Just wondering if this is supported by acados/casadi. I have a nonlinear constraint h(x) that I do not know how to write down analytically (it’s the inverse kinematics to a robot, currently solved using Gauss-Newton in our implementation). Is it possible to apply this constraint to an acados OCP, or do I need a closed-form h(x)?

From what I understand, I believe it’s not supported, but just wanted to ask the wider community.

I suggest you extend the dynamic system with this implicit equation and an (algebraic) state.
The implicit integrators in acados are iterative numerical solvers and should be able to handle this.
With such an augmentation you should be able to write the constraint wiht a closed form.