That’s a Firewall Challenge. Your Firewall Event Logs should show why that request is being challenged. Security Events · Cloudflare Web Application Firewall (WAF) docs
I am not sure if Cloudflare allow to bypass such request , in ideal world you should be able to create a WAF bypass rule specifically for requests from Vercel :
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Go to Cloudflare Dashboard > Security > WAF > Custom Rules
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Create a new rule with:
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Name: “Vercel Bypass”
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Expression:
(http.request.headers["X-API-Key"] eq "YOUR_SECRET_KEY")
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Action: “Bypass”
I will recommend reaching out to Cloudflare support team for specific guidance.