`remake` for `SDEProblem` resets and ignores `u0`

[sebapersson]

Describe the bug 🐞

When calling remake on a SDEProblem generated either from a SDESystem or a RecationSystem the remake function ignores and resets u0. This does not happen for ODEProblem.

Expected behavior

That remake should work as for ODEProblem

Minimal Reproducible Example 👇

using Catalyst, ModelingToolkit
using ModelingToolkit: t_nounits as t, D_nounits as D

rn = @reaction_network begin
    @species X(t)=50.0 Y(t)=100.0
    c1, X --> Y
    c2, Y --> X
end

u0 = [:X => 50.0, :Y => 100.0]
sprob = SDEProblem(rn, u0, [0.0, 10.0], ps)
ps = [:c1 => 0.5, :c2 => 1.0]
sprob.u0 .= [20.0, 40.0]
u0_pre = deepcopy(sprob.u0)
_sprob = remake(sprob, tspan = [0.0, 20.0], u0 = u0_pre)
println(u0_pre)
println(_sprob.u0)

Error & Stacktrace ⚠️

The output from above is:

[20.0, 40.0]
[50.0, 100.0]

When it should be

[20.0, 40.0]
[20.0, 40.0]

Environment (please complete the following information):

• Output of using Pkg; Pkg.status()

  [479239e8] Catalyst v14.4.1
  [961ee093] ModelingToolkit v9.68.1
  [789caeaf] StochasticDiffEq v6.74.1

• Output of using Pkg; Pkg.status(; mode = PKGMODE_MANIFEST)

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• Output of versioninfo()

Julia Version 1.11.4
Commit 8561cc3d68d (2025-03-10 11:36 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 14 × Intel(R) Core(TM) Ultra 7 165U
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, alderlake)
Threads: 1 default, 0 interactive, 1 GC (on 14 virtual cores)
Environment:
  JULIA_EDITOR = code
  JULIA_NUM_THREADS = 

[AayushSabharwal]

This is intentional. The SDEProblem uses the initial conditions it was provided on construction. A symbolic remake can be used to modify/add to the initial conditions, as described in the documentation. However, a non-symbolic remake doesn't update them. To change the initial condition of X and Y, Initial(X) and Initial(Y) need to be given new values.

@ChrisRackauckas I could make it so that if the user passes u0 to remake and doesn't pass p, it copies the u0 values to the Initial values. Does that seem reasonable.

[ChrisRackauckas]

Yes that seems reasonable, though I think we should also add some kind of warning if direct arrays are used anywhere.

[sebapersson]

Great, and thanks to the pointer in the documentation.

[Datseris]

@ChrisRackauckas I could make it so that if the user passes u0 to remake and doesn't pass p, it copies the u0 values to the Initial values. Does that seem reasonable.

What if a user passes u0 to remake (or reinit!) and passes p, but p does not contain any Initial entries, but rather it is a Dict of only the @parameters parameters? This is the standard way to use reinit! for DEProblems not generated by MTK. I think in this scenario again internally the Initial entries need to be updated automatically.

[AayushSabharwal]

We already handle that case if u0 is symbolic. The case when u0 is not symbolic is handled in the linked PR.

[Datseris]

Is the behavior of remake and reinit! identical when it comes to this? I can take this conversation to #3451 if you feel it is off-topic here.

[AayushSabharwal]

They are fundamentally different functions, so they won't be completely identical. That said, reinit! can (and will) be made more in line with remake for this kind of stuff.