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Fleet Pi can stream generative UI directly into chat messages. The agent emits a compact, line-oriented language called OpenUI Lang inside a fenced openui block, and Fleet Pi parses and renders it progressively as tokens arrive. Use generative UI when a card, table, chart, metric, or action row communicates more clearly than prose — for example, dashboards, status summaries, comparisons, or “what would you like to do next?” prompts that route back into the conversation.

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When it applies

Generative UI is on by default in Agent and Plan modes. The system prompt for each mode includes the OpenUI component contract, so any Pi chat response can return inline UI without extra configuration.
ModeOpenUI availableTypical use
AgentYesDashboards, status cards, metrics, tables, conversational actions
PlanYes (visual aids only)Optional summary cards alongside the Markdown numbered plan
HarnessNoWorkspace-architecture work stays in Markdown
In Plan mode the numbered plan stays in Markdown — OpenUI is reserved for compact visual summaries and decision aids, and Button actions are disabled.

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How it works

OpenUI Lang is a declarative DSL designed for LLMs. Each line assigns to an identifier, and the first line must assign to root. The renderer parses lines as they stream and shows skeletons for any forward references until they arrive. 
Component library  →  System prompt  →  LLM stream  →  Parser  →  Inline renderer
Fleet Pi defines a fixed component library (the contract between the app and the model). The agent can only emit components from that library, with positional arguments. Anything the agent generates outside the contract is rejected and surfaced as a render error inline.

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Available components

The Fleet Pi component library covers layout, content, status, data, and one safe interactive primitive.
CategoryComponents
LayoutRoot, Stack, Group, Grid, Divider
ContentHeading, Text, CodeBlock, List
StatusBadge, Callout, Metric, ProgressBar
DataCard, KeyValue, Table, BarChart
InputInput (display-only), Button (conversational)
Button is the only action primitive. Clicking a button sends its message (or its label) back into the chat as a new user turn — there are no destructive actions, URL opens, network requests, or tool calls. Buttons are disabled in Plan mode.

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Example

A status summary the agent might stream: 
Here's where we are:

```openui
root = Root([summary])
summary = Card("OpenUI status", Stack([ok, next]))
ok = Badge("Renderer wired", "success")
next = Text("Next: validate streamed fenced blocks.", "muted")
```
A conversational action row that routes the user’s choice back into chat: 
```openui
root = Root([card])
card = Card("Continue?", Stack([body, actions]))
body = Text("I can explain the implementation or run validation next.")
actions = Group([explain, validate])
explain = Button("Explain implementation", "Tell me how this OpenUI integration works", "outline")
validate = Button("Run validation", "Run the OpenUI validation checks", "default")
```
When the user clicks Run validation, Fleet Pi sends "Run the OpenUI validation checks" as a normal chat message.

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Authoring rules the agent follows

Fleet Pi’s system prompt enforces these constraints on every response:
  • Wrap each program in a fenced ```openui block. The rest of the response stays in Markdown.
  • Every program starts with root = Root(...).
  • Use only components from the library — invented names or named arguments are rejected.
  • Arguments are positional. Write Card("Title", child), not Card(title: "Title", content: child).
  • Keep blocks chat-sized; prefer Markdown for long prose, explanations, or raw code.
  • In Plan mode, never emit Button components.

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Error handling

If the model emits invalid OpenUI Lang — unknown components, unresolved references, or schema violations — Fleet Pi renders an inline diagnostic panel under the message with the parser errors and the raw output, so you can see exactly what the model produced and why it failed. Streaming responses suppress unresolved-reference errors until the stream completes.

Chat modes

How Agent, Plan, and Harness modes change tool allowlists and OpenUI rules.

OpenUI Lang reference

Full OpenUI Lang language specification from Thesys.