How to Integrate Fastio API with SvelteKit Applications

SvelteKit developers often need reliable file storage for user uploads, assets, or AI agent outputs. Fastio provides intelligent workspaces with REST API access, AI indexing, and collaboration features.

Direct integration reduces server load. Instead of proxying large files, generate presigned upload sessions server-side. Clients stream chunks in parallel, and Fastio assembles them.

SvelteKit shines here. Form actions run on the server, parsing FormData securely. Endpoints (+server.ts) list files or poll status. SSR ensures fast initial loads.

Compared to Next.js guides, this is tailored for SvelteKit routing and env handling. See Fastio docs at https://docs.fast.io/ for full API reference.

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More on this subject: Agent Integrations and APIs (96 guides)

Sign up for Fastio's 14-day Business Trial at fast.io/pricing (card required).

Generate an API key: Settings > API Keys. Store it server-only in .env:

FASTIO_API_KEY=sk_...

Create a SvelteKit app:

npm create svelte@latest my-fastio-app
cd my-fastio-app
npm install
npm run dev

Import keys with $env/static/private:

import { FASTIO_API_KEY } from '$env/static/private';

Test connectivity by listing workspaces in a load function.

Create src/lib/fastio.ts for API calls. Use Bearer tokens; keys don't expire until revoked.

import { FASTIO_API_KEY } from '$env/static/private';
import type { RequestInit } from 'undici';

export async function apiFetch(endpoint: string, options: RequestInit = {}) {
  const url = `https://api.fast.io/current/${endpoint}`;
  const headers = new Headers({
    'Authorization': `Bearer ${FASTIO_API_KEY}`,
    'Content-Type': 'application/x-www-form-urlencoded',
    ...options.headers as HeadersInit
  });

const res = await fetch(url, { ...options, headers });
  if (!res.ok) {
    const err = await res.text();
    throw new Error(`Fastio API error: ${res.status} - ${err}`);
  }
  return res.json() as Promise<any>;
}

Usage in src/routes/+page.server.ts:

import { apiFetch } from '$lib/fastio';
import type { PageServerLoad } from './$types';

export const load: PageServerLoad = async () => {
  const workspaces = await apiFetch('workspaces/');
  return { workspaces };
};

This fetches your workspaces. Handle pagination with limit and offset params.

Add Secure File Storage to Your SvelteKit App

Start with the 14-day Business Trial (card required; see /pricing/) for instant API access, uploads, and workspace management.

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Build upload UI in src/routes/upload/+page.svelte:

<script lang="ts">
  import { enhance } from '$app/forms';
  let formData = $state({});
</script>

<form method="POST" enctype="multipart/form-data" use:enhance={[{}, (data) => { formData = data; }]}>
  <input name="file" type="file" multiple accept="*/*" />
  <button type="submit">Upload Files</button>
</form>

{#if formData.success}
  <p class="success">Uploaded: {formData.filename} (ID: {formData.fileId})</p>
{/if}
{#if formData.error}
  <p class="error">{formData.error}</p>
{/if}

Handle in +page.server.ts:

import { apiFetch } from '$lib/fastio';
import { error, fail } from '@sveltejs/kit';
import type { Actions } from './$types';

const CHUNK_SIZE = 5 * 1024 * 1024; // 5MB chunks

export const actions: Actions = {
  default: async ({ request }) => {
    const data = await request.formData();
    const files = data.getAll('file') as File[];

for (const file of files) {
      if (!file.size) continue;

try {
        // Create upload session in specific workspace
        const session = await apiFetch('upload/', {
          method: 'POST',
          body: new URLSearchParams({
            name: file.name.
            size: file.size.toString(),
            action: 'create',
            instance_id: 'your_workspace_id', // Replace with dynamic ID
            folder_id: 'root'
          })
        });

const uploadId = session.response.id;

// Stream and upload chunks
        const reader = file.stream().getReader();
        let offset = 0;
        while (true) {
          const { done, value } = await reader.read();
          if (done) break;

const chunk = value.slice(0, CHUNK_SIZE);
          await apiFetch(`upload/${uploadId}/chunk/`, {
            method: 'POST',
            body: new FormData().append('chunk', new File([chunk], 'chunk'), `${offset}`)
          });
          offset += chunk.length;
        }

// Complete upload
        await apiFetch(`upload/${uploadId}/complete/`, { method: 'POST' });

// Poll until stored
        let status;
        do {
          status = await apiFetch(`upload/${uploadId}/details/?wait=10`);
          await new Promise(r => setTimeout(r, 500));
        } while (status.response.session.status !== 'stored');

return { success: true, filename: file.name, fileId: status.response.session.new_file_id };
      } catch (e) {
        return fail(500, { error: `Upload failed: ${(e as Error).message}` });
      }
    }
  }
};

This processes multiple files, chunks large ones, and polls status. Server-side keeps keys safe.

// stores.ts
export const uploadProgress = writable(0);

Create src/routes/api/files/[workspaceId]/[folderId]?/+server.ts:

import { json } from '@sveltejs/kit';
import { apiFetch } from '$lib/fastio';
import type { RequestHandler } from './$types';

export const GET: RequestHandler = async ({ params, url }) => {
  const { workspaceId, folderId = 'root' } = params;
  const limit = Number(url.searchParams.get('limit') ?? 100);

const files = await apiFetch(`workspace/${workspaceId}/storage/${folderId}/?limit=${limit}`);
  return json(files);
};

Client fetch: /api/files/my-workspace/root. Supports sorting, cursors for pagination.

For downloads, create a durable File Share: POST /current/workspace/{workspace_id}/create/fileshare/.

Fastio errors return {"result": false, "error": {...}}. Check result === true.

Common issues:

• 413: File too large. Use chunked uploads for larger files.
• 429: Rate limit. Backoff with X-Rate-Limit-Available.
• 401: Invalid key. Rotate via UI.
• Assembly failed: Poll /upload/{id}/details/?wait=10 longer.

Best practices:

Test edge cases: 0-byte files, interruptions (resume chunks via offset).

Deploy to Vercel: Serverless form actions work; ephemeral env ok with secrets.

For resumable, track uploaded chunks via /upload/{id}/details/, re-upload missing order.

Integrate Svelte stores for multi-file progress:

// Global progress across uploads
const globalProgress = derived([uploadProgress, totalSize], ([p, t]) => (p / t) * 100);

Use Fastio AI: Post-upload, query files with /ai/chat/ endpoints.

Deployment: Vercel/Netlify adapters handle serverless. Static private env for keys.