The model-facing subagent tool: delegate a self-contained task to a child agent and return its final output. Pure schema + lifecycle shaping over the ctx.subagents provider registry — an in-process, ACP, or future A2A backend swaps in without changing what the model sees.

Provider selection is config, not model-facing

This plugin binds to exactly one provider (Config.provider). The model sees only { description, prompt } — there is no provider/type parameter in the schema. To expose more than one transport, load the plugin more than once, each bound to a different provider and a distinct toolName (the tool registry rejects a duplicate name, so a second load that kept the default subagent name would throw). Keeping selection in config (not the schema) is the deliberate split: the service holds a multi-provider registry; the tool picks one.

Config key	Meaning
`provider` (required)	The `ctx.subagents` provider name to start runs on (`spawn`, `fork`, `acp`, …).
`toolName`	The model-facing tool name to register (default `subagent`). Set a distinct value per load when exposing multiple providers, e.g. `subagent` + `subagent_acp`.
`agentOptions`	Default per-child `{ model?, systemPrompt? }` applied to every spawned child.

Lifecycle (synchronous collect)

execute starts a run on the configured provider and awaits run.result inside a try/finally that always dispose()s the run — the owned child agent/session is torn down on every path (success, error, abort), never leaked. The tool's abort signal (exec.signal) is bridged to run.cancel(). A non-completed stop reason (aborted/error/max-tokens/refusal) maps to an isError tool result rather than returning partial output as success.

Background / poll collection is deferred (see the RFC); this cut blocks the parent turn until the child finishes.