Authentication

Authenticate every request with your API key in the Authorization header. No Bearer prefix. How to keep the key safe and what each authentication error means.

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How it works

Every endpoint requires your API key in the Authorization header. Send the key exactly as the dashboard shows it.

http

Authorization: YOUR_FLUENTC_KEY

bash

curl https://api.fluentc.ai/ai_agent/languages \
  -H "Authorization: $FLUENTC_KEY"

Warning: Do not add a Bearer prefix. Authorization: Bearer YOUR_FLUENTC_KEY is read as a different, unknown key and fails with 401 and "Invalid API key: Site not found." Many HTTP clients and agent frameworks add Bearer automatically when you set a token, so set the header by hand.

You get a key when you sign up. The same key works for the REST API, the MCP server, the LangChain tools, the n8n node, and Make or Zapier.

What a key controls

A key belongs to one FluentC site in your account, and the site decides what the key can do:

Languages. Only languages enabled for the key can be used as target_language or source_language. List them with Languages.
Mode. A Batch plan key can use batch mode. A Real-time plan key can use both real-time and batch. See Concepts.
Status. If the subscription is inactive, translation is disabled for the site, or a usage limit is reached, requests fail with 403.

Keeping the key safe

Call FluentC from a server, a workflow runner or an agent runtime. Never ship the key in a browser, a mobile app or a public repository.
Store it in an environment variable or a secret manager. The examples use FLUENTC_KEY.
Use a separate key per environment when you can, so a leak in one place doesn't affect the others.
If you think a key has been exposed, contact us right away.

Authentication errors

Status	Message	Cause
`401`	`API key (Site UUID) is missing`	The `Authorization` header is absent or empty.
`401`	`Invalid API key: Site not found.`	The key is wrong, has extra characters, or has a `Bearer` prefix.
`401`	`Invalid API key: Associated company not found.`	The key's account can't be resolved. Contact us.
`403`	`Access denied: Translations are disabled for this site.`	Translation is switched off for the key's site.
`403`	`Subscription error: ...`	The subscription isn't active. The message says why.
`403`	`Usage limit error: ...`	A usage limit was reached. The message says which.

The API calls the key a "Site UUID" in some messages. It is the same thing as the API key in the dashboard.

Don't retry a 401 or 403 without changing something. Fix the key or the account first. All error shapes are on the Errors page.

Something unclear, or missing? Tell us.