← Back to all guides

LLM API Errors in 2026: Practical Fixes by Status Code

A failed LLM request does not always mean the model is unavailable. Authentication, endpoint mismatches, token budgets, and application timeouts often cause the same visible symptom: no usable answer. This 2026 troubleshooting guide explains how to identify the failure layer, apply the right fix, and avoid paying for unnecessary retries.

1. Capture the evidence before changing anything

Log the HTTP status, provider error type, request ID, model identifier, endpoint path, latency, and retry count. For streaming requests, also record whether any output arrived before failure. Keep API keys, authorization headers, and sensitive prompts out of logs.

Reproduce the problem with one short message, no tools, and a small output limit. If that succeeds, restore features individually. This separates connection and authentication problems from payload-specific failures.

2. Fix 401 authentication and 403 access errors

401 usually indicates missing or invalid credentials. Check that the key belongs to the service receiving the request, contains no accidental whitespace, and is loaded in the running process. Updating an environment file will not necessarily update an already-running application.

• For OpenAI-compatible requests, verify the Authorization: Bearer header.
• For Anthropic-format requests, verify the documented API-key and version headers.
• For 403 responses, inspect permissions, model access, regional restrictions, or policy messages. Repeating the same request will not resolve an access restriction.

3. Resolve 404 endpoints and unknown models

A 404 can mean a wrong route, not just an unavailable model. Check the hostname, version prefix, endpoint path, and exact model ID. Model display names are not always valid API identifiers.

For 59API, the API host is https://api.59api.com. Confirm the required base URL for your SDK or tool: some integrations need a /v1 suffix, while others append paths themselves. Avoid duplicated segments such as /v1/v1. Also verify whether your client uses Chat Completions, Responses, or Anthropic Messages; these payloads are not interchangeable.

4. Handle 429 rate limits without creating a retry storm

A 429 may indicate request-rate limits, token-rate limits, or exhausted quota. Read the response body and available rate-limit headers before deciding what to do. Check account balance and spending limits when the error indicates quota exhaustion.

• Respect Retry-After when supplied.
• Otherwise, use exponential backoff with jitter and a maximum retry count.
• Limit concurrent requests with a queue or semaphore.
• Reduce prompt size or requested output when token throughput is the bottleneck.

Check whether your SDK already retries automatically. Stacking application retries on SDK retries can multiply traffic during an outage.

5. Repair 400 errors and context overflows

400 errors usually require changing the request. Inspect the specific validation message for unsupported parameters, invalid message roles, malformed tool definitions, or incompatible output settings. Parameters accepted by one model may be rejected by another.

For context-length failures, budget for input, requested output, and model-specific overhead. Remove redundant conversation history, retrieve only relevant document passages, and summarize older turns. Use the model's tokenizer or token-counting endpoint when available; character counts are only rough estimates. Lowering output length helps only when the remaining input fits.

6. Diagnose broken JSON and tool calls

A successful HTTP response can still fail your application. Where supported, use schema-constrained structured output, then validate the result locally. Handle refusals, truncated output, and missing fields explicitly rather than assuming every response contains valid JSON.

For tool calls, preserve call IDs and return results in the required message format. Validate arguments before execution. Never execute model-generated commands blindly, and require authorization for destructive or financial actions.

7. Recover safely from timeouts and server failures

For network failures and transient 5xx responses, use bounded retries with backoff. Distinguish connection timeouts from read timeouts. Streaming can prevent long silent waits, but a stream can still terminate mid-response or deliver an error event after HTTP 200.

Do not automatically repeat side effects after an uncertain outcome. Use application-level deduplication for tool actions and payments; a client timeout does not prove the upstream request stopped processing.

Choose economical access without weakening reliability

59API offers low-cost, pay-as-you-go access to native Claude and GPT models without downgrading model quality, with compatibility for Claude Code, Codex, and OpenAI SDKs. Its referral rebate adds another potential saving. Sign up if you want to test a lower-cost relay, then validate your exact models, endpoints, and retry behavior with a small staging workload before moving production traffic.

Pricing · Docs