diff --git a/metatomic-torch/CHANGELOG.md b/metatomic-torch/CHANGELOG.md
index 4191ca1c1..62733f346 100644
--- a/metatomic-torch/CHANGELOG.md
+++ b/metatomic-torch/CHANGELOG.md
@@ -24,6 +24,14 @@ a changelog](https://keepachangelog.com/en/1.1.0/) format. This project follows
 - 2-argument `unit_conversion_factor(from_unit, to_unit)` that parses
   arbitrary unit expressions and checks dimensional compatibility
 
+### Deprecated
+
+- `ModelOutput.quantity` field is deprecated; use the 2-argument `unit()` instead
+  for unit specification. Unit conversion now determines dimensions from the
+  expression automatically.
+- `TensorMap.set_info("quantity", ...)` convention is deprecated; quantity is
+  no longer required for unit conversion.
+
 ### Changed
 
 - 3-argument `unit_conversion_factor(quantity, from_unit, to_unit)` is
diff --git a/metatomic-torch/include/metatomic/torch/model.hpp b/metatomic-torch/include/metatomic/torch/model.hpp
index 2a0d516c6..81b11318d 100644
--- a/metatomic-torch/include/metatomic/torch/model.hpp
+++ b/metatomic-torch/include/metatomic/torch/model.hpp
@@ -55,13 +55,17 @@ class METATOMIC_TORCH_EXPORT ModelOutputHolder: public torch::CustomClassHolder
     /// description of this output, defaults to empty string of not set by the user
     std::string description;
 
-    /// quantity of the output (e.g. energy, dipole, …).  If this is an empty
+    /// quantity of the output (e.g. energy, dipole, ...).  If this is an empty
     /// string, no unit conversion will be performed.
+    /// @deprecated This field is no longer required for unit conversion.
+    ///             The unit parser determines dimensions from the expression itself.
+    [[deprecated("quantity is no longer required for unit conversion, use unit directly")]]
     const std::string& quantity() const {
         return quantity_;
     }
 
     /// set the quantity of the output
+    [[deprecated("quantity is no longer required for unit conversion, use unit directly")]]
     void set_quantity(std::string quantity);
 
     /// unit of the output. If this is an empty string, no unit conversion will
diff --git a/metatomic-torch/src/model.cpp b/metatomic-torch/src/model.cpp
index ea8c682d2..de37bc430 100644
--- a/metatomic-torch/src/model.cpp
+++ b/metatomic-torch/src/model.cpp
@@ -54,6 +54,13 @@ static void read_vector_int_json(
 /******************************************************************************/
 
 void ModelOutputHolder::set_quantity(std::string quantity) {
+    if (!quantity.empty()) {
+        TORCH_WARN_DEPRECATION(
+            "ModelOutput.quantity is deprecated and will be removed in a future version. "
+            "The quantity field is no longer required for unit conversion since the "
+            "unit parser determines dimensions from the expression itself."
+        );
+    }
     if (valid_quantity(quantity)) {
         validate_unit(quantity, unit_);
     }
diff --git a/metatomic-torch/tests/models.cpp b/metatomic-torch/tests/models.cpp
index c319b6593..4ca4528f2 100644
--- a/metatomic-torch/tests/models.cpp
+++ b/metatomic-torch/tests/models.cpp
@@ -109,11 +109,15 @@ TEST_CASE("Models metadata") {
         struct WarningHandler: public torch::WarningHandler {
             virtual ~WarningHandler() override = default;
             void process(const torch::Warning& warning) override {
-                auto expected = std::string(
-                    "unknown quantity 'unknown', only [charge energy force heat_flux "
-                    "length mass momentum pressure velocity] are supported"
-                );
-                CHECK(warning.msg() == expected);
+                // First warning is deprecation, second is unknown quantity
+                // We check for the unknown quantity warning
+                if (warning.msg().find("unknown quantity") != std::string::npos) {
+                    auto expected = std::string(
+                        "unknown quantity 'unknown', only [charge energy force heat_flux "
+                        "length mass momentum pressure velocity] are supported"
+                    );
+                    CHECK(warning.msg() == expected);
+                }
             }
         };
 
diff --git a/python/metatomic_ase/pyproject.toml b/python/metatomic_ase/pyproject.toml
index a27cc385c..faf71e8a7 100644
--- a/python/metatomic_ase/pyproject.toml
+++ b/python/metatomic_ase/pyproject.toml
@@ -44,6 +44,8 @@ python_files = ["*.py"]
 testpaths = ["tests"]
 filterwarnings = [
     "error",
+    # ModelOutput.quantity deprecation (C++ TORCH_WARN_DEPRECATION)
+    "ignore:ModelOutput.quantity is deprecated:DeprecationWarning",
     # TorchScript deprecation warnings
     "ignore:`torch.jit.script` is deprecated. Please switch to `torch.compile` or `torch.export`:DeprecationWarning",
     "ignore:`torch.jit.script_method` is deprecated. Please switch to `torch.compile` or `torch.export`:DeprecationWarning",
diff --git a/python/metatomic_ase/tests/calculator.py b/python/metatomic_ase/tests/calculator.py
index ac85d598f..7dbd25bf7 100644
--- a/python/metatomic_ase/tests/calculator.py
+++ b/python/metatomic_ase/tests/calculator.py
@@ -848,7 +848,8 @@ def test_additional_input(atoms):
         assert head == "extra"
         assert name in inputs
 
-        assert tensor.get_info("quantity") == inputs[name].quantity
+        # quantity info is no longer set (deprecated); just check unit is set
+        assert tensor.get_info("unit") == inputs[name].unit
         values = tensor[0].values.numpy()
 
         expected = ARRAY_QUANTITIES[name]["getter"](atoms).reshape(values.shape)
diff --git a/python/metatomic_ase/tests/conftest.py b/python/metatomic_ase/tests/conftest.py
index 2b9201ed0..da4d7423e 100644
--- a/python/metatomic_ase/tests/conftest.py
+++ b/python/metatomic_ase/tests/conftest.py
@@ -1,10 +1,22 @@
+import re
+
 import pytest
 
 
+# C++ TORCH_WARN_DEPRECATION writes to stderr; filter the known quantity warning.
+# Torch 2.3 emits "[W model.cpp:N] ..." (no timestamp); Torch 2.11+ emits
+# "[W<MMDD> HH:MM:SS.fractZ model.cpp:N] ...". The [^\]]* tolerates both.
+_QUANTITY_DEPRECATION_RE = re.compile(
+    r"\[W[^\]]*model\.cpp:\d+\] Warning: ModelOutput\.quantity is "
+    r"deprecated.*\(function set_quantity\)\n"
+)
+
+
 @pytest.fixture(autouse=True)
 def fail_test_with_output(capfd):
     yield
     captured = capfd.readouterr()
     # the code should not print anything to stdout or stderr
     assert captured.out == ""
-    assert captured.err == ""
+    err = _QUANTITY_DEPRECATION_RE.sub("", captured.err)
+    assert err == ""
diff --git a/python/metatomic_torch/metatomic/torch/documentation.py b/python/metatomic_torch/metatomic/torch/documentation.py
index 5dc080c05..49c10b6c2 100644
--- a/python/metatomic_torch/metatomic/torch/documentation.py
+++ b/python/metatomic_torch/metatomic/torch/documentation.py
@@ -295,6 +295,11 @@ def quantity(self) -> str:
         Quantity of the output (e.g. energy, dipole, …).  If this is an empty string, no
         unit conversion will be performed.
 
+        .. deprecated::
+            The ``quantity`` field is deprecated and will be removed.
+            Unit conversion determines dimensions from the unit expression.
+            Set ``quantity`` to an empty string to suppress deprecation warnings.
+
         The list of possible quantities is available :ref:`here
         <known-base-units>`.
         """
diff --git a/python/metatomic_torch/metatomic/torch/model.py b/python/metatomic_torch/metatomic/torch/model.py
index 22f83c0cd..566fbf4b6 100644
--- a/python/metatomic_torch/metatomic/torch/model.py
+++ b/python/metatomic_torch/metatomic/torch/model.py
@@ -265,7 +265,6 @@ class AtomisticModel(torch.nn.Module):
     >>> capabilities = ModelCapabilities(
     ...     outputs={
     ...         "energy": ModelOutput(
-    ...             quantity="energy",
     ...             unit="eV",
     ...             per_atom=False,
     ...             explicit_gradients=[],
@@ -507,16 +506,9 @@ def forward(
             for name, output in outputs.items():
                 declared = self._capabilities.outputs[name]
                 requested = options.outputs.get(name, ModelOutput())
-                if declared.quantity == "" or requested.quantity == "":
+                if declared.unit == "" or requested.unit == "":
                     continue
 
-                if declared.quantity != requested.quantity:
-                    raise ValueError(
-                        f"model produces values as '{declared.quantity}' for the "
-                        f"'{name}' output, but the engine requested "
-                        f"'{requested.quantity}'"
-                    )
-
                 conversion = unit_conversion_factor(
                     declared.unit,
                     requested.unit,
@@ -994,7 +986,9 @@ def _convert_systems_units(
                 tensor = system.get_data(name)
                 unit = tensor.get_info("unit")
 
-                if requested.quantity != "" and unit is not None:
+                # Convert units if both the tensor and requested output have units.
+                # The quantity field is deprecated; unit conversion is dimension-aware.
+                if unit is not None and requested.unit != "":
                     conversion = unit_conversion_factor(
                         unit,
                         requested.unit,
@@ -1032,7 +1026,6 @@ def _convert_systems_units(
                     blocks=new_blocks,
                 )
                 new_tensor.set_info("unit", requested.unit)
-                new_tensor.set_info("quantity", requested.quantity)
                 new_system.add_data(name, new_tensor)
 
         new_systems.append(new_system)
diff --git a/python/metatomic_torch/pyproject.toml b/python/metatomic_torch/pyproject.toml
index 249b7def1..7e2f395c0 100644
--- a/python/metatomic_torch/pyproject.toml
+++ b/python/metatomic_torch/pyproject.toml
@@ -58,6 +58,8 @@ python_files = ["*.py"]
 testpaths = ["tests"]
 filterwarnings = [
     "error",
+    # ModelOutput.quantity deprecation
+    "ignore:ModelOutput.quantity is deprecated:DeprecationWarning",
     # TorchScript deprecation warnings
     "ignore:`torch.jit.script` is deprecated. Please switch to `torch.compile` or `torch.export`:DeprecationWarning",
     "ignore:`torch.jit.save` is deprecated. Please switch to `torch.export`:DeprecationWarning",
diff --git a/python/metatomic_torch/tests/conftest.py b/python/metatomic_torch/tests/conftest.py
index 2b9201ed0..da4d7423e 100644
--- a/python/metatomic_torch/tests/conftest.py
+++ b/python/metatomic_torch/tests/conftest.py
@@ -1,10 +1,22 @@
+import re
+
 import pytest
 
 
+# C++ TORCH_WARN_DEPRECATION writes to stderr; filter the known quantity warning.
+# Torch 2.3 emits "[W model.cpp:N] ..." (no timestamp); Torch 2.11+ emits
+# "[W<MMDD> HH:MM:SS.fractZ model.cpp:N] ...". The [^\]]* tolerates both.
+_QUANTITY_DEPRECATION_RE = re.compile(
+    r"\[W[^\]]*model\.cpp:\d+\] Warning: ModelOutput\.quantity is "
+    r"deprecated.*\(function set_quantity\)\n"
+)
+
+
 @pytest.fixture(autouse=True)
 def fail_test_with_output(capfd):
     yield
     captured = capfd.readouterr()
     # the code should not print anything to stdout or stderr
     assert captured.out == ""
-    assert captured.err == ""
+    err = _QUANTITY_DEPRECATION_RE.sub("", captured.err)
+    assert err == ""
diff --git a/python/metatomic_torch/tests/examples.py b/python/metatomic_torch/tests/examples.py
index d846f0f07..5aadd9d24 100644
--- a/python/metatomic_torch/tests/examples.py
+++ b/python/metatomic_torch/tests/examples.py
@@ -47,7 +47,7 @@ def test_export_atomistic_model(tmp_path):
     )
 
     outputs = {
-        "energy": ModelOutput(quantity="energy", unit="eV", per_atom=False),
+        "energy": ModelOutput(unit="eV", per_atom=False),
     }
 
     # run bare model
diff --git a/python/metatomic_torch/tests/model.py b/python/metatomic_torch/tests/model.py
index 3937ecd11..22769212c 100644
--- a/python/metatomic_torch/tests/model.py
+++ b/python/metatomic_torch/tests/model.py
@@ -702,7 +702,6 @@ def test_not_requested_output(system):
 def test_systems_unit_conversion(system):
     requested_inputs = {
         "masses": ModelOutput(
-            quantity="mass",
             unit="kg",
             per_atom=True,
         ),
diff --git a/python/metatomic_torch/tests/units.py b/python/metatomic_torch/tests/units.py
index 95d6ffec9..e288f6c88 100644
--- a/python/metatomic_torch/tests/units.py
+++ b/python/metatomic_torch/tests/units.py
@@ -212,41 +212,42 @@ def test_overflow_division():
 
 def test_valid_units():
     # just checking that all of these are valid
-    ModelOutput(quantity="length", unit="A")
-    ModelOutput(quantity="length", unit="Angstrom")
-    ModelOutput(quantity="length", unit="Bohr")
-    ModelOutput(quantity="length", unit="meter")
-    ModelOutput(quantity="length", unit=" centimeter")
-    ModelOutput(quantity="length", unit="cm")
-    ModelOutput(quantity="length", unit="millimeter")
-    ModelOutput(quantity="length", unit="mm")
-    ModelOutput(quantity="length", unit=" micrometer")
-    ModelOutput(quantity="length", unit="um")
-    ModelOutput(quantity="length", unit="µm")
-    ModelOutput(quantity="length", unit="nanometer")
-    ModelOutput(quantity="length", unit="nm ")
-
-    ModelOutput(quantity="energy", unit="eV")
-    ModelOutput(quantity="energy", unit="meV")
-    ModelOutput(quantity="energy", unit="Hartree")
-    ModelOutput(quantity="energy", unit="kcal /  mol ")
-    ModelOutput(quantity="energy", unit="kJ/mol")
-    ModelOutput(quantity="energy", unit="Joule")
-    ModelOutput(quantity="energy", unit="J")
-    ModelOutput(quantity="energy", unit="Rydberg")
-    ModelOutput(quantity="energy", unit="Ry")
-
-    ModelOutput(quantity="force", unit="eV/Angstrom")
-    ModelOutput(quantity="force", unit="eV/A")
-
-    ModelOutput(quantity="pressure", unit="eV/Angstrom^3")
-    ModelOutput(quantity="pressure", unit="eV/A^3")
-
-    ModelOutput(quantity="momentum", unit="u * A/ fs")
-    ModelOutput(quantity="momentum", unit=" (eV*u )^(1/ 2 )")
-
-    ModelOutput(quantity="velocity", unit="A/fs")
-    ModelOutput(quantity="velocity", unit="A/s")
+    # quantity parameter is deprecated, only testing unit parsing
+    ModelOutput(unit="A")
+    ModelOutput(unit="Angstrom")
+    ModelOutput(unit="Bohr")
+    ModelOutput(unit="meter")
+    ModelOutput(unit=" centimeter")
+    ModelOutput(unit="cm")
+    ModelOutput(unit="millimeter")
+    ModelOutput(unit="mm")
+    ModelOutput(unit=" micrometer")
+    ModelOutput(unit="um")
+    ModelOutput(unit="µm")
+    ModelOutput(unit="nanometer")
+    ModelOutput(unit="nm ")
+
+    ModelOutput(unit="eV")
+    ModelOutput(unit="meV")
+    ModelOutput(unit="Hartree")
+    ModelOutput(unit="kcal /  mol ")
+    ModelOutput(unit="kJ/mol")
+    ModelOutput(unit="Joule")
+    ModelOutput(unit="J")
+    ModelOutput(unit="Rydberg")
+    ModelOutput(unit="Ry")
+
+    ModelOutput(unit="eV/Angstrom")
+    ModelOutput(unit="eV/A")
+
+    ModelOutput(unit="eV/Angstrom^3")
+    ModelOutput(unit="eV/A^3")
+
+    ModelOutput(unit="u * A/ fs")
+    ModelOutput(unit=" (eV*u )^(1/ 2 )")
+
+    ModelOutput(unit="A/fs")
+    ModelOutput(unit="A/s")
 
 
 # ---- Accumulated floating-point error with fractional powers ----
@@ -318,6 +319,7 @@ def test_micro_sign_microsecond():
 
 def test_quantity_unit_mismatch():
     # energy quantity with force unit
+    # quantity parameter is deprecated, but we still test unit validation
     with pytest.raises((ValueError, RuntimeError), match="incompatible with quantity"):
         ModelOutput(quantity="energy", unit="eV/A")
 
diff --git a/python/metatomic_torchsim/pyproject.toml b/python/metatomic_torchsim/pyproject.toml
index 9c3b119ef..5624a9841 100644
--- a/python/metatomic_torchsim/pyproject.toml
+++ b/python/metatomic_torchsim/pyproject.toml
@@ -45,6 +45,8 @@ python_files = ["*.py"]
 testpaths = ["tests"]
 filterwarnings = [
     "error",
+    # ModelOutput.quantity deprecation (C++ TORCH_WARN_DEPRECATION)
+    "ignore:ModelOutput.quantity is deprecated:DeprecationWarning",
     # TorchScript deprecation warnings
     "ignore:`torch.jit.script` is deprecated. Please switch to `torch.compile` or `torch.export`:DeprecationWarning",
     "ignore:`torch.jit.script_method` is deprecated. Please switch to `torch.compile` or `torch.export`:DeprecationWarning",
diff --git a/python/metatomic_torchsim/tests/conftest.py b/python/metatomic_torchsim/tests/conftest.py
index 2b9201ed0..da4d7423e 100644
--- a/python/metatomic_torchsim/tests/conftest.py
+++ b/python/metatomic_torchsim/tests/conftest.py
@@ -1,10 +1,22 @@
+import re
+
 import pytest
 
 
+# C++ TORCH_WARN_DEPRECATION writes to stderr; filter the known quantity warning.
+# Torch 2.3 emits "[W model.cpp:N] ..." (no timestamp); Torch 2.11+ emits
+# "[W<MMDD> HH:MM:SS.fractZ model.cpp:N] ...". The [^\]]* tolerates both.
+_QUANTITY_DEPRECATION_RE = re.compile(
+    r"\[W[^\]]*model\.cpp:\d+\] Warning: ModelOutput\.quantity is "
+    r"deprecated.*\(function set_quantity\)\n"
+)
+
+
 @pytest.fixture(autouse=True)
 def fail_test_with_output(capfd):
     yield
     captured = capfd.readouterr()
     # the code should not print anything to stdout or stderr
     assert captured.out == ""
-    assert captured.err == ""
+    err = _QUANTITY_DEPRECATION_RE.sub("", captured.err)
+    assert err == ""