# Logging in to Afnio Backend

Afnio is designed for seamless agent development: your workflow logic and forward pass run locally, while language models (LM) calls and optimization (backward pass) are securely handled by the Afnio backend, hosted on [Tellurio Studio](https://platform.tellurio.ai/). This enables scalable experiment tracking and advanced optimization features without extra setup.

To use Afnio—whether for inference, evaluation, or optimization—you’ll need to log in with your [Tellurio API key](https://platform.tellurio.ai/settings/api-keys). Your credentials are securely stored in your system keyring after login.

For more details on experiment tracking and optimization, see [Runs and Experiments](runs_and_experiments).

## Login via CLI (Recommended)

The fastest way to log in is using the command line:

```bash
afnio login --relogin
```

_Output:_

```output
Please enter your Tellurio API key and hit enter, or press ctrl+c to quit:
[afnio] API key provided and stored securely in local keyring.
[afnio] Currently logged in as 'username' to 'https://platform.tellurio.ai'. Use `afnio login --relogin` to force relogin.
```

You will be prompted for your Tellurio API key (available on the Tellurio Studio overview page or under [API Keys](https://platform.tellurio.ai/settings/api-keys)).
Once logged in, your credentials are securely stored in your system keyring for future sessions.

## Login via Python Code

You can also log in programmatically from your script or notebook:

```python
import afnio.tellurio as te

te.login(api_key="YOUR_TELLURIO_API_KEY", relogin=True)
```

_Output:_

```output
INFO     : API key provided and stored securely in local keyring.
INFO     : Currently logged in as 'username' to 'https://platform.tellurio.ai'. Use `afnio login --relogin` to force relogin.
```

You only need to log in once per session.

If you change your API key or want to force a new login, use `relogin=True`.

## Login via Environment Variable

You can also log in by setting the `TELLURIO_API_KEY` environment variable before starting your Python session or running CLI commands:

```bash
export TELLURIO_API_KEY="YOUR_TELLURIO_API_KEY"
```

## Credential Storage and Resolution

- Credentials and preferences (such as your most recent username and consent to share your API key with the server) are stored in `~/.tellurio_config.json` in your home directory.
- When logging in via Python, Afnio looks for your API key in the following order:
  1. The `api_key` argument passed directly to `te.login()`
  2. The `TELLURIO_API_KEY` environment variable
  3. Your system keyring (if previously stored)

## Troubleshooting

- If you see authentication errors, check that your API key is set correctly.
- You can store your API key securely in your environment or use a password manager.

## Further Reading

- [Runs and Experiments](runs_and_experiments)