What does “Anthropic API compatible” actually mean?

“Compatible” should mean more than accepting one JSON request. For a coding assistant, the boring details are the product.

The minimum contract

A useful gateway accepts Anthropic Messages requests at /v1/messages, understands system prompts, content blocks and tool definitions, and returns the response shape the client expects.

Streaming is not optional

Claude Code relies on server-sent events so text and tool activity arrive incrementally. A proxy that only returns a final JSON blob may technically answer, but the terminal experience will feel broken.

Tools must round-trip cleanly

Read, Bash and edit tools carry structured input and results. A good adapter preserves tool names, schemas, IDs and stop reasons. Losing one of those fields can cause duplicate calls or stalled sessions.

How to evaluate a gateway

• Run a simple text request.
• Run a long streaming request.
• Ask the agent to use a tool.
• Force an error and check that it is actionable.
• Measure latency and cost on your real workload.

Related guides

• Anthropic API Alternative: Same Models, Saner Billing
• How to use Claude Code with a custom API endpoint