I understand what you're saying with the difference between default and initial_value, but cannot exactly reproduce your error, nor your fix to it.

The closest I can get to an similar error, is for example by attempting to directly use an IntegerParameter as an argument, but that's not allowed.

Note there's a distinction between a command's arguments and a command's entries:

• An argument is what the user sees in the form. There is no possibility to use parameter types for arguments. Argument types and parameter types are separate things in XTCE, and I kept that distinction in this library (while very similar, there are various differences between TM and TC data types).

• A command's entries describes what is ultimately encoded into the generated command binary. For example, it's perfectly valid to have arguments in a different order than you have argument entries, or to add some extra bits between two encoded arguments (using FixedValueEntry, or relative offsets).

A ParameterEntry may also be used as a special kind of argument entry, however the purpose of this is to directly encode the current value of that parameter in the command binary, without any user involvement (not linked to a command argument). I do confirm there an issue in this library with this scenario, which is TODO: https://github.com/yamcs/pymdb/blob/v1.0.12/src/yamcs/pymdb/xtce.py#L446

If I have misunderstood you, maybe you can adapt below example to illustrate the issue that you are seeing:

import yamcs.pymdb as Y

system = Y.System("myproject")
ccsds_header = Y.ccsds.add_ccsds_header(system)

command_id = Y.IntegerArgument(
    name="command_id",
    signed=False,
    encoding=Y.uint16_t,
)
base_command = Y.Command(
    system=system,
    name="Base",
    abstract=True,
    base=ccsds_header.tc_command,
    assignments={
        ccsds_header.tc_secondary_header.name: "Not Present",
        ccsds_header.tc_apid.name: 101,
    },
    arguments=[
        command_id,
    ],
)

arg = Y.IntegerArgument(
    name="arg",
    encoding=Y.uint16_t,
)
par = Y.IntegerParameter(
    system=system,
    name="par",
    data_source=Y.DataSource.LOCAL,
    encoding=Y.int32_t,
)

Y.Command(
    system=system,
    base=base_command,
    name="SomeCommand",
    assignments={command_id.name: 3},
    arguments=[
        arg,
        # par,  # Not allowed
    ],
    entries=[
        Y.ArgumentEntry(arg),
        # Y.ParameterEntry(par),  # Currently broken
    ],
)

print(system.dumps())

since that would cause that argument to be hidden inside the default dropdown menu when trying to Send a Command in YAMCS.

See option collapseInitializedArguments in yamcs-web configuration settings:
https://docs.yamcs.org/yamcs-server-manual/web-interface/configuration/

A ParameterEntry may also be used as a special kind of argument entry, however the purpose of this is to directly encode the current value of that parameter in the command binary, without any user involvement (not linked to a command argument).

Oh I see, I was not aware of this intended meaning of ParameterEntry. I'll close this PR then, since it was made in error.

Y.Command(
system=system,
base=base_command,
name="SomeCommand",
assignments={command_id.name: 3},
arguments=[
arg,
# par, # Not allowed -- wasn't aware of this
],
entries=[
Y.ArgumentEntry(arg),
# Y.ParameterEntry(par), # This is what I was trying to get working, but without the part where it refers to the current parameter value
],
)