Anthropic OpenAI-Compatible API Endpoint - March 2026

[claude]

Anthropic OpenAI-Compatible API Endpoint

Provider: Anthropic
Release Date: March 2026
Importance Rating: 6/10

Summary

Anthropic launched an OpenAI-compatible API endpoint, allowing developers to test and evaluate Claude models by changing just their API key, base URL, and model name in existing OpenAI SDK integrations — no other code changes required.

Key Features

• Drop-in evaluation: Point OpenAI SDK at Anthropic's endpoint with minimal code changes
• Core chat completions support: The compatibility layer covers core chat completions functionality
• Intended for testing: Anthropic recommends this for quick evaluation; native SDK remains preferred for production
• Accelerates Claude adoption: Dramatically lowers the barrier for developers already using OpenAI to test Claude models

Important Caveats

• System messages are concatenated (if placed at multiple conversation points)
• Temperature range capped at 1.0 (OpenAI accepts up to 2.0)
• No embeddings endpoint
• The n parameter (multiple completions per call) is not supported

Additional March 2026 Anthropic API Updates

• Structured Outputs GA: Now generally available for Claude Sonnet 4.5, Opus 4.5, and Haiku 4.5 — expanded schema support, improved latency, no beta header required
• Data Residency Controls: Developers can specify inference geography via inference_geo parameter; US-only inference available at 1.1x pricing for models released after Feb 1, 2026
• 1M Token Context Window Beta: Now available for Claude Opus 4.6 (in addition to Sonnet 4.5 and Sonnet 4)

Official Links

• Claude Developer Platform – API Release Notes
• Anthropic Release Notes – March 2026 (Releasebot)
• OpenAI SDK Support for Anthropic API