child
Detail that is progressively disclosed: hidden until the LLM invokes this tool, then appended verbatim to the unfolded message returned by call (after the "Tools now available: …" preamble).
Because only the LLM that just descended into this tool pays for it, childToolUsageNotes is the right home for content that would be wasteful in the parent description — workflow guidance, when-to-use routing between siblings, an agent skill's full body, or any other detail the LLM needs only after committing to use this tool.
Keep the parent description short (it costs every turn); put long-form context here (it costs only on use).
Null or blank ⇒ unfolded message is just the preamble.