Skills
skills/jezweb/claude-skills/vercel-blob

vercel-blob

SKILL.md

Vercel Blob

Last Updated: 2026-01-21 Version: @vercel/blob@2.0.0 Skill Version: 2.1.0

Quick Start

# Create Blob store: Vercel Dashboard → Storage → Blob
vercel env pull .env.local  # Creates BLOB_READ_WRITE_TOKEN
npm install @vercel/blob

Server Upload:

'use server';
import { put } from '@vercel/blob';

export async function uploadFile(formData: FormData) {
  const file = formData.get('file') as File;
  const blob = await put(file.name, file, { access: 'public' });
  return blob.url;
}

CRITICAL: Never expose BLOB_READ_WRITE_TOKEN to client. Use handleUpload() for client uploads.

Client Upload (Secure)

Server Action (generates presigned token):

'use server';
import { handleUpload } from '@vercel/blob/client';

export async function getUploadToken(filename: string) {
  return await handleUpload({
    body: {
      type: 'blob.generate-client-token',
      payload: { pathname: `uploads/${filename}`, access: 'public' }
    },
    request: new Request('https://dummy'),
    onBeforeGenerateToken: async (pathname) => ({
      allowedContentTypes: ['image/jpeg', 'image/png'],
      maximumSizeInBytes: 5 * 1024 * 1024
    })
  });
}

Client Component:

'use client';
import { upload } from '@vercel/blob/client';

const tokenResponse = await getUploadToken(file.name);
const blob = await upload(file.name, file, {
  access: 'public',
  handleUploadUrl: tokenResponse.url
});

File Management

List/Delete:

import { list, del } from '@vercel/blob';

// List with pagination
const { blobs, cursor } = await list({ prefix: 'uploads/', cursor });

// Delete
await del(blobUrl);

Multipart (>500MB):

import { createMultipartUpload, uploadPart, completeMultipartUpload } from '@vercel/blob';

const upload = await createMultipartUpload('large-video.mp4', { access: 'public' });
// Upload chunks in loop...
await completeMultipartUpload({ uploadId: upload.uploadId, parts });

Critical Rules

Always:

  • ✅ Use handleUpload() for client uploads (never expose BLOB_READ_WRITE_TOKEN)
  • ✅ Validate file type/size before upload
  • ✅ Use pathname organization (avatars/, uploads/)
  • ✅ Add timestamp/UUID to filenames (avoid collisions)

Never:

  • ❌ Expose BLOB_READ_WRITE_TOKEN to client
  • ❌ Upload >500MB without multipart
  • ❌ Skip file validation

Known Issues Prevention

This skill prevents 16 documented issues:

Issue #1: Missing Environment Variable

Error: Error: BLOB_READ_WRITE_TOKEN is not defined Source: https://vercel.com/docs/storage/vercel-blob Why It Happens: Token not set in environment Prevention: Run vercel env pull .env.local and ensure .env.local in .gitignore.

Issue #2: Client Upload Token Exposed

Error: Security vulnerability, unauthorized uploads Source: https://vercel.com/docs/storage/vercel-blob/client-upload Why It Happens: Using BLOB_READ_WRITE_TOKEN directly in client code Prevention: Use handleUpload() to generate client-specific tokens with constraints.

Issue #3: File Size Limit Exceeded

Error: Error: File size exceeds limit (500MB) Source: https://vercel.com/docs/storage/vercel-blob/limits Why It Happens: Uploading file >500MB without multipart upload Prevention: Validate file size before upload, use multipart upload for large files.

Issue #4: Wrong Content-Type

Error: Browser downloads file instead of displaying (e.g., PDF opens as text) Source: Production debugging Why It Happens: Not setting contentType option, Blob guesses incorrectly Prevention: Always set contentType: file.type or explicit MIME type.

Issue #5: Public File Not Cached

Error: Slow file delivery, high egress costs Source: Vercel Blob best practices Why It Happens: Using access: 'private' for files that should be public Prevention: Use access: 'public' for publicly accessible files (CDN caching).

Issue #6: List Pagination Not Handled

Error: Only first 1000 files returned, missing files Source: https://vercel.com/docs/storage/vercel-blob/using-blob-sdk#list Why It Happens: Not iterating with cursor for large file lists Prevention: Use cursor-based pagination in loop until cursor is undefined.

Issue #7: Delete Fails Silently

Error: Files not deleted, storage quota fills up Source: https://github.com/vercel/storage/issues/150 Why It Happens: Using wrong URL format, blob not found Prevention: Use full blob URL from put() response, check deletion result.

Issue #8: Upload Timeout (Large Files) + Server-Side 4.5MB Limit

Error: Error: Request timeout for files >100MB (server) OR file upload fails at 4.5MB (serverless function limit) Source: Vercel function timeout limits + 4.5MB serverless limit + Community Discussion Why It Happens:

  • Serverless function timeout (10s free tier, 60s pro) for server-side uploads
  • CRITICAL: Vercel serverless functions have a hard 4.5MB request body limit. Using put() in server actions/API routes fails for files >4.5MB.

Prevention: Use client-side upload with handleUpload() for files >4.5MB OR use multipart upload.

// ❌ Server-side upload fails at 4.5MB
export async function POST(request: Request) {
  const formData = await request.formData();
  const file = formData.get('file') as File; // Fails if >4.5MB
  await put(file.name, file, { access: 'public' });
}

// ✅ Client upload bypasses 4.5MB limit (supports up to 500MB)
const blob = await upload(file.name, file, {
  access: 'public',
  handleUploadUrl: '/api/upload/token',
  multipart: true, // For files >500MB, use multipart
});

Issue #9: Filename Collisions

Error: Files overwritten, data loss Source: Production debugging Why It Happens: Using same filename for multiple uploads Prevention: Add timestamp/UUID: `uploads/${Date.now()}-${file.name}` or addRandomSuffix: true.

Issue #10: Missing Upload Callback

Error: Upload completes but app state not updated Source: https://vercel.com/docs/storage/vercel-blob/client-upload#callback-after-upload Why It Happens: Not implementing onUploadCompleted callback Prevention: Use onUploadCompleted in handleUpload() to update database/state.

Issue #11: Client Upload Token Expiration for Large Files

Error: Error: Access denied, please provide a valid token for this resource Source: GitHub Issue #443 Why It Happens: Default token expires after 30 seconds. Large files (>100MB) take longer to upload, causing token expiration before validation. Prevention: Set validUntil parameter for large file uploads.

// For large files (>100MB), extend token expiration
const jsonResponse = await handleUpload({
  body,
  request,
  onBeforeGenerateToken: async (pathname) => {
    return {
      maximumSizeInBytes: 200 * 1024 * 1024,
      validUntil: Date.now() + 300000, // 5 minutes
    };
  },
});

Issue #12: v2.0.0 Breaking Change - onUploadCompleted Requires callbackUrl (Non-Vercel Hosting)

Error: onUploadCompleted callback doesn't fire when not hosted on Vercel Source: Release Notes @vercel/blob@2.0.0 Why It Happens: v2.0.0 removed automatic callback URL inference from client-side location.href for security. When not using Vercel system environment variables, you must explicitly provide callbackUrl. Prevention: Explicitly provide callbackUrl in onBeforeGenerateToken for non-Vercel hosting.

// v2.0.0+ for non-Vercel hosting
await handleUpload({
  body,
  request,
  onBeforeGenerateToken: async (pathname) => {
    return {
      callbackUrl: 'https://example.com', // Required for non-Vercel hosting
    };
  },
  onUploadCompleted: async ({ blob, tokenPayload }) => {
    // Now fires correctly
  },
});

// For local development with ngrok:
// VERCEL_BLOB_CALLBACK_URL=https://abc123.ngrok-free.app

Issue #13: ReadableStream Upload Not Supported in Firefox

Error: Upload never completes in Firefox Source: GitHub Issue #881 Why It Happens: The TypeScript interface accepts ReadableStream as a body type, but Firefox does not support ReadableStream as a fetch body. Prevention: Convert stream to Blob or ArrayBuffer for cross-browser support.

// ❌ Works in Chrome/Edge, hangs in Firefox
const stream = new ReadableStream({ /* ... */ });
await put('file.bin', stream, { access: 'public' }); // Never completes in Firefox

// ✅ Convert stream to Blob for cross-browser support
const chunks: Uint8Array[] = [];
const reader = stream.getReader();
while (true) {
  const { done, value } = await reader.read();
  if (done) break;
  chunks.push(value);
}
const blob = new Blob(chunks);
await put('file.bin', blob, { access: 'public' });

Issue #14: Pathname Cannot Be Modified in onBeforeGenerateToken

Error: File uploaded to wrong path despite server-side pathname override attempt Source: GitHub Issue #863 Why It Happens: The pathname parameter in onBeforeGenerateToken cannot be changed. It's set at upload(pathname, ...) time on the client side. Prevention: Construct pathname on client, validate on server. Use clientPayload to pass metadata.

// Client: Construct pathname before upload
await upload(`uploads/${Date.now()}-${file.name}`, file, {
  access: 'public',
  handleUploadUrl: '/api/upload',
  clientPayload: JSON.stringify({ userId: '123' }),
});

// Server: Validate pathname matches expected pattern
await handleUpload({
  body,
  request,
  onBeforeGenerateToken: async (pathname, clientPayload) => {
    const { userId } = JSON.parse(clientPayload || '{}');

    // Validate pathname starts with expected prefix
    if (!pathname.startsWith(`uploads/`)) {
      throw new Error('Invalid upload path');
    }

    return {
      allowedContentTypes: ['image/jpeg', 'image/png'],
      tokenPayload: JSON.stringify({ userId }), // Pass to onUploadCompleted
    };
  },
});

Issue #15: Multipart Upload Minimum Chunk Size (5MB)

Error: Manual multipart upload fails with small chunks Source: Official Docs + Community Discussion Why It Happens: Each part in manual multipart upload must be at least 5MB (except the last part). This conflicts with Vercel's 4.5MB serverless function limit, making manual multipart uploads impossible via server-side routes. Prevention: Use automatic multipart (multipart: true in put()) or client uploads.

// ❌ Manual multipart upload fails (can't upload 5MB chunks via serverless function)
const upload = await createMultipartUpload('large.mp4', { access: 'public' });
// uploadPart() requires 5MB minimum - hits serverless limit

// ✅ Use automatic multipart via client upload
await upload('large.mp4', file, {
  access: 'public',
  handleUploadUrl: '/api/upload',
  multipart: true, // Automatically handles 5MB+ chunks
});

Issue #16: Missing File Extension Causes Access Denied Error

Error: Error: Access denied, please provide a valid token for this resource Source: GitHub Issue #664 Why It Happens: Pathname without file extension causes non-descriptive access denied error. Prevention: Always include file extension in pathname.

// ❌ Fails with confusing error
await upload('user-12345', file, {
  access: 'public',
  handleUploadUrl: '/api/upload',
}); // Error: Access denied

// ✅ Extract extension and include in pathname
const extension = file.name.split('.').pop();
await upload(`user-${userId}.${extension}`, file, {
  access: 'public',
  handleUploadUrl: '/api/upload',
});

Common Patterns

Avatar Upload with Replacement:

'use server';
import { put, del } from '@vercel/blob';

export async function updateAvatar(userId: string, formData: FormData) {
  const file = formData.get('avatar') as File;
  if (!file.type.startsWith('image/')) throw new Error('Only images allowed');

  const user = await db.query.users.findFirst({ where: eq(users.id, userId) });
  if (user?.avatarUrl) await del(user.avatarUrl); // Delete old

  const blob = await put(`avatars/${userId}.jpg`, file, { access: 'public' });
  await db.update(users).set({ avatarUrl: blob.url }).where(eq(users.id, userId));
  return blob.url;
}

Protected Upload (access: 'private'):

const blob = await put(`documents/${userId}/${file.name}`, file, { access: 'private' });
Weekly Installs
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Repository
jezweb/claude-skills
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