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Claude Code Connection & Auth Errors: 2026 Fix Guide

Start by identifying which connection is failing

Claude Code connection and authentication errors usually involve one of four things: credentials, the API destination, network access, or account permissions. The fastest troubleshooting method in 2026 is to check these separately. Repeatedly signing in will not fix an incorrect relay URL, and replacing an API key will not repair a corporate proxy.

Run claude --version and claude doctor, then open Claude Code and run /status. Record the exact error, active authentication method, and provider information shown. Use claude update if your installation supports it; otherwise, update through the package manager or deployment method that installed Claude Code.

Resolve conflicting authentication settings

Choose the access method you actually intend to use: a Claude account login, a direct API key, or a relay credential. These credentials are not interchangeable. A Claude subscription does not automatically provide API credits, and a relay key cannot authenticate against Anthropic's direct API.

• For account login: remove unintended API and relay environment variables from the launching shell, restart Claude Code, and use /login if authentication is required.
• For direct API access: check that your intended API key and account have access and available billing. Remove stale relay settings.
• For relay access: use the relay's credential and base URL together. Follow its Claude Code instructions for the authentication variable.

Check ANTHROPIC_API_KEY, ANTHROPIC_AUTH_TOKEN, and ANTHROPIC_BASE_URL locally without sharing their values. Also inspect shell startup files, project settings, user settings, and IDE launch environments. Multiple credential sources can produce unexpected behavior. After changing variables, fully restart the terminal or IDE that launches Claude Code.

Check the 59API base URL and credential pairing

For 59API, the API base URL is https://api.59api.com. Configure the Claude Code integration using 59API's current instructions. Do not substitute the dashboard address or append /v1/messages to the base URL: that is a request endpoint, and the client constructs request paths itself.

If the integration specifies ANTHROPIC_AUTH_TOKEN, place your 59API key there and remove an unintended competing ANTHROPIC_API_KEY. On macOS or Linux, environment changes made with export apply to that shell and its child processes. Windows PowerShell uses $env:VARIABLE_NAME; Bash export commands will not work there.

Also verify the selected model. Human-readable names and API model IDs can differ. For example, when selecting Claude Sonnet 5.5 or Claude Haiku 4.5, copy the corresponding supported identifier from 59API's current model listing instead of inventing one from the display name.

Interpret the response before changing anything

• 401 Unauthorized: check for an expired, revoked, incomplete, or incorrectly supplied key. Confirm that the credential belongs to the configured API host.
• 403 Forbidden: inspect the response body for account restrictions, model permissions, or provider access policies.
• 404 Not Found: check the base path, endpoint compatibility, and model ID. A successful connection does not prove the requested route exists.
• 429 Too Many Requests: check concurrency, rate limits, and quota or balance messages. Reduce parallel sessions and respect any retry delay.
• 5xx errors: check provider status information and retry with increasing delays. Repeated rapid retries can worsen throttling.

Separate network failures from API failures

To test basic connectivity without sending credentials, run curl --connect-timeout 10 --max-time 20 -sS -o /dev/null -w '%{http_code}' https://api.59api.com. An HTTP response, even 404, shows that this request reached an HTTP server; it does not validate authentication or Claude Code compatibility.

DNS failures, connection timeouts, and certificate errors point toward networking. Check VPNs, firewall rules, proxy variables, and corporate certificate requirements. Avoid disabling TLS verification. If ordinary requests work but responses stall during generation, investigate proxy buffering and streaming connection timeouts.

Retest with one small request

Restart Claude Code and send a short prompt using one supported model. If it still fails, share the timestamp, client version, sanitized error, and request ID with support. Never include keys or complete environment dumps.

59API combines low-cost, pay-as-you-go access with native, official-quality models and no downgrades. Its Claude Code compatibility and referral rebate make it worth considering for ongoing development. Sign up for 59API and test a small request before expanding your usage.

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