// Type is always parseable (Z6)
			if ( $targetTypeZid === ZTypeRegistry::Z_STRING ) {
				continue;
			}

			// Type is generic (Z1) and we treat it as a Z6
			// TODO (T390678): Adjust API filter code to allow Z1s as well?
			if ( $targetTypeZid === ZTypeRegistry::Z_OBJECT ) {
				continue;
			}

I feel like FunctionCallHandler already accept Z1's

Change #1133907 had a related patch set uploaded (by Daphne Smit; author: Daphne Smit):

[mediawiki/extensions/WikiLambda@master] wikilambdasearch_functions API: Allow functions that use Z1 for inputs and outputs.

https://gerrit.wikimedia.org/r/1133907

Needs a Product decision if we want to do this.

Change #1133907 abandoned by Daphne Smit:

[mediawiki/extensions/WikiLambda@master] wikilambdasearch_functions API: Allow functions that use Z1 for inputs and outputs.

https://gerrit.wikimedia.org/r/1133907