Recipes

Predictability

An expression has no clock and no random numbers. Without host functions, the same scopes give the same result: in the browser and on the server, today and next year. Two results still depend on the machine:

• The locale methods (toLocaleString, toLocaleUpperCase, toLocaleLowerCase, localeCompare) follow its default locale when the expression passes none.
• Date.parse reads a date-time without an offset, such as "2026-09-30T10:00", in its time zone.

A random value and the time come from host functions. The prompt tells the model to call one in a step and store the result, so the value is taken once, and the elements that read it re-run only when it changes.

Random values

import { standardTool } from "standard-tool";
import z from "zod";
 
export const random = standardTool({
  name: "random",
  description: "A random number from 0 up to, not including, 1.",
  outputSchema: z.number(),
  execute: () => Math.random(),
});

Bind it like any other host function, in the evaluator and in the prompt:

const evaluator = new Evaluator({ functions: [random, ...tools] });
const functionsPrompt = getFunctionsPartialPrompt({ functions: [random, ...tools] });

A tip picked once, when the element mounts:

{ "key": "tip", "component": "Typography",
  "seed": [{ "set": "scopes.root.pick", "expr": "random()" }],
  "props": { "expr": "({ text: scopes.root.tips[Math.floor(scopes.root.pick * scopes.root.tips.length)] })" } }

The time

export const now = standardTool({
  name: "now",
  description: "The current time: milliseconds since 1970-01-01 UTC.",
  outputSchema: z.number(),
  execute: () => Date.now(),
});

An order’s age in days, from the time taken when the element mounts:

{ "key": "age", "component": "Badge",
  "seed": [{ "set": "scopes.root.now", "expr": "now()" }],
  "props": { "expr": "({ text: Math.round((scopes.root.now - Date.parse(scopes.root.order.createdAt)) / 86400000) + 'd' })" } }

The stored time does not move. A display that counts (“3 minutes ago”) belongs in a component that updates itself, such as the reference catalog’s DateTime with format: "relative".