
ID	0174
Class	KNOWN-LIMITATION
Severity	1
Status	ready

Models: —

Found: 2026-10-26, r2-newton-solvers (Q4a) verification, 4dcb3a9d

Evidence

SolverPlan.cpp:410: ForEngineImplicit calls RefuseSelfConnection for BackwardEuler_i whenever the Jacobian source is not Numeric (FHSIM-0172, r2-D5). The Engine has no setting that makes BackwardEuler_i use a numeric Jacobian.

Effect

A model with a self-connection and any analytical Jacobian block can run only with an explicit method, or with SUNDIALS and <Jacobian Type="none"/>.

Possible fix

Add an Engine numeric-Jacobian switch when a real model needs it. Owner ruling: file only, no code now.

Test that would prove it

A BackwardEuler_i run of a self-connected model with the switch on reaches the end.

Risk

Low. No known model needs it.