enforce systems can only have brownians or noise equations

Author: isaacsas

lib/ModelingToolkitBase/src/systems/system.jl

@@ -326,6 +326,13 @@ struct System <: IntermediateDeprecationSystem
                 error()
             end
             N1 == Neq || throw(IllFormedNoiseEquationsError(N1, Neq))
+            if noise_eqs !== nothing && !isempty(brownians)
+                throw(ArgumentError(
+                    "A system cannot have both `noise_eqs` and `brownians` specified. " *
+                    "Use either `noise_eqs` (a matrix of noise coefficients) or " *
+                    "`brownians` (symbolic brownian variables in equations), but not both."
+                ))
+            end
             check_equations(equations(continuous_events), iv)
             check_subsystems(systems)
         end

lib/ModelingToolkitBase/src/systems/systems.jl

@@ -120,19 +120,21 @@ function scalarized_vars(vars)
 end
 
 function _mtkcompile(sys::AbstractSystem; kwargs...)
-    # TODO: convert noise_eqs to brownians for simplification
-    if has_noise_eqs(sys) && get_noise_eqs(sys) !== nothing
-        sys = noise_to_brownians(sys; names = :αₘₜₖ)
-    end
+    # For systems with jumps, skip full structural simplification to preserve
+    # variables that only appear in jumps.
     if !isempty(jumps(sys))
-        # For systems with jumps, skip full structural simplification to preserve
-        # variables that only appear in jumps. But if brownians are present,
-        # we still need to extract them into noise_eqs for SDEProblem construction.
+        # If brownians are present, extract them to noise_eqs for SDEProblem construction.
+        # If noise_eqs is already set, return as-is (no need to convert).
         if !isempty(brownians(sys))
             return extract_brownians_to_noise_eqs(sys)
         end
         return sys
     end
+
+    # For non-jump systems, convert noise_eqs to brownians for simplification
+    if has_noise_eqs(sys) && get_noise_eqs(sys) !== nothing
+        sys = noise_to_brownians(sys; names = :αₘₜₖ)
+    end
     if isempty(equations(sys)) && !is_time_dependent(sys) && !_iszero(cost(sys))
         return simplify_optimization_system(sys; kwargs...)::System
     end