# Agent Note: Child agents join their parent's preset composition

Status: implemented

English | [中文](2026-08-10-child-agents-join-their-parent-preset.zh.md)

## Problem

Tool and prompt-section visibility is inherited along `dsh-scope`'s parent chain, and an agent's scope key is minted with no parent. [Per-session agent presets](../architecture/2026-08-03-per-session-agent-presets.md) moved every model-facing row onto the agent plane and made `AgentPresets.mount()` the one thing that binds that parent link — from the api-proxy's session create, resume, and fork paths. The two in-process subagent drivers compose their children through `applyChildComposition()`, which installed only the per-child persona and tool filter, so a child's scope chain had length one and its registry view resolved the global layer alone.

That layer is now empty in any deployment with a preset roster: the web-app patch layer disables every host-plane tool row. A one-shot child therefore reached the model with zero tools, a continuable child with only the host-plane `report`, and neither carried its parent's persona, workspace context, plan-mode section, or skill catalog. The fork path had already been given the same treatment for the same reason; delegation had not.

The child's durable header compounded it. `childSessionMeta()` recorded no preset, so a cold read of a child session resolved the deployment default — a tool set the child never ran under, which is exactly what the model-visible ⟺ logged rule exists to prevent.

## Decision

`AgentPresets.composeFrom(agentCtx, parentCtx)` joins one agent to the standing composition another already runs on, and returns the preset id joined. It locates the parent's mount through `standingMountFor()` — the agent's key is parented to its preset's standing key, the same relation `serviceForAgent()` reads — and binds the child's key to that same standing key, keeping the binding under the roster's sole re-link authority. A parent that joined no preset yields no join and no error, which is the rosterless deployment: its model-facing rows sit in the host composition, where the child already resolves them through the global layer.

This is a bind, not a mount, and both differences are load-bearing. The child gets its parent's exact generation, so a composition file edited since the parent started cannot hand the child a different one than its parent's history was produced under, and a preset deleted since cannot fail a child whose parent keeps running. It is also synchronous, which is what lets the child creation windows use it — both in-process drivers compose inside a synchronous `setup`.

`applyChildComposition(childCtx, parent, composition)` takes the parent and performs the join before applying the child's own registrations. The parameter is the point: it makes composing a child without the join unrepresentable at the call sites, rather than leaving each new driver to remember a second step. `childSessionMeta()` records the joined id through `AgentPresets.composedPreset()`, read from the parent's live scope chain rather than its header, because a parent that switched preset while blank runs on the newer composition while its header still names the older one.

`dsh-subagent` reaches the roster through `ctx.get('agentPresets')` with a type-only import and an optional peer dependency — the documented opportunistic-consumption pattern it already uses for `sandboxPolicy` and `approval`.

Giving the child its parent's tools exposed a second defect the same agent-plane move introduced: `ToolRuntime` exempted SCOPED registrations from a restriction and filtered only the global layer, so once every model-facing row became an ancestor contribution, a child's `toolFilter` stopped constraining anything — and, with the global layer empty, `restrict()` rejected every name it was given as unknown, failing the child outright. The exempt set is the tools a scope registers ITSELF, not the tools that happen to live in the global layer; reading it the second way held only while those two sets coincided. `view()` now filters everything a scope inherits — the global layer and every ancestor layer — and exempts only its own. The own-layer exemption is load-bearing rather than incidental: the delegation runtime registers a child's `report` and structured-output tools into the child's own layer, and a filter naming the capabilities the child may use must not strip the machinery it answers through.

## Alternatives considered

**Re-mount the parent's preset by id in the child's setup.** Rejected on both semantics and mechanics. It re-reads the roster and re-stats the composition file, so an edit since the parent started forks the child onto a different generation, and a preset deleted since fails the child while its parent runs on. `mount()` is also asynchronous, which the synchronous creation windows cannot accept without restructuring both drivers.

**Bind the child's key to the PARENT's key rather than to the standing mount.** Rejected because it changes what a child inherits: the parent's own scope layer carries its per-agent restrictions, which would then intersect into every descendant, and a child outliving its parent would hang off a disposed agent's key. Joining the standing mount gives the child its parent's composition and nothing else.

**Extend the continuable activation setup registry to cover one-shot children.** Rejected because that registry's contribution type is synchronous `(childCtx) => () => void` with per-installation revocation, modelling deployment capabilities that come and go, while a preset join is a one-time bind with no revocation of its own. Widening it would have made the omission possible again for any driver that skipped the registry.

**Let `dsh-subagent` import `resolveSessionPreset` and mount by the resolved id.** Rejected because it makes the preset roster a hard module edge for a package that must work without one, and it lands back on the remount semantics above.

**Filter every layer on the chain, including the scope's own.** Rejected because it makes a per-child capability filter delete that child's reporting and structured-output tools, which the delegation runtime registers into the child's own layer — an `allow` naming the capabilities a child may use would leave it unable to answer at all.

**Leave the durable header alone and fix only the live join.** Rejected because the live child and the same child read cold would then disagree about which composition produced its history — the same class of defect, moved rather than fixed.

## Testing

`packages/preset/agent-presets/tests/mount.spec.ts` covers the join against real fixture compositions: the child sees its parent's tools and prompt sections, no second generation is mounted, the join survives the parent's disposal (a background child outliving its parent), the reported id matches, a parent without a preset joins nothing, and an unscoped context is refused.

`packages/core/tools/tests/scoped.spec.ts` covers the restriction rule directly: a child's filter removes a tool it inherited from an ancestor scope, the child's own registrations survive its own filter, and an ancestor's restriction still reaches every scope nested inside it.

`packages/subagent/subagent-in-process-driver/tests/preset-inheritance.spec.ts` asserts the model-visible result through `startInProcessRun()` on a host composition carrying no model-facing rows: the schemas in the child's own request, its parent's prompt section, the recorded header preset, a `toolFilter` applied over the inherited preset tools, and a parent that switched preset while blank — to a DIFFERENT preset, so the assertion distinguishes reading the parent's live scope chain from reading its creation header.

The assembled-transcript layer is the shipped Web composition's e2e rather than a keyless snapshot. Every runnable example this repo ships composes no preset roster, so the defect is not observable in the snapshot harness at all: a snapshot scenario would first need an example that mounts a roster AND delegates. The Web e2e boots the real `base` + `web-app` patch layers with both shipped presets, which is the assembled evidence the testing policy asks for; the Web browser lane's subagent goldens carry the visible consequence, since a child that records its preset now shows the preset badge its parent shows.

## Consequences

Delegation now costs a scope-parent bind per child and nothing else — no extra plugin instances, no roster read, no failure mode. A child's capabilities are exactly its parent's, minus whatever its own `toolFilter` removes; a per-subagent preset ("agent types") remains unbuilt and would be a new request field rather than a change to this join.

`applyChildComposition()` changed shape, so any future out-of-tree in-process driver must supply the parent. That is the intended cost: the previous signature let a caller compose a capability-less child and get no error.

A cold-resumed continuable child joins its parent's CURRENT composition rather than the one its own header records. The window is narrow — the parent must create the child, stay blank, switch preset, and only then wake it, since a resident child never re-joins and a one-shot child never resumes — and the alternative is worse: resolving the child's own recorded id would re-read the roster and hand back the preset-deleted failure mode this join exists to avoid. The child's header still records what it started under, so the divergence is observable rather than silent.

`ToolRuntime` now reads a restriction's exempt set as "what this scope registers itself" rather than "the global layer", which changes one documented behavior beyond delegation: a tool an ANCESTOR scope contributes is now subject to a descendant's filter, where before only global-layer tools were. Nothing else on the chain loses its exemption — a scope's own registrations stay outside its own filter, which is the property the delegation runtime depends on.
