FMI3 capability flag providesDirectionalDerivatives not supported

[CarstenOldemeyer]

From FMI2 to FMI3 the capability flag providesDirectionalDerivative has been renamed to providesDirectionalDerivatives.

See https://fmi-standard.org/docs/3.0.2/#common-capability-flags
and table in 4.3.1 of FMI-Specification-2.0.5-2.pdf

Since only the FMI2 spelling is contained in the InterfaceType, the attribute isnt copied from the xml of a FMI3-FMU to the fmpy model description.

FMPy/src/fmpy/model_description.py

Line 51 in 637dee0

providesDirectionalDerivative: bool = field(default=False, repr=False)

Accessing the directional derivatives works fine.

[Fstarnb]

I looked into this and can reproduce the root cause.

FMI 3.0 uses providesDirectionalDerivatives (plural), while FMPy’s InterfaceType keeps the FMI 2 field name providesDirectionalDerivative. On the FMI 3 read path, _copy_attributes only copies attributes whose XML names match Python field names, so the plural flag is never applied. The FMI 3 write path currently emits the singular name as well.

A minimal fix that matches the existing FMU-state attribute renames:

1. Read (FMI 3 ME/CS/SE): after _copy_attributes, map providesDirectionalDerivatives → providesDirectionalDerivative (accept true/1).
2. Write (FMI 3 only): emit providesDirectionalDerivatives="true" when the field is set; omit when false. Leave the FMI 2 singular path unchanged.

I also added unit tests that:

• read ME/CS/SE with the plural flag set/unset
• write the plural attribute (and not the singular one)
• round-trip write → read for Model Exchange

Happy to open a PR if external contributions are acceptable again; otherwise the above should be enough to land a one-file change in model_description.py. Happy to rework if you’d prefer a different API shape (e.g. an explicit plural field).