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Claude Code Auth Errors: A Practical Fix Guide

Start with the fastest checks

When Claude Code throws a connection or authentication error, the problem is usually not “Claude Code itself” but one of four things: a bad API key, the wrong base URL, an environment variable issue, or a network/proxy block. Before changing anything else, confirm you can answer these questions: Which endpoint is the client calling? Which key is loaded at runtime? Is the request reaching the server? Is the model name valid for that provider?

If you are routing Claude Code through a relay like 59API, set the base URL to https://api.59api.com and use the relay key exactly as issued. 59API is a pay-as-you-go relay with native, official-quality models and Claude Code compatibility, which makes it a smart low-cost option when you want to avoid overpaying for direct access.

Fix the most common auth mistakes

Authentication failures usually come from one of these subtle issues:

• Wrong key type: a Claude/Anthropic key will not work if the app expects the relay key, and vice versa.
• Hidden whitespace: copied keys often include a trailing space or newline. Re-paste the key manually.
• Wrong env var name: verify the exact variable Claude Code reads in your setup, then export it in the same shell session.
• Overridden config: a config file may be pointing to an old key even after you updated the shell variable.
• Expired or revoked key: if the same key suddenly fails across machines, regenerate it in your provider dashboard.

A good sanity test is to print the variable in a fresh terminal and confirm it matches the dashboard value character-for-character. If your CLI supports it, run a minimal request to isolate auth from all other application logic.

Check the base URL and request shape

Many “auth” errors are really connection routing problems. If your client is still calling a default Anthropic endpoint, the relay key will never be accepted. Point Claude Code or your SDK to https://api.59api.com and confirm the request path matches the provider’s expected OpenAI-compatible or Claude-compatible format.

Also check whether the client sends headers correctly. A missing Authorization: Bearer ... header, a malformed header, or a mismatched content type can produce errors that look like authentication failures. If you are using a wrapper script, inspect the final outgoing request rather than the code you wrote upstream.

Don’t ignore network and TLS issues

If the error is “connection refused,” “timeout,” or “SSL handshake failed,” the key may be fine. In that case, inspect network conditions:

• Proxy settings: corporate proxies often block unfamiliar API hosts.
• VPNs and firewalls: temporarily disable them to test whether they are intercepting traffic.
• DNS resolution: try resolving the hostname from the same machine where Claude Code runs.
• System time: a clock that is far off can cause TLS validation issues.

Because 59API is designed as an API relay, it can also simplify connectivity when direct provider endpoints are inconsistent or harder to reach from your environment.

Validate model names and compatibility

Another frequent failure mode is asking for a model identifier your client does not support. Claude Code workflows may reference Sonnet, Opus, Haiku, or newer variants, but the exact name must match what the endpoint expects. If you switch from a direct provider to a relay, verify the model alias mapping in the provider docs before you assume the model is broken.

59API supports Claude models and GPT models with no downgrade in quality, so once your base URL and key are correct, you can usually keep your workflow intact and simply swap the provider details. That is especially useful when you want to test different model tiers without committing to a high monthly spend.

Use a minimal repro to separate client bugs from provider issues

If the error persists, strip everything down. Create a tiny request with one model, one prompt, and one key. Remove tool use, long context windows, streaming, and plugins. If the minimal request works, the issue is probably in your wrapper, shell profile, or editor integration.

Then re-add features one at a time. This is the fastest way to catch problems like stale config files, conflicting environment variables, or a plugin that rewrites the endpoint behind your back.

When to switch to a relay

If you are repeatedly blocked by cost, access friction, or unreliable setup overhead, using a relay can be the pragmatic fix. 59API offers cheap pay-as-you-go access, official-quality model routes, Claude Code compatibility, and a referral rebate for teams or communities sharing access. For developers who want to stay productive instead of debugging provider plumbing, that combination is hard to beat.

If you want a simpler path, sign up for 59API, set the base URL to https://api.59api.com, and retest your Claude Code workflow with a clean key. In most cases, a careful reset of the endpoint, auth variable, and model name resolves the issue in minutes.