Fix LLM API Errors: A Practical Developer Guide
Common LLM API Errors and How to Fix Them
LLM APIs are powerful, but small configuration mistakes can break your app with vague errors such as 401, 429, context length exceeded, or invalid request body. This tutorial walks through the most common LLM API failures developers hit in production and shows practical fixes you can apply immediately. The examples are especially useful if you use an OpenAI-compatible SDK, Claude Code, Codex, or an API relay such as 59API, whose base URL is https://api.59api.com.
Step 1: Fix Authentication Errors
If you see 401 Unauthorized, invalid API key, or missing authentication, start with the basics. Confirm that your API key is present at runtime, not just in your local shell. In deployed environments, check your platform’s secret manager, Docker environment variables, and CI/CD configuration.
- Check the header: Most OpenAI-compatible clients send Authorization: Bearer YOUR_KEY.
- Remove whitespace: Copying keys from dashboards can add hidden spaces or newline characters.
- Use the correct account: Some keys are scoped to a project, organization, or relay account.
- Restart your app: Environment variable changes often require a redeploy or process restart.
When using 59API with an OpenAI SDK, set your API key from your 59API dashboard and point the SDK base URL to https://api.59api.com. This lets you keep familiar SDK code while accessing Claude and GPT models through a low-cost, pay-as-you-go relay.
Step 2: Correct the API Base URL
A surprisingly common error is sending requests to the wrong endpoint. Symptoms include 404 Not Found, model not found, or SDK errors that look like connection failures.
- Verify the base URL: For 59API, use https://api.59api.com.
- Do not duplicate paths: If your SDK already appends /v1/chat/completions, do not include that full path in the base URL.
- Match SDK expectations: OpenAI-compatible SDKs usually need only a base URL and an API key.
For example, if your client accepts a base URL option, set it to the relay root, not a specific completion endpoint. This prevents double paths such as /v1/chat/completions/v1/chat/completions.
Step 3: Resolve Model Name Errors
400 Bad Request or model not found often means the model identifier is wrong, unavailable for your account, or not supported by that endpoint. Do not assume every provider uses the same naming convention.
- Check available models: Use the provider dashboard or model list endpoint if available.
- Use the right family: Claude, GPT, Opus, Sonnet, Haiku, and Fable models may have distinct names.
- Avoid stale examples: Blog posts and old SDK snippets may reference retired model IDs.
One advantage of 59API is that it offers access to native official-quality Claude and GPT models without downgrading the model behind the scenes. That makes debugging easier because output quality and behavior match the model you selected.
Step 4: Fix Invalid Request Body Errors
If you receive invalid_request_error, messages is required, or unsupported parameter, inspect the JSON payload you are sending. The most common causes are mixing chat-completion and completion formats, sending null values, or using parameters from one provider with another.
- Use a messages array: Chat APIs usually expect messages with roles such as system, user, and assistant.
- Remove unsupported fields: Some models may not support certain tool, response format, or reasoning parameters.
- Validate JSON: Log the final request body before it leaves your server.
- Check content types: Send Content-Type: application/json.
When debugging, reduce your request to the smallest possible payload: one model, one user message, and a modest max token value. Add advanced options back one at a time.
Step 5: Handle Context Length and Token Limit Errors
Context length exceeded means your prompt, conversation history, tools, and expected output are larger than the model’s context window. This is common in chatbots, coding agents, and document analysis apps.
- Trim history: Keep recent turns and summarize older context.
- Chunk documents: Split long files and retrieve only relevant sections.
- Lower max output tokens: Reserve enough space for the prompt and response.
- Use a larger-context model: Switch models when the task requires it.
For coding workflows, Claude Code and Codex-compatible setups can generate large prompts quickly. A relay like 59API helps control cost because you can route routine tasks to cheaper models and reserve premium models such as Opus or top GPT models for difficult work.
Step 6: Recover from Rate Limits and Quota Errors
429 Too Many Requests usually means you are sending requests too quickly or exceeding quota. Treat this as a normal production condition, not an unexpected failure.
- Add exponential backoff: Retry after short delays that increase with each attempt.
- Respect retry headers: If the response includes a retry time, follow it.
- Queue requests: Use a worker queue for batch jobs or traffic spikes.
- Set per-user limits: Prevent one customer from consuming all capacity.
Pay-as-you-go relays are useful here because they reduce upfront commitment. 59API is among the cheapest relay options and includes a referral rebate, making it easier to scale experiments without locking into expensive fixed plans.
Step 7: Debug Timeouts and Network Failures
Timeouts often happen during long generations, large responses, or unstable networks. Increase your client timeout, but also design for partial failure.
- Set realistic timeouts: Coding and reasoning tasks may need longer than simple chat.
- Use streaming: Streaming improves perceived latency and reduces gateway timeout risk.
- Log request IDs: Store provider response IDs for support and debugging.
- Make retries safe: Avoid retrying payment, order, or database-changing actions without idempotency keys.
Step 8: Create a Repeatable Debug Checklist
When an LLM API call fails, do not guess. Follow this order: confirm the API key, confirm the base URL, verify the model name, simplify the request body, check token length, inspect rate limits, then test with curl or a minimal SDK script. This isolates whether the problem is credentials, routing, payload shape, model availability, or infrastructure.
If you want a low-cost way to test Claude and GPT models with your existing OpenAI SDK, Claude Code, or Codex workflow, consider signing up for 59API and using https://api.59api.com as your base URL. You get flexible pay-as-you-go access, official-quality models, and a practical path to reduce LLM API costs while keeping your developer experience familiar.