Lovable File Upload Fails with 403? Fix Supabase Storage RLS

If a file picker works in your Lovable interface but the upload request returns 403 Forbidden, new row violates row-level security policy, or simply fails after you connected Supabase Storage, do not start by rebuilding the upload component.

The useful question is: which Storage operation is being denied, for which user, on which bucket and object path?

That distinction matters because a Supabase bucket can exist, be visible in the dashboard and even be public for downloads while browser uploads are still correctly denied.

The short answer

Supabase Storage uses Row Level Security on storage.objects to control file operations. According to Supabase’s Storage access-control documentation, Storage does not allow uploads to a bucket until an RLS policy permits the operation.

For a normal new upload, the important permission is usually INSERT.

For an overwrite or upsert, Supabase also requires SELECT and UPDATE permissions.

So a 403 is often not evidence that Lovable generated a broken file picker. It can mean the frontend successfully reached Storage and Storage refused an operation your current policy does not authorize.

Debug the upload as a chain

Treat the workflow as separate boundaries:

selected file → signed-in session → bucket name → object path → Storage request → RLS policy → stored object → returned URL

Find the first boundary that fails.

If you only tell an AI builder “fix file upload”, it may rewrite UI code that was already doing its job.

1. Read the failing network request first

Open the deployed application, reproduce the failure and inspect the browser Network panel.

Look for a request to Supabase Storage, normally containing /storage/v1/.

Record:

• the HTTP status
• the response body
• the bucket being targeted
• the object path
• whether the request is a new upload or overwrite
• whether the user is signed in when it happens

A message mentioning row-level security is a strong signal to inspect authorization before changing the component.

Do not hide the useful error behind a generic “upload failed” toast while debugging.

2. Public bucket does not mean public uploads

This is one of the easiest assumptions to get wrong.

Supabase’s bucket documentation distinguishes public and private buckets. A public bucket makes retrieval of its assets public, but access control still applies to operations such as uploading, deleting, moving and copying.

That means this reasoning is wrong:

The avatar bucket is public, so every user should be able to upload to it.

A better design is:

Avatar images may be publicly readable, but only the correct authenticated user should be able to create or replace their own object.

Read access and write access are different decisions.

3. Check the exact operation your code performs

A simple upload and an upsert are not equivalent from an authorization perspective.

Supabase documents that uploading a new object requires an INSERT policy on storage.objects. If you overwrite with upsert, the operation also needs SELECT and UPDATE permissions.

So if first-time uploads work but replacing a profile photo fails, inspect the uploader for an upsert option before broadening the policy blindly.

Do not respond by creating a policy that allows every authenticated user to update every object in the bucket.

Give the operation only the access it actually needs.

4. Compare the policy with the real bucket and path

Storage policies can be scoped to a bucket and to folders inside the object name.

For example, a policy might intentionally permit an authenticated user to upload only into a folder based on their user ID.

If the policy expects a path like:

<user-id>/avatar.png

but the Lovable-generated code uploads to:

public/avatar.png

the policy can be perfectly valid and the request can still be correctly rejected.

Before editing SQL, compare:

1. the bucket ID in the request
2. the object path generated by the application
3. the bucket and folder conditions in the policy
4. the identity available in the user’s JWT

Fix the mismatch rather than weakening the rule until the error disappears.

5. Confirm the user is actually authenticated at upload time

A policy written to authenticated cannot authorize a request that is effectively anonymous.

If the upload is supposed to happen after login, confirm the production application has restored the user’s session before it sends the Storage request.

This is especially important if you recently changed authentication, custom domains or redirect behavior. A screen can look logged in while the request path is being triggered before session state is ready.

If your app is looping or losing sessions after OAuth, debug that separately with my Lovable login redirect-loop guide.

6. Do not use a privileged key as the browser-side fix

Supabase explicitly documents that service keys bypass Storage RLS. Its S3 credentials with full access are likewise intended for server-side use.

That makes them useful for trusted backend operations and dangerous as a shortcut in frontend code.

If someone suggests pasting a service-role key into the Lovable client because “it makes the upload work”, stop there.

You have not fixed authorization. You have bypassed it.

For user-driven browser uploads, prefer a user-scoped design enforced by RLS. If a privileged server process genuinely needs to perform the operation, keep that credential in trusted server-side code.

This is the same client/server boundary behind my guide on whether a Supabase anon key is safe to expose.

7. Test with two users, not only your admin account

A successful upload proves functionality. It does not prove isolation.

Before launch, create two ordinary test accounts.

User A should be able to upload the files User A is meant to own.

User B should not gain write or delete access to User A’s private objects merely by changing a path or object identifier.

Then test the actions your product actually supports:

• first upload
• replacement/upsert
• download or display
• delete
• listing files, if applicable

This catches the dangerous opposite of a 403: a policy so permissive that the product works for everyone because everyone can access everyone else’s objects.

My broader AI-built app production checklist covers the same principle across authentication, database permissions, payments and deployment.

8. Separate authorization errors from file-size and upload-method problems

Not every failed upload is RLS.

Supabase’s current standard upload guidance recommends standard uploads for smaller files and its resumable upload guidance recommends resumable uploads when files are larger than about 6 MB or network reliability matters.

Bucket configuration can also restrict file size and allowed MIME types.

So classify the failure before fixing it:

The goal is to avoid turning five different failure modes into one vague “Supabase upload problem”.

A safer prompt for Lovable

Once you know the failing boundary, give the AI builder a constrained task rather than asking for a rewrite.

For example:

Audit the existing Supabase Storage upload flow without replacing the working UI. Identify the bucket, object path, authenticated role and whether the call uses upload or upsert. Compare those with the current storage.objects RLS policies. Propose the smallest policy or path correction that lets each signed-in user manage only the files they are meant to own. Do not expose service-role or S3 secret credentials in client code. Preserve existing working behavior and list the tests I should run with two separate users.

That prompt makes the security boundary explicit.

When the problem is bigger than one policy

If fixing one Storage rule reveals inconsistent user ownership, privileged keys in frontend code, missing authorization across tables or a product whose permissions exist only in UI conditions, stop treating each 403 as an isolated bug.

At that point the app needs a permissions audit.

A production-ready system should be able to answer:

• who owns this resource?
• who may read it?
• who may create, replace or delete it?
• where is that rule actually enforced?
• can a second user bypass the interface and still perform the action?

That is product architecture, not just prompt engineering.

If your Lovable or Supabase app is already built but Storage, authentication, permissions or production behavior is unreliable, see my MVP development service or contact me. The objective is not to rewrite working software. It is to make the authority boundaries dependable.