Q: What does a 401 Unauthorized error mean? A: A 401 occurs when the server requires valid credentials but the request carries none or carries credentials that are invalid, expired, or blocked. It differs from a 403 because a 403 means the server knows who you are but refuses access. Q: What quick browser checks can help me resolve a 401? A: If you need to know how to fix 401 unauthorized error fast, start by confirming the URL, logging out and back in, clearing cookies and cache, or trying a private/incognito window. Also turn off VPNs, proxies, or ad blockers, check device date and time, try another browser or network, and reset your password if necessary. Q: How should developers verify Authorization headers to stop 401s? A: Ensure your client sends the exact Authorization header the server expects, for example Authorization: Bearer YOUR_TOKEN for bearer tokens or a Base64 username:password in Authorization: Basic …, or place the API key in the required header or query parameter. Also check that proxies or servers are not stripping the Authorization header and forward it in Nginx with proxy_set_header Authorization $http_authorization; and in Apache ensure the header reaches the app. Q: How do token expiry and refresh tokens affect 401 errors? A: Tokens cause 401s when JWT exp or nbf claims show the token is expired, invalid, or blocked. Implement a refresh token flow before access tokens expire, check exp and nbf claims, and add small leeway for clock skew between services. Q: Can CORS or preflight requests trigger a 401 and how should I handle them? A: Yes, browsers can send an OPTIONS preflight that returns 401 if the server requires auth for preflight, so allow anonymous OPTIONS responses or return the proper CORS headers like Access-Control-Allow-Origin, Methods, and Headers. Do not require authentication on preflight requests so the actual call can proceed. Q: Why do cookies, sessions, or CSRF mismatches cause 401 responses? A: Misconfigured Set-Cookie attributes (domain, path, Secure, HttpOnly, SameSite) or an unreachable session store can prevent the server from seeing a valid session and lead to 401s. Missing or mismatched CSRF tokens and WordPress nonce failures also present as 401 during REST calls, so send the expected CSRF header and cookie pair or refresh the nonce. Q: What diagnostic steps can I run to locate where authentication breaks? A: Read the response status, WWW-Authenticate header, and any error body, and compare the same request sent by the browser and a tool like curl with and without Authorization. Use the DevTools Network tab to see which headers or cookies the browser sends, inspect server, proxy, load balancer and WAF logs, and isolate the issue by trying another user, device, or network. Q: How can I prevent recurring 401 errors going forward? A: Renew tokens proactively and implement clear refresh or re-login flows, use sliding sessions or keep-alive pings, and monitor 401 rates by route to catch broken deploys quickly. Document required headers, scopes, and sample requests, and add health checks for clock drift and key rotation.