UI Selection

Choose which Angular components to show for an input, or narrow a chat, with a decision.

1. Describe each option in plain language.
2. The uiDecisionResource resource asks a decision model which option fits the input.
3. Render only the options it chooses, or let a chat answer with only the components it chooses.

Two Modes

uiDecisionResource has two modes. Which one you get depends on the options you pass.

Both modes take these options:

The resource requires provideHashbrown and an injection context. Set up the decision endpoint as described in Decisions.

Candidates Mode

Use candidates when your code already knows what each option shows. The model never writes props. It only picks.

Candidates mode also takes these options:

import { Component, computed, input } from '@angular/core';
import { UiDecisionComponent, uiDecisionResource } from '@hashbrownai/angular';

@Component({
  selector: 'app-answer',
  imports: [UiDecisionComponent],
  template: `<hb-ui-decision [resource]="answer" />`,
})
export class AnswerComponent {
  readonly query = input('');
  readonly callsign = input('UAL123');

  readonly answer = uiDecisionResource({
    input: computed(() => (this.query() === '' ? null : this.query())),
    select: 'one',
    minConfidence: 0.7,
    candidates: [
      {
        id: 'flight',
        description: 'The user asks about one aircraft',
        component: FlightCardComponent,
        inputs: computed(() => ({ callsign: this.callsign() })),
      },
      {
        id: 'airport',
        description: 'The user asks about an airport',
        component: AirportCardComponent,
      },
    ],
  });
}

1. Each candidate has a unique id that is not empty and has no ., a description that tells the model when it fits, a component, and optional inputs. inputs is a plain object or a signal of one. A signal keeps the rendered component in step with app state: when it changes, the chosen component re-renders with the new inputs and no new decision is made. The id and description stay fixed.
2. <hb-ui-decision [resource]> renders the chosen components in order. ids() and chosen() hold what was picked.
3. Candidates mode renders nothing while the decision is pending, when there are no candidates, and when confidence is below minConfidence.

With select: 'one', confidence is the model's confidence in its pick. With { max }, each candidate is a yes or no question. Candidates at 0.5 probability or higher are kept, sorted by probability, and capped at max. minConfidence is compared to the best probability.

Narrowing a Chat

Use chat mode when a chat writes the answer and you want each answer to use only the components that fit the message. The app keeps its own chat, with its own system prompt and tools, and sends messages through the resource instead. A decision first picks from the chat's components. Then the chat answers, seeing only the chosen components. This keeps the prompt small and the choices focused.

import { Component, inject } from '@angular/core';
import { s } from '@hashbrownai/core';
import {
  createTool,
  exposeComponent,
  RenderMessageComponent,
  uiChatResource,
  uiDecisionResource,
} from '@hashbrownai/angular';

@Component({
  selector: 'app-assistant',
  imports: [RenderMessageComponent, ComposerComponent],
  template: `
    @for (message of chat.value(); track $index) {
      @if (message.role === 'assistant') {
        <hb-render-message [message]="message" />
      } @else if (message.role === 'user') {
        <p>{{ message.content }}</p>
      }
    }
    <app-composer
      [disabled]="narrowing.isDeciding()"
      (send)="narrowing.sendMessage($event)"
    />
  `,
})
export class AssistantComponent {
  // 1. The app's own chat: its system prompt, components and tools.
  readonly chat = uiChatResource({
    system:
      'Answer questions about live flights. Use the cards to show results.',
    tools: [
      createTool({
        name: 'getFlights',
        description: 'Get the flights near an airport',
        schema: s.object('Airport', {
          ident: s.string('The airport identifier'),
        }),
        handler: ({ ident }) => inject(FlightService).near(ident),
      }),
    ],
    components: [
      exposeComponent(FlightCardComponent, {
        name: 'FlightCard',
        description: 'Shows one aircraft',
        input: { callsign: s.string('The aircraft callsign') },
      }),
      exposeComponent(AirportCardComponent, {
        name: 'AirportCard',
        description: 'Shows an airport',
        input: { ident: s.string('The airport identifier') },
      }),
    ],
  });

  // 2. Narrows the chat's components for each message.
  readonly narrowing = uiDecisionResource({
    chat: this.chat,
    select: { max: 2 },
    minConfidence: 0.7,
    system: 'A flight-tracking app. The input is a chat message.',
  });
}

1. chat is the ref from uiChatResource. The resource needs only its components signal, setComponents, sendMessage and isLoading signal, described by the UiDecisionChat type.
2. sendMessage(content) asks the decision about content, sets the chat's components to the chosen ones, and then sends content to the chat. The send always uses the narrowed components.
3. Render the chat as usual, with <hb-render-message>. <hb-ui-decision> is only for candidates mode. The transcript, tools, isLoading() and stop() stay on the chat. The resource returns only sendMessage, decision, isDeciding() and decisionError().
4. The decision sees each component's name and description. UI kits are decided over by member: each kit is expanded into its components, including the children they declare, and duplicate names count once.
5. A chosen component brings the components declared in its children with it, recursively, so the chat can always render a chosen parent's children. The chat sees the chosen components in the order they were declared.
6. Component names become option ids, so the flattened list must fit the selection limits: no name may be empty, contain ., or be __proto__, constructor or prototype, and select: 'one' allows at most 255 components.

Always-on components

Some components, such as Markdown for prose, should be available on every turn. Put them in a kit created with selectable: false, next to the kit the decision picks from.

const cards = createUiKit({ components: [flightCard, airportCard] });
const prose = createUiKit({
  components: [exposeMarkdown({ citations: true })],
  selectable: false,
});

const chat = uiChatResource({
  system: 'Answer questions about live flights.',
  components: [cards, prose],
});
const narrowing = uiDecisionResource({ chat, select: { max: 2 } });

The decision is asked only about cards: FlightCard and AirportCard. Markdown is never an option, and it is always sent with whatever is picked. If every component is non-selectable, there is nothing to decide and the message is sent with all of them.

Two prompts

Chat mode uses two models, and each one has its own prompt. Neither prompt defaults to the other.

The resource's system is context for the decision. It is sent with each message, so describe the situation rather than give instructions. The chat never sees it, and the decision never sees the chat's system prompt or tools.

Fallback

Decide when sure, offer everything otherwise. The message is sent with all of the chat's components when:

• confidence is below minConfidence;
• the decision fails, or decision.stop() aborts it (stopping never cancels the send);
• the components break the selection limits above (no decision is asked).

Failures are exposed as decisionError(). A DecisionSchemaError there means the components could not be decided over. isDeciding() is true while a decision is in flight.

Restoring after the turn

When the narrowed turn finishes, errors or is stopped, the resource sets the chat's components back to the full list. The same happens when the chat never starts the turn, and when the resource's injection context is destroyed while the chat is narrowed. A follow-up sent directly with chat.sendMessage sees every component. If your app replaces the chat's components during the turn, the resource leaves your list in place.

A UI chat keeps every component it has ever offered, one per name, and the newest definition of a name replaces older ones. Earlier answers render with that set, so they keep rendering after a narrowed turn or a setComponents call. New responses are parsed against it too, so a provider that does not enforce the response schema strictly can still render a component that is no longer offered.

Errors

sendMessage throws at once, as the chat's own sendMessage would, when the chat cannot take a message, for example while an interrupt batch is pending. If the chat refuses the message after the decision, the resource restores every component and exposes the error as decisionError().

Concurrency

Calling sendMessage while a decision is still in flight aborts that decision and starts a new one, so only the latest message is sent. Once a message is sent, the chat owns it: sending while the chat is still generating behaves exactly like the chat's own sendMessage.

Pinning the Questions on the Server

The browser builds the selection questions from your candidates or the chat's components. A public decision route answers whatever questions it is sent, unless the server pins them. To pin them, build the same schema on the server with createSelectionSchema from @hashbrownai/core, and pass it to the adapter's schema option. Pass the same select value the browser uses.

createSelectionSchema takes the same list the resource decides over, in the same order: your candidates, or the components you pass to uiChatResource. Each entry can be:

• a candidate item, { id, description }, as in candidates mode;
• a component, such as an exposeComponent result or a component definition (UiComponentDefinition) with a name, a description and optional children;
• a UI kit.

UI kits and declared children are flattened exactly as the resource flattens them: each kit's members in place, each component followed by its declared children, and every name once. An entry with an id is always a candidate item, even if it also has a name; an entry with a name and no id is a component, and its option id is its name. So a chat component must not carry an id at the top level, or createSelectionSchema treats it as a candidate item and the pinned questions differ. Kit members and declared children are always components.

import { createSelectionSchema } from '@hashbrownai/core';
import { HashbrownTypeSafe } from '@hashbrownai/typesafe';
// Shared with the browser: the components and kits passed to uiChatResource.
import { flightCard, cardsKit } from './cards';

const decision = await HashbrownTypeSafe.decide({
  apiKey: process.env.TYPESAFE_API_KEY!,
  model: 'jev-1.14.0',
  schema: createSelectionSchema([flightCard, cardsKit], { max: 2 }),
  request,
});

createSelectionSchema skips non-selectable members, so pass the same kits the browser passes. To pin the questions without sharing the kits, use a plain group, { components, selectable? }. A group is treated like a kit: its members are flattened in place, and with selectable: false they are never asked about.

const schema = createSelectionSchema(
  [
    flightCard,
    airportCard,
    { components: [markdownDefinition], selectable: false },
  ],
  { max: 2 },
);

A group is recognized only when its keys are exactly components and, optionally, selectable.

For candidates mode, pass the candidates (or just their { id, description } items), such as createSelectionSchema(cards, 'one'). If the server can't import your components or kits, pass { name, description, children? } definitions with the same names, descriptions and children, in the same order.

A pinned schema replaces the questions the browser sends, so optionOrder: 'shuffled' has no effect once the questions are pinned. The server always asks in the order it built.

Writing Good Descriptions

• Describe when an option fits, not what it is. "The user asks about an airport" works better than "Airport card".
• Keep descriptions distinct. Overlapping options split confidence between them.
• In chat mode, child components are options too, and select: 'one' can pick a child alone. Say where a child belongs in its description, such as "A row, only inside a Table".
• Add an option for "none of these" when it makes sense. In candidates mode, simply render nothing when confidence is low.
• Pass short, plain input. See the model limits in Decisions.

Next Steps

• Decisions

  Ask typed closed questions with decisionResource.

• Serve decisions with TypeSafe

  Add the server route that answers decision requests.