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TypeError: Failed to fetch (Supabase Edge Function / CORS)

Exact error

Access to fetch at 'https://<ref>.supabase.co/functions/v1/...' has been blocked by CORS policy

What it means

The browser blocked the response because the Edge Function did not return CORS headers, often on the OPTIONS preflight request.

Likely causes

• Edge Function doesn't handle the OPTIONS method.
• CORS headers missing on success or error responses.

How to fix it

1. Handle preflight and add headers to every response:

   const corsHeaders = {
     'Access-Control-Allow-Origin': '*',
     'Access-Control-Allow-Headers': 'authorization, x-client-info, apikey, content-type',
   };
   
   Deno.serve(async (req) => {
     if (req.method === 'OPTIONS') return new Response('ok', { headers: corsHeaders });
     const data = { ok: true };
     return new Response(JSON.stringify(data), {
       headers: { ...corsHeaders, 'Content-Type': 'application/json' },
     });
   });

2. Redeploy the function: `supabase functions deploy <name>`.

Prevent this next time

• Add timeouts to outbound requests and return clear error responses instead of crashing.
• Set up uptime checks and log alerts on critical endpoints so you hear about failures before users do.

Guide: catch outages before your users do.

Related errors

• Invalid JWT / JWT expired