# OptimizationState in Optim.jl does not have expected fields

**URL:** <https://discourse.julialang.org/t/optimizationstate-in-optim-jl-does-not-have-expected-fields/136212>\
**Category:** Optimization (Mathematical)\
**Created:** [March 16, 2026, 1:51am UTC](https://discourse.julialang.org/t/optimizationstate-in-optim-jl-does-not-have-expected-fields/136212 "2026-03-16T01:51:29Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![pjssilva](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pjssilva/32/4238_2.png) [@pjssilva](https://discourse.julialang.org/u/pjssilva)\
**Post date:** [March 16, 2026, 1:51am UTC](https://discourse.julialang.org/t/optimizationstate-in-optim-jl-does-not-have-expected-fields/136212/1 "2026-03-16T01:51:30Z")

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Hello. I am trying to use the callback functionality Optim.jl’s solvers (CG and LBFGS). However, the state passed to the callback function does not seem to have the expected fields. In fact, the state only has the fields `iteration` (iteration number), `value` (probably the function value at the current iterate), `g_norm` (probably the gradient norm at the current iterate) and `metadata` (a dictionary that only seems to have the entry `time`).

I would like to access the current iterate and the objective gradient at that iterate. Is that possible?

To verify the behavior I cited above, you can run the code:

```julia
using Optim
# function my_callback(state)
# print(" Objective Value: ", state.f_x)
# println(" at state x: ", state.x)
# return false # Return true to stop the optimization
# end
function my_callback(state)
    @show propertynames(state)
    @show keys(state.metadata)
    return true
end
function objective(x)
    return (x[1]-2)^2 + (x[2]-3)^2
end

initial_x = [0.0, 0.0]
method = BFGS()
options = Optim.Options(callback=my_callback)
d = OnceDifferentiable(objective, initial_x)

optstate = Optim.initial_state(method, options, d, initial_x)
result = optimize(d, initial_x, method, options, optstate)

```

This code was adapted from [Optim.jl documentation](https://julianlsolvers.github.io/Optim.jl/stable/user/callbacks/#Callback-Function-Example), where I changed the definition `my_callback` to show the fields of the state and the keys of the metadata. The original code ends with an error saying that the field `f_x` is not available.

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<div class="post-metadata">

**Author:** ![kellertuer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kellertuer/32/220707_2.png) [@kellertuer](https://discourse.julialang.org/u/kellertuer)\
**Post date:** [March 16, 2026, 2:15am UTC](https://discourse.julialang.org/t/optimizationstate-in-optim-jl-does-not-have-expected-fields/136212/2 "2026-03-16T02:15:33Z")

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Which version of Optim.jl are you using? I think there was a mayor rework towards version 2.0 on this.

When I try to run your code on Optim 2.0.1 I get that the state has no field metadata from you second line in the callback, i.e.

```julia
julia> result = optimize(d, initial_x, method, options, optstate)
propertynames(state) = (:x, :g_x, :f_x, :x_previous, :g_x_previous, :f_x_previous, :dx, :dg, :u, :invH, :s, :x_ls, :alpha)
ERROR: FieldError: type Optim.BFGSState has no field `metadata`, available fields: `x`, `g_x`, `f_x`, `x_previous`, `g_x_previous`, `f_x_previous`, `dx`, `dg`, `u`, `invH`, `s`, `x_ls`, `alpha`
Stacktrace:

```

But one can see that there are fields lie `x` for the current iterate, `f_x` is probably the cost, `g_x` is probably the current gradient,…

Running the example you linked also works fine on Optim 2.0.1 and prints something like

```Julia
julia> result = optimize(d, initial_x, method, options, optstate)
 Objective Value: 13.0 at state x: [0.0, 0.0]
 Objective Value: 4.1479071968772985e-21 at state x: [1.9999999999606561, 3.00000000005099]
 * Status: success
[... rest of status print skipped here for space reasons ...]

```

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<div class="post-metadata">

**Author:** ![pjssilva](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pjssilva/32/4238_2.png) [@pjssilva](https://discourse.julialang.org/u/pjssilva)\
**Post date:** [March 16, 2026, 12:39pm UTC](https://discourse.julialang.org/t/optimizationstate-in-optim-jl-does-not-have-expected-fields/136212/3 "2026-03-16T12:39:44Z")

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I am using v. 1.13.4. That is probably the problem. I will have to hunt down what is precluding the use of the latest Optim.jl. Sorry for that.

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<div class="post-metadata">

**Author:** ![kellertuer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kellertuer/32/220707_2.png) [@kellertuer](https://discourse.julialang.org/u/kellertuer)\
**Post date:** [March 16, 2026, 11:33pm UTC](https://discourse.julialang.org/t/optimizationstate-in-optim-jl-does-not-have-expected-fields/136212/4 "2026-03-16T23:33:03Z")

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No need to be sorry, it is just by chance that I knew, this area was a major breaking part of the 2.0 release – and I have not yet found the changelog of “breaking things” for that release.

But I do think that that is then the reason. If I remember correctly, the amount one could access in a callback in Optim 1.x was very limited; in 2.x you should be able to access everything from the state of a solver.