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HTTP errors

Our API follows a predictable HTTP error code format:
  • 400 - invalid_request_error: There was an issue with the format or content of your request. We may also use this error type for other 4XX status codes not listed below.
  • 401 - authentication_error: There’s an issue with your API key.
  • 403 - permission_error: Your API key does not have permission to use the specified resource.
  • 404 - not_found_error: The requested resource was not found.
  • 413 - request_too_large: Request exceeds the maximum allowed number of bytes. The maximum request size is 32 MB for standard API endpoints.
  • 429 - rate_limit_error: Your account has hit a rate limit.
  • 500 - api_error: An unexpected error has occurred internal to Anthropic’s systems.
  • 529 - overloaded_error: The API is temporarily overloaded.
    529 errors can occur when APIs experience high traffic across all users.In rare cases, if your organization has a sharp increase in usage, you might see 429 errors due to acceleration limits on the API. To avoid hitting acceleration limits, ramp up your traffic gradually and maintain consistent usage patterns.
When receiving a streaming response via SSE, it’s possible that an error can occur after returning a 200 response, in which case error handling wouldn’t follow these standard mechanisms.

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Request size limits

The API enforces request size limits to ensure optimal performance:
Endpoint TypeMaximum Request Size
Messages API32 MB
Token Counting API32 MB
Batch API256 MB
Files API500 MB
If you exceed these limits, you’ll receive a 413 request_too_large error. The error is returned from Cloudflare before the request reaches our API servers.

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Error shapes

Errors are always returned as JSON, with a top-level error object that always includes a type and message value. The response also includes a request_id field for easier tracking and debugging. For example:
JSON
{
  "type": "error",
  "error": {
    "type": "not_found_error",
    "message": "The requested resource could not be found."
  },
  "request_id": "req_011CSHoEeqs5C35K2UUqR7Fy"
}
In accordance with our versioning policy, we may expand the values within these objects, and it is possible that the type values will grow over time.

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Request id

Every API response includes a unique request-id header. This header contains a value such as req_018EeWyXxfu5pfWkrYcMdjWG. When contacting support about a specific request, please include this ID to help us quickly resolve your issue. Our official SDKs provide this value as a property on top-level response objects, containing the value of the request-id header: 
import anthropic

client = anthropic.Anthropic()

message = client.messages.create(
    model="claude-sonnet-4-5",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "Hello, Claude"}
    ]
)
print(f"Request ID: {message._request_id}")

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Long requests

We highly encourage using the streaming Messages API or Message Batches API for long running requests, especially those over 10 minutes.
We do not recommend setting a large max_tokens values without using our streaming Messages API or Message Batches API:
  • Some networks may drop idle connections after a variable period of time, which can cause the request to fail or timeout without receiving a response from Anthropic.
  • Networks differ in reliability; our Message Batches API can help you manage the risk of network issues by allowing you to poll for results rather than requiring an uninterrupted network connection.
If you are building a direct API integration, you should be aware that setting a TCP socket keep-alive can reduce the impact of idle connection timeouts on some networks. Our SDKs will validate that your non-streaming Messages API requests are not expected to exceed a 10 minute timeout and also will set a socket option for TCP keep-alive.