The return value of a tool call, fed back to the model.
usage is an optional contributed token usage (e.g. from a delegated
sub-agent run) that the agent loop merges into the run's accumulated
usage. It is nil for ordinary tools and is not part of the serialized
message history (usage is accounted for at runtime).
Summary
Types
@type t() :: %ExAgent.Message.Part.ToolReturn{ content: term(), tool_call_id: String.t(), tool_name: String.t(), usage: ExAgent.Message.Usage.t() | nil }