fix(subagent): compose children from their parent's preset

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 moved every model-facing row onto the agent plane and made AgentPresets.mount() the one thing that binds that link, from the api-proxy's session create, resume, and fork paths. The two in-process subagent drivers 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 — which is empty wherever a preset roster is composed. One-shot children reached the model with no tools, continuable ones with only the host-plane `report`, and neither carried its parent's persona, workspace context, or skill catalog. AgentPresets.composeFrom() joins one agent to the standing composition another already runs on. It is a bind, not a mount: the child gets its parent's exact generation, so a composition edited since the parent started cannot fork it onto another one, and it is synchronous, which is what lets a child creation window use it. applyChildComposition() now takes the parent and performs the join first, making a child composed without it unrepresentable at the call sites. childSessionMeta() records the joined id so a cold read rebuilds the composition the child actually ran under. The audit that followed found two api-proxy readers on the wrong authority: presenterScopeFor() and the live-agent branch of assertPresetUnchanged() both read header.agentPreset, which goes stale the moment a blank session switches preset. A switched session's cold transcript resolved presenters in the older composition's layer and silently degraded to generic cards, and the gateway refused to adopt a live session under the preset it actually runs while accepting the one it left. Both now resolve through resolveSessionPreset(), matching the resume branch fifteen lines above. The owning architecture Agent Note carried the stale claim that the header records what a session runs; it is corrected to name the header/log pair and its three readers. Fixes #2165
2026-08-10 17:46:34 +08:00
parent 3c7e7262c1
commit e53f448650
36 changed files with 698 additions and 41 deletions
@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write .agents/notes/implemented/architecture/2026-08-03-per-session-agent-presets.md
-2026-08-03-per-session-agent-presets.md: 6f1643c25008c3363cb10adb7fbff7afeea31cbe
-2026-08-03-per-session-agent-presets.zh.md: 7afe9ade5c98fadb96384a7e0acd47531c370e0c
+2026-08-03-per-session-agent-presets.md: c39117ab0de001650a95f98ccf3e42f3a5034c92
+2026-08-03-per-session-agent-presets.zh.md: 5e98a2865013a355c134317ded8a4f2ddaccf42c
@@ -31,7 +31,7 @@ Which preset an unnamed session gets is a user setting (`agent-presets.default`)

 ## Consequences

-**The effective default is read per resolution, never snapshotted.** A cached value would need a `watch` subscription and a reload path to stay honest, and the resolved scope already re-reads a hot-reloaded document. Reading through is also what makes the boundary correct rather than merely cheap: the new value applies to the next session created, and every running session keeps the composition it was built from. That invariant is the same one the session header enforces from the other side — the header records the id a session actually runs, so a resume rebuilds that composition rather than today's default, and the gateway rejects an attempt to adopt a live session under a different one. A snapshot would make the two disagree at exactly the moment the setting changes.
+**The effective default is read per resolution, never snapshotted.** A cached value would need a `watch` subscription and a reload path to stay honest, and the resolved scope already re-reads a hot-reloaded document. Reading through is also what makes the boundary correct rather than merely cheap: the new value applies to the next session created, and every running session keeps the composition it was built from. That invariant is the same one the session log enforces from the other side — the header records the id a session was CREATED with and an `agent-preset/selected` event records any later blank-session switch, so a reader resolves the pair (`resolveSessionPreset`) and never the header alone: a resume rebuilds the composition its history was produced under rather than today's default, a cold transcript's presenters resolve in that composition's layer, and the gateway rejects an attempt to adopt a live session under a preset other than the one it currently runs. A snapshot would make the two disagree at exactly the moment the setting changes.

 **A directly-plugged subtree is invisible to the boot audit.** It never links itself to an `Entry`, so it is absent from `ctx.loader.entries()` and `assertEntriesActivated` cannot see it. The mount audits its own rows instead, reading the tree through an `Include` subclass that publishes it.

@@ -31,7 +31,7 @@ Status: implemented

 ## 后果

-**有效默认值在每次解析时读取，从不快照。** 缓存下来就需要一个 `watch` 订阅和一条重载路径才能保持诚实，而解析后的 scope 本来就会重读热重载过的文档。读穿也不只是省事，它让边界本身是对的：新值作用于**下一个新建的会话**，每个运行中的会话保持它被构建时的那份组装。这条不变量正是 session header 从另一侧执行的同一条——header 记录会话实际运行的 id，因此恢复重建的是那份组装而不是当下的默认值，网关也会拒绝把一个活着的会话收编到另一个 preset 之下。快照会让两者恰好在设置改变的那一刻各说各话。
+**有效默认值在每次解析时读取，从不快照。** 缓存下来就需要一个 `watch` 订阅和一条重载路径才能保持诚实，而解析后的 scope 本来就会重读热重载过的文档。读穿也不只是省事，它让边界本身是对的：新值作用于**下一个新建的会话**，每个运行中的会话保持它被构建时的那份组装。这条不变量正是 session 日志从另一侧执行的同一条——header 记录会话**创建时**的 id，此后空白期的任何切换由 `agent-preset/selected` 事件记录，因此读取方解析的是两者之和（`resolveSessionPreset`）、绝不单看 header：恢复重建的是其历史所产出的那份组装而不是当下的默认值，冷读记录的 presenter 在那份组装的层里解析，网关也会拒绝把一个活着的会话收编到它当前运行的 preset 以外的 preset 之下。快照会让两者恰好在设置改变的那一刻各说各话。


 **直接挂载的子树对启动审计不可见。** 它不会把自己关联到 `Entry`，因此不在 `ctx.loader.entries()` 中，`assertEntriesActivated` 也看不到它。改由挂载过程自行校验各行，通过一个会公开自身 tree 的 `Include` 子类读取。
@@ -0,0 +1,6 @@
+# Bilingual-pair consistency record (docs/i18n/README.md): the git blob hash of each
+# side as of the last confirmed-consistent state. Both languages carry equal authority;
+# after editing either side, bring the other along and re-record with:
+#   pnpm run verify-translation-pairing --write .agents/notes/implemented/bug-fix/2026-08-10-child-agents-join-their-parent-preset.md
+2026-08-10-child-agents-join-their-parent-preset.md: c9917c48d10c2b2515284405ea52aed8b476f8b1
+2026-08-10-child-agents-join-their-parent-preset.zh.md: 09e4de5292b65e50bb3973704fd803c819be9f1c
@@ -0,0 +1,47 @@
+# 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`.
+
+## 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.
+
+**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/subagent/subagent-inprocess/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, and a parent that switched preset while blank.
+
+## 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.
@@ -0,0 +1,47 @@
+# Agent Note: Child agents join their parent's preset composition
+
+Status: implemented
+
+[English](2026-08-10-child-agents-join-their-parent-preset.md) | 中文
+
+## Problem
+
+工具与提示段的可见性沿 `dsh-scope` 的父链继承，而 agent 的 scope key 铸造出来时没有父。[逐会话 agent preset](../architecture/2026-08-03-per-session-agent-presets.md) 把所有面向模型的行搬到了 agent 平面，并让 `AgentPresets.mount()` 成为绑定那条父链的唯一途径——调用点在 api-proxy 的会话创建、恢复与 fork 路径上。两个进程内 subagent 驱动通过 `applyChildComposition()` 组装子 agent，而它只安装了逐子 agent 的 persona 与工具限制，于是子 agent 的 scope 链长度为一，其注册表视图只能解析到全局层。
+
+在任何配置了 preset roster 的部署里，那一层现在是空的：web-app 补丁层禁用了全部宿主平面工具行。因此一次性子 agent 抵达模型时工具为零，可继续子 agent 只剩宿主平面的 `report`，两者都不带父方的 persona、工作区上下文、plan-mode 段与技能目录。fork 路径此前已因同一理由做过相同处理；委派没有。
+
+子 agent 的持久化 header 让问题更进一步。`childSessionMeta()` 不记录任何 preset，于是冷读一个子会话解析到的是部署默认值——一套该子 agent 从未运行过的工具集，而这正是"模型可见 ⟺ 已记录"规则要杜绝的情形。
+
+## Decision
+
+`AgentPresets.composeFrom(agentCtx, parentCtx)` 让一个 agent 加入另一个 agent 已在运行的常驻组装，并返回所加入的 preset id。它通过 `standingMountFor()` 定位父方的挂载——agent 的 key 认父到其 preset 的常驻 key，正是 `serviceForAgent()` 读取的同一关系——再把子 agent 的 key 绑到同一个常驻 key 上，绑定句柄仍归 roster 独有的重链权威持有。未加入任何 preset 的父方不产生加入、也不报错，那就是无 roster 的部署：它面向模型的行位于宿主组装中，子 agent 已经能通过全局层解析到它们。
+
+这是认父而非挂载，两处差别都要紧。子 agent 拿到的是父方那个确切的代际，因此父方启动后被编辑过的组装文件不可能把与父方历史所产出时不同的另一个代际交给它，此后被删除的 preset 也不可能让一个父方仍在运行的子 agent 失败。它还是同步的，这正是子 agent 创建窗口能够使用它的前提——两个进程内驱动都在同步的 `setup` 中完成组装。
+
+`applyChildComposition(childCtx, parent, composition)` 接收父方，并在应用子 agent 自身注册之前完成加入。这个参数正是要点所在：它让"组装子 agent 却不做该加入"在各调用点无法表达，而不是把第二个步骤留给每个新驱动去记住。`childSessionMeta()` 通过 `AgentPresets.composedPreset()` 记录所加入的 id，该值从父方**活着的** scope 链读取而不是从其 header 读取，因为在空白期切换过 preset 的父方运行在更新的那份组装上，而它的 header 仍写着旧的那个。
+
+`dsh-subagent` 以类型级导入加可选 peer 依赖的方式，通过 `ctx.get('agentPresets')` 触达 roster——这正是它对 `sandboxPolicy` 与 `approval` 已在使用的、有明确文档的机会性消费模式。
+
+## Alternatives considered
+
+**在子 agent 的 setup 里按 id 重新挂载父方的 preset。** 语义与机制两方面都不成立而被否决。它会重读 roster 并重新 stat 组装文件，因此父方启动后的一次编辑就会把子 agent 分叉到另一个代际，而此后被删除的 preset 会让子 agent 失败、父方却照常运行。`mount()` 还是异步的，同步的创建窗口无法在不重构两个驱动的前提下接受它。
+
+**把子 agent 的 key 绑到**父方的** key 而不是常驻挂载上。** 否决，因为这改变了子 agent 继承的内容：父方自己的 scope 层携带其逐 agent 限制，那些限制会就此与每个后代求交，而活得比父方久的子 agent 会挂在一个已 dispose 的 agent key 上。加入常驻挂载给到子 agent 的是父方的组装，仅此而已。
+
+**扩展可继续 activation setup 注册表以覆盖一次性子 agent。** 否决，因为该注册表的贡献类型是同步的 `(childCtx) => () => void` 并带有逐次安装的撤销，建模的是会来会走的部署能力，而 preset 加入是一次性认父、自身没有撤销可言。扩展它反而会让任何绕过该注册表的驱动重新具备遗漏的可能。
+
+**让 `dsh-subagent` 导入 `resolveSessionPreset` 并按解析出的 id 挂载。** 否决，因为这会给一个必须在没有 roster 时也能工作的包引入硬模块边，而且最终仍落回上述的重新挂载语义。
+
+**只修活着的加入，不动持久化 header。** 否决，因为那样活着的子 agent 与冷读同一个子 agent 会对"哪份组装产出了这段历史"给出不同答案——同一类缺陷，只是被搬了个地方而不是被修掉。
+
+## Testing
+
+`packages/preset/agent-presets/tests/mount.spec.ts` 用真实 fixture 组装覆盖该加入：子 agent 看到父方的工具与提示段、不会挂载出第二个代际、加入在父方 dispose 后依然成立（活得比父方久的后台子 agent）、上报的 id 一致、没有 preset 的父方不产生加入、以及无 scope 的上下文被拒绝。
+
+`packages/subagent/subagent-inprocess/tests/preset-inheritance.spec.ts` 在一个不含任何面向模型行的宿主组装上，通过 `startInProcessRun()` 断言模型可见的结果：子 agent 自身请求中的 schema、父方的提示段、记录下来的 header preset，以及在空白期切换过 preset 的父方。
+
+## Consequences
+
+委派现在的成本是每个子 agent 一次 scope 认父，再无其他——没有额外的插件实例、没有 roster 读取、没有新的失败模式。子 agent 的能力恰好等于父方的能力，减去它自己的 `toolFilter` 所移除的部分；逐 subagent 的 preset（"agent 类型"）仍未构建，那会是一个新的请求字段，而不是对这次加入的改动。
+
+`applyChildComposition()` 的形态变了，因此将来任何仓库外的进程内驱动都必须提供父方。这是刻意付出的代价：此前的签名允许调用方组装出一个毫无能力的子 agent 而不报任何错。
@@ -77,6 +77,7 @@
    "@deepseek-ai/dsh-loader-smoke": "workspace:^",
    "@deepseek-ai/dsh-session": "workspace:^",
    "@deepseek-ai/dsh-settings": "workspace:^",
+    "@deepseek-ai/dsh-subagent": "workspace:^",
    "@deepseek-ai/dsh-system-prompt": "workspace:^",
    "@deepseek-ai/dsh-tools": "workspace:^",
    "@types/js-yaml": "^4.0.9",
@@ -11,6 +11,7 @@ import type { PatchOptions } from '@cordisjs/plugin-include'
 import { beforeAll, describe, expect, it } from 'vitest'
 import { settingsNamespace } from '@deepseek-ai/dsh-settings'
 import { resolveSessionPreset, SETTINGS_NAMESPACE } from '@deepseek-ai/dsh-agent-presets'
+import { applyChildComposition, childSessionMeta } from '@deepseek-ai/dsh-subagent'
 import { CallId } from '@deepseek-ai/dsh-llm'
 import type {} from '@deepseek-ai/dsh-skill'
 import type {} from '@deepseek-ai/dsh-tools'
@@ -422,6 +423,59 @@ describe('a forked session', () => {
  })
 })

+describe('a delegated child', () => {
+  it('runs on the composition its parent runs on', async () => {
+    const parent = await ctx.agents.create({
+      sessionId: SessionId('preset-child-parent'),
+      meta: { agentPreset: 'standard' },
+      setup: agentCtx => ctx.agentPresets.mount(agentCtx, 'standard').then(() => undefined),
+    })
+    // Exactly what an in-process subagent driver's creation window does.
+    const child = await parent.agent.ctx.agents.create({
+      sessionId: SessionId('preset-child'),
+      meta: childSessionMeta(parent.agent, 1, 0),
+      setup: (agentCtx) => {
+        applyChildComposition(agentCtx, parent.agent, {})
+      },
+    })
+    try {
+      expect(toolNames(ctx, child.agent)).toEqual(toolNames(ctx, parent.agent))
+      // The shipped `standard` preset is the whole coding agent; an empty
+      // child here is the defect, and equality alone would not catch it.
+      expect(toolNames(ctx, child.agent)).toContain('bash')
+      expect(child.agent.session.header.agentPreset).toBe('standard')
+    } finally {
+      await child.dispose()
+      await parent.dispose()
+    }
+  })
+
+  it('follows a parent that switched preset while blank', async () => {
+    const parent = await ctx.agents.create({
+      sessionId: SessionId('preset-child-switch-parent'),
+      meta: { agentPreset: 'standard' },
+      setup: agentCtx => ctx.agentPresets.mount(agentCtx, 'standard').then(() => undefined),
+    })
+    await ctx.agentPresets.recompose(parent.agent.ctx, 'minimal')
+    const child = await parent.agent.ctx.agents.create({
+      sessionId: SessionId('preset-child-switch'),
+      meta: childSessionMeta(parent.agent, 1, 0),
+      setup: (agentCtx) => {
+        applyChildComposition(agentCtx, parent.agent, {})
+      },
+    })
+    try {
+      // The live scope chain is the authority, not the parent's creation
+      // header — which still names `standard`.
+      expect(toolNames(ctx, child.agent)).toEqual(toolNames(ctx, parent.agent))
+      expect(child.agent.session.header.agentPreset).toBe('minimal')
+    } finally {
+      await child.dispose()
+      await parent.dispose()
+    }
+  })
+})
+
 describe('authoring a preset on the shipped composition', () => {
  let authorCtx: Context
  let userRoot: string
@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/module-graph.md
-module-graph.md: a2407f5d394020834172288e3d03518d1e8045db
-module-graph.zh.md: 364033c29d773a764ce3f8f0036edeac7c9e0b21
+module-graph.md: 398b49ff2aa795c377abe19bf7a8649078aa0585
+module-graph.zh.md: 5cfcc612b27d64e098a51fda326190fdae1d7e7e
@@ -822,6 +822,7 @@ flowchart TD
  pkg_command_compact --> pkg_compact
  pkg_command_compact --> pkg_invariants
  pkg_subagent --> pkg_agent
+  pkg_subagent --> pkg_agent_presets
  pkg_subagent --> pkg_brand
  pkg_subagent --> pkg_invariants
  pkg_subagent --> pkg_llm
@@ -1386,7 +1387,7 @@ flowchart TD
 | [`tool-str-replace-editor`](../packages/fs/tool-str-replace-editor) | `fs` | [`fs`](../packages/fs/fs), [`invariants`](../packages/support/invariants), [`sandbox`](../packages/sandbox/sandbox), [`sandbox-policy`](../packages/sandbox/sandbox-policy), [`tools`](../packages/core/tools) |
 | [`tool-skill`](../packages/skill/tool-skill) | `skill` | [`agent`](../packages/core/agent), [`invariants`](../packages/support/invariants), [`llm`](../packages/llm/llm), [`skill`](../packages/skill/skill), [`tools`](../packages/core/tools) |
 | [`command-compact`](../packages/compact/command-compact) | `compact` | [`commands`](../packages/interaction/commands), [`compact`](../packages/compact/compact), [`invariants`](../packages/support/invariants) |
-| [`subagent`](../packages/subagent/subagent) | `subagent` | [`agent`](../packages/core/agent), [`brand`](../packages/util/brand), [`invariants`](../packages/support/invariants), [`llm`](../packages/llm/llm), [`scope`](../packages/core/scope), [`session`](../packages/core/session), [`session-persistence`](../packages/session/session-persistence), [`session-projection`](../packages/session/session-projection), [`session-projection-cache`](../packages/session/session-projection-cache), [`tasks`](../packages/tasks/tasks), [`tools`](../packages/core/tools) |
+| [`subagent`](../packages/subagent/subagent) | `subagent` | [`agent`](../packages/core/agent), [`agent-presets`](../packages/preset/agent-presets), [`brand`](../packages/util/brand), [`invariants`](../packages/support/invariants), [`llm`](../packages/llm/llm), [`scope`](../packages/core/scope), [`session`](../packages/core/session), [`session-persistence`](../packages/session/session-persistence), [`session-projection`](../packages/session/session-projection), [`session-projection-cache`](../packages/session/session-projection-cache), [`tasks`](../packages/tasks/tasks), [`tools`](../packages/core/tools) |
 | [`tool-web`](../packages/web/tool-web) | `web` | [`invariants`](../packages/support/invariants), [`llm`](../packages/llm/llm), [`system-prompt`](../packages/core/system-prompt), [`tools`](../packages/core/tools), [`web`](../packages/web/web) |
 | [`spill-policy`](../packages/spill/spill-policy) | `spill` | [`invariants`](../packages/support/invariants), [`llm`](../packages/llm/llm), [`retention`](../packages/util/retention), [`session`](../packages/core/session), [`spill`](../packages/spill/spill), [`tools`](../packages/core/tools) |
 | [`tool-todo`](../packages/todo/tool-todo) | `todo` | [`agent`](../packages/core/agent), [`invariants`](../packages/support/invariants), [`session`](../packages/core/session), [`session-projection`](../packages/session/session-projection), [`tools`](../packages/core/tools) |
@@ -824,6 +824,7 @@ flowchart TD
  pkg_command_compact --> pkg_compact
  pkg_command_compact --> pkg_invariants
  pkg_subagent --> pkg_agent
+  pkg_subagent --> pkg_agent_presets
  pkg_subagent --> pkg_brand
  pkg_subagent --> pkg_invariants
  pkg_subagent --> pkg_llm
@@ -1388,7 +1389,7 @@ flowchart TD
 | [`tool-str-replace-editor`](../packages/fs/tool-str-replace-editor) | `fs` | [`fs`](../packages/fs/fs), [`invariants`](../packages/support/invariants), [`sandbox`](../packages/sandbox/sandbox), [`sandbox-policy`](../packages/sandbox/sandbox-policy), [`tools`](../packages/core/tools) |
 | [`tool-skill`](../packages/skill/tool-skill) | `skill` | [`agent`](../packages/core/agent), [`invariants`](../packages/support/invariants), [`llm`](../packages/llm/llm), [`skill`](../packages/skill/skill), [`tools`](../packages/core/tools) |
 | [`command-compact`](../packages/compact/command-compact) | `compact` | [`commands`](../packages/interaction/commands), [`compact`](../packages/compact/compact), [`invariants`](../packages/support/invariants) |
-| [`subagent`](../packages/subagent/subagent) | `subagent` | [`agent`](../packages/core/agent), [`brand`](../packages/util/brand), [`invariants`](../packages/support/invariants), [`llm`](../packages/llm/llm), [`scope`](../packages/core/scope), [`session`](../packages/core/session), [`session-persistence`](../packages/session/session-persistence), [`session-projection`](../packages/session/session-projection), [`session-projection-cache`](../packages/session/session-projection-cache), [`tasks`](../packages/tasks/tasks), [`tools`](../packages/core/tools) |
+| [`subagent`](../packages/subagent/subagent) | `subagent` | [`agent`](../packages/core/agent), [`agent-presets`](../packages/preset/agent-presets), [`brand`](../packages/util/brand), [`invariants`](../packages/support/invariants), [`llm`](../packages/llm/llm), [`scope`](../packages/core/scope), [`session`](../packages/core/session), [`session-persistence`](../packages/session/session-persistence), [`session-projection`](../packages/session/session-projection), [`session-projection-cache`](../packages/session/session-projection-cache), [`tasks`](../packages/tasks/tasks), [`tools`](../packages/core/tools) |
 | [`tool-web`](../packages/web/tool-web) | `web` | [`invariants`](../packages/support/invariants), [`llm`](../packages/llm/llm), [`system-prompt`](../packages/core/system-prompt), [`tools`](../packages/core/tools), [`web`](../packages/web/web) |
 | [`spill-policy`](../packages/spill/spill-policy) | `spill` | [`invariants`](../packages/support/invariants), [`llm`](../packages/llm/llm), [`retention`](../packages/util/retention), [`session`](../packages/core/session), [`spill`](../packages/spill/spill), [`tools`](../packages/core/tools) |
 | [`tool-todo`](../packages/todo/tool-todo) | `todo` | [`agent`](../packages/core/agent), [`invariants`](../packages/support/invariants), [`session`](../packages/core/session), [`session-projection`](../packages/session/session-projection), [`tools`](../packages/core/tools) |
@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/subsystems/core.md
-core.md: af27484160769156836f377e5b3aba2521280005
-core.zh.md: 12935f4d881f371cfe2c3c5bed85ef88f57ec71a
+core.md: b66d07194cbdb44037d4ea3a972f4646b7854c52
+core.zh.md: ce95df4d88160e0ffc9c0031a7aad47e72a4b4f1
@@ -419,6 +419,43 @@ async resolve(id?: string): Promise<AgentPreset>
 */
 async mount(agentCtx: Context, id?: string): Promise<AgentPreset>

+/**
+ * Join one agent to the SAME standing composition another already runs on.
+ *
+ * This is how a child agent inherits its parent's capabilities. It is a bind,
+ * not a mount: the parent's generation is already composed, so the child gets
+ * that exact instance — the same plugin objects, the same tool registrations,
+ * the same prompt sections. Re-resolving the parent's preset by id instead
+ * would re-read the roster, and a composition file edited since the parent
+ * started would hand the child a DIFFERENT generation than the one its
+ * parent's history was produced under (and a preset deleted since would fail
+ * the child outright while its parent keeps running).
+ *
+ * Synchronous and infallible for that reason, which is what lets a child
+ * creation window use it: the two in-process subagent drivers compose their
+ * children inside a synchronous `setup`.
+ *
+ * A parent that joined no preset — a rosterless deployment — yields no join
+ * and no error: there, the model-facing rows sit in the host composition and
+ * the child already sees them through the global layer.
+ * @param agentCtx - the joining agent's scope context.
+ * @param parentCtx - the scope context of the agent whose composition to join.
+ * @returns the preset id joined, or undefined when the parent joined none.
+ * @throws when `agentCtx` carries no scope, or has already joined a preset.
+ */
+composeFrom(agentCtx: Context, parentCtx: Context): string | undefined
+
+/**
+ * The preset one live agent runs on.
+ *
+ * Read from the live scope chain rather than from the session, so it answers
+ * for an agent whose session has not recorded a preset yet — a child agent
+ * whose durable header is being built from its parent's composition.
+ * @param agentCtx - the agent's scope context.
+ * @returns the preset id, or undefined when the agent joined none.
+ */
+composedPreset(agentCtx: Context): string | undefined
+
 /**
 * Read one preset's composition text.
 * @param id - the preset id.
@@ -427,6 +427,43 @@ async resolve(id?: string): Promise<AgentPreset>
 */
 async mount(agentCtx: Context, id?: string): Promise<AgentPreset>

+/**
+ * Join one agent to the SAME standing composition another already runs on.
+ *
+ * This is how a child agent inherits its parent's capabilities. It is a bind,
+ * not a mount: the parent's generation is already composed, so the child gets
+ * that exact instance — the same plugin objects, the same tool registrations,
+ * the same prompt sections. Re-resolving the parent's preset by id instead
+ * would re-read the roster, and a composition file edited since the parent
+ * started would hand the child a DIFFERENT generation than the one its
+ * parent's history was produced under (and a preset deleted since would fail
+ * the child outright while its parent keeps running).
+ *
+ * Synchronous and infallible for that reason, which is what lets a child
+ * creation window use it: the two in-process subagent drivers compose their
+ * children inside a synchronous `setup`.
+ *
+ * A parent that joined no preset — a rosterless deployment — yields no join
+ * and no error: there, the model-facing rows sit in the host composition and
+ * the child already sees them through the global layer.
+ * @param agentCtx - the joining agent's scope context.
+ * @param parentCtx - the scope context of the agent whose composition to join.
+ * @returns the preset id joined, or undefined when the parent joined none.
+ * @throws when `agentCtx` carries no scope, or has already joined a preset.
+ */
+composeFrom(agentCtx: Context, parentCtx: Context): string | undefined
+
+/**
+ * The preset one live agent runs on.
+ *
+ * Read from the live scope chain rather than from the session, so it answers
+ * for an agent whose session has not recorded a preset yet — a child agent
+ * whose durable header is being built from its parent's composition.
+ * @param agentCtx - the agent's scope context.
+ * @returns the preset id, or undefined when the agent joined none.
+ */
+composedPreset(agentCtx: Context): string | undefined
+
 /**
 * Read one preset's composition text.
 * @param id - the preset id.
@@ -33,6 +33,7 @@ import {
  PresetNotWritableError, resolveSessionPreset,
  SETTINGS_NAMESPACE as AGENT_PRESET_SETTINGS_NAMESPACE, UnknownPresetError,
 } from '@deepseek-ai/dsh-agent-presets'
+import type { PresetBearingSession } from '@deepseek-ai/dsh-agent-presets'
 import type {} from '@deepseek-ai/dsh-tools'
 import type {
  ApiProxy, ConfigurableProviderView, CredentialView, GoalRef, HistoryEntry, HostFrame,
@@ -1350,17 +1351,26 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
   * The registry view scope a transcript's presenters resolve in.
   *
   * A live agent is that scope itself (its chain passes through its preset's
-   * standing layer). A cold session names its preset on the header, and the
+   * standing layer). A cold session resolves its preset from the LOG, and the
   * preset's STANDING key serves without resuming anything — ensuring the
   * mount composes plugins but starts no agent, session, or turn. No roster,
   * no recorded preset, or a preset the roster no longer supplies all fall
   * back to the global layer: the transcript still serves, with the generic
   * cards a viewless entry renders.
+   *
+   * Reading the header alone would render a session that switched while blank
+   * through the composition it was CREATED with. Every tool only the newer
+   * preset registers resolves to no presenter there, and the transcript
+   * silently degrades to generic cards for exactly the calls its history is
+   * made of.
   * @param sessionId - the transcript being read.
-   * @param header - that session's header (attached or inspected).
+   * @param session - that session's header and log (attached or inspected).
   * @returns the scope to pass to presenter lookups, or undefined for global.
   */
-  async function presenterScopeFor(sessionId: SessionId, header: SessionHeader): Promise<ScopeKey | undefined> {
+  async function presenterScopeFor(
+    sessionId: SessionId,
+    session: PresetBearingSession,
+  ): Promise<ScopeKey | undefined> {
    const live = ctx.get('agents')?.get(sessionId)
    if (live !== undefined) return live
    const presets = ctx.get('agentPresets')
@@ -1370,7 +1380,7 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
      // through the DEFAULT preset's standing layer: that is the composition
      // an unnamed session composes today, and presenters are pure display,
      // so the worst a mismatch produces is the generic card it had anyway.
-      return await presets.standingKeyFor(header.agentPreset)
+      return await presets.standingKeyFor(resolveSessionPreset(session))
    } catch {
      // Swallows only the unknown/unusable-preset rejection from the roster:
      // a deleted or broken preset must degrade this read, never fail it.
@@ -1463,7 +1473,7 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
    // Beside the cwd check for the same reason, and after the await so it
    // covers every path that yields a live agent — freshly created, adopted
    // live, resumed from disk, or recovered by the concurrent-creation catch.
-    assertPresetUnchanged(sessionId, presetId, agent.session.header.agentPreset)
+    assertPresetUnchanged(sessionId, presetId, resolveSessionPreset(agent.session))
    if (agent.session.header.cwd !== cwd) {
      throw new SessionCwdConflict(sessionId, cwd, agent.session.header.cwd)
    }
@@ -2003,7 +2013,7 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
            details: {},
          })
        }
-        const page = historyPage(ctx, state.events, beforeSeq, maxMessages, await presenterScopeFor(sessionId, state.header))
+        const page = historyPage(ctx, state.events, beforeSeq, maxMessages, await presenterScopeFor(sessionId, state))
        return ok(request, {
          events: page.events,
          hasMore: page.hasMore,
@@ -2982,7 +2992,7 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
        // The scope presenters resolve in — the live agent, else the recorded
        // preset's standing key, else the global layer — so a cold session's
        // '/' popup lists the catalog its composition actually serves.
-        const scope = await presenterScopeFor(sessionId, session.header)
+        const scope = await presenterScopeFor(sessionId, session)
        try {
          const skills = (await skillRegistry.list({ cwd, scope })).filter(isUserInvocable)
          return ok(request, {
@@ -186,6 +186,24 @@ describe('session.create with an agent preset', () => {
    })
  })

+  it('adopts a live session under the preset it SWITCHED to', async () => {
+    const { api, ctx } = await harness(['standard', 'minimal'])
+    await api.sessions.create(request({ sessionId: SessionId('s4b'), agentPreset: 'standard' }))
+    // Exactly what `agentPreset.select` leaves behind on a blank session: the
+    // header keeps the creation fact, the log states what the agent runs.
+    ctx.sessions.get(SessionId('s4b'))?.append('agent-preset/selected', { agentPreset: 'minimal' })
+
+    const adopted = await api.sessions.create(request({ sessionId: SessionId('s4b'), agentPreset: 'minimal' }))
+    const stale = await api.sessions.create(request({ sessionId: SessionId('s4b'), agentPreset: 'standard' }))
+
+    // Comparing against the header would invert both answers: the preset the
+    // session actually runs would be refused, and the one it left would pass.
+    expect(adopted.result.ok).toBe(true)
+    expect(stale.result.ok).toBe(false)
+    if (stale.result.ok) throw new Error('unreachable')
+    expect(stale.result.error.details).toMatchObject({ existingPreset: 'minimal' })
+  })
+
  it('adopts a live session unchanged when the caller names no preset', async () => {
    const { api } = await harness(['standard', 'minimal'])
    await api.sessions.create(request({ sessionId: SessionId('s5'), agentPreset: 'minimal' }))
@@ -660,6 +678,27 @@ describe('session.history presenter scope', () => {
    expect(standingKeyRequests).toEqual([])
  })

+  it('resolves a switched session from the LOG, not its creation header', async () => {
+    // The header is a creation fact; a switch while blank is a logged event,
+    // and every turn after it ran under the newer composition. Reading the
+    // header would render that history through the older preset's layer,
+    // where the tools it is made of have no presenter at all.
+    const meta = { id: SessionId('p4'), createdAt: 1, cwd: '/tmp/p4', agentPreset: 'standard' }
+    const { api } = await harness(['standard', 'minimal'], {
+      list: () => Promise.resolve([meta]),
+      inspect: () => Promise.resolve({
+        meta,
+        events: [{ type: 'agent-preset/selected', seq: 1, time: 0, data: { agentPreset: 'minimal' } }],
+      }),
+    })
+
+    standingKeyRequests.length = 0
+    const response = await api.sessions.history(request({ sessionId: SessionId('p4') }))
+
+    expect(response.result.ok).toBe(true)
+    expect(standingKeyRequests).toEqual(['minimal'])
+  })
+
  it('serves a COLD transcript whose standing mount is no longer usable', async () => {
    // A genuinely cold session: persistence knows it, no live agent exists.
    const meta = { id: SessionId('p3'), createdAt: 1, cwd: '/tmp/p3', agentPreset: 'standard' }
@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/preset/agent-presets/README.md
-README.md: b6d469b26a0254adc654e5cc49d3df2d10817b2d
-README.zh.md: 60c7bc695c27bf2c0169a0e405aa84aa711b9b21
+README.md: 5ccf1d7b224d0e3a67b3aeb9dc6679d6e802063f
+README.zh.md: ed79cf48b96ed927feec8860b6211cedc369cdda
@@ -14,6 +14,8 @@ Discovery is unmemoized: `list()` and `resolve()` re-read the roots on every cal
 - `ctx.agentPresets.list(): Promise<AgentPreset[]>` Every preset the configured roots currently supply, earlier root winning a duplicate id; broken presets included, each carrying its reason.
 - `ctx.agentPresets.resolve(id?): Promise<AgentPreset>` One preset by id, defaulting to `defaultId`. Throws naming the available ids when no root supplies it. A broken preset resolves — deleting, reading, and reporting one all need the row.
 - `ctx.agentPresets.mount(agentCtx, id?): Promise<AgentPreset>` Compose one agent from a preset — ensure its standing mount (single-flight) and parent the agent's scope key to it — returning the preset for the caller to record. Refuses a broken preset up front with its discovery-reported reason, so every unloadable shape fails the same way before the loader is involved.
+- `ctx.agentPresets.composeFrom(agentCtx, parentCtx): string | undefined` Join one agent to the standing composition another already runs on, returning the preset id joined — `undefined` when the parent joined none, which is the rosterless deployment and not an error. A bind rather than a mount, so it is synchronous and cannot fail.
+- `ctx.agentPresets.composedPreset(agentCtx): string | undefined` The preset one LIVE agent runs on, read from its scope chain rather than from its session — the only answer available for an agent whose durable header is still being built.
 - `ctx.agentPresets.recompose(agentCtx, id): Promise<AgentPreset>` Re-link one agent to a different preset's standing composition. Valid only while the agent has produced nothing — **the caller owns that check**; the new mount is ensured before the link moves, so a failure leaves the agent as it was. Refuses a broken preset like `mount()`.
 - `ctx.agentPresets.standingKeyFor(id?): Promise<ScopeKey>` The standing scope key a host reader with no agent (a cold transcript read) resolves preset registrations in; ensures the mount without starting an agent, session, or turn. Refuses a broken preset like `mount()`.
 - `ctx.agentPresets.authorable: boolean` Whether any configured root has `user` trust, and therefore whether a preset can be created at all.
@@ -27,6 +29,14 @@ Discovery is unmemoized: `list()` and `resolve()` re-read the roots on every cal

 The agent factory's `setup(agentCtx)` hook is the one supported call site. Only there is the join installed while the agent is still unpublished, so a rejected composition rolls the whole creation back rather than leaving a half-composed session. The standing subtree is owned by the roster service's own fiber — deliberately its UNTRACED context, because a subtree minted from a traced `this.ctx` resolves every service through the caller's shadow fiber instead of each entry's own inject store — so it survives every agent and unwinds only with the whole tree. Each generation records its composition file's stamp (mtime and size): a session that finds the stamp stale starts the next generation, while every session already joined keeps the one it runs on — the composition a running session joined outlives its file changing or disappearing underneath it, and files are the only composition editor, so the stamp is what carries an edit to later sessions.

+### Composing a child agent
+
+A subagent's child joins its parent's standing composition through `composeFrom()`, never through `mount()`. Every model-facing row lives on the agent plane, so the tool registry's global layer is empty and a child that joins nothing reaches the model with no tools at all and none of its parent's prompt sections.
+
+Re-mounting the parent's preset by id would differ from the bind in two ways that both matter. A composition file edited since the parent started would hand the child a DIFFERENT generation than the one its parent's history was produced under, and a preset deleted since would fail the child outright while its parent keeps running. The bind is also synchronous, which is what lets the in-process subagent drivers use it at all — they compose their children inside a synchronous creation window.
+
+The child records the joined id on its own durable header ([`dsh-subagent`](../../subagent/subagent/README.md)), so a cold read of the child's history rebuilds the composition it actually ran under rather than the deployment default.
+
 ### Which preset a session runs

 The creation header names the preset a session STARTED with; `resolveSessionPreset(session)` names the one it RUNS. They differ whenever a blank session switched, so every reconstruction path — the summary a picker reads, a resume, a fork — resolves rather than reading the header.
@@ -14,6 +14,8 @@
 - `ctx.agentPresets.list(): Promise<AgentPreset[]>` 当前各根目录提供的全部 preset；id 重复时靠前的根目录胜出；损坏的 preset 也在其中，各自携带原因。
 - `ctx.agentPresets.resolve(id?): Promise<AgentPreset>` 按 id 取一个 preset，缺省取 `defaultId`。没有任何根目录提供该 id 时抛错，并列出可用 id。损坏的 preset 照样解析——删除、读取与上报都需要这一行。
 - `ctx.agentPresets.mount(agentCtx, id?): Promise<AgentPreset>` 用一个 preset 组装一个 agent——确保其常驻挂载（并发去重）并把 agent 的 scope key 认父到它——返回该 preset 供调用方记录。对损坏的 preset 直接以发现时记下的原因拒绝，所以每种不可加载的形态都在加载器介入之前以同一方式失败。
+- `ctx.agentPresets.composeFrom(agentCtx, parentCtx): string | undefined` 让一个 agent 加入另一个 agent 已在运行的常驻组装，返回所加入的 preset id——父方未加入任何 preset 时返回 `undefined`，那是无 roster 的部署，不是错误。这是认父而非挂载，因此同步且不会失败。
+- `ctx.agentPresets.composedPreset(agentCtx): string | undefined` 某个**活着的** agent 正在运行的 preset，从其 scope 链读取而不是从其会话读取——对于持久化 header 尚在构建中的 agent，这是唯一能拿到的答案。
 - `ctx.agentPresets.recompose(agentCtx, id): Promise<AgentPreset>` 把一个 agent 重链到另一个 preset 的常驻组装。仅在该 agent 尚无任何产出时合法——**由调用方负责该检查**；新挂载在链移动之前确保完成，失败时 agent 原封不动。与 `mount()` 一样拒绝损坏的 preset。
 - `ctx.agentPresets.standingKeyFor(id?): Promise<ScopeKey>` 没有 agent 的宿主读取方（冷读记录）解析 preset 注册所用的常驻 scope key；确保挂载而不启动任何 agent、会话或轮次。与 `mount()` 一样拒绝损坏的 preset。
 - `ctx.agentPresets.authorable: boolean` 是否有任一配置根目录具备 `user` 信任级别，因而 preset 是否可创建。
@@ -27,6 +29,14 @@

 agent 工厂的 `setup(agentCtx)` 钩子是唯一受支持的调用点。只有在那里，认父是在 agent 尚未发布时完成的，因此组装被拒绝会让整次创建回滚，而不会留下一个组装到一半的会话。常驻子树归 roster 服务自己的 fiber 所有——刻意用其未追踪的上下文，因为从被追踪的 `this.ctx` 派生的子树会经调用方的 shadow fiber 解析一切服务、无视各 entry 自己的 inject store——所以它比任何 agent 都活得久，只随整棵树卸载。每个代际记录其组装文件的 stamp（mtime 与大小）：发现 stamp 过期的会话会开启下一个代际，而所有已加入的会话保持各自正在运行的那个——正在运行的会话所加入的组装在其文件被修改或删除后继续存活；文件是唯一的组装编辑器，stamp 正是把编辑送达后续会话的机制。

+### 组装子 agent
+
+subagent 的子 agent 通过 `composeFrom()` 加入其父方的常驻组装，绝不走 `mount()`。所有面向模型的行都在 agent 平面，工具注册表的全局层是空的，因此没有加入任何组装的子 agent 抵达模型时既没有任何工具，也没有父方的任何提示段。
+
+按 id 重新挂载父方的 preset 与认父有两处差别，且两处都要紧。父方启动后被编辑过的组装文件会把与父方历史所产出时**不同**的一个代际交给子 agent；而此后被删除的 preset 会让子 agent 直接失败，尽管其父方仍在正常运行。认父还是同步的，这正是进程内 subagent 驱动能够使用它的前提——它们在同步的创建窗口里组装子 agent。
+
+子 agent 会把所加入的 id 记在自己的持久化 header 上（见 [`dsh-subagent`](../../subagent/subagent/README.md)），因此冷读子 agent 的历史时重建的是它实际运行过的组装，而不是部署默认值。
+
 ### 会话实际运行的是哪个 preset

 创建头部记录的是会话**以什么开始**，`resolveSessionPreset(session)` 给出的才是它**实际运行的**。空白会话一旦切换过，两者就不同，因此所有重建路径——选择器读取的摘要、resume、fork——都走解析，而非直接读头部。
@@ -28,7 +28,7 @@ import { bindScopeParent, createScope, scopeOf, type Scope, type ScopeKey, type
 import { settingsNamespace, type SettingsScope, type default as SettingsService } from '@deepseek-ai/dsh-settings'
 import { discoverPresets } from './discovery.ts'
 import { copyComposition, deleteComposition, readComposition } from './authoring.ts'
-import { mountPreset, serviceForAgent } from './mount.ts'
+import { mountPreset, serviceForAgent, standingMountFor } from './mount.ts'
 import { PresetExistsError } from './authoring.ts'
 import { PresetMountError, UnknownPresetError, type AgentPreset, type Config } from './types.ts'

@@ -51,8 +51,8 @@ export {
  METADATA_FILE, readPresetMetadata, renderPresetMetadata, type PresetMetadata,
 } from './metadata.ts'
 export {
-  inactiveRows, leakedServices, livePresetMounts, mountPreset, serviceForAgent,
-  type PresetMount,
+  inactiveRows, leakedServices, livePresetMounts, mountPreset, serviceForAgent, standingMountFor,
+  type JoinedPresetMount, type PresetMount,
 } from './mount.ts'
 export {
  copyComposition, deleteComposition, InvalidPresetIdError, PresetExistsError,
@@ -238,6 +238,54 @@ export class AgentPresets extends Service {
    return preset
  }

+  /**
+   * Join one agent to the SAME standing composition another already runs on.
+   *
+   * This is how a child agent inherits its parent's capabilities. It is a bind,
+   * not a mount: the parent's generation is already composed, so the child gets
+   * that exact instance — the same plugin objects, the same tool registrations,
+   * the same prompt sections. Re-resolving the parent's preset by id instead
+   * would re-read the roster, and a composition file edited since the parent
+   * started would hand the child a DIFFERENT generation than the one its
+   * parent's history was produced under (and a preset deleted since would fail
+   * the child outright while its parent keeps running).
+   *
+   * Synchronous and infallible for that reason, which is what lets a child
+   * creation window use it: the two in-process subagent drivers compose their
+   * children inside a synchronous `setup`.
+   *
+   * A parent that joined no preset — a rosterless deployment — yields no join
+   * and no error: there, the model-facing rows sit in the host composition and
+   * the child already sees them through the global layer.
+   * @param agentCtx - the joining agent's scope context.
+   * @param parentCtx - the scope context of the agent whose composition to join.
+   * @returns the preset id joined, or undefined when the parent joined none.
+   * @throws when `agentCtx` carries no scope, or has already joined a preset.
+   */
+  composeFrom(agentCtx: Context, parentCtx: Context): string | undefined {
+    const agentKey = scopeOf(agentCtx)
+    if (agentKey === undefined) {
+      throw new Error('agent-presets: refusing to compose an unscoped context; the scope key is what joins an agent to its preset')
+    }
+    const standing = standingMountFor(parentCtx)
+    if (standing === undefined) return undefined
+    this.bindings.set(agentKey, bindScopeParent(agentKey, standing.key))
+    return standing.presetId
+  }
+
+  /**
+   * The preset one live agent runs on.
+   *
+   * Read from the live scope chain rather than from the session, so it answers
+   * for an agent whose session has not recorded a preset yet — a child agent
+   * whose durable header is being built from its parent's composition.
+   * @param agentCtx - the agent's scope context.
+   * @returns the preset id, or undefined when the agent joined none.
+   */
+  composedPreset(agentCtx: Context): string | undefined {
+    return standingMountFor(agentCtx)?.presetId
+  }
+
  /** Whether this deployment configures a root locally authored presets go to. */
  get authorable(): boolean {
    return this.config.roots.some(root => root.trust === 'user')
@@ -202,6 +202,33 @@ export function leakedServices(ctx: Context, mount: Fiber): string[] {
  return leaked.sort((left, right) => left.localeCompare(right))
 }

+/** A live standing mount located through one agent already joined to it. */
+export type JoinedPresetMount = PresetMount & {
+  /** The standing key, definite because it is what the lookup matched on. */
+  readonly key: ScopeKey
+}
+
+/**
+ * The standing composition one agent is joined to.
+ *
+ * The agent's own key is parented to its preset's standing key, so the mount
+ * is found by matching that parent rather than by walking up from the agent —
+ * the mount is not under the agent's fiber. An agent that joined no preset —
+ * a deployment composing no roster, or a child agent before its join — has no
+ * parent link and resolves to undefined.
+ * @param agentCtx - the agent's scope context.
+ * @returns the mount the agent joined, or undefined when it joined none.
+ */
+export function standingMountFor(agentCtx: Context): JoinedPresetMount | undefined {
+  const agentKey = scopeOf(agentCtx)
+  if (agentKey === undefined) return undefined
+  const standingKey = scopeParentOf(agentKey)
+  if (standingKey === undefined) return undefined
+  return livePresetMounts().find(
+    (candidate): candidate is JoinedPresetMount => candidate.key === standingKey,
+  )
+}
+
 /**
 * One agent's instance of a service its preset mounted.
 *
@@ -231,14 +258,7 @@ export function serviceForAgent<K extends string & keyof Context>(
  agent: { ctx: Context },
  name: K,
 ): Context[K] | undefined {
-  // The agent's own key is parented to its preset's standing key; the mount
-  // is no longer under the agent's fiber, so the search roots at the standing
-  // mount instead of walking up from the agent.
-  const agentKey = scopeOf(agent.ctx)
-  if (agentKey === undefined) return undefined
-  const standingKey = scopeParentOf(agentKey)
-  if (standingKey === undefined) return undefined
-  const mount = livePresetMounts().find(candidate => candidate.key === standingKey)
+  const mount = standingMountFor(agent.ctx)
  if (mount === undefined) return undefined
  const store = ctx.reflect.store
  for (const key of Object.getOwnPropertySymbols(store)) {
@@ -153,6 +153,79 @@ describe('composing an agent from a preset', () => {
  })
 })

+describe('composing a child agent from its parent', () => {
+  /** Create one agent joined to `parent`'s composition, as a child creation window does. */
+  async function childOf(ctx: Context, id: string, parent: Agent): Promise<Agent> {
+    const handle = await ctx.agents.create({
+      sessionId: SessionId(id),
+      setup: (childCtx: Context) => void ctx.agentPresets.composeFrom(childCtx, parent.ctx),
+    })
+    return handle.agent
+  }
+
+  it('gives the child its parent\'s tools and prompt sections', async () => {
+    const parent = await agentOn(ctx, 'sess-parent', 'standard')
+
+    const child = await childOf(ctx, 'sess-child', parent)
+
+    expect(toolNames(ctx, child)).toEqual(['alpha'])
+    const prompt = await ctx.systemPrompt.assemble(assembleContextFor(child))
+    expect(prompt.sections.map(section => section.name)).toContain('preset:alpha')
+  })
+
+  it('joins the parent\'s own generation rather than remounting its preset', async () => {
+    const parent = await agentOn(ctx, 'sess-shared', 'standard')
+    const before = livePresetMounts().length
+
+    await childOf(ctx, 'sess-shared-child', parent)
+
+    // A remount would compose a second copy of every row in the preset; the
+    // child must run on the plugin instances its parent already runs on.
+    expect(livePresetMounts()).toHaveLength(before)
+  })
+
+  it('keeps the child composed after its parent is disposed', async () => {
+    const parentHandle = await ctx.agents.create({
+      sessionId: SessionId('sess-dying-parent'),
+      setup: async (agentCtx: Context) => void await ctx.agentPresets.mount(agentCtx, 'standard'),
+    })
+    const child = await childOf(ctx, 'sess-orphan', parentHandle.agent)
+
+    await parentHandle.dispose()
+
+    // Standing mounts outlive the agents that joined them, so a child outliving
+    // its parent — a background subagent — keeps the composition it started on.
+    expect(toolNames(ctx, child)).toEqual(['alpha'])
+  })
+
+  it('reports the preset id the child joined, for the durable header', async () => {
+    const parent = await agentOn(ctx, 'sess-named', 'minimal')
+
+    const child = await childOf(ctx, 'sess-named-child', parent)
+
+    expect(ctx.agentPresets.composedPreset(parent.ctx)).toBe('minimal')
+    expect(ctx.agentPresets.composedPreset(child.ctx)).toBe('minimal')
+  })
+
+  it('composes nothing when the parent joined no preset', async () => {
+    // The rosterless deployment: model-facing rows sit in the host composition
+    // and the child already resolves them through the registry's global layer.
+    const bare = (await ctx.agents.create({ sessionId: SessionId('sess-bare-parent') })).agent
+
+    const child = await childOf(ctx, 'sess-bare-child', bare)
+
+    expect(ctx.agentPresets.composedPreset(bare.ctx)).toBeUndefined()
+    expect(ctx.agentPresets.composeFrom(child.ctx, bare.ctx)).toBeUndefined()
+    expect(toolNames(ctx, child)).toEqual([])
+  })
+
+  it('refuses to compose an unscoped context', async () => {
+    const parent = await agentOn(ctx, 'sess-unscoped-parent', 'standard')
+
+    expect(() => ctx.agentPresets.composeFrom(ctx, parent.ctx)).toThrow(/unscoped context/)
+  })
+})
+
 describe('rejecting a composition that cannot be used', () => {
  it('refuses to mount into a context that carries no agent scope', async () => {
    await expect(ctx.agentPresets.mount(ctx, 'standard'))
@@ -110,6 +110,14 @@ export const SERVICE_API: readonly ServiceApiEntry[] = [
        signature: 'async mount(agentCtx: Context, id?: string): Promise<AgentPreset>',
        jsDoc: '/**\n * Compose one agent from a preset: ensure the preset\'s standing mount, then\n * parent the agent\'s scope key to it so the mount\'s registrations and\n * listeners cover this agent.\n *\n * Call from the agent factory\'s `setup(agentCtx)`; a rejection there rolls\n * the agent creation back, so a broken preset never yields a half-composed\n * session.\n * @param agentCtx - the agent\'s scope context.\n * @param id - the preset id, or `undefined` for {@link defaultId}.\n * @returns the preset that was composed, for the caller to record.\n * @throws when the preset is unknown or its composition is unusable.\n */',
      },
+      {
+        signature: 'composeFrom(agentCtx: Context, parentCtx: Context): string | undefined',
+        jsDoc: '/**\n * Join one agent to the SAME standing composition another already runs on.\n *\n * This is how a child agent inherits its parent\'s capabilities. It is a bind,\n * not a mount: the parent\'s generation is already composed, so the child gets\n * that exact instance — the same plugin objects, the same tool registrations,\n * the same prompt sections. Re-resolving the parent\'s preset by id instead\n * would re-read the roster, and a composition file edited since the parent\n * started would hand the child a DIFFERENT generation than the one its\n * parent\'s history was produced under (and a preset deleted since would fail\n * the child outright while its parent keeps running).\n *\n * Synchronous and infallible for that reason, which is what lets a child\n * creation window use it: the two in-process subagent drivers compose their\n * children inside a synchronous `setup`.\n *\n * A parent that joined no preset — a rosterless deployment — yields no join\n * and no error: there, the model-facing rows sit in the host composition and\n * the child already sees them through the global layer.\n * @param agentCtx - the joining agent\'s scope context.\n * @param parentCtx - the scope context of the agent whose composition to join.\n * @returns the preset id joined, or undefined when the parent joined none.\n * @throws when `agentCtx` carries no scope, or has already joined a preset.\n */',
+      },
+      {
+        signature: 'composedPreset(agentCtx: Context): string | undefined',
+        jsDoc: '/**\n * The preset one live agent runs on.\n *\n * Read from the live scope chain rather than from the session, so it answers\n * for an agent whose session has not recorded a preset yet — a child agent\n * whose durable header is being built from its parent\'s composition.\n * @param agentCtx - the agent\'s scope context.\n * @returns the preset id, or undefined when the agent joined none.\n */',
+      },
      {
        signature: 'async read(id: string): Promise<string>',
        jsDoc: '/**\n * Read one preset\'s composition text.\n * @param id - the preset id.\n * @returns the composition exactly as stored.\n * @throws when no configured root supplies that id.\n */',
@@ -45,9 +45,12 @@
    }
  },
  "devDependencies": {
+    "@cordisjs/plugin-include": "^1.0.4",
+    "@cordisjs/plugin-loader": "^1.0.0-rc.5",
    "@deepseek-ai/dsh-agent": "workspace:^",
    "@deepseek-ai/dsh-agent-loop": "workspace:^",
    "@deepseek-ai/dsh-agent-loop-testkit": "workspace:^",
+    "@deepseek-ai/dsh-agent-presets": "workspace:^",
    "@deepseek-ai/dsh-fs-sandbox": "workspace:^",
    "@deepseek-ai/dsh-invariants": "workspace:^",
    "@deepseek-ai/dsh-llm": "workspace:^",
@@ -125,7 +125,7 @@ export async function startInProcessRun(
    if (inheritedPolicy !== undefined) {
      childSession.append('approval/policy', { policy: inheritedPolicy, source: 'delegation' })
    }
-    applyChildComposition(childCtx, {
+    applyChildComposition(childCtx, parent, {
      persona: request.persona,
      toolFilter: request.toolFilter,
    })
@@ -0,0 +1,20 @@
+// A preset row standing in for the agent-plane tool rows a real preset mounts.
+// Import-free on purpose — the Loader resolves entry modules through Node's ESM
+// resolver, which cannot see this workspace's TypeScript sources.
+export const name = 'preset-tool'
+export const inject = ['tools', 'systemPrompt']
+
+export function apply(ctx, config) {
+  ctx.effect(() => ctx.tools.register({
+    name: config.tool,
+    description: `fixture tool ${config.tool}`,
+    parameters: { type: 'object', properties: {}, additionalProperties: false },
+    output: { schema: { type: 'string' }, render: (_args, value) => [{ type: 'text', text: String(value) }] },
+    execute: () => Promise.resolve(config.tool),
+  }))
+  ctx.effect(() => ctx.systemPrompt.section({
+    name: `preset:${config.tool}`,
+    order: 10,
+    text: `section for ${config.tool}`,
+  }))
+}
@@ -0,0 +1,5 @@
+# Agent-plane composition: the model-facing row lives here, not in the host.
+- id: only
+  name: ../../plugins/preset-tool.js
+  config:
+    tool: preset_only
@@ -0,0 +1,116 @@
+/**
+ * Composition inheritance: a child runs on the preset its parent runs on.
+ *
+ * With every model-facing row on the agent plane, the tool registry's global
+ * layer is empty, so a child that joins no preset reaches the model with no
+ * tools at all. These assert the model-visible result — the schemas in the
+ * child's own request — rather than the join that produces it.
+ */
+
+import { afterEach, describe, expect, it } from 'vitest'
+import { dirname, join } from 'node:path'
+import { fileURLToPath, pathToFileURL } from 'node:url'
+import { Context } from 'cordis'
+import Loader from '@cordisjs/plugin-loader'
+import Include from '@cordisjs/plugin-include'
+import type { Agent } from '@deepseek-ai/dsh-agent'
+import AgentLoop from '@deepseek-ai/dsh-agent-loop'
+import { mountAgentLoopTestDependencies } from '@deepseek-ai/dsh-agent-loop-testkit'
+import AgentPresets from '@deepseek-ai/dsh-agent-presets'
+import { SessionId } from '@deepseek-ai/dsh-session'
+import { snapshotSubagentDescriptor } from '@deepseek-ai/dsh-subagent'
+import { MockAdapter, textResponse } from '../../../core/agent-loop/tests/mock-adapter.ts'
+import { startInProcessRun } from '../src/index.ts'
+
+const FIXTURES = join(dirname(fileURLToPath(import.meta.url)), 'fixtures')
+const ROOTS = [{ path: join(FIXTURES, 'presets'), trust: 'system' as const }]
+
+const contexts: Context[] = []
+
+afterEach(async () => {
+  for (const ctx of contexts.splice(0).reverse()) await ctx.fiber.dispose()
+})
+
+/** A host composition carrying no model-facing rows, plus the preset roster. */
+async function setupPresetHost(): Promise<{ ctx: Context; adapter: MockAdapter; parent: Agent }> {
+  const ctx = new Context()
+  contexts.push(ctx)
+  ctx.baseUrl = pathToFileURL(FIXTURES).href + '/'
+  await ctx.plugin(Loader)
+  ctx.loader.builtins.include = Include
+  await mountAgentLoopTestDependencies(ctx)
+  await ctx.plugin(AgentLoop, { agents: [] })
+  await ctx.plugin(AgentPresets, { default: 'coding', roots: ROOTS })
+  const adapter = new MockAdapter([textResponse('parent idle'), textResponse('child done')])
+  ctx.llm.registerAdapter(['mock'], adapter)
+  const handle = await ctx.agents.create({
+    sessionId: SessionId('parent'),
+    agentOptions: { provider: 'mock', model: 'mock' },
+    setup: async (agentCtx: Context) => void await ctx.agentPresets.mount(agentCtx, 'coding'),
+  })
+  return { ctx, adapter, parent: handle.agent }
+}
+
+/** The one-shot spawn request shape both in-process providers build. */
+function spawnRequest(parent: Agent) {
+  return {
+    label: 'child task',
+    prompt: [{ type: 'text' as const, text: 'child task' }],
+    parent,
+    signal: new AbortController().signal,
+    descriptor: snapshotSubagentDescriptor({
+      mode: 'one-shot' as const,
+      provider: 'spawn',
+      label: 'child task',
+    }),
+  }
+}
+
+describe('a child agent composed in-process', () => {
+  it('reaches the model with its parent\'s preset tools', async () => {
+    const { ctx, adapter, parent } = await setupPresetHost()
+
+    const run = await startInProcessRun(spawnRequest(parent), {})
+    await run.result
+
+    const childRequest = adapter.requests.at(-1)
+    expect(childRequest?.tools?.map(tool => tool.name)).toEqual(['preset_only'])
+    expect(ctx.tools.schemas(run.localAgent).map(schema => schema.name)).toEqual(['preset_only'])
+    await run.dispose()
+  })
+
+  it('carries its parent\'s prompt sections', async () => {
+    const { parent } = await setupPresetHost()
+
+    const run = await startInProcessRun(spawnRequest(parent), {})
+    await run.result
+
+    expect(run.localAgent?.session.events.some(event =>
+      event.type === 'request/header'
+      && JSON.stringify(event.data).includes('section for preset_only'))).toBe(true)
+    await run.dispose()
+  })
+
+  it('records the composition it ran under on the child header', async () => {
+    const { parent } = await setupPresetHost()
+
+    const run = await startInProcessRun(spawnRequest(parent), {})
+    await run.result
+
+    // Without this the child's own history reads back under the deployment
+    // default, which is a different tool set than the one it actually used.
+    expect(run.localAgent?.session.header.agentPreset).toBe('coding')
+    await run.dispose()
+  })
+
+  it('follows a parent that switched preset while blank', async () => {
+    const { ctx, parent } = await setupPresetHost()
+    await ctx.agentPresets.recompose(parent.ctx, 'coding')
+
+    const run = await startInProcessRun(spawnRequest(parent), {})
+    await run.result
+
+    expect(ctx.tools.schemas(run.localAgent).map(schema => schema.name)).toEqual(['preset_only'])
+    await run.dispose()
+  })
+})
@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/subagent/subagent/README.md
-README.md: 762030629c09305c48adebc71244655a5faa6585
-README.zh.md: 535cc25895e04e82b6667e6d2769f2dcbfa49cff
+README.md: b69428e4af7d1f53adb22be1e59beb79c054713f
+README.zh.md: 9f5eb5f1c508135c21bf3923f60f4f872de8e9f6
@@ -40,6 +40,10 @@ Start-time features are advertised in `provider.capabilities` because the servic
 - `toolFilter` — apply the requested child tool restriction.
 - `persona` — apply a per-child persona.

+Every in-process child is composed by one call, `applyChildComposition(childCtx, parent, composition)`, which joins the parent's agent-preset composition before applying the child's own persona and tool filter. The join is what gives the child its capabilities: with every model-facing row on the agent plane, a child that joined nothing would reach the model with an empty tool registry ([`dsh-agent-presets`](../../preset/agent-presets/README.md)). Taking the parent as a parameter is deliberate — it makes composing a child WITHOUT that join unrepresentable at the call sites, which is the defect the one call exists to prevent. A deployment composing no preset roster joins nothing and needs nothing: its model-facing rows sit in the host composition, where the child already resolves them through the tool registry's global layer.
+
+`childSessionMeta()` records the joined preset id on the child's durable header for the same reason a top-level session records its own: the preset decides the tool schemas and prompt sections the model saw, so a cold read of the child's history has to rebuild that composition rather than the deployment default. It is read from the parent's live scope chain, not from the parent header, because a parent that switched preset while blank runs on the newer composition while its header still names the older one.
+
 Continuable creation is the optional `SubagentProvider.prepareContinuable?()` method: its presence is the capability check, so the service rejects a configured continuable start on a provider without it, while a provider that has it may still serve ordinary one-shot delegations. The method returns only a detached `ContinuableCreateSpec` (`{ seed? }`) — data, never a capability: it carries no Agent, `AgentHandle`, prompt delivery, result, disposal, or resume operation, because the continuation manager owns identity reservation, composition, Agent creation, prompt delivery, cold resume, ownership, and disposal after preparation. A one-shot `SubagentRun` represents one disposable foreground delegation with one result and no cold-resume operation.

 ## The durable descriptor
@@ -40,6 +40,10 @@ subagent seam 允许一个 agent（智能体）通过具名提供方把工作委
 - `toolFilter`：应用请求的子 agent 工具限制；
 - `persona`：应用每个子 agent 独立的 persona。

+每个进程内子 agent 都由一次调用完成组装：`applyChildComposition(childCtx, parent, composition)` 先加入父方的 agent-preset 组装，再应用该子 agent 自己的 persona 与工具限制。加入组装正是子 agent 获得能力的途径：所有面向模型的行都在 agent 平面，没有加入任何组装的子 agent 抵达模型时工具注册表是空的（见 [`dsh-agent-presets`](../../preset/agent-presets/README.md)）。把父方作为参数是刻意的——这让"组装一个子 agent 却不做该加入"在各调用点无法表达，而这正是这一次调用所要杜绝的缺陷。未组装 preset roster 的部署不加入任何组装、也不需要加入：它的面向模型的行位于宿主组装中，子 agent 已经能通过工具注册表的全局层解析到它们。
+
+`childSessionMeta()` 把所加入的 preset id 记在子 agent 的持久化 header 上，理由与顶层会话记录自己的那一个相同：preset 决定了模型所见的工具 schema 与提示段，因此冷读子 agent 的历史时必须重建那份组装，而不是部署默认值。该值从父方**活着的** scope 链读取，而不是从父方 header 读取，因为在空白期切换过 preset 的父方运行在更新的那份组装上，而它的 header 仍写着旧的那个。
+
 可继续创建对应可选的 `SubagentProvider.prepareContinuable?()` 方法：方法是否存在就是能力检查，因此服务会在没有该方法的提供方上拒绝已配置的可继续启动，而具备该方法的提供方仍可服务普通一次性委派。该方法只返回分离的 `ContinuableCreateSpec`（`{ seed? }`）——这是数据，绝非能力：它不携带任何 Agent、`AgentHandle`、提示词投递、结果、dispose 或恢复操作，因为准备之后，继续执行管理器拥有身份预留、组合、Agent 创建、提示词投递、冷恢复、所有权和 dispose。一次性 `SubagentRun` 表示一次可 dispose 的前台委派，只有一个结果，且没有冷恢复操作。

 ## 持久化描述符
@@ -34,6 +34,7 @@
  },
  "peerDependencies": {
    "@deepseek-ai/dsh-agent": "^0.0.1",
+    "@deepseek-ai/dsh-agent-presets": "^0.0.1",
    "@deepseek-ai/dsh-brand": "^0.0.1",
    "@deepseek-ai/dsh-invariants": "^0.0.1",
    "@deepseek-ai/dsh-llm": "^0.0.1",
@@ -47,6 +48,9 @@
    "cordis": "^4.0.0-rc.7"
  },
  "peerDependenciesMeta": {
+    "@deepseek-ai/dsh-agent-presets": {
+      "optional": true
+    },
    "@deepseek-ai/dsh-session-persistence": {
      "optional": true
    },
@@ -62,6 +66,7 @@
  },
  "devDependencies": {
    "@deepseek-ai/dsh-agent": "workspace:^",
+    "@deepseek-ai/dsh-agent-presets": "workspace:^",
    "@deepseek-ai/dsh-brand": "workspace:^",
    "@deepseek-ai/dsh-invariants": "workspace:^",
    "@deepseek-ai/dsh-llm": "workspace:^",
@@ -12,6 +12,12 @@ import type { Context } from 'cordis'
 import type { Agent, AgentOptions, CreateAgentOptions } from '@deepseek-ai/dsh-agent'
 import type { SessionId } from '@deepseek-ai/dsh-session'
 import type { ToolRestriction } from '@deepseek-ai/dsh-tools'
+// Type-only: make `ctx.get('agentPresets')` resolve to the preset roster when
+// composed — a child inherits its parent's composition opportunistically (the
+// documented `ctx.get` pattern), never as a hard dep. A rosterless deployment
+// keeps its model-facing rows on the host plane, where the child already sees
+// them through the tool registry's global layer.
+import type {} from '@deepseek-ai/dsh-agent-presets'
 import { delegationDepthOf } from './depth.ts'

 /** Thrown when starting a child would exceed the requested depth cap. */
@@ -72,8 +78,15 @@ export function resolveChildAgentOptions(
 /**
 * Build the child session's durable creation metadata: the parent's workspace,
 * its direct lineage, coarse product origin, the recursion budget that must
- * survive persistence, and the seed boundary that separates inherited parent
- * history from child work.
+ * survive persistence, the seed boundary that separates inherited parent
+ * history from child work, and the composition the child runs under.
+ *
+ * The preset is read from the parent's LIVE scope chain rather than from its
+ * header, because a parent that switched preset while blank runs on the newer
+ * composition and its header still names the older one. Recording it is what
+ * makes a child's history reconstructable: without it a cold read of the child
+ * resolves the deployment default and rebuilds turns under a tool set the
+ * child never had.
 * @param parent - the delegating parent agent.
 * @param childDepth - the resolved delegation depth to persist.
 * @param lineageSeedLength - how many leading events came from the parent's log.
@@ -85,8 +98,10 @@ export function childSessionMeta(
  lineageSeedLength: number,
 ): NonNullable<CreateAgentOptions['meta']> {
  const parentHeader = parent.session.header
+  const agentPreset = parent.ctx.get('agentPresets')?.composedPreset(parent.ctx)
  return {
    ...parentHeader.cwd !== undefined ? { cwd: parentHeader.cwd } : {},
+    ...agentPreset === undefined ? {} : { agentPreset },
    parentSession: parentHeader.id,
    // Navigation classification only; the descriptor remains the authority
    // for mode and continuation capability.
@@ -106,13 +121,31 @@ export interface ChildComposition {
 }

 /**
- * Apply one child's scoped composition inside its creation window: a shadowing
- * persona section and a tool restriction, both owned by the child's scope and
- * therefore invisible to its parent and siblings.
+ * Compose one child inside its creation window: join its parent's preset, then
+ * apply the child's own shadowing persona section and tool restriction, both
+ * owned by the child's scope and therefore invisible to its parent and
+ * siblings.
+ *
+ * The join comes first and the child's own registrations second, which is the
+ * order the layering already implies — the nearest scope wins a name, and a
+ * per-child restriction intersects with everything its chain admits — but
+ * stating it here keeps the two steps from being read as independent.
+ *
+ * Both steps live in ONE call because a child composed with only the second is
+ * exactly the defect this function exists to prevent: with every model-facing
+ * row on the agent plane, a child that joins no preset sees an empty tool
+ * registry and none of its parent's prompt sections. Taking the parent as a
+ * parameter is what makes that omission unrepresentable at the call sites.
 * @param childCtx - the child agent's scoped creation context.
- * @param composition - the persona and tool filter to install.
+ * @param parent - the delegating parent whose composition the child joins.
+ * @param composition - the per-child persona and tool filter to install.
 */
-export function applyChildComposition(childCtx: Context, composition: ChildComposition): void {
+export function applyChildComposition(
+  childCtx: Context,
+  parent: Agent,
+  composition: ChildComposition,
+): void {
+  childCtx.get('agentPresets')?.composeFrom(childCtx, parent.ctx)
  if (composition.persona !== undefined) {
    childCtx.systemPrompt.section({ name: 'deployment:persona', order: 0, text: composition.persona })
  }
@@ -885,7 +885,7 @@ export class SubagentContinuationManager {
    // some other owner holds — a duplicate would reject there with rollback.
    inputs.signal.throwIfAborted()
    const setup = (childCtx: Context): AgentSetupCommit => {
-      applyChildComposition(childCtx, inputs.composition)
+      applyChildComposition(childCtx, parent, inputs.composition)
      return this.setupRegistry.apply(childCtx)
    }
    const observer = this.host.observeActivation(provider, childId, parent)
@@ -26,6 +26,9 @@
    {
      "path": "../../core/scope"
    },
+    {
+      "path": "../../preset/agent-presets"
+    },
    {
      "path": "../../session/session-persistence"
    },
@@ -309,6 +309,9 @@ importers:
      '@deepseek-ai/dsh-settings':
        specifier: workspace:^
        version: link:../../packages/settings/settings
+      '@deepseek-ai/dsh-subagent':
+        specifier: workspace:^
+        version: link:../../packages/subagent/subagent
      '@deepseek-ai/dsh-system-prompt':
        specifier: workspace:^
        version: link:../../packages/core/system-prompt
@@ -6285,6 +6288,9 @@ importers:
      '@deepseek-ai/dsh-agent':
        specifier: workspace:^
        version: link:../../core/agent
+      '@deepseek-ai/dsh-agent-presets':
+        specifier: workspace:^
+        version: link:../../preset/agent-presets
      '@deepseek-ai/dsh-brand':
        specifier: workspace:^
        version: link:../../util/brand
@@ -6539,6 +6545,12 @@ importers:

  packages/subagent/subagent-inprocess:
    devDependencies:
+      '@cordisjs/plugin-include':
+        specifier: ^1.0.4
+        version: link:../../../vendor/include
+      '@cordisjs/plugin-loader':
+        specifier: ^1.0.0-rc.5
+        version: link:../../../vendor/loader
      '@deepseek-ai/dsh-agent':
        specifier: workspace:^
        version: link:../../core/agent
@@ -6548,6 +6560,9 @@ importers:
      '@deepseek-ai/dsh-agent-loop-testkit':
        specifier: workspace:^
        version: link:../../support/agent-loop-testkit
+      '@deepseek-ai/dsh-agent-presets':
+        specifier: workspace:^
+        version: link:../../preset/agent-presets
      '@deepseek-ai/dsh-fs-sandbox':
        specifier: workspace:^
        version: link:../../fs/fs-sandbox