0327 — HasJacobians of NetStructure and Disk depends on the contact state of the last OdeFcn call

Class	KNOWN-LIMITATION
Severity	1
Status	ready

Models: Net/NetStructure, Net/Disk

Found: 2026-09-30 review of f42360e..a6492d6 (four parallel correctness reviews)

Decision needed: —

Evidence

src/net/NetStructure.cpp:1010 return !m_inBottomContact && HasJacobianConfiguration(); and src/net/Disk.cpp:240 return !m_onSeabed;; both flags are set in the last OdeFcn call (Disk.cpp:186), which may be a perturbed or different-stage state, and are false before the first call (MARE-0134 design).

Effect

Contact that starts between that call and the Jacobian evaluation gets an analytic Jacobian without the contact block; a net or disk that starts on the seabed begins with one. Only the Newton matrix is affected; it may slow implicit convergence at touchdown.

Possible fix

Evaluate the contact test at the state passed to OdeJacobian (a cheap per-node depth check), or keep it and document it.

Test that would prove it

A net resting on the seabed at T = 0 with an implicit integrator: HasJacobians must be false at the first Jacobian evaluation.

Risk

Low.