What
In coordinator sessions the Agent tool's model parameter description is built at runtime: with CLAUDE_CODE_COORDINATOR_FORCE_WORKER_INHERIT_MODEL set, it tells the model the parameter is ignored and must not be set; otherwise it warns to set it only when the user explicitly asks. Outside coordinator sessions nothing is appended.
Evidence
CLAUDE_CODE_COORDINATOR_FORCE_WORKER_INHERIT_MODEL
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
Names in the bundleCLAUDE_CODE_COORDINATOR_FORCE_WORKER_INHERIT_MODEL
One source agreesOne thing we can check says the same as this entry.
The name it cites is new in this build
New in this build: CLAUDE_CODE_COORDINATOR_FORCE_WORKER_INHERIT_MODEL