The mode parameter for the Agent tool in teammate/subagent contexts is now marked as deprecated: "Deprecated; ignored. Subagents inherit the parent session's permission mode; agent-definition frontmatter may override it." Previously this produced different error text referencing mode as unavailable.
Evidence
"Deprecated; ignored. Subagents inherit the parent session's permission mode" (search for the string directly)
Strings lifted out of the shipped bundle, so the claim above can be checked against them.