Anthropic

Repoint your Anthropic Messages client at AnyRouter by changing the base URL and API key. Your messages.create calls keep the native Anthropic body shape, and the anthropic-version header stays required. This guide is for teams already on the Anthropic API who want unified billing and access to every Claude model through one key.

Before you start

• An AnyRouter API key (prefixed sk-ar-). Create one in the dashboard.
• An existing Anthropic Messages client (@anthropic-ai/sdk or raw HTTP).

What changes

Before and after

Only the base URL, key, and model id change — the request body is native Anthropic.

import Anthropic from "@anthropic-ai/sdk"

const client = new Anthropic({
  baseURL: "https://api.anthropic.com",
  apiKey: process.env.ANTHROPIC_API_KEY,
})

const message = await client.messages.create({
  model: "claude-sonnet-4-5",
  max_tokens: 1024,
  messages: [{ role: "user", content: "Review this incident summary." }],
})

import Anthropic from "@anthropic-ai/sdk"

const client = new Anthropic({
  baseURL: "https://anyrouter.dev/api",
  apiKey: process.env.ANYROUTER_API_KEY,
})

const message = await client.messages.create({
  model: "anthropic/claude-sonnet-4.6",
  max_tokens: 1024,
  messages: [{ role: "user", content: "Review this incident summary." }],
})

Migrate

Verify

Send one request against the Messages endpoint with your new key:

curl https://anyrouter.dev/api/v1/messages \
  -H "x-api-key: sk-ar-your-key" \
  -H "anthropic-version: 2023-06-01" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "anthropic/claude-haiku-4.5",
    "max_tokens": 512,
    "messages": [{"role": "user", "content": "Say hello in one sentence."}]
  }'

The Messages endpoint accepts native Anthropic request bodies — same shape, same fields. For cross-provider routing instead, use Chat Completions or Responses.

Migrate with an agent prompt

Paste this into a coding agent (Claude Code, Cursor, Codex, …) to repoint the Anthropic client across the project:

Migrate this project from the Anthropic API to AnyRouter.

1. Change the Anthropic SDK base URL to https://anyrouter.dev/api
   (the SDK appends /v1/messages — the base URL must stop at /api).
2. Swap ANTHROPIC_API_KEY for ANYROUTER_API_KEY (prefix sk-ar-). Never hardcode it.
3. Keep the `anthropic-version: 2023-06-01` header — it is still required.
4. Rewrite model ids to the anthropic/ form:
     claude-sonnet-4-5 → anthropic/claude-sonnet-4.6
     claude-haiku      → anthropic/claude-haiku-4.5
5. messages.create calls stay unchanged — native Anthropic body shape.
6. Keep streaming and error handling exactly as they are.
7. Add a smoke test that sends one short prompt and prints the reply.

Only swap the provider config — do not change UI or business logic.

Troubleshooting

Related

• Messages API — native Anthropic endpoint reference.
• Claude Code — route the Claude Code CLI through AnyRouter.
• Models API — list every available model.