fix!: reject keyword arguments in @model signatures instead of warning

src/model_macro.jl

@@ -957,7 +957,13 @@ function model_macro_interior(backend_type, model_specification)
 
     num_interfaces = Base.length(ms_args)
     if !isnothing(ms_kwargs) && length(ms_kwargs) > 0
-        @warn("Model specification language does not support keyword arguments. Ignoring $(length(ms_kwargs)) keyword arguments.")
+        # Keyword arguments in a model signature were previously parsed and then silently dropped, with only
+        # a warning. The body never saw them, so the only real signal was a later `UndefVarError`. Fail here
+        # instead, consistent with how unsupported positional arguments are rejected at the call site.
+        kwargs_names = map(kwarg -> (kwarg isa Expr && kwarg.head === :kw) ? kwarg.args[1] : kwarg, ms_kwargs)
+        error(
+            "The `$(ms_name)` model macro does not support keyword arguments in the model signature, but got $(length(ms_kwargs)): $(join(kwargs_names, ", ")). Declare all model interfaces as positional arguments, `$(ms_name)($(join(ms_args, ", ")))`, they are passed by name at the call site."
+        )
     end
 
     boilerplate_functions = GraphPPL.get_boilerplate_functions(backend_type, ms_name, ms_args, num_interfaces)

test/model_macro_tests.jl

@@ -2070,3 +2070,28 @@ end
         somemodel()
     )
 end
+
+@testitem "`@model` should reject keyword arguments in the model signature" begin
+    import GraphPPL: model_macro_interior
+
+    include("testutils.jl")
+
+    # Keyword arguments used to be parsed and then silently dropped with only a warning, so the body
+    # never saw them and the user's only signal was a later `UndefVarError`
+    kwargs_spec = :(function model_with_kwargs(x; a = 1, b = 2)
+        y ~ Normal(x, a)
+    end)
+
+    @test_throws "does not support keyword arguments in the model signature" model_macro_interior(
+        TestUtils.TestGraphPPLBackend, kwargs_spec
+    )
+    # the offending keyword arguments are named, and the supported form is shown
+    @test_throws "but got 2: a, b" model_macro_interior(TestUtils.TestGraphPPLBackend, kwargs_spec)
+    @test_throws "model_with_kwargs(x)" model_macro_interior(TestUtils.TestGraphPPLBackend, kwargs_spec)
+
+    # positional-only signatures are unaffected
+    positional_spec = :(function model_without_kwargs(x, a)
+        y ~ Normal(x, a)
+    end)
+    @test model_macro_interior(TestUtils.TestGraphPPLBackend, positional_spec) isa Expr
+end