How to use it.

§ 01 — What this is

What this is.

OpusX is an Anthropic-compatible proxy. It speaks the same `/v1/messages` protocol Claude does, accepts the same headers, and returns the same SSE events.

https://opusx.vercel.app/api
(Anthropic clients append /v1/messages → /api/v1/messages)

§ 02 — Quick start

Quick start.

curl -fsSL https://opusx.vercel.app/setup.sh | bash
irm https://opusx.vercel.app/setup.ps1 | iex

§ 03 — Authentication

Authentication.

curl https://opusx.vercel.app/api/v1/messages \
  -H "Content-Type: application/json" \
  -H "x-api-key: sk-ant-ox-..." \
  -H "anthropic-version: 2023-06-01" \
  -d '{"model":"claude-sonnet-4-6","max_tokens":256,"messages":[{"role":"user","content":"Hello"}]}'

§ 04 — Models

Models.

Any model Anthropic publishes can be requested through OpusX.

curl https://opusx.vercel.app/api/v1/models -H "x-api-key: sk-ant-ox-..."

§ 05 — Messages API

Messages API.

Identical shape to Anthropic's `/v1/messages` with prompt caching support.

§ 06 — Streaming

Streaming.

event: message_start
data: {"type":"message_start","message":{"id":"msg_..."}}

event: message_delta
data: {"type":"message_delta","delta":{"text":"Hello"}}

§ 07 — IDE setup

IDE setup.

Claude Code: set ANTHROPIC_BASE_URL=https://opusx.vercel.app/api. Cursor OpenAI-compatible mode: base URL https://opusx.vercel.app/api/v1 (requests go to /api/v1/chat/completions).

§ 08 — OpenAI-compatible

OpenAI-compatible.

POST /v1/chat/completions

§ 09 — Errors & limits

Errors & limits.

• 401 — Key missing, invalid, or disabled
• 429 — Per-minute rate limit (check `Retry-After`)
• 400 — Malformed request
• Window/budget limit — 200 with budget-exceeded style message