diff --git a/AGENTS.md b/AGENTS.md
index 7f7a89f..0f8656b 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -39,6 +39,7 @@
 8. **e2e scratch profile 的 `minimumReleaseAgeExclude` 含 `'@deepseek-ai/*'`**（`scripts/e2e-mount.sh` / `e2e-aggregate-mount.sh`，与仓库根 `pnpm-workspace.yaml` 同策）：alpha 版本常在发布后 24h 内跑 lane，pnpm 11 的 `minimumReleaseAge` 默认会拒装新鲜包。
 9. **插件开发树的 dsh-* 传递 peer 需提升为 devDependencies**（alpha.3 首见）：`dsh-subagent` 等 npm 包把 `dsh-attachment` 等 dsh-* 姊妹包全部声明为 peerDependencies（由宿主 bundle 树统一提供，宿主侧无此问题），插件仓库若只直接依赖其中一部分，其余 peer 在 pnpm 下会解析到树上残留的旧版——如 `dsh-attachment@0.1.1-rc.1` 缺 `admitPromptContent` 导出、`dsh-subagent` 产物 import 它时测试加载即崩。因此 devDependencies 需涵盖 dev 树实际触达的全部 peer（attachment / code-runtime / scope / session-projection / system-prompt / user-approval / util-time 七个即为此提升，与直接依赖同款精确钉版）；`@deepseek-ai/cordis` peer 自 alpha.3 起要求 `^4.0.2`（上游全线 peer 已升）。适配新 alpha 版本时先跑 `pnpm peers check`，把新失配的传递 peer 一并提升进 devDependencies。例外：**`dsh-client-locale` 的 peer/devDep 允许落后于基线**（alpha.5 时上游停在 0.1.2-alpha.3 未发新版）——peer 下限 `^0.1.2-alpha.3` 天然容纳同 tuple 的 alpha.5 运行时，此时保持旧钉版并在 `pnpm-workspace.yaml` 注明，待上游发版再追平；rc.1 起上游恢复发版，peer/devDep 已与基线一并钉 `0.1.2-rc.1`，该例外消除。
 10. **聊天文件打开漏斗是 `remote.session.openWorkspacePath`**（`ctx.workspaces.openPath` 已随 alpha 删除，`IWorkspaces` 无此方法）：「聊天区文件在侧边栏打开」拦截在 `ctx.inject(['remote.session'], …)` 内以 **defineProperty 数据属性遮蔽**该命名空间方法（gateway client 的方法是 accessor 属性、异步挂载、contribution 重挂载时服务重建——inject 回调重跑即自愈）；实现见 `src/client/openpath-intercept.ts`，设计见 [docs/plans/2026-08-31-openpath-intercept-alpha-design.md](docs/plans/2026-08-31-openpath-intercept-alpha-design.md)。
+11. **自定义种子必须带 fork 标记对（`meta.isSeeded: true` + `inheritedEventCount`）**：dsh-session 契约「只给 seed 不标 isSeeded，种子算重放历史而非继承前缀」——缺标记时 `Session.ownEvents()` 含整个种子，子会话 `Inbox` 构造器重放种子里的 `agent/inbox/spliced`，**继承父会话切割时刻未领取的 inbox 输入**（排队用户消息 next-turn、长回合中工具结果上下文/steering next-step——后者即「上下文很长时侧边对话先把之前的 User msg 发出去」的根因，幽灵消息排在 boundary 之前最先发给模型）。宿主 `session.fork`（api-session-controller）的调用即规范形态；回归由 `tests/sidechat-seed-validation.spec.ts`（真实 `Session.create` + `Inbox`）守护。0.1.2-rc.1 复评：无更优雅的 sidechat 宿主 API（`ContinuableStartSpec` 仍无 seed 字段），`AgentRegistry.create` 接缝补齐标记即为规范用法。
 
 ---
 
diff --git a/docs/plans/2026-08-20-sidechat-tab-design.md b/docs/plans/2026-08-20-sidechat-tab-design.md
index 26461ad..18bd9d3 100644
--- a/docs/plans/2026-08-20-sidechat-tab-design.md
+++ b/docs/plans/2026-08-20-sidechat-tab-design.md
@@ -96,3 +96,5 @@
   3. 重开线程 Tab 丢工具行：根因**不在宿主**（`session.history` cold 路径原样返回 `tool/call`，见 `api-proxy.ts:749` 与 `chunk-rows.ts` 的无损 chunk 压缩）而在首挂回源窗口——旧实现 8 事件/页 × 32 页 = 256 事件上限，而 cold 读会把 chunk-rows 展开回每个流式 delta 一条事件，一轮回答动辄数百事件，较早的 `tool/call` 掉出窗口（settled 文本靠最终 `assistant/message` 幸存）。修复 = 抽出 `collectOwnEvents`（200 事件/页 × 40 页帽、日志穷尽/无标记时落定 `seedBoundary:0` 停止每轮重走），单测覆盖四分支。
   4. Subagent 页「目录损坏」行：见 §5 修正——种子追加 descriptor + SubagentView/subagent-detect 按前缀过滤（`isSideThreadSummary` 同时修好 autoOpenSubagent 误触发与计数膨胀）。旧版创建的线程仍是 corrupt 诊断行，由 SubagentView 的标题前缀过滤兜底隐藏。
 - **转录传输迁至自有路由（DSH 0.1.2-alpha.1 适配）**：§6 原设计的时间线轮询走通用 `connection.api.sessions.history` RPC——该客户端面（`ctx.connection.api`）在 0.1.2-alpha.1 的 Remote-gateway 迁移中被整体移除，且其继任者（`session/follow`/`session/page`）对 `origin:'subagent'` 会话强制 subagent 地址寻址（普通 `{kind:'session'}` 地址被 `agent-busy` 拒绝），纯分页也拿不到最新尾页（`throughSeq` 不得超当前游标）。修复 = 新增 **`sidechat.events`** 路由（`src/sidechat-routes.ts`）：host 侧读线程日志（live 走 `agent.session.events`，冷线程走 `ctx.get('sessionPersistence').inspect`——两版 DSH 同形，alpha.1 的 inspect 返回展开后的逻辑日志而非磁盘上的打包 chunk-row），服务端切 `session/end-seed`（`threadOwnLogEvents`，`collectOwnEvents` 的 walk 语义下沉），`afterSeq` 增量 + 8000 事件尾部帽（对齐旧 walk 上限 40×200）。客户端 `fetchThread` 单路径化，删除 `collectOwnEvents` 与 `ctx.connection` 镜像/inject。种子切割仍由 `transcriptRows` 客户端二次防御。挂载冒烟补 live+冷读两条断言。
+- **种子缺 fork 标记 → 子会话继承父会话未领取的 inbox 消息（2026-09-05，用户实测「上下文很长时侧边对话会把之前的 User msg 先发送出去」）**：`sidechat.start` 原只传 `seed` 不传 `meta.isSeeded: true` + `inheritedEventCount`。dsh-session 契约：**只给 seed 不标 isSeeded，种子算重放历史而非继承前缀** → `Session.ownEvents()` 含整个种子 → 子会话 `Inbox` 构造器（重放 `ownEvents()` 中的 `agent/inbox/spliced`）继承父会话切割时刻**任何未领取的 inbox 输入**。两个触发窗口（均以真实 0.1.2-rc.1 包 `Session.create` + `Inbox` 实验证实）：① next-turn——父会话排队中的用户消息（长回合运行中补发）；② **next-step（用户实测场景：父会话运行中、用户队列已清空）**——长回合里工具结果上下文/steering/运行时快照在步骤边界之间插入 next-step、尚未领取；上下文越长回合越久窗口越宽。后果：首条 `sidechat.prompt` 的 claim 顺序为 `[幽灵消息, boundary, question]`——幽灵消息排在 boundary 之前最先发给模型（工具上下文常为「工具完成，接下来做 X」，模型先继续父会话任务），并在子会话日志落成 `user/message`。修复 = create 补 `meta.isSeeded: true` + `inheritedEventCount: SessionLogOffset(seed.length)`（宿主自己的 `session.fork`——api-session-controller——的规范调用形态；`createAgent` 工厂逐字段透传 `sessions.prepare`）。`threadOwnLogEvents` 按 end-seed 标记切割，标记位置不变，转录行为不变；冷恢复按持久化 header 切割，一致。回归测试进 `tests/sidechat-seed-validation.spec.ts`（真实 Session + Inbox 双向锁定：旧形态继承幽灵 / 新形态两队列皆空）。用户拍板：不迁移既有线程、不加 inbox.clear() 兜底（修复后新线程为无操作；旧「从未发过消息」空线程首次使用可能仍先答一次旧消息，之后自愈）。
+- **「更优雅 API」复评（2026-09-05，0.1.2-rc.1 = npm latest）**：无 sidechat/sidechain 宿主 API；`ContinuableStartSpec`（`ctx.subagents.startContinuable`）仍无 seed 字段；`session/create` RPC 只建全新会话。§3 的自建结论继续成立，`AgentRegistry.create` 接缝即宿主 fork 自用的公开接缝——补齐 fork 标记即为规范用法。
diff --git a/src/sidechat-routes.ts b/src/sidechat-routes.ts
index acdeadb..a0252b3 100644
--- a/src/sidechat-routes.ts
+++ b/src/sidechat-routes.ts
@@ -27,6 +27,7 @@ import type { Agent, AgentSetup, CreateAgentOptions, ResumeAgentOptions } from '
 import { snapshotSubagentDescriptor } from '@deepseek-ai/dsh-subagent'
 import type { Context as CordisContext } from '@deepseek-ai/cordis'
 import type { SessionEvent, SessionId } from '@deepseek-ai/dsh-session'
+import { SessionLogOffset } from '@deepseek-ai/dsh-session'
 import type {
   Context,
   SidebarAgentPresetsService,
@@ -238,16 +239,28 @@ export function buildSidechatApi(ctx: Context): SidechatRoutes {
         data: descriptor as unknown as Record<string, unknown>,
       }
       const seed = [...inheritance.seed, descriptorEvent]
+      // Fork-marker fields (the exact shape the host's own session.fork uses,
+      // api-session-controller): without `isSeeded` + `inheritedEventCount` the
+      // session treats the whole seed as the child's OWN events, so the child's
+      // Inbox constructor replays the parent's `agent/inbox/spliced` events and
+      // inherits whatever input sat UNCLAIMED in the parent at the click moment
+      // (a queued follow-up, or a tool-result context spliced into next-step
+      // between step boundaries of a long-running turn). The first side prompt
+      // would then claim and send that stale message BEFORE the boundary +
+      // question. The marker keeps `ownEvents()` at the end-seed boundary, so
+      // the inherited inbox replays to empty.
       const options: CreateAgentOptions = {
         sessionId: childId,
         meta: {
           ...(parentSession.header.cwd === undefined ? {} : { cwd: parentSession.header.cwd }),
           parentSession: parentSession.id,
+          isSeeded: true,
           origin: 'subagent',
           delegationDepth: (parentSession.header.delegationDepth ?? 0) + 1,
           ...(agentPreset === undefined ? {} : { agentPreset }),
         },
         seed: seed as unknown as readonly SessionEvent[],
+        inheritedEventCount: SessionLogOffset(seed.length),
         agentOptions: { ...parent.options },
         setup,
         signal: AbortSignal.timeout(CREATE_TIMEOUT_MS),
diff --git a/tests/sidechat-routes.spec.ts b/tests/sidechat-routes.spec.ts
index ddce8af..4f939c6 100644
--- a/tests/sidechat-routes.spec.ts
+++ b/tests/sidechat-routes.spec.ts
@@ -113,6 +113,7 @@ describe('sidechat.start', () => {
       sessionId: string
       meta: Record<string, unknown>
       seed: readonly { type: string; data: Record<string, unknown> }[]
+      inheritedEventCount: number
       agentOptions: { provider: string; model: string }
       setup: unknown
     }
@@ -120,10 +121,15 @@ describe('sidechat.start', () => {
     expect(options.meta).toMatchObject({
       parentSession: 'parent',
       origin: 'subagent',
+      isSeeded: true,
       delegationDepth: 1,
       agentPreset: 'preset-a',
       cwd: '/p',
     })
+    // The fork-marker pair is load-bearing: without it the child's Inbox
+    // replays the parent's seed inbox splices and inherits whatever input sat
+    // unclaimed in the parent (see sidechat-seed-validation.spec.ts).
+    expect(options.inheritedEventCount).toBe(options.seed.length)
     expect(options.agentOptions).toEqual({ provider: 'test', model: 'model-x' })
     // The child carries the parent's completed turns as a verbatim seed,
     // closed by the durable subagent descriptor (honest catalog citizenship:
@@ -221,6 +227,16 @@ describe('sidechat.start', () => {
 
     const { childId } = await api['sidechat.start']({ sessionId: 'parent', question: '   ' })
 
+    // The empty thread carries the same fork-marker pair (isSeeded +
+    // inheritedEventCount = seed length) as the prompted create.
+    const options = services.create.mock.calls[0]![0] as {
+      meta: Record<string, unknown>
+      seed: readonly { type: string }[]
+      inheritedEventCount: number
+    }
+    expect(options.meta.isSeeded).toBe(true)
+    expect(options.inheritedEventCount).toBe(options.seed.length)
+
     // No prompt yet: the composer owns the first message; the placeholder
     // label is pinned and the in-progress snapshot is parked for it.
     expect(child.inject).not.toHaveBeenCalled()
diff --git a/tests/sidechat-seed-validation.spec.ts b/tests/sidechat-seed-validation.spec.ts
index 3f929d4..f828701 100644
--- a/tests/sidechat-seed-validation.spec.ts
+++ b/tests/sidechat-seed-validation.spec.ts
@@ -10,8 +10,9 @@
  * not a mock, so the class of bug cannot silently return.
  */
 import { describe, expect, it } from 'vitest'
-import { Session } from '@deepseek-ai/dsh-session'
+import { Session, SessionLogOffset } from '@deepseek-ai/dsh-session'
 import type { SessionId } from '@deepseek-ai/dsh-session'
+import { Inbox } from '@deepseek-ai/dsh-agent'
 import { snapshotSubagentDescriptor } from '@deepseek-ai/dsh-subagent'
 import type { SidebarSessionEvent } from '../src/context-types.ts'
 import { buildSidechatInheritance } from '../src/sidechat-core.ts'
@@ -111,3 +112,115 @@ describe('sidechat seed against the real dsh-session validator', () => {
     expect(types.at(-1)).toBe('session/end-seed')
   })
 })
+
+describe('sidechat seed fork markers vs the real dsh-agent Inbox replay', () => {
+  /**
+   * Regression: the thread-create call originally passed `seed` WITHOUT the
+   * fork-marker pair (`meta.isSeeded: true` + `inheritedEventCount`). A seed
+   * without them is REPLAY history, not an inherited prefix, so the child's
+   * ownEvents() includes the whole seed and its Inbox constructor replays the
+   * parent's `agent/inbox/spliced` events — inheriting whatever input sat
+   * UNCLAIMED in the parent at the click moment (a queued follow-up, or a
+   * tool-result context spliced into next-step between step boundaries of a
+   * long-running turn). The first side prompt then claimed and SENT that stale
+   * message before the boundary + question. These tests run the REAL
+   * dsh-session Session and dsh-agent Inbox.
+   */
+  const pendingMessage = (id: string, kind: string, text: string) => ({
+    id,
+    role: 'user',
+    content: [{ type: 'text', text }],
+    source: { kind },
+  })
+  const inboxOver = (session: Session): { nextTurn: number; nextStep: number } => {
+    const inbox = new Inbox(session, { inserted() {}, discarded() {}, claimed() {} })
+    return { nextTurn: inbox.nextTurn.length, nextStep: inbox.nextStep.length }
+  }
+  /** The shape the routes pass since the fix: isSeeded + inheritedEventCount. */
+  const forkMarkedSession = (id: string, seed: readonly SidebarSessionEvent[]): Session =>
+    Session.create(
+      id as SessionId,
+      seed as never,
+      { version: 0, id: id as SessionId, createdAt: Date.now(), isSeeded: true },
+      SessionLogOffset(seed.length),
+    )
+  /** Real loop order: inbox insert → turn/start → claim deletion → step/start
+   *  → user/message. One completed, fully claimed turn (seq 0-7). */
+  const completedTurnLog = (): SidebarSessionEvent[] => [
+    ev('agent/inbox/spliced', 0, { target: 'next-turn', start: 0, inserted: [pendingMessage('m-q', 'user', 'q')] }),
+    ev('turn/start', 1, { turn: 1 }),
+    ev('agent/inbox/spliced', 2, { target: 'next-turn', start: 0, removedCount: 1, inserted: [] }),
+    ev('step/start', 3, { turn: 1, step: 1 }),
+    ev('user/message', 4, pendingMessage('m-q', 'user', 'q')),
+    ev('assistant/message', 5, { turn: 1, step: 1, message: assistantMessage('a') }),
+    ev('step/end', 6, { turn: 1, step: 1 }),
+    ev('turn/end', 7, { turn: 1, reason: { kind: 'completed' } }),
+  ]
+  /** Mid-turn with an UNCLAIMED tool-result context in next-step (seq 15). */
+  const midTurnLog = (): SidebarSessionEvent[] => [
+    ...completedTurnLog(),
+    ev('agent/inbox/spliced', 8, { target: 'next-turn', start: 0, inserted: [pendingMessage('m-q2', 'user', 'q2')] }),
+    ev('turn/start', 9, { turn: 2 }),
+    ev('agent/inbox/spliced', 10, { target: 'next-turn', start: 0, removedCount: 1, inserted: [] }),
+    ev('step/start', 11, { turn: 2, step: 1 }),
+    ev('user/message', 12, pendingMessage('m-q2', 'user', 'q2')),
+    ev('tool/call', 13, { turn: 2, step: 1, callId: 'c1', name: 'bash', arguments: '{}' }),
+    ev('tool/result', 14, {
+      turn: 2, step: 1,
+      message: {
+        id: 'm-r',
+        role: 'user',
+        content: [{ type: 'tool-result', toolCallId: 'c1', content: [{ type: 'text', text: 'ok' }] }],
+        source: { kind: 'tool', callId: 'c1' },
+      },
+    }),
+    ev('agent/inbox/spliced', 15, {
+      target: 'next-step',
+      start: 0,
+      inserted: [pendingMessage('m-ctx', 'tool', 'tool bash finished: exit 0 ... now continue the plan')],
+    }),
+  ]
+
+  it('inherits the parent\'s UNCLAIMED next-step tool context without the fork markers (root cause)', () => {
+    // Parent mid-turn (user queue empty): the bash tool returned and spliced
+    // its follow-up context into next-step; the next step boundary has not
+    // claimed it yet. The reported long-context scenario.
+    const { seed } = buildSidechatInheritance(midTurnLog())
+    // OLD call shape (seed only): the whole seed counts as own events and the
+    // phantom context is inherited as pending input.
+    const unmarked = Session.create('session-inbox-unmarked' as SessionId, seed as never)
+    expect(unmarked.ownEvents().length).toBe(seed.length + 1)
+    expect(inboxOver(unmarked)).toEqual({ nextTurn: 0, nextStep: 1 })
+  })
+
+  it('inherits the parent\'s QUEUED follow-up (next-turn) without the fork markers', () => {
+    const queued = pendingMessage('m-queued', 'user', 'queued second message')
+    const log: SidebarSessionEvent[] = [
+      ...completedTurnLog(),
+      ev('agent/inbox/spliced', 8, { target: 'next-turn', start: 0, inserted: [queued] }),
+    ]
+    const { seed } = buildSidechatInheritance(log)
+    const unmarked = Session.create('session-inbox-unmarked-turn' as SessionId, seed as never)
+    expect(inboxOver(unmarked)).toEqual({ nextTurn: 1, nextStep: 0 })
+  })
+
+  it('the fork-marker pair (isSeeded + inheritedEventCount) leaves the inherited inbox empty', () => {
+    // Same mid-turn log as the root-cause case, plus the descriptor the
+    // routes append — exactly what sidechat.start passes to agents.create.
+    const { seed } = buildSidechatInheritance(midTurnLog())
+    const withDescriptor = [
+      ...seed,
+      {
+        type: 'subagent/descriptor',
+        seq: seed.length,
+        time: Date.now(),
+        data: snapshotSubagentDescriptor({ mode: 'continuable', provider: 'sidechat', label: 'Side: test' }),
+      } as unknown as SidebarSessionEvent,
+    ]
+    const marked = forkMarkedSession('session-inbox-marked', withDescriptor)
+    // Only the end-seed marker is own; the transcript cut is unchanged.
+    expect(marked.ownEvents().map(event => event.type)).toEqual(['session/end-seed'])
+    expect(marked.snapshotEvents().map(event => event.type).at(-1)).toBe('session/end-seed')
+    expect(inboxOver(marked)).toEqual({ nextTurn: 0, nextStep: 0 })
+  })
+})