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A hooks bridge translating SubagentStart/SubagentStop needs to know WHICH kind of subagent ran and WHAT it produced — Claude Code's hooks carry subagent_type and the child's final message. Enrich the existing lifecycle emits to match, observe-only: - agentType: an optional caller-supplied subagent-kind label (CC's subagent_type), added to SubagentStartRequest and carried VERBATIM onto both subagent/start (SubagentRunInfo) and subagent/end (SubagentRunEndInfo). The seam never interprets it. dsh-tool-subagent threads it from a new optional Config.agentType, so a deployment exposing multiple subagent kinds (one tool load per kind) labels each. - lastAssistantMessage: the child's final output (SubagentResult.output), added to SubagentRunEndInfo on the settle path so an observer sees what the subagent produced without holding the run. Absent on the reject path (no result produced). Strictly observe-only: both events stay plain emits (subagent/end fires from a detached .then and awaits no listener). A control-flow subagent/end (awaited waterfall returning a decision) would need the emit→waterfall reshape, awaiting listeners before settling, and a provider resume capability — deferred to the background/steering redesign (FIXME(subagent-continuation) anchors it). RFC: implemented/feature/2026-06-30-subagent-observe-enrich.md.
subagent/ — subagent capability family
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) |
subagent-acp/ |
Out-of-process backend: a child agent in a spawned subprocess, driven over ACP | (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 the out-of-process subagent-acp backend ships alongside them here; the test-only dsh-subagent-mock (in support) is separate. All product packages except the mock.
The proposal and design rationale: docs/rfc/implemented/feature/2026-06-21-subagent-capability-seam.md.