[Bug] Subagent model-selection policy is never recorded: firstLiveSeq is 5, not 0

[bjoterpe]

What happens

The documented model-selection feature for subagents does not work at all. The
README for @deepseek-ai/dsh-tool-subagent says:

When enabled, its exact route list is recorded in each fresh top-level
Session, inherited by child Sessions, and preserved on restore; later
settings edits affect only new Sessions.

In practice, no session ever records a policy. As a result, every explicit
child route is rejected, even when it is configured and even when it equals the
parent route:

Error: child LLM route "gemini-api/gemini-3.5-flash" is not allowed for this
Session; without a recorded model-selection policy, explicit selection
(including reasoning_effort) is limited to the parent route

Environment

• @deepseek-ai/dsh 0.2.2-alpha.2, commit f3803c9697
• Setting: subagent-model-selection-settings with enabled: true and an
  allowedModels list, provided through the profile patch

Root cause

packages/subagent/tool-subagent/src/model-selection-settings.ts, in the
agent/created handler:

} else if (target.firstLiveSeq === 0
  // oxlint-disable-next-line typescript/no-deprecated -- …
  && target.eventAt(SessionSeq(0))?.type !== 'session/end-seed') {

firstLiveSeq is the offset of the first live event, i.e. the number of seed
events. A fresh session in the real harness always starts with five of them:

permission/preset       seq 0
sandbox/mode            seq 1
approval/policy         seq 2
agent-preset/selected   seq 3
session/end-seed        seq 4

So firstLiveSeq is 5, the condition === 0 is never true, and the policy is
never appended.

Why the existing test does not catch it

tests/model-selection-initialization.spec.ts creates its session through
ctx.agents.create({ sessionId }) — without the initial events the
application writes at session start. In that environment firstLiveSeq is 0,
the condition matches, and the test is green. The testkit builds something the
app does not.

Evidence

Four sessions on 0.2.2-alpha.2, two of them with gemini-api as the parent:
none carried a subagent/model-selection-policy event. The symptom is
symmetric — DeepSeek→Gemini and Gemini→DeepSeek both fail with the same
"without a recorded model-selection policy" message.

Suggested fix

The header already carries the right field: isSeeded is false for a fresh
session and true for one created from a seed (resume/fork). The else branch
is already restricted to non-subagents by the parentId check above, so:

-      } else if (target.firstLiveSeq === 0
-        // oxlint-disable-next-line typescript/no-deprecated -- …
-        && target.eventAt(SessionSeq(0))?.type !== 'session/end-seed') {
+      } else if (target.header.isSeeded === false) {
         const current = this.current()
         allowedModels = current.enabled ? current.allowedModels : undefined
       }

The now-unused SessionSeq import goes with it.

Verified locally

After the change (built with pnpm exec tsdown in the package and a full
restart), fresh sessions record the configured routes, and a subagent on an
allowed route runs; a route outside the list is still rejected — but with the
shorter "is not allowed" message, without the "without a recorded
model-selection policy" suffix.

Regression test needed

A test that creates a session with the initial events (or at least with
isSeeded: false and a live offset > 0) and asserts that the policy is
appended. Without it, the same refactor will silently reintroduce this.

[sanxi33]

在 0.2.1-rc.5（macOS，Web profile）上也能复现，但触发途径和上面不同：Web 界面点「新会话」时，会带上一个之前保存的草稿会话 ID；claim() 发现这条会话还没有 turn/start，就直接复用它。这条会话是从磁盘恢复的，firstLiveSeq > 0，所以策略不会写入，subagent 发布出来没有 provider/model 参数，也没有 list_subagent_models（和 #10889 下 plaask 的评论描述一致）。

所以修复时除了真正新建的会话，还要覆盖被复用的草稿会话。header.isSeeded === false 应该能同时覆盖两种情况；我本地改成「还没记录过任何请求」（target.requestHeader() === undefined），两种情况都能生效：

-const freshSession = target.firstLiveSeq === 0 && target.eventAt(SessionSeq(0))?.type !== "session/end-seed";
+const freshSession = target.firstLiveSeq === 0 && target.eventAt(SessionSeq(0))?.type !== "session/end-seed" || target.requestHeader?.() === undefined;

另外一个相关的易用性问题：如果在 subagent 和 subagent_fork 两行都开 modelSelectionSettings: true，所有新会话都会创建失败，报错是 gateway/internal: failed to create session: Error: tool "list_subagent_models" is already registered in this scope。README 里确实写了只能有一个实例负责模型选择，但如果能在加载配置时就拒绝，并在报错里点出是哪两行冲突，会好排查得多。