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-conversation/src/client/apply.ts

@@ -15,6 +15,8 @@ import { resolveToolPath } from './contract/tool-call-model.ts'
 import { createChatStore } from './stores.ts'
 import { ConversationService } from './service.ts'
 import type { IConversation } from './service.ts'
+import { ComposerBlockRegistry } from './input/blocks.ts'
+import type { ComposerBlock } from './input/blocks.ts'
 import { InputHub } from './input/hub.ts'
 import { ComposerSubmissionPolicy } from './input/submission-policy.ts'
 import { InputBar } from './skeleton/InputBar.tsx'
@@ -54,6 +56,11 @@ const ABSENT_NOTICES = {
   getSnapshot: (): InputNotice | null => null,
   subscribe: () => () => {},
 }
+/** No session, therefore nothing to block; same one-identity rule as above. */
+const ABSENT_BLOCK = {
+  getSnapshot: (): ComposerBlock | undefined => undefined,
+  subscribe: () => () => {},
+}
 const EMPTY_LEXICON: ReadonlyMap<'/' | '@', readonly string[]> = new Map()
 const ABSENT_LEXICON = {
   getSnapshot: () => EMPTY_LEXICON,
@@ -133,6 +140,12 @@ export function apply(ctx: Context): void {
   // ctx.conversation.input by the service below sharing this one instance).
   const inputHub = new InputHub(ctx)
 
+  // The composer-block registry: a plugin that knows a session cannot send —
+  // ui-model, when no adapter serves the session's route — raises a block
+  // here, and the bar reads its own session's store. It cannot flow the other
+  // way: this package must not import the plugins that would know.
+  const composerBlocks = new ComposerBlockRegistry()
+
   // Decision 19/20: the input machine feeds every session-scope slot
   // component through the standard provide channel — the 'input' hook plus
   // the two public actions. Materialization is the shell creation trigger
@@ -167,6 +180,7 @@ export function apply(ctx: Context): void {
       'conversation.hero.workspace': { kind: 'single', scope: 'root' },
     },
     inject: (sessionId: SessionId | undefined): ConversationInjected => ({
+      hooks: { composerBlock: sessionId === undefined ? ABSENT_BLOCK : composerBlocks.storeFor(sessionId) },
       selectWorkspace: async (workspaceId) => {
         const nextId = await workspaces.connectWorkspace(workspaceId)
         if (sessionId !== undefined && nextId !== sessionId) {
@@ -351,7 +365,7 @@ export function apply(ctx: Context): void {
   // registers itself as `conversation` and lives on its own child fiber.
   // Presentation registrants depend directly on their slot declarations;
   // this service remains only where conversation actions are required.
-  ctx.plugin(ConversationService, { input: inputHub })
+  ctx.plugin(ConversationService, { input: inputHub, blocks: composerBlocks })
 
   // The bash sample rides the same declaration seam, in third-party posture
   // (ToolRow-matching Bash · {description} chrome).

packages/client/ui-conversation/src/client/contract/slots.ts

@@ -5,6 +5,7 @@ import type {
 } from '@deepseek-ai/dsh-client-ui-slots'
 import type { CommandNode, ConversationSnapshot, ObservableSnapshot, PendingInteraction, PendingWait, SessionId, ToolCallBlock, WorkspaceId } from '@deepseek-ai/dsh-client-runtime/client'
 import type {} from '@deepseek-ai/dsh-client-ui-layout/client'
+import type { ComposerBlock } from '../input/blocks.ts'
 import type { ComposerKeyboard, EditSelection, InputActions, InputNotice, InputState } from '../input/contract.ts'
 import type { createChatStore } from '../stores.ts'
 import type { ComposerSubmitGesture, InputSubmitMode } from './composer-submission.ts'
@@ -223,6 +224,12 @@ export interface ConversationInjected {
    * When a blank session is already current, carry its draft to the target.
    */
   selectWorkspace: (workspaceId: WorkspaceId) => Promise<void>
+  /**
+   * Framework-bound sources. `composerBlock` is this session's block when a
+   * plugin raised one; the reason is the blocker's own localized copy, which
+   * the root renders as the inert composer's placeholder.
+   */
+  hooks: { composerBlock: ObservableSnapshot<ComposerBlock | undefined> }
 }
 
 /** Business callbacks injected into the strict Session body seat. */
@@ -356,7 +363,7 @@ export type ConversationSlotProps =
     | 'conversation.input.left' | 'conversation.input.right'
     | 'conversation.hero.workspace'
   >
-  & ConversationInjected
+  & InjectFace<ConversationInjected>
   & PropsLocale<'conversation'>
 
 /** Full strict-session body props: per-session store, view ring, and draft mirror. */

packages/client/ui-conversation/src/client/input/blocks.ts

@@ -0,0 +1,77 @@
+/**
+ * Composer blocks: the one way another plugin stops a session's input.
+ *
+ * The composer cannot read the plugins that would know — the dependency runs
+ * ui-model → ui-conversation, never back — so a blocker pushes here and the
+ * bar reads its own session's store. A block carries the localized reason it
+ * exists, because the plugin that raised it owns that copy; the composer only
+ * knows how to render an inert textarea with a placeholder, exactly as it
+ * already does for a session with no workspace.
+ *
+ * This is an affordance, not enforcement: the Host refuses a prompt it cannot
+ * route regardless of what any client disables.
+ */
+
+import { createSnapshotStore } from '@deepseek-ai/dsh-client-runtime/client'
+import type { SessionId, SnapshotStore } from '@deepseek-ai/dsh-client-runtime/client'
+
+/** Why one session's composer is inert. */
+export interface ComposerBlock {
+  /**
+   * Localized placeholder replacing the composer's own, owned by the plugin
+   * that raised the block.
+   */
+  readonly reason: string
+}
+
+/** The registry face other plugins reach through `ctx.conversation.blocks`. */
+export interface ComposerBlocks {
+  /**
+   * Raise or clear this session's block. Idempotent: setting a block equal to
+   * the current one, or clearing an absent one, notifies nobody.
+   * @param sessionId - the session whose composer is affected.
+   * @param block - the block to raise, or undefined to clear it.
+   */
+  set(sessionId: SessionId, block: ComposerBlock | undefined): void
+  /**
+   * The store the composer subscribes to for one session. Created on first
+   * read from either side, so a blocker may raise a block before the session's
+   * composer mounts and the composer still sees it.
+   * @param sessionId - the session to observe.
+   * @returns that session's block store (undefined value = not blocked).
+   */
+  storeFor(sessionId: SessionId): SnapshotStore<ComposerBlock | undefined>
+  /**
+   * Drop one session's store. The session scope's disposer calls this; a
+   * blocker never needs to.
+   * @param sessionId - the session being torn down.
+   */
+  forget(sessionId: SessionId): void
+}
+
+/** The per-session composer-block registry (one instance per plugin fiber). */
+export class ComposerBlockRegistry implements ComposerBlocks {
+  private readonly stores = new Map<SessionId, SnapshotStore<ComposerBlock | undefined>>()
+
+  /** @inheritdoc */
+  set(sessionId: SessionId, block: ComposerBlock | undefined): void {
+    const store = this.storeFor(sessionId)
+    const current = store.getSnapshot()
+    if (current?.reason === block?.reason) return
+    store.set(block)
+  }
+
+  /** @inheritdoc */
+  storeFor(sessionId: SessionId): SnapshotStore<ComposerBlock | undefined> {
+    const existing = this.stores.get(sessionId)
+    if (existing !== undefined) return existing
+    const created = createSnapshotStore<ComposerBlock | undefined>(undefined)
+    this.stores.set(sessionId, created)
+    return created
+  }
+
+  /** @inheritdoc */
+  forget(sessionId: SessionId): void {
+    this.stores.delete(sessionId)
+  }
+}

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

@@ -14,6 +14,7 @@ import type { Context } from 'cordis'
 // method) instead of the standalone helper.
 import type { ISessions, SessionFace, SessionId } from '@deepseek-ai/dsh-client-runtime/client'
 import type { QueueAction, QueueItemId } from './contract/queue.ts'
+import type { ComposerBlocks } from './input/blocks.ts'
 import type { InputService } from './input/contract.ts'
 
 /**
@@ -24,6 +25,11 @@ import type { InputService } from './input/contract.ts'
 export interface IConversation {
   /** The per-session input machine registry (InputService face). */
   readonly input: InputService
+  /**
+   * The per-session composer-block registry: how a plugin the composer
+   * cannot import makes a session's input inert with its own reason.
+   */
+  readonly blocks: ComposerBlocks
   /**
    * Send a prompt into the caller scope's session (queued turn).
    * @param text - prompt text, sent verbatim as one text block.
@@ -53,16 +59,20 @@ export interface IConversation {
 export class ConversationService extends Service implements IConversation {
   /** The per-session input machine registry (InputService face, design §5.2). */
   readonly input: InputService
+  /** The per-session composer-block registry. */
+  readonly blocks: ComposerBlocks
 
   /**
    * @param ctx - owning root context (the plugin apply context; the service
    * registers itself and follows that fiber's lifetime).
-   * @param config - carries the InputService instance constructed by the
-   * plugin apply (the same InputHub the slot inject factories close over).
+   * @param config - carries the InputService and composer-block registry
+   * constructed by the plugin apply (the same instances the slot inject
+   * factories close over).
    */
-  constructor(ctx: Context, config: { input: InputService }) {
+  constructor(ctx: Context, config: { input: InputService; blocks: ComposerBlocks }) {
     super(ctx, 'conversation')
     this.input = config.input
+    this.blocks = config.blocks
   }
 
   /**

packages/client/ui-conversation/src/client/skeleton/ConversationRoot.tsx

@@ -13,7 +13,7 @@ import css from './ConversationRoot.module.css'
 export type ConversationRootProps = ConversationSlotProps
 
 export function ConversationRoot({
-  sessionId, useSession, useSessions, useWorkspaces, useInput,
+  sessionId, useSession, useSessions, useWorkspaces, useInput, useComposerBlock,
   renderSlot, renderSlotChain, selectWorkspace, t,
 }: ConversationRootProps) {
   const openState = useSession(s => s.openState)
@@ -24,6 +24,9 @@ export function ConversationRoot({
   const cwd = useSessions(s => sessionId === undefined ? undefined : s.byId[sessionId]?.cwd)
   const summaryBlank = useSessions(s => sessionId === undefined ? undefined : s.byId[sessionId]?.blank)
   const workspaces = useWorkspaces(s => s)
+  // A plugin this package cannot import (ui-model) says this session cannot
+  // send; its reason is already localized by whoever raised it.
+  const composerBlock = useComposerBlock(block => block)
 
   const [pickerOpen, setPickerOpen] = useState(false)
   const [pendingWorkspaceId, setPendingWorkspaceId] = useState<WorkspaceId | undefined>()
@@ -126,11 +129,17 @@ export function ConversationRoot({
   // bar is ONE session-maybe slot rendered unconditionally — inert is a prop,
   // not a different tree, so the textarea DOM survives the transition.
   const inert = sessionId === undefined || (hero && chipTitle === undefined)
+  // A raised block is the same inert posture with the blocker's own reason:
+  // one disabled textarea, never a second tree. The no-workspace state wins
+  // when both hold — picking a workspace is the earlier prerequisite.
+  const blocked = !inert && composerBlock !== undefined
   const inputBar = renderSlot('conversation.composer.bar', {
     variant: hero ? 'hero' : 'composer',
     ...(inert
       ? { disabled: true, placeholder: t('placeholder.workspace') }
-      : hero ? { placeholder: t('placeholder.hero') } : {}),
+      : blocked
+        ? { disabled: true, placeholder: composerBlock.reason }
+        : hero ? { placeholder: t('placeholder.hero') } : {}),
     overlay: renderSlot('conversation.input.overlay', {}),
     leftItems: zone === undefined ? null : renderSlot('conversation.input.left', zone),
     rightItems: zone === undefined ? null : renderSlot('conversation.input.right', zone),