This package holds the shared foundation: the form schema, validation, the conditional-logic engine, response prefill/piping, and a JS client with a resumable upload protocol. It has zero framework dependencies.

Most apps don't install this directly — you install a renderer (@usefillo/react or @usefillo/dom), which re-exports everything here you need for embedding. Reach for @usefillo/core when you're building your own renderer or working with forms on the server.

npm i @usefillo/core

import { createClient, defineForm, validateResponse, visibleBlocks } from "@usefillo/core";

const client = createClient({ key: "pk_live_…" }); // for syncing code-defined forms

const form = defineForm({
  id: "cust-feedback",
  pages: [{ id: "p1", blocks: [
    { id: "email", kind: "email", label: "Work email", required: true },
    { id: "msg", kind: "long_text", label: "What should we know?" },
  ] }],
});

Published renderer embeds can fetch by formId without a key. Use a client object for custom API origins, uploads, or submissions from your own renderer; add a publishable key only when syncing defineForm() schemas from code.

Schema settings include submitMode: "auto" for one-tap forms that should submit after a complete discrete answer instead of rendering the first submit button, and submissionLimit: "once_per_visitor" for browser-scoped duplicate prevention. Themes include colorScheme: "light" | "dark" | "auto" for renderer defaults.

Key exports: createClient / FilloClient, defineForm, validateResponse, validateField, visibleBlocks, and the schema types (FormSchema, Field, FieldKind, FormTheme, ResponseData, …).