Definition
A subagent is an agent invoked by another agent or orchestrator to perform a bounded part of a larger task. The relationship defines the term: a subagent is subordinate in scope and control, not necessarily smaller, cheaper, or less capable.
Delegation needs a contract. The caller should provide the objective, relevant context, allowed tools, resource budget, acceptance criteria, and the form of the result. It should also say whether the subagent may create more work or affect shared state.
The caller remains responsible for integration unless control is formally handed off. A subagent's confident report is an input, not proof that its work is correct.
Distinguish it from nearby terms
A subagent receives bounded delegated work and returns a result. A handoff transfers next-action authority. A tool call may perform one capability without running an agent loop.
Check your understanding
A coding subagent is asked to "fix authentication" with write access to the whole repository and no acceptance test. The task has been delegated, but it is not bounded well enough to verify or contain.