@nchaulet I tried to have a fully custom part in this config here. When I try to fill it in with foo: bar in the UI I get the following error:

can not read an implicit mapping pair; a colon is missed at line 7, column 13: "##custom##" ^

If would assume it replaced {{custom}} by foo: bar so it should be valid?

This a good scenario that we did not cover, I did the compilation of yaml to be able to support values like custom: {{custom}}

Thinking of the solution we have:

1. We can probably add a new concept in the registry variables, to flag a variable that should be spread to the agent yaml.
2. we can try to detect in the template that custom is a root level yaml value and do another special treatment, I have a POC here [Ingest] Support root level yaml variables in agent stream template kibana#66120

++ on option 2 as I would prefer to have as few "types" as possible on the package level.