Observed variables fail with world age error when `eval_expression = true`

[sablonl]

Describe the bug 🐞

Accessing a variable that was removed during the tearing process and thus defined via an observed function fails with a 'method too new' error when eval_expression=true in the ODEProblem. The runtime evaluation of these functions makes them inaccessible to the current world age, breaking post-simulation analysis for all non-state variables.

Minimal Reproducible Example 👇

using Pkg
Pkg.activate(temp=true)
Pkg.add(["ModelingToolkitBase", "SymbolicIndexingInterface", "OrdinaryDiffEqTsit5"])

using ModelingToolkitBase, OrdinaryDiffEqTsit5
using ModelingToolkitBase: t_nounits as t, D_nounits as D
using SymbolicIndexingInterface: getp

function build_model(eval_expression)
	@variables x(t) y(t) z(t) obs(t)
	@discretes d(t)
	@parameters σ=10.0 ρ=28.0 β=8/3

	# Define equations
	eqs = [
		D(x) ~ σ * (y - x) + d,
		D(y) ~ x * (ρ - z) - y,
		D(z) ~ x * y - β * z,
		obs ~ x + y + z + d,
	]

	# Create the system
	@named lorenz_sys = System(eqs, t)
	lorenz = mtkcompile(lorenz_sys)

	# Initial conditions
	u0_p = merge(
		Dict(x => 1.0, y => 0.0, z => 0.0, d => 1.0),
		Dict(σ => 10.0, ρ => 28.0, β => 8/3),
	)

	# Time span
	tspan = (0.0, 100.0)

	# Create ODE problem WITH eval_expression = true
	# This is where the issue occurs
	prob = ODEProblem(
		lorenz,
		u0_p,
		tspan,
		eval_expression = eval_expression,
	)
    var = (; x, y, z, obs, d)
	return prob, var
end

function getx1(prob, sol, var, d)
    ret1 = sol[var][:]
    getd = getp(prob, d)
    return ret1, getd
end

prob_true, var_true = build_model(true);
sol_true = solve(prob_true, Tsit5());
obs_true, _ = getx1(prob_true, sol_true, var_true.obs, var_true.d); # This errors !
x1_true, d_true = getx1(prob_true, sol_true, var_true.x, var_true.d); # This works
d_true(sol_true) 

prob_false, var_false = build_model(false);
sol_false = solve(prob_false, Tsit5());
obs_false, _ = getx1(prob_false, sol_false, var_false.obs, var_false.d); # This works
x1_false, d_false = getx1(prob_false, sol_false, var_false.x, var_false.d); # This works
d_false(sol_false)

Error & Stacktrace ⚠️

julia> obs_true, _ = getx1(prob_true, sol_true, var_true.obs, var_true.d); # This errors !
ERROR: MethodError: no method matching (::ModelingToolkitBase.var"#1982#1983")(::Vector{…}, ::MTKParameters{…}, ::Float64)
The function `#1982` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  (::ModelingToolkitBase.var"#1982#1983")(::Any, ::Any, ::Any) (method too new to be called from this world context.)
   @ ModelingToolkitBase none:0

Stacktrace:
  [1] macro expansion
    @ ~/.julia/packages/ModelingToolkitBase/Ylh9H/src/systems/codegen_utils.jl:-1 [inlined]
  [2] _generated_call(::ModelingToolkitBase.GeneratedFunctionWrapper{…}, ::Vector{…}, ::MTKParameters{…}, ::Float64)
    @ ModelingToolkitBase ~/.julia/packages/ModelingToolkitBase/Ylh9H/src/systems/codegen_utils.jl:396
  [3] (::ModelingToolkitBase.GeneratedFunctionWrapper{…})(::Vector{…}, ::Vararg{…})
    @ ModelingToolkitBase ~/.julia/packages/ModelingToolkitBase/Ylh9H/src/systems/codegen_utils.jl:393
  [4] _broadcast_getindex_evalf
    @ ./broadcast.jl:699 [inlined]
  [5] _broadcast_getindex
    @ ./broadcast.jl:672 [inlined]
  [6] _getindex
    @ ./broadcast.jl:620 [inlined]
  [7] getindex
    @ ./broadcast.jl:616 [inlined]
  [8] copy
    @ ./broadcast.jl:933 [inlined]
  [9] materialize
    @ ./broadcast.jl:894 [inlined]
 [10] TimeDependentObservedFunction
    @ ~/.julia/packages/SymbolicIndexingInterface/D9aQf/src/state_indexing.jl:84 [inlined]
 [11] TimeDependentObservedFunction
    @ ~/.julia/packages/SymbolicIndexingInterface/D9aQf/src/state_indexing.jl:80 [inlined]
 [12] (::SymbolicIndexingInterface.TimeDependentObservedFunction{…})(prob::ODESolution{…})
    @ SymbolicIndexingInterface ~/.julia/packages/SymbolicIndexingInterface/D9aQf/src/value_provider_interface.jl:166
 [13] _getindex
    @ ~/.julia/packages/RecursiveArrayTools/RnETD/src/vector_of_array.jl:476 [inlined]
 [14] getindex
    @ ~/.julia/packages/RecursiveArrayTools/RnETD/src/vector_of_array.jl:502 [inlined]
 [15] getx1(prob::ODEProblem{…}, sol::ODESolution{…}, var::Num, d::Num)
    @ Main /cyfast/lsablon/dev/Geoclim.jl/mwe_eval_expression_error.jl:48
 [16] top-level scope
    @ /cyfast/lsablon/dev/Geoclim.jl/mwe_eval_expression_error.jl:55
Some type information was truncated. Use `show(err)` to see complete types.

Environment (please complete the following information):

• Output of using Pkg; Pkg.status()

(jl_Luse6X) pkg> st
Status `/tmp/jl_Luse6X/Project.toml`
  [7771a370] ModelingToolkitBase v1.2.0
⌃ [b1df2697] OrdinaryDiffEqTsit5 v1.5.0
  [2efcf032] SymbolicIndexingInterface v0.3.46

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

(jl_Luse6X) pkg> st --manifest
Status `/tmp/jl_Luse6X/Manifest.toml`
  [47edcb42] ADTypes v1.20.0
  [6e696c72] AbstractPlutoDingetjes v1.3.2
  [1520ce14] AbstractTrees v0.4.5
  [7d9f7c33] Accessors v0.1.43
  [79e6a3ab] Adapt v4.4.0
  [ec485272] ArnoldiMethod v0.4.0
  [4fba245c] ArrayInterface v7.22.0
  [4c555306] ArrayLayouts v1.12.2
  [e2ed5e7c] Bijections v0.2.2
  [caf10ac8] BipartiteGraphs v0.1.4
  [62783981] BitTwiddlingConvenienceFunctions v0.1.6
  [8e7c35d0] BlockArrays v1.9.3
  [70df07ce] BracketingNonlinearSolve v1.6.0
  [2a0fbf3d] CPUSummary v0.2.7
  [fb6a15b2] CloseOpenIntervals v0.1.13
⌅ [861a8166] Combinatorics v1.0.2
  [38540f10] CommonSolve v0.2.4
  [bbf7d656] CommonSubexpressions v0.3.1
  [f70d9fcc] CommonWorldInvalidations v1.0.0
  [34da2185] Compat v4.18.1
  [b152e2b5] CompositeTypes v0.1.4
  [a33af91c] CompositionsBase v0.1.2
  [2569d6c7] ConcreteStructs v0.2.3
  [187b0558] ConstructionBase v1.6.0
  [adafc99b] CpuId v0.3.1
  [864edb3b] DataStructures v0.19.3
  [2b5f629d] DiffEqBase v6.192.0
  [459566f4] DiffEqCallbacks v4.10.1
  [163ba53b] DiffResults v1.1.0
  [b552c78f] DiffRules v1.15.1
  [a0c0ee7d] DifferentiationInterface v0.7.12
  [ffbed154] DocStringExtensions v0.9.5
  [5b8099bc] DomainSets v0.7.16
  [7c1d4256] DynamicPolynomials v0.6.4
  [4e289a0a] EnumX v1.0.5
  [f151be2c] EnzymeCore v0.8.17
  [e2ba6199] ExprTools v0.1.10
  [55351af7] ExproniconLite v0.10.14
  [7034ab61] FastBroadcast v0.3.5
  [9aa1b823] FastClosures v0.3.2
  [a4df4552] FastPower v1.2.0
  [1a297f60] FillArrays v1.15.0
  [64ca27bc] FindFirstFunctions v1.4.2
  [6a86dc24] FiniteDiff v2.29.0
  [f6369f11] ForwardDiff v1.3.0
  [069b7b12] FunctionWrappers v1.1.3
  [77dc65aa] FunctionWrappersWrappers v0.1.3
  [46192b85] GPUArraysCore v0.2.0
  [86223c79] Graphs v1.13.2
  [615f187c] IfElse v0.1.1
⌃ [3263718b] ImplicitDiscreteSolve v1.2.0
  [d25df0c9] Inflate v0.1.5
  [18e54dd8] IntegerMathUtils v0.1.3
  [8197267c] IntervalSets v0.7.13
  [3587e190] InverseFunctions v0.1.17
  [92d709cd] IrrationalConstants v0.2.6
  [82899510] IteratorInterfaceExtensions v1.0.0
  [692b3bcd] JLLWrappers v1.7.1
  [ae98c720] Jieko v0.2.1
  [ccbc3e58] JumpProcesses v9.19.3
  [10f19ff3] LayoutPointers v0.1.17
  [87fe0de2] LineSearch v0.1.4
  [2ab3a3ac] LogExpFunctions v0.3.29
  [e6f89c97] LoggingExtras v1.2.0
  [1914dd2f] MacroTools v0.5.16
  [d125e4d3] ManualMemory v0.1.8
  [bb5d69b7] MaybeInplace v0.1.4
  [7771a370] ModelingToolkitBase v1.2.0
  [2e0e35c7] Moshi v0.3.7
  [46d2c3a1] MuladdMacro v0.2.4
  [102ac46a] MultivariatePolynomials v0.5.13
  [d8a4904e] MutableArithmetics v1.6.7
  [77ba4419] NaNMath v1.1.3
  [be0214bd] NonlinearSolveBase v2.5.0
  [6fe1bfb0] OffsetArrays v1.17.0
  [bac558e1] OrderedCollections v1.8.1
⌅ [bbf590c4] OrdinaryDiffEqCore v1.36.0
⌃ [b1df2697] OrdinaryDiffEqTsit5 v1.5.0
  [e409e4f3] PoissonRandom v0.4.7
  [f517fe37] Polyester v0.7.18
  [1d0040c9] PolyesterWeave v0.2.2
  [d236fae5] PreallocationTools v0.4.34
  [aea7be01] PrecompileTools v1.3.3
  [21216c6a] Preferences v1.5.0
  [27ebfcd6] Primes v0.5.7
  [988b38a3] ReadOnlyArrays v0.2.0
  [795d4caa] ReadOnlyDicts v1.0.0
  [3cdcf5f2] RecipesBase v1.3.4
  [731186ca] RecursiveArrayTools v3.41.0
  [189a3867] Reexport v1.2.2
  [ae029012] Requires v1.3.1
  [7e49a35a] RuntimeGeneratedFunctions v0.5.16
  [94e857df] SIMDTypes v0.1.0
  [0bca4576] SciMLBase v2.128.0
  [19f34311] SciMLJacobianOperators v0.1.11
  [a6db7da4] SciMLLogging v1.7.1
  [c0aeaf25] SciMLOperators v1.14.1
  [431bcebd] SciMLPublic v1.0.0
  [53ae85a6] SciMLStructures v1.7.0
  [efcf1570] Setfield v1.1.2
  [727e6d20] SimpleNonlinearSolve v2.9.0
  [699a6c99] SimpleTraits v0.9.5
  [ce78b400] SimpleUnPack v1.1.0
  [276daf66] SpecialFunctions v2.6.1
  [aedffcd0] Static v1.3.1
  [0d7ed370] StaticArrayInterface v1.8.0
  [90137ffa] StaticArrays v1.9.15
  [1e83bf80] StaticArraysCore v1.4.4
  [10745b16] Statistics v1.11.1
  [7792a7ef] StrideArraysCore v0.5.8
  [2efcf032] SymbolicIndexingInterface v0.3.46
  [19f23fe9] SymbolicLimits v0.2.4
  [d1185830] SymbolicUtils v4.9.1
  [0c5d862f] Symbolics v7.2.0
  [ed4db957] TaskLocalValues v0.1.3
  [8ea1fca8] TermInterface v2.0.0
  [8290d209] ThreadingUtilities v0.5.5
  [a759f4b9] TimerOutputs v0.5.29
  [781d530d] TruncatedStacktraces v1.4.0
  [3a884ed6] UnPack v1.0.2
  [d30d5f5c] WeakCacheSets v0.1.0
  [efe28fd5] OpenSpecFun_jll v0.5.6+0
  [0dad84c5] ArgTools v1.1.2
  [56f22d72] Artifacts v1.11.0
  [2a0f44e3] Base64 v1.11.0
  [ade2ca70] Dates v1.11.0
  [8ba89e20] Distributed v1.11.0
  [f43a241f] Downloads v1.7.0
  [7b1f6079] FileWatching v1.11.0
  [9fa8497b] Future v1.11.0
  [b77e0a4c] InteractiveUtils v1.11.0
  [ac6e5ff7] JuliaSyntaxHighlighting v1.12.0
  [b27032c2] LibCURL v0.6.4
  [76f85450] LibGit2 v1.11.0
  [8f399da3] Libdl v1.11.0
  [37e2e46d] LinearAlgebra v1.12.0
  [56ddb016] Logging v1.11.0
  [d6f4376e] Markdown v1.11.0
  [ca575930] NetworkOptions v1.3.0
  [44cfe95a] Pkg v1.12.1
  [de0858da] Printf v1.11.0
  [3fa0cd96] REPL v1.11.0
  [9a3f8284] Random v1.11.0
  [ea8e919c] SHA v0.7.0
  [9e88b42a] Serialization v1.11.0
  [6462fe0b] Sockets v1.11.0
  [2f01184e] SparseArrays v1.12.0
  [f489334b] StyledStrings v1.11.0
  [fa267f1f] TOML v1.0.3
  [a4e569a6] Tar v1.10.0
  [8dfed614] Test v1.11.0
  [cf7118a7] UUIDs v1.11.0
  [4ec0a83e] Unicode v1.11.0
  [e66e0078] CompilerSupportLibraries_jll v1.3.0+1
  [deac9b47] LibCURL_jll v8.15.0+0
  [e37daf67] LibGit2_jll v1.9.0+0
  [29816b5a] LibSSH2_jll v1.11.3+1
  [14a3606d] MozillaCACerts_jll v2025.5.20
  [4536629a] OpenBLAS_jll v0.3.29+0
  [05823500] OpenLibm_jll v0.8.7+0
  [458c3c95] OpenSSL_jll v3.5.4+0
  [bea87d4a] SuiteSparse_jll v7.8.3+2
  [83775a58] Zlib_jll v1.3.1+2
  [8e850b90] libblastrampoline_jll v5.15.0+0
  [8e850ede] nghttp2_jll v1.64.0+1
  [3f19e933] p7zip_jll v17.7.0+0

• Output of versioninfo()

julia> versioninfo()
Julia Version 1.12.3
Commit 966d0af0fdf (2025-12-15 11:20 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 64 × AMD Ryzen Threadripper 3970X 32-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, znver2)
  GC: Built with stock GC
Threads: 1 default, 1 interactive, 1 GC (on 64 virtual cores)
Environment:
  JULIA_EDITOR = code
  JULIA_VSCODE_REPL = 1

[ChrisRackauckas]

That's precisely the semantics of eval in Julia. This is why RGFs exist. Don't use eval if you need to evaluate in the same world-age.

[sablonl]

It's not intended from my side, perhaps should ODEProblem throw an error/warning when eval_expression = true and some variables are moved to observed ?

[ChrisRackauckas]

Not necessarily. It depends on how it's used. But generally this argument is only for very specific cases because eval doesn't generally have the right semantics for MTK