Add generic Stop and Queue UX for long-running chat turns

[chubes4]

Problem

@extrachill/chat currently disables the chat input while isLoading is true. That is safe for today’s synchronous backend contract, but it creates a poor UX for long-running agent turns: users cannot stop generation and cannot type the next instruction while the agent is working.

The backend contract gap is tracked in Automattic/agents-api#236. The WordPress REST adapter follow-up is Extra-Chill/frontend-agent-chat#47.

Current behavior

• useChat.sendMessage() returns early when isLoading is true.
• ChatInput is rendered with disabled={chat.isLoading}.
• Message suggestions are disabled while loading.
• There is no generic cancel or queue UI contract.

Desired behavior

Add optional, backend-agnostic UI hooks for two capabilities:

1. Stop/cancel the active run.
2. Queue a follow-up message while the active run is still running.

The package should not assume Agents API specifically; it should expose generic props/API hooks that adapters can wire up.

Proposed API direction

Possible additions to Chat / useChat:

type ChatRunCapabilities = {
  cancel?: boolean;
  queue?: boolean;
};

type ChatProps = {
  runCapabilities?: ChatRunCapabilities;
  onCancelRun?: (runId: string, sessionId: string) => Promise<void>;
  onQueueMessage?: (input: {
    sessionId: string;
    content: string;
    files?: File[];
  }) => Promise<void>;
};

Exact names can change, but the behavior should be:

• If cancel is available and chat is loading, show a Stop/Cancel control.
• If queue is available and chat is loading, keep the input usable and send through queue semantics.
• If neither is available, preserve the current disabled-input behavior.

Acceptance criteria

• Existing consumers keep current behavior by default.
• Stop control appears only when cancellation support is supplied.
• Input remains enabled during loading only when queue support is supplied.
• Queued messages render optimistically with a queued/pending visual state, or otherwise clearly indicate they are queued.
• The hook exposes active runId when supplied by the backend adapter.
• The API is backend-neutral and does not hardcode WordPress or Agents API specifics.
• Tests or story/demo coverage exists for:
  • no run-control support
  • cancel-only support
  • queue-only support
  • cancel + queue support

Upstream/downstream

• Upstream contract: Add canonical chat run control: run IDs, status, cancellation, and queued messages Automattic/agents-api#236
• WordPress adapter: Support Agents API chat run control in the REST adapter frontend-agent-chat#47
• Studio Web should inherit this through the adapter once available.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafting the issue from the observed disabled-input UX and upstream contract gap. Chris remains responsible for prioritization and implementation.