diff --git a/kwave/compat.py b/kwave/compat.py
index a077c8fc..70f39391 100644
--- a/kwave/compat.py
+++ b/kwave/compat.py
@@ -39,7 +39,13 @@ def options_to_kwargs(simulation_options=None, execution_options=None):
         kwargs["use_kspace"] = opts.use_kspace
         kwargs["smooth_p0"] = opts.smooth_p0
         if opts.data_path is not None:
-            kwargs["data_path"] = opts.data_path
+            import os
+            from tempfile import gettempdir
+
+            normalized_data_path = os.path.realpath(os.path.normpath(os.fspath(opts.data_path)))
+            normalized_tempdir = os.path.realpath(os.path.normpath(os.fspath(gettempdir())))
+            if normalized_data_path != normalized_tempdir:
+                kwargs["data_path"] = opts.data_path
         if opts.save_to_disk_exit:
             kwargs["save_only"] = True
 
diff --git a/kwave/solvers/cpp_simulation.py b/kwave/solvers/cpp_simulation.py
index fe465c7c..2f17f747 100644
--- a/kwave/solvers/cpp_simulation.py
+++ b/kwave/solvers/cpp_simulation.py
@@ -4,6 +4,7 @@
 Handles HDF5 serialization and C++ binary execution without
 depending on kWaveSimulation or the legacy options classes.
 """
+
 import os
 import shutil
 import stat
@@ -46,6 +47,11 @@ def prepare(self, data_path=None):
         input_file = os.path.join(data_path, "kwave_input.h5")
         output_file = os.path.join(data_path, "kwave_output.h5")
 
+        if os.path.exists(input_file):
+            raise FileExistsError(
+                f"{input_file!r} already exists. Delete it or choose a different data_path to avoid overwriting previous simulation inputs."
+            )
+
         self._write_hdf5(input_file)
         return input_file, output_file
 
@@ -56,7 +62,16 @@ def run(self, *, device="cpu", num_threads=None, device_num=None, quiet=False, d
         input_file, output_file = self.prepare(data_path=data_path)
         data_dir = os.path.dirname(input_file)
         try:
-            self._execute(input_file, output_file, device=device, num_threads=num_threads, device_num=device_num, quiet=quiet, debug=debug, binary_path=binary_path)
+            self._execute(
+                input_file,
+                output_file,
+                device=device,
+                num_threads=num_threads,
+                device_num=device_num,
+                quiet=quiet,
+                debug=debug,
+                binary_path=binary_path,
+            )
             result = self._parse_output(output_file)
             result = self._fix_output_order(result)
             return result
diff --git a/tests/test_compat.py b/tests/test_compat.py
index 498b2001..9995af89 100644
--- a/tests/test_compat.py
+++ b/tests/test_compat.py
@@ -91,3 +91,15 @@ def test_both_options(self):
     def test_none_options(self):
         kwargs = options_to_kwargs()
         assert kwargs == {}
+
+    def test_default_data_path_not_forwarded(self):
+        # Before fix: data_path defaulted to gettempdir() and was always forwarded,
+        # so every run targeted /tmp/kwave_input.h5 and crashed on the second call.
+        kwargs = options_to_kwargs(simulation_options=SimulationOptions())
+        assert "data_path" not in kwargs
+
+    def test_custom_data_path_is_forwarded(self, tmp_path):
+        opts = SimulationOptions()
+        opts.data_path = str(tmp_path)
+        kwargs = options_to_kwargs(simulation_options=opts)
+        assert kwargs["data_path"] == str(tmp_path)
diff --git a/tests/test_cpp_simulation.py b/tests/test_cpp_simulation.py
index c2171028..b99a0062 100644
--- a/tests/test_cpp_simulation.py
+++ b/tests/test_cpp_simulation.py
@@ -10,6 +10,16 @@
 from kwave.solvers.cpp_simulation import CppSimulation
 
 
+class TestPrepare:
+    def test_raises_when_input_already_exists(self, tmp_path):
+        # Before fix: h5py raised a cryptic ValueError('name already exists')
+        # instead of a clear FileExistsError.
+        (tmp_path / "kwave_input.h5").write_bytes(b"")
+        sim = CppSimulation.__new__(CppSimulation)
+        with pytest.raises(FileExistsError, match="already exists"):
+            sim.prepare(data_path=str(tmp_path))
+
+
 class TestResolveBinaryPath:
     """Tests for CppSimulation._resolve_binary_path().