feat(ui): make a session that cannot send refuse to accept one

A default naming a route the Models page has since removed left the
composer saying 选择模型 while the input still accepted a message, which
then failed inside the adapter mid-turn.

`session.prompt` now refuses with `model-unavailable` before opening a
turn. That is the enforcement boundary: the method stays callable no
matter what a client disables. `session.models` reports the same fact as
`routable`, and ui-model pushes a block through the new
`ctx.conversation.blocks` registry so the bar renders the disabled
textarea it already renders without a workspace, carrying the blocker's
own reason. The push direction is forced — ui-model already depends on
ui-conversation, so ui-conversation cannot read it back.

The gate is `routable`, not "matches no advertised group": catalog
membership is advisory, so a route serving a model it stopped advertising
is missing from the groups yet perfectly usable, and `null` before the
first load never blocks so a slow Host cannot lock a working composer.

The scaffold gains a route-only adapter for fixture-less keyless
scenarios. Registering zero providers is a test artifact — every product
composition mounts one — and the goldens that froze the seat's fallback
label now show the model those scenarios actually route to.

.agents/notes/implemented/feature/2026-08-07-default-model-follows-the-picker.i18n.yaml

@@ -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/feature/2026-08-07-default-model-follows-the-picker.md
-2026-08-07-default-model-follows-the-picker.md: 5174b224a17728b65f7fd69d7f72388d50e8e825
-2026-08-07-default-model-follows-the-picker.zh.md: 6ead561b928572a479f2c7b19e409b3845363ed6
+2026-08-07-default-model-follows-the-picker.md: 4142b3aea6a807001831df62c2038ddf57bbd6ad
+2026-08-07-default-model-follows-the-picker.zh.md: c3566567781edac12cd9269d63f86528139c8796

.agents/notes/implemented/feature/2026-08-07-default-model-follows-the-picker.md

@@ -10,7 +10,7 @@ The route a new session started from was frozen into the gateway's composition e
 
 ## Decision
 
-`ApiProxyService` registers its `{provider, model, reasoningEffort?}` slice as the `api-gateway` settings section: the composition entry is the `base` layer and `settings.yaml` layers the user's choice over it. `workspaceRoot` stays outside the section — a launcher fact, not a preference. The section schema is picked out of `static Config` rather than restated, because the configuration-catalog generator reads that literal statically and a spread breaks it.
+`ApiProxyService` registers its `{provider, model, reasoningEffort?}` slice as the `api-gateway` settings section: the composition entry is the `base` layer and `settings.yaml` layers the user's choice over it. `workspaceRoot` stays outside the section — a launcher fact, not a preference. `reasoningEffort` is the mirror case: it lives in the section but NOT in the plugin config, because the seam merges the user layer over the composition entry per field and an absent key cannot override a present one. A composition-set effort would therefore survive every later switch to a model without one — precisely the stranding the wholesale `replace` exists to prevent. Effort is a per-model fact anyway; a deployment default for it belongs on the adapter profile, which resolves per model.
 
 `session.selectModel` records an accepted switch as the new default. There is no separate gesture: switching models in the composer IS how the default is chosen. The write is `replace`, not `update` — switching to a model with no reasoning effort has to clear a stored one, and a merged patch would strand it for the next session to fail on. A storage failure is logged without undoing the switch, which already applies to its own session, and a deployment with no settings provider keeps the entry with the switch staying process-local.
 
@@ -24,6 +24,16 @@ The stored route is not validated against the registry. A default naming a route
 
 `ApiProxyDefaults` changed shape, updating ~40 test construction sites. `host.describe` now reports the live default rather than a captured one, which is what it always meant. `settings.yaml` gains an `api-gateway:` section the moment a user switches models; the `api-gateway` namespace is deliberately NOT added to the gateway's exposed-namespace allowlist, so the Settings page neither reads nor writes it — the model picker is its editor.
 
+## Follow-up: blocking a session that cannot send
+
+A default naming a route the Models page has since removed leaves the composer saying "Select model" while the input still accepts a message, which then fails inside the adapter mid-turn. Two changes close it.
+
+The Host refuses. `session.prompt` checks whether an adapter serves the session's route and answers `model-unavailable` before opening a turn. This is the enforcement boundary: a client that disables its composer is an affordance, and the method stays callable regardless.
+
+The composer goes inert. `session.models` reports `routable`, and ui-model pushes a block through the new `ctx.conversation.blocks` registry; the bar renders the same disabled textarea it already renders without a workspace, with the blocker's own localized reason as the placeholder. The push direction is forced — ui-model already depends on ui-conversation, so ui-conversation cannot read it back.
+
+The gate is `routable`, NOT "the current target matches no advertised group". Catalog membership is advisory by design: a route serving a model it stopped advertising is absent from the groups yet perfectly usable, and blocking there would break a supported configuration (a narrowed `models` list over a live route). `routable` is also three-valued on the client — `null` before the first load or after a failed one never blocks, so a slow or unreachable Host cannot lock a working composer.
+
 ## Alternatives considered
 
 - **Falling back to the composition entry when the stored route is unregistered.** Rejected: the composer would then name the shipped DeepSeek model instead of prompting, which is both a silent switch to a provider the user did not pick and the opposite of the requested behavior.

.agents/notes/implemented/feature/2026-08-07-default-model-follows-the-picker.zh.md

@@ -10,7 +10,7 @@ Status: implemented
 
 ## 决定
 
-`ApiProxyService` 把自己的 `{provider, model, reasoningEffort?}` 切片注册为 `api-gateway` 设置段：组合条目是 `base` 层，`settings.yaml` 把用户的选择叠加其上。`workspaceRoot` 留在段外——它是启动器事实，不是偏好。段 schema 从 `static Config` 里挑出来而不是重述一遍，因为配置目录生成器是静态读取那个字面量的，展开语法会让它失败。
+`ApiProxyService` 把自己的 `{provider, model, reasoningEffort?}` 切片注册为 `api-gateway` 设置段：组合条目是 `base` 层，`settings.yaml` 把用户的选择叠加其上。`workspaceRoot` 留在段外——它是启动器事实，不是偏好。`reasoningEffort` 则是镜像的一例：它在段里、但**不在**插件配置里，因为 seam 是按字段把用户层合并到组合条目之上的，缺席的键覆盖不了存在的键。组合层设的推理等级因此会在此后每一次切到不支持推理的模型时继续存活——正是整段 `replace` 想要杜绝的那种滞留。何况推理等级本就是按模型的事实，它的部署级默认值属于适配器 profile，那里是按模型解析的。
 
 `session.selectModel` 把被接受的切换记录为新的默认值。没有另一个单独的手势：在输入框切模型**就是**选定默认值的方式。写入用 `replace` 而非 `update`——切到一个不支持推理的模型必须清掉已存的等级，而合并补丁会把它滞留下来，让下一个会话在它上面失败。存储失败只记日志，不撤销这次切换（它对自己所在的会话已经生效）；没有设置提供方的部署保留组合条目，切换只停留在进程内。
 
@@ -24,6 +24,16 @@ Status: implemented
 
 `ApiProxyDefaults` 形状变了，约 40 处测试构造点随之更新。`host.describe` 现在报告的是活的默认值而非捕获的快照，这本就是它一直想表达的含义。用户一旦切换模型，`settings.yaml` 就会多出一个 `api-gateway:` 段；`api-gateway` 这个 namespace 刻意**没有**加进网关的暴露名单，因此设置页既不读也不写它——模型选择器就是它的编辑器。
 
+## 后续：让发不出消息的会话禁止输入
+
+默认值指向一条模型页已删除的路由时，编辑器显示「选择模型」，输入框却仍接受消息，然后这一轮在适配器内部失败。两处改动关掉这个口子。
+
+宿主拒绝。`session.prompt` 检查是否有适配器服务该会话的路由，在开启轮次之前就以 `model-unavailable` 应答。这是执行边界：客户端禁用编辑器只是提示性设计，这个方法始终可被调用。
+
+编辑器变惰性。`session.models` 报告 `routable`，ui-model 经新的 `ctx.conversation.blocks` 注册表推送一个 block；输入栏渲染的仍是它在没有 Workspace 时就会渲染的那个禁用 textarea，只是把抬起方自己的本地化理由作为 placeholder。推送方向是被迫的——ui-model 本就依赖 ui-conversation，因此 ui-conversation 读不回去。
+
+闸门是 `routable`，**不是**「当前目标匹配不到任何已公布分组」。目录成员关系按设计是咨询性的：一条仍在服务、只是不再公布该模型的路由不在分组里，却完全可用，在那里阻断会破坏一种受支持的配置（对一条活着的路由收窄 `models` 列表）。`routable` 在客户端还是三值的——首次加载之前或加载失败之后的 `null` 绝不阻断，因此慢的或够不着的宿主锁不死一个本来能用的编辑器。
+
 ## 考虑过的替代方案
 
 - **存下来的路由未注册时回落到组合条目。** 否决：那样输入框会显示出厂的 DeepSeek 模型而不是提示选择，既是静默切到用户没选的提供方，也与要求的行为正好相反。