OpenAI SDK compatibility (beta)

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Before you begin

This compatibility layer is intended to test and compare model capabilities with minimal development effort and is not considered a long-term or production-ready solution for most use cases. For the best experience and access to Anthropic API full feature set (PDF processing, citations, extended thinking, and prompt caching), we recommend using the native Anthropic API.

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Getting started with the OpenAI SDK

To use the OpenAI SDK compatibility feature, you’ll need to:

1. Use an official OpenAI SDK
2. Change the following
   • Update your base URL to point to Anthropic’s API
   • Replace your API key with an Anthropic API key
   • Update your model name to use a Claude model
3. Review the documentation below for what features are supported

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Quick start example

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Important OpenAI compatibility limitations

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API behavior

Here are the most substantial differences from using OpenAI:

• The strict parameter for function calling is ignored, which means the tool use JSON is not guaranteed to follow the supplied schema.
• Audio input is not supported; it will simply be ignored and stripped from input
• Prompt caching is not supported, but it is supported in the Anthropic SDK
• System/developer messages are hoisted and concatenated to the beginning of the conversation, as Anthropic only supports a single initial system message.

Most unsupported fields are silently ignored rather than producing errors. These are all documented below.

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Output quality considerations

If you’ve done lots of tweaking to your prompt, it’s likely to be well-tuned to OpenAI specifically. Consider using our prompt improver in the Anthropic Console as a good starting point.

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System / Developer message hoisting

Most of the inputs to the OpenAI SDK clearly map directly to Anthropic’s API parameters, but one distinct difference is the handling of system / developer prompts. These two prompts can be put throughout a chat conversation via OpenAI. Since Anthropic only supports an initial system message, we take all system/developer messages and concatenate them together with a single newline (\n) in between them. This full string is then supplied as a single system message at the start of the messages.

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Extended thinking support

You can enable extended thinking capabilities by adding the thinking parameter. While this will improve Claude’s reasoning for complex tasks, the OpenAI SDK won’t return Claude’s detailed thought process. For full extended thinking features, including access to Claude’s step-by-step reasoning output, use the native Anthropic API.

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Rate limits

Rate limits follow Anthropic’s standard limits for the /v1/messages endpoint.

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Detailed OpenAI Compatible API Support

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Request fields

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Simple fields

Field	Support status
model	Use Claude model names
max_tokens	Fully supported
max_completion_tokens	Fully supported
stream	Fully supported
stream_options	Fully supported
top_p	Fully supported
parallel_tool_calls	Fully supported
stop	All non-whitespace stop sequences work
temperature	Between 0 and 1 (inclusive). Values greater than 1 are capped at 1.
n	Must be exactly 1
logprobs	Ignored
metadata	Ignored
response_format	Ignored
prediction	Ignored
presence_penalty	Ignored
frequency_penalty	Ignored
seed	Ignored
service_tier	Ignored
audio	Ignored
logit_bias	Ignored
store	Ignored
user	Ignored
modalities	Ignored
top_logprobs	Ignored
Reasoning_effort	Ignored

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tools / functions fields

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messages array fields

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Response fields

Field	Support status
id	Fully supported
choices[]	Will always have a length of 1
choices[].finish_reason	Fully supported
choices[].index	Fully supported
choices[].message.role	Fully supported
choices[].message.content	Fully supported
choices[].message.tool_calls	Fully supported
object	Fully supported
created	Fully supported
model	Fully supported
finish_reason	Fully supported
content	Fully supported
usage.completion_tokens	Fully supported
usage.prompt_tokens	Fully supported
usage.total_tokens	Fully supported
usage.completion_tokens_details	Always empty
usage.prompt_tokens_details	Always empty
choices[].message.refusal	Always empty
choices[].message.audio	Always empty
logprobs	Always empty
service_tier	Always empty
system_fingerprint	Always empty

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Error message compatibility

The compatibility layer maintains consistent error formats with the OpenAI API. However, the detailed error messages will not be equivalent. We recommend only using the error messages for logging and debugging.

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Header compatibility

While the OpenAI SDK automatically manages headers, here is the complete list of headers supported by Anthropic’s API for developers who need to work with them directly.

Header	Support Status
x-ratelimit-limit-requests	Fully supported
x-ratelimit-limit-tokens	Fully supported
x-ratelimit-remaining-requests	Fully supported
x-ratelimit-remaining-tokens	Fully supported
x-ratelimit-reset-requests	Fully supported
x-ratelimit-reset-tokens	Fully supported
retry-after	Fully supported
x-request-id	Fully supported
openai-version	Always 2020-10-01
authorization	Fully supported
openai-processing-ms	Always empty