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Fix Claude Code Auth and Connection Errors Fast

Claude Code connection errors usually come from a small set of causes: the wrong authentication method, a mismatched API endpoint, network interference, or an account limit. Start by identifying which service your terminal is actually contacting. Then change one thing at a time so you can tell what fixed it.

1. Identify the failing connection

Run claude --version to confirm the executable works, then start Claude Code and run /status. Check the reported authentication and configuration information. Use claude doctor to investigate installation problems, especially if the command fails before making any request.

Decide which access path you intend to use: an Anthropic account, an Anthropic API key, or a relay such as 59API. Credentials belong to their issuing service. An Anthropic subscription login does not authenticate requests to a relay, and a relay key does not authenticate requests to Anthropic.

2. Match the endpoint and credentials

For 59API, the API base URL is https://api.59api.com. Claude Code uses ANTHROPIC_BASE_URL for a custom Anthropic-compatible endpoint. Avoid automatically adding /v1: the client constructs API paths, and an incorrect prefix can produce duplicate paths and 404 errors.

In Bash or Zsh, set the endpoint with export ANTHROPIC_BASE_URL='https://api.59api.com'. Follow the relay's Claude Code setup instructions for its credential variable. Claude Code supports ANTHROPIC_AUTH_TOKEN for bearer-token authentication and ANTHROPIC_API_KEY for API-key authentication; these send different headers and are not interchangeable for every gateway.

If your setup specifies token authentication, use export ANTHROPIC_AUTH_TOKEN='YOUR_59API_KEY'. Remove a stale ANTHROPIC_API_KEY from that shell if it conflicts with the intended setup. Restart Claude Code afterward: an already-running process will not inherit later environment changes. Keep real credentials out of screenshots, shared logs, and committed configuration.

3. Read the error before changing settings

• 401 Unauthorized: Check for an expired, revoked, mistyped, or wrong-provider credential. Also check whether the gateway expects a different authentication header.
• 403 Forbidden: Investigate account permissions, model access, gateway policy, or network restrictions. Repeated login attempts rarely resolve a permission issue.
• 404 Not Found: Inspect the base URL, API route, and model identifier. Read the response body to distinguish a missing route from an unavailable model.
• 429 Too Many Requests: Check the response for rate-limit or quota details. Reduce parallel requests, honor Retry-After when supplied, and check account balance.
• 500, 502, 503, or 504: Retry briefly with increasing delays, then check service notices. Persistent failures may involve the gateway or upstream provider.

4. Separate network failures from authentication

Run curl -sS -o /dev/null -w '%{http_code}' --connect-timeout 10 --max-time 20 https://api.59api.com from the same environment that runs Claude Code. This checks DNS, TLS, and basic HTTP reachability. A response such as 404 still confirms that an HTTP server answered; it does not validate your credentials or model access.

If curl reports a DNS failure, investigate your resolver or VPN. A connection timeout suggests routing, firewall, or proxy trouble. For certificate errors, check your system clock, certificate store, and corporate TLS inspection configuration. Keep certificate verification enabled.

Inspect HTTPS_PROXY, HTTP_PROXY, ALL_PROXY, and NO_PROXY locally. An old proxy setting can break an otherwise working connection. Remember that containers, WSL, SSH sessions, and IDE terminals may have different network settings and environments.

5. Verify model access and billing

Once authentication works, confirm the exact API model identifier in your provider's catalog. Display names such as Claude Sonnet 5.5 and Claude Opus 5.5 are not necessarily valid request identifiers. Use claude --model YOUR_MODEL_ID with the documented identifier, then send a short prompt.

59API offers inexpensive, pay-as-you-go access to native, official-quality models without downgrading them. Its Claude Code compatibility makes it a practical option for developers managing usage costs. Check current pricing and balance; sign up when you are ready to test a small workload. A referral rebate is also available.

6. Escalate with useful evidence

If the problem persists, collect the Claude Code version, timestamp with timezone, HTTP status, sanitized error text, requested model identifier, and request ID if provided. Send those details to the relevant provider. Exclude authorization headers, tokens, and private project content so support can investigate safely.

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