Anthropic /v1/messages call fails on RelayRouter: header and version issues

When an Anthropic /v1/messages call fails on RelayRouter, the cause is usually the request base and authentication header rather than the model. RelayRouter exposes the Anthropic protocol at POST /v1/messages with base https://relayrouter.io, and authentication uses Authorization: Bearer YOUR_API_KEY. Keep your existing Anthropic SDK, point the base URL at RelayRouter, and swap the key. Failed or errored requests are generally not billed, so you can retry safely.

Which endpoint and base does the Anthropic protocol use

The Anthropic protocol on RelayRouter is served at POST /v1/messages with the base https://relayrouter.io (note there is no /v1 segment in the base). RelayRouter is "Compatible with both the OpenAI and Anthropic protocols" (relayrouter.io), so a 404 on /v1/messages often means the OpenAI base (https://relayrouter.io/v1, used for POST /v1/chat/completions) was set instead. Confirm you are targeting the Anthropic base, then send your /v1/messages body unchanged. Live per-model routing details are listed at relayrouter.io/models. Mismatched bases are the most common reason a call returns an error before it reaches a model.

Which header format resolves authentication and version errors

Use Authorization: Bearer YOUR_API_KEY to authenticate the Anthropic /v1/messages call on RelayRouter. If you copy a native Anthropic setup, you may have supplied an x-api-key header or an incorrect version header, which can produce header or version errors. RelayRouter guidance is to "Keep your existing SDK, change base_url and the key, no other code changes" (relayrouter.io/docs), meaning the migration is limited to base URL and key. Create or rotate keys at your dashboard, then set the Authorization header exactly as shown. See relayrouter.io/docs for the migration steps. A 401 or 403 typically indicates a missing Bearer prefix or an invalid key.

How to fix a failing /v1/messages call step by step

Correct the base, header, and model reference in that order to resolve most failures.

1. Set the base URL to https://relayrouter.io for the Anthropic protocol.
2. Send the request as POST /v1/messages.
3. Set the header Authorization: Bearer YOUR_API_KEY (include the Bearer prefix).
4. Reference a supported model, for example a Claude line model such as claude-opus-4-8.
5. Retry: failed or errored requests are generally not billed.

How the two protocols compare on RelayRouter

The Anthropic and OpenAI protocols differ in base and path, which matters when a call fails.

Both protocols draw from the same model coverage, including the Claude family, gpt-5.5 and Gemini 3.5, plus DeepSeek, GLM, MiniMax and Moonshot. Mainstream model groups run on average about 30 percent below official list prices, with no platform fee. Streaming is supported on both.

FAQ

Why does my /v1/messages call return 404? The base is likely set to https://relayrouter.io/v1 (the OpenAI base). For the Anthropic protocol, use base https://relayrouter.io with path /v1/messages.

Which authentication header should I send? Send Authorization: Bearer YOUR_API_KEY, including the Bearer prefix. A missing prefix or invalid key produces authentication errors.

Am I charged if the request errors during debugging? No. Failed or errored requests are generally not billed, so you can retry while fixing header or version issues.