# Prompt to Add Cloudflare R2 File Upload

> Copy-paste AI prompt to add Cloudflare R2 presigned upload URLs to a Next.js App Router project with server-side validation.

**Type:** Prompt  
**Tools:** Cursor, Claude Code, Codex, Windsurf  
**Stack:** Next.js, Cloudflare, TypeScript  
**Difficulty:** medium  
**Updated:** 2026-06-08

---

Give this prompt to your agent to implement direct-to-R2 file uploads using presigned
URLs — keeping secrets server-side and avoiding the common mistake of routing file
bytes through your Next.js server.

## Main Prompt

```txt title="Main Prompt"
You are working in a Next.js App Router project using TypeScript.

Task: add Cloudflare R2 file upload via presigned PUT URLs.

Requirements:
- Install `@aws-sdk/client-s3` and `@aws-sdk/s3-request-presigner` (R2 is S3-compatible).
- Create a Server Action in `src/lib/actions/get-upload-url.ts` that:
  - Accepts `{ filename: string; contentType: string; sizeBytes: number }`.
  - Validates: `sizeBytes` must be <= 10_485_760 (10 MB); `contentType` must start with `image/`.
  - Uses `PutObjectCommand` + `getSignedUrl` to generate a 60-second presigned URL.
  - Returns `{ uploadUrl: string; publicUrl: string; key: string }`.
- Store credentials in `.env`: `R2_ACCOUNT_ID`, `R2_ACCESS_KEY_ID`, `R2_SECRET_ACCESS_KEY`,
  `R2_BUCKET_NAME`, `R2_PUBLIC_URL`.
- Create a client component `src/components/FileUploader.tsx` that:
  - Renders a file `<input accept="image/*" />`.
  - On change: calls the Server Action to get the presigned URL, then does a `fetch` PUT directly
    to R2, then shows the `publicUrl` as a preview.
  - Shows upload progress via `XMLHttpRequest` `progress` events (not fetch, which lacks progress).
- Do not add any other dependencies. Do not modify `next.config.ts` unless required for image domains.

Stop and list all planned file changes before writing any code.
```

## Implementation Notes

- R2 uses the S3 SDK with `endpoint: https://<accountId>.r2.cloudflarestorage.com` and
  `region: 'auto'`. Forgetting `region: 'auto'` causes a signature error.
- Presigned URLs are generated server-side; the client never sees `R2_SECRET_ACCESS_KEY`.
- For public bucket access, set the R2 bucket's "Public access" toggle in the Cloudflare dashboard
  and supply the resulting `r2.dev` URL as `R2_PUBLIC_URL`.

## Expected File Changes

```txt
src/lib/actions/get-upload-url.ts      (new)
src/lib/r2.ts                          (new — S3Client singleton)
src/components/FileUploader.tsx        (new — Client Component)
.env.example                           (edited)
package.json                           (edited)
next.config.ts                         (edited — add r2.dev to images.remotePatterns)
```

## Acceptance Criteria

- A file selected in `FileUploader` triggers a presigned URL fetch, then uploads directly to R2.
- Files larger than 10 MB are rejected before any network request.
- Non-image MIME types are rejected server-side and return a 400-equivalent error.
- The public URL preview renders the uploaded image after upload completes.

## Test Commands

```bash
bun run typecheck
bun run dev
# upload a PNG < 10 MB via the component and confirm it appears in R2 bucket
# attempt a 15 MB file and confirm rejection
# attempt a .pdf and confirm rejection
```

## Common AI Mistakes

- Setting `region: 'us-east-1'` instead of `'auto'`, causing `SignatureDoesNotMatch` errors.
- Routing the file bytes through the Next.js Server Action instead of fetching directly to R2.
- Forgetting to call `'use server'` at the top of the actions file.
- Using `fetch` for the upload and losing progress events.

## Fix Prompt

```txt title="Fix Prompt"
The upload fails with `SignatureDoesNotMatch` or the file routes through the server.
Fix in order:
1. In `src/lib/r2.ts`, set `region: 'auto'` and `endpoint: https://${R2_ACCOUNT_ID}.r2.cloudflarestorage.com`.
2. In `FileUploader.tsx`, use `XMLHttpRequest` with a `progress` event to PUT directly to the
   presigned URL — not fetch, and not a Server Action for the actual upload bytes.
Show corrected diff only.
```