Guide to the assistant-ui runtime system, the state and action layer behind every chat surface. Use when creating a runtime (useLocalRuntime with a ChatModelAdapter, useExternalStoreRuntime for Redux or Zustand backed stores, useRemoteThreadListRuntime, useCloudThreadListRuntime, useAssistantTransportRuntime), wiring AssistantRuntimeProvider, or reading and mutating thread, message, composer, and attachment state. Covers useAui, useAuiState, and useAuiEvent with the stable selector rule, the scope accessors (aui.thread, aui.message, aui.composer, and siblings), the AuiConfig plus AuiProvider grammar, RuntimeCapabilities, and the attachment, speech, dictation, feedback, suggestion, and history adapters. Route here for a provider error ('Cannot read property of undefined'), a removed v0.12 hook import, an infinite re-render from an object literal selector, or state not updating. For multi-thread list UI use thread-list instead; for toolkit definitions and tool call UI use tools instead.