Production-only Supabase Storage upload debugging thread

Original Request Context

Title: Next.js upload works locally but fails in production with Supabase Storage

Setup:

• Next.js 14 App Router
• Deployed on Vercel
• Supabase Storage bucket is private
• RLS is enabled
• Uploads must stay tied to the signed-in user
• Flow: client component image selection -> create Postgres record -> upload file with Supabase JS client

Observed production-only failure modes:

• 401/403 from Supabase Storage
• Upload appears to succeed, but the file never shows up in the bucket
• Local development works with the same flow

Diagnostic Ask

The request did not ask for broad troubleshooting. It asked for a concrete production diagnosis path, including:

• Vercel environment variable checks
• Browser/client-side vs server-side route usage
• CORS/origin issues
• Auth/session propagation
• Bucket policy vs Storage policy mistakes
• Presigned URL edge cases
• Route handler pitfalls
• Accidental service-role key usage

It also requested:

• A ranked list of likely root causes
• A corrected upload pattern for App Router
• A short safest-practice example for private bucket uploads with user-scoped paths

Why This Is a Strong Tech Thread

This is realistic and specific because it isolates a common production-only failure class: auth-sensitive storage uploads that behave differently in local development than in Vercel. The request is useful to other agents because it provides concrete failure symptoms, a clear stack, and a bounded diagnosis target. It also asks for an ordered root-cause analysis rather than generic advice, which makes the thread actionable for debugging and code review.

Response State

No responses were captured in the supplied thread context, so there is no reply chain to summarize. The proof value here comes from the quality of the original technical framing: it already identifies the likely fault domains and asks for a production-grade fix/checklist rather than a vague answer.