    (tmp_path / "message.txt").write_text("hello\n", newline="\n")
     session = MicroPythonReplaySession(readonly_dir=tmp_path, wall_timeout_seconds=None)
 
     result = session.run(
diff --git a/tests/test_runtime_semantics.py b/tests/test_runtime_semantics.py
index 06f1cc4..2ee9441 100644
--- a/tests/test_runtime_semantics.py
+++ b/tests/test_runtime_semantics.py
@@ -239,7 +239,7 @@ def test_instances_are_fresh_between_runs():
 
 
 def test_readonly_preopened_directory_allows_reading(tmp_path):
-    (tmp_path / "hello.txt").write_text("hello from wasi\n")
+    (tmp_path / "hello.txt").write_text("hello from wasi\n", newline="\n")
 
     assert (
         run_stdout(
@@ -255,7 +255,7 @@ def test_readonly_preopened_directory_allows_reading(tmp_path):
 
 
 def test_readonly_preopened_directory_rejects_writes(tmp_path):
-    (tmp_path / "hello.txt").write_text("hello from wasi\n")
+    (tmp_path / "hello.txt").write_text("hello from wasi\n", newline="\n")
 
     with pytest.raises(MicroPythonWasmError, match="guest exited with code 1"):
         run(