note that if split-true you can't use integer access

src/core/dynamicalsystem_interface.jl

@@ -58,10 +58,12 @@ See also the DynamicalSystems.jl tutorial online for an example.
 
 !!! warn "ModelingToolkit.jl v9"
     In ModelingToolkit.jl v9 the default `split` behavior of the parameter container
-    is `true`. This leads to various accessing errors, so the integration
-    only works if you explicitly pass `structural_simplify(sys; split = false)`
-    when structurally simplifying the equations systems before making the
-    `...Problem` instance.
+    is `true`. This means that the parameter container is no longer a `Vector{Float64}`
+    by default, which means that you cannot use integers to access parameters.
+    In this case, the `index` given to `current_parameter/set_parameter!`
+    must be the symbolic parameter binding.
+    Use `structural_simplify(sys; split = false)` to allow accessing parameters
+    with integers again.
 
 ## API
 

test/mtk_integration.jl

@@ -95,6 +95,22 @@ set_parameter!(pmap, fol_1.τ, 4.0)
 @test current_parameter(pmap, fol_1.τ) == 4.0
 @test observe_state(pmap, fol_1.x) ≈ 0 atol = 1e-3 rtol = 0
 
+# test with split
+sys = structural_simplify(connected; split = true)
+
+u0 = [fol_1.x => -0.5,
+    fol_2.x => 1.0]
+
+p = [fol_1.τ => 2.0,
+    fol_2.τ => 4.0]
+
+prob = ODEProblem(sys, u0, (0.0, 10.0), p)
+ds = CoupledODEs(prob)
+
+@test current_parameter(ds, fol_1.τ) == 2.0
+set_parameter!(ds, fol_1.τ, 3.0)
+@test current_parameter(ds, fol_1.τ) == 3.0
+
 # test without sys
 function lorenz!(du, u, p, t)
     du[1] = p[1] * (u[2] - u[1])