How to display the differential equations from Catalyst.jl?

Specific Domains Modelling & Simulations
1

I can not get this example in Introduction to Catalyst · Catalyst.jl to work.

It gives the following error:

julia> odesys = convert(ODESystem, rn)
ERROR: MethodError: Cannot `convert` an object of type ReactionSystem{Catalyst.NetworkProperties{Int64, SymbolicUtils.BasicSymbolicImpl.var"typeof(BasicSymbolicImpl)"{SymReal}}} to an object of type ModelingToolkitBase.IntermediateDeprecationSystem
The function `convert` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  ModelingToolkitBase.IntermediateDeprecationSystem(::Any...; kwargs...)
   @ ModelingToolkitBase ~/.julia/packages/ModelingToolkitBase/yP1tD/src/deprecations.jl:20
  convert(::Type{T}, ::T) where T
   @ Base Base_compiler.jl:133

Stacktrace:
 [1] top-level scope
   @ REPL[18]:1

Here’s a mwe giving the error:

using Catalyst, DifferentialEquations
rn = @reaction_network begin
    b, 0 --> N      
    d, N + N --> N 
end
convert(ODESystem, rn)

Is this ModelingToolkit v11 related or am I just holding it wrong?

I have:


pkg> st
Status `/tmp/jl_8MPXrp/Project.toml`
  [479239e8] Catalyst v16.0.0
  [0c46a032] DifferentialEquations v7.17.0


julia> versioninfo()
Julia Version 1.12.4
Commit 01a2eadb047 (2026-01-06 16:56 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 24 × AMD Ryzen 9 7900 12-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, znver4)
  GC: Built with stock GC
Threads: 24 default, 1 interactive, 24 GC (on 24 virtual cores)
Environment:
  JULIA_EDITOR = emacs
2

Sorry, I was looking at the stable (v15) documentation.

v16: Introduction to Catalyst · Catalyst.jl says:

odesys = ode_model(rn)

Giving this output:

Model ##ReactionSystem#285:
Equations (1):
  1 standard: see equations(##ReactionSystem#285)
Unknowns (1): see unknowns(##ReactionSystem#285)
  N(t)
Parameters (2): see parameters(##ReactionSystem#285)
  b
  d

and

julia> equations( ode_model(rn))
1-element Vector{Equation}:
 Differential(t, 1)(N(t)) ~ b - (1//2)*d*(N(t)^2)
\begin{align} \frac{\mathrm{d} ~ N\left( t \right)}{\mathrm{d}t} &= b - \frac{1}{2} ~ \left( N\left( t \right) \right)^{2} ~ d \end{align}