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.

packages/client/ui-model/src/client/directory.ts

@@ -15,6 +15,14 @@ import { createSnapshotStore } from '@deepseek-ai/dsh-client-runtime/client'
 export interface ModelDirectoryState {
   /** Target the host reports for the next assembled step; null before the first load. */
   current: ModelTarget | null
+  /**
+   * Whether an adapter serves the current target's route, as the host reports
+   * it — null before the first load, which is NOT the same as blocked. Read
+   * this rather than "current matches no group": catalog membership is
+   * advisory, so a route serving a model it stopped advertising is missing
+   * from the groups yet perfectly usable.
+   */
+  routable: boolean | null
   /** Successfully loaded provider groups (last good load). */
   groups: readonly ModelProviderGroup[]
   /** Provider-local failures from the last load; usable groups stay usable. */
@@ -29,7 +37,7 @@ export interface ModelDirectoryState {
 export class ModelDirectory {
   /** The shared snapshot both entries render from (uSES-safe store). */
   readonly store: SnapshotStore<ModelDirectoryState> = createSnapshotStore<ModelDirectoryState>({
-    current: null, groups: [], failures: [], status: 'idle', error: null,
+    current: null, routable: null, groups: [], failures: [], status: 'idle', error: null,
   })
 
   /** Latest operation wins; an older response never overwrites a newer one. */
@@ -65,9 +73,10 @@ export class ModelDirectory {
       this.store.update((s) => { s.status = 'error'; s.error = `${result.error.code}: ${result.error.message}` })
       throw new Error(`session.models failed: ${result.error.code}: ${result.error.message}`)
     }
-    const { current, groups, failures } = result.value
+    const { current, routable, groups, failures } = result.value
     this.store.update((s) => {
       s.current = current
+      s.routable = routable
       s.groups = groups
       s.failures = failures
       s.status = 'ready'
@@ -102,7 +111,14 @@ export class ModelDirectory {
       this.store.update((s) => { s.status = 'error'; s.error = `${result.error.code}: ${result.error.message}` })
       throw new Error(`session.selectModel failed: ${result.error.code}: ${result.error.message}`)
     }
-    this.store.update((s) => { s.current = result.value.selected; s.status = 'ready'; s.error = null })
+    // The Host validated the route before accepting it, so a selection that
+    // landed is by construction one it can serve.
+    this.store.update((s) => {
+      s.current = result.value.selected
+      s.routable = true
+      s.status = 'ready'
+      s.error = null
+    })
   }
 
   /**
@@ -115,6 +131,7 @@ export class ModelDirectory {
     ++this.generation
     this.store.update((s) => {
       s.current = null
+      s.routable = null
       s.groups = []
       s.failures = []
       s.status = 'idle'

packages/client/ui-model/src/client/index.ts

@@ -105,14 +105,16 @@ export const inject = ['command', 'connection', 'locale', 'sessions', 'slots']
  * @param ctx - client root context.
  */
 export function apply(ctx: ClientContext): void {
-  ctx.plugin(ModelService)
-
   ctx.effect(() => ctx.locale.register(NS, { zh, en }), 'ui-model: dictionaries')
 
   // Non-slot faces (the command description, the popup option builder) read
   // through the bound translate; the seat component reads the standard seat.
   const t = ctx.locale.bind(NS)
 
+  // The composer-block reason is this plugin's own copy, read at raise time so
+  // a locale change reaches the next publish.
+  ctx.plugin(ModelService, { blockReason: () => t('blocked.composer') })
+
   // Entry 1: the /model popupSelect over the shared directory. The command
   // description is registry-held text: it reads t() once at registration and
   // refreshes only on re-registration, not on locale change.

packages/client/ui-model/src/client/locales.ts

@@ -25,6 +25,7 @@ export const zh = {
   'action.reload': '重新加载',
   'warning.groupLoad': '{name} 加载失败：{message}',
   'empty.models': '没有可用的模型。',
+  'blocked.composer': '当前模型不可用，请先选择模型',
   'empty.efforts': '当前模型未提供推理等级。',
 } satisfies Record<string, string>
 
@@ -48,5 +49,6 @@ export const en = {
   'action.reload': 'Reload',
   'warning.groupLoad': '{name} failed to load: {message}',
   'empty.models': 'No models available.',
+  'blocked.composer': 'This model is unavailable — select one to continue',
   'empty.efforts': 'This model provides no reasoning effort levels.',
 } satisfies Record<ModelKey, string>

packages/client/ui-model/src/client/service.ts

@@ -36,11 +36,16 @@ export class ModelService extends Service {
 
   private readonly live: LiveState = { directories: new Map() }
 
+  /** Localized composer-block copy; this plugin owns the string it raises. */
+  private readonly blockReason: () => string
+
   /**
    * @param ctx - owning root context (the service registers itself as `models`).
+   * @param config - the bound translator for this plugin's own dictionary.
    */
-  constructor(ctx: Context) {
+  constructor(ctx: Context, config: { blockReason: () => string }) {
     super(ctx, 'models')
+    this.blockReason = config.blockReason
     ctx.on('connection/reset', () => {
       for (const directory of this.live.directories.values()) directory.resetConnected()
     })
@@ -74,6 +79,27 @@ export class ModelService extends Service {
       () => sessions.subagentAddress(sessionId) === undefined,
     )
     live.directories.set(sessionId, directory)
+    // The composer cannot read this plugin (the dependency runs one way), so
+    // the block is pushed: the Host says whether an adapter serves the
+    // session's route, and only a definite `false` makes the input inert.
+    // `null` — before the first load, or after one failed — must not, or a
+    // slow or unreachable Host would lock a working composer.
+    const conversation = this.ctx.get('conversation')
+    if (conversation !== undefined) {
+      const publish = (): void => {
+        conversation.blocks.set(sessionId, directory.store.getSnapshot().routable === false
+          ? { reason: this.blockReason() }
+          : undefined)
+      }
+      publish()
+      actx.effect(() => {
+        const stop = directory.store.subscribe(publish)
+        return () => {
+          stop()
+          conversation.blocks.set(sessionId, undefined)
+        }
+      }, 'ui-model: composer block')
+    }
     actx.effect(() => () => {
       directory.dispose()
       live.directories.delete(sessionId)