The subagent seam: an agent delegating work to a child agent. Like the bash and llm families this is a capability seam (see capability seams) — but with one defining difference: multiple provider implementations coexist in one context, registered by name, rather than the single-implementation bash shape. The registry mirrors the LLM adapter registry.

Package	Role	ctx key
`subagent/`	Abstract subagent seam: named-provider registry + vocabulary	`ctx.subagents`
`subagent-inprocess/`	Shared in-process run driver (pure lib; registers nothing)	—
`subagent-spawn/`	In-process backend: a fresh child agent	(registers on `ctx.subagents`)
`subagent-fork/`	In-process backend: a child seeded with the parent's completed-turn prefix	(registers on `ctx.subagents`)
`tool-subagent/`	Model-facing `subagent` delegation tool over `ctx.subagents`	(registers on `ctx.tools`)

The interface lives at subagent/subagent/. The in-process subagent-spawn / subagent-fork backends share the subagent-inprocess driver (a pure library — both depend on it, neither on the other) and ship here; the out-of-process dsh-subagent-acp and the test-only dsh-subagent-mock (in support) are separate. All product packages except the mock.

The proposal and design rationale: docs/rfc/proposed/feature/2026-06-21-subagent-capability-seam.md.