Raise a clear error when a no-input model targets below iOS18

coremltools/converters/mil/converter.py

@@ -8,6 +8,7 @@
 from typing import Optional, Text, Tuple
 
 import coremltools as ct
+from coremltools import _SPECIFICATION_VERSION_IOS_18
 from coremltools.converters._profile_utils import _profile
 from coremltools.converters.mil import input_types
 from coremltools.converters.mil.mil import Builder as mb
@@ -303,6 +304,28 @@ def mil_convert_to_proto(
 
     prog._check_early_error_out_for_invalid_program()
 
+    # Models with no inputs require specification version >= iOS18; otherwise
+    # the resulting mlpackage fails to compile downstream with
+    #   "Empty input is only valid in specification version >= 9".
+    # Surface that as an explicit error here rather than letting the user
+    # discover it at compile time.
+    spec_version = kwargs.get("specification_version")
+    if (
+        convert_to == "mlprogram"
+        and spec_version is not None
+        and spec_version < _SPECIFICATION_VERSION_IOS_18
+    ):
+        main_func = prog.functions.get(prog.default_function_name)
+        if main_func is not None and len(main_func.inputs) == 0:
+            raise ValueError(
+                "Model has no inputs, which requires a deployment target of "
+                "iOS18 / macOS15 or later (specification version "
+                f"{_SPECIFICATION_VERSION_IOS_18}); the requested target maps "
+                f"to specification version {spec_version}. Re-run "
+                "`ct.convert(...)` with "
+                "`minimum_deployment_target=ct.target.iOS18` (or a later target)."
+            )
+
     backend_converter_type = converter_registry.backends.get(convert_to.lower())
     if not backend_converter_type:
         raise NotImplementedError(

coremltools/converters/mil/frontend/torch/test/test_torch_export_conversion_api.py

@@ -333,6 +333,31 @@ def forward(self, x):
         )
         assert mlmodel.user_defined_metadata[_METADATA_SOURCE_DIALECT] == dialect_name
 
+    @pytest.mark.parametrize("frontend", frontends)
+    def test_no_input_model_requires_ios18(self, frontend):
+        # Regression test for #2578: a model with no inputs requires
+        # specification version >= iOS18, otherwise the produced mlpackage
+        # fails to compile with "Empty input is only valid in specification
+        # version >= 9". Verify both the happy path (iOS18 succeeds) and the
+        # error path (lower target raises a clear ValueError pointing the
+        # user at minimum_deployment_target).
+        class Model(torch.nn.Module):
+            def forward(self):
+                return torch.ones(5, 5)
+
+        exported_model = export_torch_model_to_frontend(Model().eval(), (), frontend)
+
+        # Default target (iOS15) must fail with an actionable error message.
+        with pytest.raises(ValueError, match=r"minimum_deployment_target=ct\.target\.iOS18"):
+            ct.convert(exported_model)
+
+        # Explicit iOS18 target must succeed and produce a runnable mlpackage.
+        mlmodel = ct.convert(exported_model, minimum_deployment_target=ct.target.iOS18)
+        spec = mlmodel.get_spec()
+        assert len(spec.description.input) == 0
+        assert spec.specificationVersion >= ct.target.iOS18.value
+        verify_prediction(mlmodel)
+
 
 @pytest.mark.skipif((version_info.major, version_info.minor) == (3, 13), reason="rdar://158079341")
 class TestExecuTorchExamples(TorchBaseTest):