---
title: "Building a Chat UI"
description: "Design a client that renders universal session events consistently across providers."
---

This guide explains how to build a chat UI that works across all agents using the universal event
stream.

## High-level flow

1. List agents and read their capabilities.
2. Create a session for the selected agent.
3. Send user messages.
4. Subscribe to events (polling or SSE).
5. Render items and deltas into a stable message timeline.

## Use agent capabilities

Capabilities tell you which features are supported for the selected agent:

- `tool_calls` and `tool_results` indicate tool execution events.
- `questions` and `permissions` indicate HITL flows.
- `plan_mode` indicates that the agent supports plan-only execution.
- `reasoning` and `status` indicate that the agent can emit reasoning/status content parts.

Use these to enable or disable UI affordances (tool panels, approval buttons, etc.).

## Event model

Every event includes:

- `event_id`, `sequence`, and `time` for ordering.
- `session_id` for the universal session.
- `native_session_id` for provider-specific debugging.
- `type` with one of:
  - `session.started`, `session.ended`
  - `item.started`, `item.delta`, `item.completed`
  - `permission.requested`, `permission.resolved`
  - `question.requested`, `question.resolved`
  - `error`, `agent.unparsed`
- `data` which holds the payload for the event type.
- `synthetic` and `source` to show daemon-generated events.
- `raw` (optional) when `include_raw=true`.

## Rendering items

Items are emitted in three phases:

- `item.started`: first snapshot of a message or tool item.
- `item.delta`: incremental updates (token streaming or synthetic deltas).
- `item.completed`: final snapshot.

Recommended render flow:

```ts
type ItemState = {
  item: UniversalItem;
  deltas: string[];
};

const items = new Map<string, ItemState>();
const order: string[] = [];

function applyEvent(event: UniversalEvent) {
  if (event.type === "item.started") {
    const item = event.data.item;
    items.set(item.item_id, { item, deltas: [] });
    order.push(item.item_id);
  }

  if (event.type === "item.delta") {
    const { item_id, delta } = event.data;
    const state = items.get(item_id);
    if (state) {
      state.deltas.push(delta);
    }
  }

  if (event.type === "item.completed") {
    const item = event.data.item;
    const state = items.get(item.item_id);
    if (state) {
      state.item = item;
    }
  }
}
```

When rendering, combine the item content with accumulated deltas. If you receive a delta before a
started event (should not happen), treat it as an error.

## Content parts

Each `UniversalItem` has `content` parts. Your UI can branch on `part.type`:

- `text` for normal chat text.
- `tool_call` and `tool_result` for tool execution.
- `file_ref` for file read/write/patch previews.
- `reasoning` if you display public reasoning text.
- `status` for progress updates.
- `image` for image outputs.

Treat `item.kind` as the primary layout decision (message vs tool call vs system), and use content
parts for the detailed rendering.

## Questions and permissions

Question and permission events are out-of-band from item flow. Render them as modal or inline UI
blocks that must be resolved via:

- `POST /v1/sessions/{session_id}/questions/{question_id}/reply`
- `POST /v1/sessions/{session_id}/questions/{question_id}/reject`
- `POST /v1/sessions/{session_id}/permissions/{permission_id}/reply`

If an agent does not advertise these capabilities, keep those UI controls hidden.

## Error and unparsed events

- `error` events are structured failures from the daemon or agent.
- `agent.unparsed` indicates the provider emitted something the converter could not parse.

Treat `agent.unparsed` as a hard failure in development so you can fix converters quickly.

## Event ordering

Prefer `sequence` for ordering. It is monotonic for a given session. The `time` field is for
timestamps, not ordering.

## Handling session end

`session.ended` includes the reason and who terminated it. Disable input after a terminal event.

## Optional raw payloads

If you need provider-level debugging, pass `include_raw=true` when streaming or polling events
(including one-turn streams) to receive the `raw` payload for each event.

## SSE vs polling vs turn streaming

- SSE gives low-latency updates and simplifies streaming UIs.
- Polling is simpler to debug and works in any environment.
- Turn streaming (`POST /v1/sessions/{session_id}/messages/stream`) is a one-shot stream tied to a
  single prompt. The stream closes automatically once the turn completes.

Both yield the same event payloads.

## Mock agent for UI testing

Use the built-in `mock` agent to exercise UI behaviors without external credentials:

```bash
curl -X POST http://127.0.0.1:2468/v1/sessions/demo-session \
  -H "content-type: application/json" \
  -d '{"agent":"mock"}'
```

The mock agent sends a prompt telling you what commands it accepts. Send messages like `demo`,
`markdown`, or `permission` to emit specific event sequences. Any other text is echoed back as an
assistant message so you can test rendering, streaming, and approval flows on demand.

## Reference implementation

The [Inspector chat UI](https://github.com/rivet-dev/sandbox-agent/blob/main/frontend/packages/inspector/src/App.tsx)
is a complete reference implementation showing how to build a chat interface using the universal event
stream. It demonstrates session management, event rendering, item lifecycle handling, and HITL approval
flows.