---

name: runtime description: Guide for assistant-ui runtime system and state management. Use when working with runtimes, accessing state, or managing thread/message data. version: 0.0.1 license: MIT

assistant-ui Runtime

Always consult assistant-ui.com/llms.txt for latest API.

References

• ./references/local-runtime.md -- useLocalRuntime deep dive
• ./references/external-store.md -- useExternalStoreRuntime deep dive
• ./references/thread-list.md -- Thread list management
• ./references/state-hooks.md -- State access hooks
• ./references/types.md -- Type definitions

Runtime Hierarchy

CopyAssistantRuntime
├── ThreadListRuntime (thread management)
│   ├── ThreadListItemRuntime (per-thread item)
│   └── ...
└── ThreadRuntime (current thread)
    ├── ComposerRuntime (input state)
    └── MessageRuntime[] (per-message)
        └── MessagePartRuntime[] (per-content-part)

State Access (Modern API)

Copyimport { useAssistantApi, useAssistantState, useAssistantEvent } from "@assistant-ui/react";

function ChatControls() {
  const api = useAssistantApi();
  const messages = useAssistantState(s => s.thread.messages);
  const isRunning = useAssistantState(s => s.thread.isRunning);

  useAssistantEvent("message-added", (e) => {
    console.log("New message:", e.message);
  });

  return (
    <div>
      <button onClick={() => api.thread().append({
        role: "user",
        content: [{ type: "text", text: "Hello!" }],
      })}>
        Send
      </button>
      {isRunning && (
        <button onClick={() => api.thread().cancelRun()}>Cancel</button>
      )}
    </div>
  );
}

Thread Operations

Copyconst api = useAssistantApi();
const thread = api.thread();

// Append message
thread.append({ role: "user", content: [{ type: "text", text: "Hello" }] });

// Cancel generation
thread.cancelRun();

// Get current state
const state = thread.getState();  // { messages, isRunning, ... }

Message Operations

Copyconst message = api.thread().message(0);  // By index

message.edit({ role: "user", content: [{ type: "text", text: "Updated" }] });
message.reload();

Events

CopyuseAssistantEvent("thread-started", () => {});
useAssistantEvent("thread-ended", () => {});
useAssistantEvent("message-added", ({ message }) => {});
useAssistantEvent("run-started", () => {});
useAssistantEvent("run-ended", () => {});

Capabilities

Copyconst caps = useAssistantState(s => s.thread.capabilities);
// { cancel, edit, reload, copy, speak, attachments }

Quick Reference

Copy// Get messages
const messages = useAssistantState(s => s.thread.messages);

// Check running state
const isRunning = useAssistantState(s => s.thread.isRunning);

// Append message
api.thread().append({ role: "user", content: [{ type: "text", text: "Hi" }] });

// Cancel generation
api.thread().cancelRun();

// Edit message
api.thread().message(index).edit({ ... });

// Reload message
api.thread().message(index).reload();

Common Gotchas

"Cannot read property of undefined"

• Ensure hooks are called inside AssistantRuntimeProvider

State not updating

• Use selectors with useAssistantState to prevent unnecessary re-renders

Messages array empty

• Check runtime is configured
• Verify API response format