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CORS Errors Debugging: Preflight OPTIONS, Allow-Origin, Allow-Headers and Testing Servers with curl or Python

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Self-Hosting & Infra intermediate 6 min read $0.01 Updated 2026-10-20

Executed against a local server: an OPTIONS preflight with Origin, Access-Control-Request-Method and Request-Headers returning allow headers for one origin and nothing for another, plus guidance on Vary, credentials and why servers cannot fix CORS from the client.

A browser blocks your fetch with a CORS error while curl works fine. Learn what the browser checks, reproduce the preflight request from the command line, and read exactly which header is missing.

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#cors#http#browser#api#debugging

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