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Add in-process subagent backends: spawn (fresh) and fork (seeded)

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The second PR of the subagent seam: the two in-process backends that run a
child agent on the same cordis context, reusing the agent factory's quiescent
AgentHandle teardown. Both register on ctx.subagents (PR1's named-provider
registry) and share one run driver.

- dsh-subagent-spawn: a FRESH child via ctx.agents.create — own session, the
  parent's model by default (overridable), zero inherited conversation. Also
  exports the shared in-process run driver (startInProcessRun): mint ids, stamp
  cwd/parentSession-lineage/depth, drive the one-shot (send → whenIdle), read
  the last assistant/message + turn/end reason, dispose to quiescence.
- dsh-subagent-fork: a child SEEDED with the parent's balanced completed-turn
  prefix (the log up to and including its last turn/end), so the child inherits
  context. The in-flight unbalanced turn is excluded — a raw seed would fail the
  invariants replay. Proven: a regression test goes red if the boundary seeds
  the open turn.
- Seam extension: CreateAgentOptions.seed, threaded through AgentLoop.createAgent
  → ctx.sessions.prepare({ seed }) (the primitive resume already used). This is
  the fork-lineage path the TODO(sub-agents) markers anticipated.
- Depth: a merge-extensible AgentOptions.subagentDepth (0 top-level, parent+1 for
  a child); the depthLimit capability refuses a spawn past request.maxDepth.

Tests: real-loop unit tests for both backends (mock MODEL only, real loop +
invariants), a multi-subagent test (one parent drives a fork AND a spawn child
then keeps working), and a with-key e2e (a real parent delegates via the
`subagent` tool to a real child that writes a file on disk — world-verified).
100% per-file coverage. The coding-agent demo wires the spawn backend + tool.

Snapshot coverage of nested agents is deferred to a stacked follow-up
(TODO(subagent-snapshots)): dsh-llm-replay is a single global positional cursor
that cannot route calls to a parent vs. a child on one context. Recorded in the
RFC's deferrals and a new AGENTS.md rule: designing a subsystem must design its
test infrastructure END TO END up front, verifying the snapshot/e2e harness can
express the new shape — a gap this plan hit.
This commit is contained in:
Tianyi Cui
2026-06-22 05:58:40 +08:00
parent 861791d2d8
commit 7aabd2a3df
28 changed files with 1296 additions and 13 deletions
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packages/subagent/README.md
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@@ -5,8 +5,10 @@ The subagent seam: an agent delegating work to a child agent. Like the [bash](..
| Package | Role | ctx key |
|---|---|---|
| `subagent/` | Abstract subagent seam: named-provider registry + vocabulary | `ctx.subagents` |
| `subagent-spawn/` | In-process backend: a fresh child agent (+ the shared run driver) | (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/`. Provider implementations live in their own packages — the in-process `dsh-subagent-spawn` / `dsh-subagent-fork` and the out-of-process `dsh-subagent-acp` — plus the test-only `dsh-subagent-mock` in [support](../support/README.md). All **product** packages except the mock.
The interface lives at `subagent/subagent/`. The in-process `subagent-spawn` / `subagent-fork` backends ship here; the out-of-process `dsh-subagent-acp` and the test-only `dsh-subagent-mock` (in [support](../support/README.md)) are separate. All **product** packages except the mock.
The proposal and design rationale: [docs/rfc/proposed/feature/2026-06-21-subagent-capability-seam.md](../../docs/rfc/proposed/feature/2026-06-21-subagent-capability-seam.md).