diff --git a/cuda_pathfinder/cuda/pathfinder/_headers/find_nvidia_headers.py b/cuda_pathfinder/cuda/pathfinder/_headers/find_nvidia_headers.py
index f5f56817141..a7af7b07467 100644
--- a/cuda_pathfinder/cuda/pathfinder/_headers/find_nvidia_headers.py
+++ b/cuda_pathfinder/cuda/pathfinder/_headers/find_nvidia_headers.py
@@ -31,13 +31,13 @@
 # ---------------------------------------------------------------------------
 
 
-@dataclass
+@dataclass(frozen=True)
 class LocatedHeaderDir:
     abs_path: str | None
     found_via: str
 
     def __post_init__(self) -> None:
-        self.abs_path = _abs_norm(self.abs_path)
+        object.__setattr__(self, "abs_path", _abs_norm(self.abs_path))
 
 
 #: Type alias for a header find step callable.
diff --git a/cuda_pathfinder/docs/source/release/1.6.1-notes.rst b/cuda_pathfinder/docs/source/release/1.6.1-notes.rst
index 4b6e7a0a55e..9a4da0ab77c 100644
--- a/cuda_pathfinder/docs/source/release/1.6.1-notes.rst
+++ b/cuda_pathfinder/docs/source/release/1.6.1-notes.rst
@@ -35,6 +35,12 @@ Highlights
   architecture-specific executable targets using the native Windows machine
   architecture.
 
+* Make ``LocatedHeaderDir`` immutable, matching ``LocatedStaticLib`` and
+  ``LocatedBitcodeLib``. ``locate_nvidia_header_directory`` is
+  ``functools.cache``-backed, so writing to a returned object previously
+  changed what every later lookup of that ``libname`` returned. The type is
+  now hashable as well.
+
 Internal maintenance
 --------------------
 
diff --git a/cuda_pathfinder/tests/test_find_nvidia_headers.py b/cuda_pathfinder/tests/test_find_nvidia_headers.py
index 3e045dae265..6136011f180 100644
--- a/cuda_pathfinder/tests/test_find_nvidia_headers.py
+++ b/cuda_pathfinder/tests/test_find_nvidia_headers.py
@@ -11,6 +11,7 @@
 # sudo apt install libnvshmem3-cuda-12 libnvshmem3-dev-cuda-12
 # sudo apt install libnvshmem3-cuda-13 libnvshmem3-dev-cuda-13
 
+import dataclasses
 import functools
 import glob
 import importlib.metadata
@@ -264,3 +265,33 @@ def test_locate_ctk_headers_canary_probe_errors_are_not_masked(monkeypatch, mock
         locate_nvidia_header_directory("cudart")
     with pytest.raises(RuntimeError, match="canary probe failed"):
         find_nvidia_header_directory("cudart")
+
+
+@pytest.mark.agent_authored(model="claude-opus-5")
+@pytest.mark.usefixtures("clear_locate_nvidia_header_cache")
+def test_located_header_dir_is_immutable(tmp_path, monkeypatch, mocker):
+    cuda_home = tmp_path / "cuda-home"
+    expected_hdr_dir = _create_ctk_header(cuda_home, "cudart")
+
+    monkeypatch.delenv("CONDA_PREFIX", raising=False)
+    monkeypatch.setenv("CUDA_HOME", str(cuda_home))
+    monkeypatch.delenv("CUDA_PATH", raising=False)
+    mocker.patch.object(find_nvidia_headers_module, "find_sub_dirs_all_sitepackages", return_value=[])
+
+    first = locate_nvidia_header_directory("cudart")
+    assert first is not None
+    assert first.abs_path == expected_hdr_dir
+
+    with pytest.raises(dataclasses.FrozenInstanceError):
+        first.abs_path = str(tmp_path / "somewhere-else")
+
+    assert locate_nvidia_header_directory("cudart").abs_path == expected_hdr_dir
+    assert find_nvidia_header_directory("cudart") == expected_hdr_dir
+
+
+@pytest.mark.agent_authored(model="claude-opus-5")
+def test_located_header_dir_is_hashable(tmp_path):
+    include_dir = str(tmp_path / "include")
+    hdr_dir = LocatedHeaderDir(abs_path=include_dir, found_via="conda")
+    assert hdr_dir in {hdr_dir}
+    assert hdr_dir == LocatedHeaderDir(abs_path=include_dir, found_via="conda")