Claude API Setup and Error Reference
A compact reference for Claude API setup, required headers, common error types, request-size limits, streaming, batches, 529, 429 and debugging checks.
| Item | Value | Debug check |
|---|---|---|
| Base URL | https://api.anthropic.com | Use this host for direct Claude API / Anthropic API calls. |
| Messages API | POST /v1/messages | Most chat, generation and tool-use workflows start here. |
| Message Batches API | POST /v1/messages/batches | For large asynchronous workloads; official docs state a 50% cost reduction, but it is not real-time. |
| Token Counting API | POST /v1/messages/count_tokens | Count tokens before sending to manage cost, context and rate-limit risk. |
| Models API | GET /v1/models | List available models and details instead of hard-coding stale names. |
| Authentication | credential header | For 401, check credentials, forwarding proxy and workspace. |
| Version header | anthropic-version: 2023-06-01 | Required on all requests; missing or wrong versions can fail validation. |
| Content-Type | application/json | Required for JSON requests; send valid JSON matching the model/request shape. |
| Request tracking | request-id / request_id | Keep the request ID for support, incident reports and log correlation. |
| Long requests | streaming or Message Batches | Long non-streaming requests are more exposed to network or SDK timeouts. |
| HTTP | error.type | Meaning | Check first |
|---|---|---|---|
400 | invalid_request_error | Invalid request shape, parameter or body. | Validate JSON, model, messages, max_tokens and tool params. |
400 | invalid_request_error | Newer models do not support prefilling assistant messages. | Do not send a prefilled final assistant message; use structured outputs, system instructions or output_config. |
401 | authentication_error | Authentication failed. | Check credentials, forwarding proxy and workspace. |
402 | billing_error | Billing or payment problem. | Check Console billing, credits, limits or Marketplace status. |
403 | permission_error | No permission for the resource. | Check workspace, model, file or resource access. |
404 | not_found_error | Resource not found. | Check endpoint, model, file, batch or message ID. |
413 | request_too_large | Request exceeds size limits. | Reduce input, split requests or use Batches/Files. |
429 | rate_limit_error | Rate or acceleration limit hit. | Back off, lower concurrency, ramp traffic gradually and check limits. |
500 | api_error | Server-side API error. | Retry with backoff and log the request ID. |
504 | timeout_error | Processing timed out. | Use streaming or Message Batches for long requests. |
529 | overloaded_error | Temporary Anthropic-side overload. | Retry with backoff; this may reflect global traffic. |
| API | Limit | Notes |
|---|---|---|
| Messages API | 32 MB | Standard message requests; exceeding it can return 413. |
| Token Counting API | 32 MB | Token-count requests. |
| Message Batches API | 256 MB | For larger asynchronous batch work. |
| Files API | 500 MB | File upload workflows. |
| Sessions / Agents / Environments | 32 MB | Related session/container APIs. |
- Common community issues cluster around 429, 529, streaming, long-request timeouts, assistant prefill, large requests and request_id debugging.
- Streaming responses use SSE and can return an in-stream error after the initial HTTP 200, so clients should handle error events.
- Use streaming for long-running requests; use Message Batches for very long asynchronous work to avoid non-streaming timeouts.
- 429 can come from normal rate limits or organization-level acceleration limits; avoid sudden traffic spikes.
- 529 is server-side overload, not a model refusal; use backoff with jitter and avoid synchronized retries across workers.
- Error responses usually include top-level type, error.type, error.message and request_id; responses also include a request-id header.
- Newer models do not support assistant prefill; do not send a final assistant message as a prefilled output.
Source: official Anthropic API documentation, error documentation, Claude pricing page and recurring community troubleshooting topics. Community posts are used as topic signals only; official docs and console limits remain authoritative.
This Claude API setup and Anthropic API error reference helps debug Claude API authentication_error, rate_limit_error, request_too_large, overloaded_error, timeout_error, prefill incompatibility, streaming failures after HTTP 200, long-running request failures and request_id tracing.
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FAQ
What headers does the Claude API need?
Direct Anthropic API calls commonly need credentials, anthropic-version, and content-type: application/json for JSON requests. For 401, check workspace ownership and whether a proxy stripped headers.
What is the difference between 429 and 529?
429 usually means your organization or traffic pattern hit a rate or acceleration limit. 529 means temporary Anthropic-side overload. Both need backoff, but 429 also requires checking limits, concurrency and traffic ramp-up.
Is 529 overloaded_error a model refusal?
No. 529 is a server-side overload/capacity condition. Log request-id, retry with exponential backoff and jitter, and avoid synchronized retries across all workers.
Why can a streaming call fail after HTTP 200?
SSE streams can emit an error event after the connection is established, so clients must handle stream-level errors, not only the initial status code.
How do I fix “Prefilling assistant messages is not supported”?
It is a 400 invalid_request_error. Do not send a final assistant message as prefill for newer models; use structured outputs, system instructions or output_config instead.
Why can a multi-turn tool call suddenly return 400?
A common cause is a proxy, logger or message compaction layer filtering, reordering or reconstructing model-returned special content. Verify that your middleware preserves the model response shape across turns.
What should I do when the request is too large?
Reduce context, split messages or compress inputs first; for batch or file-heavy workflows consider Message Batches API or Files API. Raising max_tokens will not fix an oversized request body.
What should I do with long non-streaming timeouts?
Use streaming for long-running requests. For multi-minute or batch workloads, prefer Message Batches and poll results instead of holding one connection open.
Where should I log request_id?
Log it in application logs, error reports and user-copyable diagnostics. For 500, 529, timeouts or provider support, request_id is usually safer and more useful than full prompt logs.
Can Message Batches replace realtime chat?
No. Batches are for offline, high-volume, waitable work. Realtime chat and interactive tool-use should stay on the normal Messages API path.