fix: split mount prefixes with trailing space into separate arguments

src/container_magic/core/runner.py

@@ -204,7 +204,11 @@ def _add_mount_volumes(
             args.extend(["-v", f"{resolved}:{container_path}:z"])
 
         manifest_lines.append(f"{resolved}:{container_path}")
-        command_fragments.append(f"{spec.prefix}{container_path}")
+        if spec.prefix and spec.prefix != spec.prefix.rstrip():
+            prefix_parts = spec.prefix.rstrip().split()
+            command_fragments.extend(prefix_parts + [container_path])
+        else:
+            command_fragments.append(f"{spec.prefix}{container_path}")
 
     return command_fragments, manifest_lines
 

tests/unit/test_runner.py

@@ -31,6 +31,7 @@
 
 from container_magic.core.config import ContainerMagicConfig, CustomCommand
 from container_magic.core.runner import (
+    _add_mount_volumes,
     _build_feature_flags,
     _detect_container_home,
     _detect_shell,
@@ -216,6 +217,46 @@ def test_unknown_name_value_stays_in_remaining(self):
         assert remaining == ["unknown=value"]
 
 
+class TestAddMountVolumes:
+    def _make_command(self, mounts=None):
+        data = {"command": "test-cmd"}
+        if mounts:
+            data["mounts"] = mounts
+        return CustomCommand(**data)
+
+    def test_space_prefix_splits_into_separate_fragments(self, tmp_path):
+        test_file = tmp_path / "input.txt"
+        test_file.write_text("test")
+        cmd = self._make_command(mounts={"data": {"mode": "ro", "prefix": "--data "}})
+        args = []
+        fragments, _ = _add_mount_volumes(args, cmd, {"data": str(test_file)})
+        assert fragments == ["--data", f"/mnt/data/{test_file.name}"]
+
+    def test_equals_prefix_stays_as_one_fragment(self, tmp_path):
+        test_file = tmp_path / "input.txt"
+        test_file.write_text("test")
+        cmd = self._make_command(mounts={"data": {"mode": "ro", "prefix": "--data="}})
+        args = []
+        fragments, _ = _add_mount_volumes(args, cmd, {"data": str(test_file)})
+        assert fragments == ["--data=/mnt/data/input.txt"]
+
+    def test_no_prefix_produces_path_only(self, tmp_path):
+        test_file = tmp_path / "input.txt"
+        test_file.write_text("test")
+        cmd = self._make_command(mounts={"data": {"mode": "ro"}})
+        args = []
+        fragments, _ = _add_mount_volumes(args, cmd, {"data": str(test_file)})
+        assert fragments == [f"/mnt/data/{test_file.name}"]
+
+    def test_rw_mount_with_space_prefix(self, tmp_path):
+        output_dir = tmp_path / "output"
+        output_dir.mkdir()
+        cmd = self._make_command(mounts={"results": {"mode": "rw", "prefix": "--output "}})
+        args = []
+        fragments, _ = _add_mount_volumes(args, cmd, {"results": str(output_dir)})
+        assert fragments == ["--output", "/mnt/results"]
+
+
 class TestParseRunArgs:
     def test_no_args(self):
         detach, passthrough, remaining = _parse_run_args([])