## TL;DR

Twilio 20003 means the request carried no credentials at all: the Account SID or auth token is missing from the client config. Set both and retry.

## Error

```text
Twilio error 20003: Authentication Error - No credentials provided
```

## Steps

1. Check the client init for Account SID and auth token Expected: you find the missing one.
2. Set both from config (never hardcode) Expected: the client is credentialed.
3. Retry the SMS send Expected: it succeeds.
4. Add a startup check that fails fast when credentials are absent Expected: it cannot regress.

## When to use

- Twilio 20003 on send
- Credentials are missing from the config

## When not to use

- Credentials present but wrong (that is a different error code)
- Subaccount confusion (then check which account the number belongs to)

## Tool compatibility

- Twilio auth
- SMS integrations

## Variant phrasings

### twilio 20003

No credentials were sent. Set the Account SID and auth token.

### twilio no credentials provided

Same: the client init is missing auth.

## Why it happens

Agents scaffold the send call and fill credentials 'later', then later never comes. Twilio distinguishes missing from wrong, which actually helps.

## Edge cases

- API keys can substitute for the auth token; either works
- Test credentials only work with test endpoints

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_zo7B5uwRiBat2jjFbO3HhA