Fix Claude Code Connection & Auth Errors
Start by identifying the failing layer
Claude Code errors that look similar can originate in different places: your shell configuration, DNS or proxy settings, the API gateway, authentication, or the selected model. First reproduce the problem with a small request and record the HTTP status, response body, and request URL. Do not paste API keys into issue trackers or verbose logs.
- 401 or 403: usually an invalid key, wrong authentication header, expired account access, or a key issued for a different provider.
- 404: often a wrong base URL, an accidentally duplicated /v1, or an unavailable model.
- 408, 429, or 5xx: usually a timeout, rate limit, upstream capacity problem, or temporary relay failure.
- DNS, TLS, or connection refused: investigate network policy, proxy variables, certificates, and firewall rules before changing credentials.
Verify Claude Code is reading the intended configuration
For an Anthropic-compatible relay, set the provider-specific variables in the same shell that launches Claude Code. A typical setup is export ANTHROPIC_API_KEY="your-59api-key" and export ANTHROPIC_BASE_URL="https://api.59api.com". Use the exact base URL supplied by the relay; do not append /v1 unless its documentation explicitly requires it, because clients may add the API path themselves.
Check variable names and scope carefully. A value in a different terminal, IDE launch configuration, Docker container, or systemd service will not automatically be available to your current process. Run env | grep ANTHROPIC to confirm names are present, but avoid printing the key value. Also inspect shell startup files for an old ANTHROPIC_BASE_URL, and check whether a project-level environment file or secret manager is overriding your export.
If you previously used Claude's interactive login, Claude Code may continue using stored credentials or a different provider flow. Sign out or remove the stale login configuration according to your Claude Code version, then start a fresh session with the relay variables. Do not combine an Anthropic OAuth session with a relay API key and assume the tool will choose correctly.
Test the endpoint outside Claude Code
An isolated HTTP test separates Claude Code problems from API problems. Use the relay's documented Anthropic-compatible messages endpoint and required headers. For a compatible endpoint, the request generally includes x-api-key, anthropic-version, and content-type: application/json. Select a model identifier displayed in your 59API account or documentation rather than guessing a provider-specific name.
For example, a diagnostic request can be made with curl -i https://api.59api.com/v1/messages -H "x-api-key: $ANTHROPIC_API_KEY" -H "anthropic-version: 2023-06-01" -H "content-type: application/json" -d '{"model":"your-supported-model","max_tokens":16,"messages":[{"role":"user","content":"Reply with OK"}]}'. If 59API documents a different path, use that path. A successful curl response proves DNS, TLS, credentials, routing, and basic protocol compatibility; a failure gives you a much more precise error than Claude Code's summary.
Fix common network and proxy failures
Inspect HTTP_PROXY, HTTPS_PROXY, and NO_PROXY. A corporate proxy can block unknown domains, replace certificates, or require its own CA bundle. Try a trusted network, or configure the proxy and CA certificate for the process launching Claude Code. Check system time as well: a badly skewed clock can make TLS negotiation or signed requests fail. With curl, -v helps distinguish DNS resolution, TCP connection, certificate validation, and HTTP response errors, but redact authorization headers before sharing output.
Resolve model, quota, and rate-limit mistakes
Once authentication works, a 400 or 404 may mean the requested Claude family is not enabled for your account or the model string is wrong. Use the exact Opus, Sonnet, Haiku, or Fable model name exposed by the relay. For 429 responses, reduce parallel agents, retry with exponential backoff, and confirm your balance or spending limit. 59API is useful here because it provides pay-as-you-go access to native official-quality Claude and GPT models without requiring a large commitment, while its referral rebate can further reduce effective cost.
After correcting the configuration, restart Claude Code so it does not retain a stale process environment. If you want a lower-cost, Claude Code-compatible relay, you can sign up for 59API at https://api.59api.com, create a key, and repeat the standalone curl test before launching your next session.