# Anthropic and OpenAI Side by Side — Claude API / OpenAI API Basics

Source: https://www.geekswithgeeks.com/en/llm-apis/b-compare

> Learn the main differences so you can read either provider's docs.

## Same ideas, different spellings

Both APIs take a model name, a list of role-tagged messages and settings; both return a reply, a reason the generation stopped and token usage. Typical differences: **authentication header** (`x-api-key` plus `anthropic-version` for Anthropic; `Authorization: Bearer` for OpenAI); **system prompt** (a separate top-level `system` field for Anthropic; a message with role `system`, or `instructions` in the newer Responses API, for OpenAI); **max_tokens** (required for Anthropic Messages; optional for Chat Completions); **reply location** (`content[0].text` versus `choices[0].message.content`); **stop reason names** (`end_turn`, `max_tokens`, `tool_use` versus `stop`, `length`, `tool_calls`); and **tool formats** (`input_schema` versus a `function` with `parameters`). Model names and features change often, so read the current official docs and keep the model name in configuration.

## A comparison table

Field names as of the SDK versions used for this course; check the docs for your version.

```text
                     Anthropic (Messages)           OpenAI (Chat Completions)
endpoint             /v1/messages                   /v1/chat/completions   (newer: /v1/responses)
auth                 x-api-key + anthropic-version  Authorization: Bearer <key>
system prompt        top-level "system"             message with role "system"
max_tokens           required                       optional
reply                content[0].text                choices[0].message.content
stop reasons         end_turn / max_tokens / tool_use   stop / length / tool_calls
usage                input_tokens, output_tokens    prompt_tokens, completion_tokens, total_tokens
tool schema          input_schema                   function.parameters
```

## Keep a thin adapter

Wrap provider calls in one small module of your own so switching providers or upgrading an SDK touches one file.

**Quiz:** Where does Anthropic's Messages API put the system prompt?

- [ ] In the URL
- [x] In a separate top-level system field
- [ ] In the API key
- [ ] Nowhere; it is not supported

*Answer:* In a separate top-level system field. Anthropic separates the system prompt from the messages list.