RelayRouter 401 invalid API key error: causes and fixes

A 401 invalid API key error on RelayRouter means the request reached the gateway but the credential failed authentication. The common causes are a missing or malformed Authorization header, a wrong or revoked key, or a key sent to the wrong base URL after migration. Fix it by supplying a valid Authorization: Bearer YOUR_API_KEY, confirming the key at your dashboard, and pointing the base URL correctly. Failed or errored requests are generally not billed.

What causes the 401 error

The 401 comes from a credential problem, not from your model choice or payload. Most cases fall into three groups: the header is absent or misspelled (it must be Authorization: Bearer YOUR_API_KEY), the key value is wrong, revoked, or copied with extra whitespace, or the request goes to a base URL that does not match your intended protocol. RelayRouter supports two protocol surfaces: OpenAI compatible at /v1/chat/completions and Anthropic compatible at /v1/messages. According to the official relayrouter.io docs, RelayRouter is 「Compatible with both the OpenAI and Anthropic protocols」, so verify that your SDK targets the correct endpoint for the key in use.

How to fix it, step by step

Resolve the 401 by checking the credential and base URL in order.

1. Create or copy a valid key from your RelayRouter dashboard and remove any leading or trailing spaces.
2. Set the header exactly as Authorization: Bearer YOUR_API_KEY.
3. Confirm the base URL matches the protocol: OpenAI at /v1/chat/completions or Anthropic at /v1/messages.
4. Retry a minimal request against your chosen model listed at relayrouter.io/models.

According to the official relayrouter.io/docs docs, migration is simple: 「Keep your existing SDK, change base_url and the key, no other code changes」. If a step is skipped during that swap (often the key), a 401 is the typical result. Because failed or errored requests are generally not billed, retrying while you debug does not add charges.

Endpoint and protocol checklist

A 401 often traces to a mismatch between the key, the SDK, and the endpoint, so confirm all three together.

Both surfaces accept models across the Claude family, GPT-5.5, and Gemini 3.5, plus DeepSeek, GLM, MiniMax, and Moonshot. See relayrouter.io/docs for the migration reference. When you keep your existing SDK and change only the base URL and the key, the two changes that matter for authentication are the base URL (which selects the protocol) and the key value (which passes the 401 check).

Frequently asked questions

Does a 401 error cost me anything? No. Failed or errored requests are generally not billed, so authentication failures during debugging do not add charges.

Which endpoints does the key work with? The same key format is used across both the OpenAI compatible /v1/chat/completions and the Anthropic compatible /v1/messages endpoints; match the base URL to your SDK.

Do I need to rewrite my code to fix migration 401s? No. Keep your existing SDK and change only the base URL and the key, with no other code changes.