Documentation update for the UI Agent Protocol that shipped across these four PRs:

• pipecat-ai/pipecat#4407 — wire format as first-class RTVI message types (5 new message types, paired Data/Message envelopes, 5 new pipeline frames, on_ui_message handler, payload models, PROTOCOL_VERSION 1.2.0 → 1.3.0).
• pipecat-ai/pipecat-client-web#203 — UIAgentClient plus React idioms (UIAgentProvider, UITasksProvider, hooks, standard handlers, A11ySnapshotStreamer).
• pipecat-ai/pipecat-ai-subagents#18 — UIAgent (now extends LLMContextAgent), expanded action helpers, ReplyToolMixin, start_user_task_group for streaming worker fan-out, keep_history flag, attach_ui_bridge rewired to on_ui_message.
• pipecat-ai/pipecat-music-player#1 — voice-driven music player as the canonical reference app.

The wire format moved into Pipecat as the canonical authority, so single-LLM apps can target it directly without taking a subagents dependency. Subagents builds the agent abstractions on top.

Changes

Pipecat (server)

• client/rtvi-standard.mdx — bumped version note to 1.3, added a UI Agent Protocol section documenting all 5 message types (ui-event, ui-snapshot, ui-cancel-task, ui-command, ui-task) with shapes, plus a Standard Command Payloads table for the 8 standard payload models.
• api-reference/server/rtvi/rtvi-processor.mdx — added a UI Agent Protocol section documenting the new on_ui_message event handler and the inbound / outbound frame conventions.
• api-reference/server/rtvi/rtvi-observer.mdx — extended the frame-translation table for RTVIUICommandFrame and RTVIUITaskFrame.
• api-reference/server/frames/system-frames.mdx — added a RTVI UI Frames section documenting all 5 new pipeline frames (RTVIUICommandFrame, RTVIUITaskFrame, RTVIUIEventFrame, RTVIUISnapshotFrame, RTVIUICancelTaskFrame).

Subagents

• api-reference/pipecat-subagents/ui-agent.mdx — fixed parent class to LLMContextAgent, refreshed bridge frame references, added keep_history, added the 5 action helpers (scroll_to, highlight, select_text, click, set_input_value), added user_task_group / start_user_task_group / reset_context, added on_task_cancelled lifecycle hook, documented single-flight task lock semantics.
• api-reference/pipecat-subagents/ui-tools.mdx — full rewrite for ReplyToolMixin (the prior ScrollToToolMixin / HighlightToolMixin documentation no longer matched the source).
• api-reference/pipecat-subagents/ui-commands.mdx — fixed import path to point at pipecat.processors.frameworks.rtvi.models (canonical home), added Click, SetInputValue, SelectText.
• api-reference/pipecat-subagents/ui-prompts.mdx — fixed import path, added <selection> block bullet.
• api-reference/pipecat-subagents/messages.mdx — added 4 task-lifecycle bus messages (BusUITaskGroupStartedMessage, BusUITaskUpdateMessage, BusUITaskCompletedMessage, BusUITaskGroupCompletedMessage), refreshed attach_ui_bridge to reflect the new wiring (subscribes to on_ui_message, emits RTVIUICommandFrame / RTVIUITaskFrame), removed stale UI_*_MESSAGE_TYPE constants section.
• api-reference/pipecat-subagents/overview.mdx — added LLMContextAgent row to the agent hierarchy and fixed UIAgent's parent.
• NEW: subagents/learn/ui-agent.mdx — consolidated UI Agent guide. Sections: Why, Architecture (voice + UI agent split), How information flows (4 loops), Sequence diagram (voice + UI for action), Action vocabulary (pointing / reading / writing / presentation), Choosing a tool shape (ReplyToolMixin vs custom @tool reply vs per-domain tools), Async tasks and lifecycle (start_user_task_group + useUITasks + cancel), When to use (deployment shapes), Reference app, API reference. Sourced from the internal design doc and the music-player demo.
• subagents/examples/overview.mdx — added a Reference Applications section linking pipecat-music-player.
• docs.json — registered subagents/learn/ui-agent in the navigation.

Client (js + react)

• api-reference/client/js/ui-agent-client.mdx — refreshed to RTVI v1.3 (RTVIEvent.UICommand instead of RTVIEvent.ServerMessage, kebab-case wire types), added addTaskListener / removeTaskListener / removeAllTaskListeners / cancelTask.
• api-reference/client/js/a11y-snapshot.mdx — replaced the dropped UI_SNAPSHOT_EVENT_NAME section with A11ySelection shape docs and a See-also section noting the streamer dispatches via RTVIMessageType.UI_SNAPSHOT.
• api-reference/client/js/client-methods.mdx — added sendRTVIMessage(msgType, data?) section under Messages.
• api-reference/client/react/hooks.mdx — added useStandardSelectTextHandler, useStandardSetInputValueHandler, useStandardClickHandler, useUITasks; updated useStandardCommandHandlers to reflect that it installs all six handlers.
• api-reference/client/react/components.mdx — added UITasksProvider; refreshed UIAgentProvider description for RTVIEvent.UICommand / RTVIEvent.UITask.

Verification

• mint broken-links — no broken links found across the entire docs site.
• npx prettier --write — applied to all touched files.