Documentation incorrectly states Vertex AI supports API key authentication

[sergenX1]

Description

The documentation at /docs/cli/authentication.md states that Vertex AI supports Google Cloud API keys, but the actual Vertex AI service rejects API keys with "API keys are not supported by this API" error.

Current Documentation Says

Under the "Vertex AI" section:

Obtain your Google Cloud API key: Get an API Key
Set the GOOGLE_API_KEY environment variable...

This links to https://cloud.google.com/vertex-ai/generative-ai/docs/start/api-keys

Actual Behavior

When using a Google Cloud API key with Vertex AI endpoint:

curl -X POST \
  "https://us-central1-aiplatform.googleapis.com/v1/projects/PROJECT_ID/locations/us-central1/publishers/google/models/gemini-2.0-flash-exp:generateContent?key=API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"contents": [{"role": "user", "parts": [{"text": "test"}]}]}'

Returns:

{
  "error": {
    "code": 401,
    "message": "API keys are not supported by this API. Expected OAuth2 access token or other authentication credentials that assert a principal. See https://cloud.google.com/docs/authentication",
    "status": "UNAUTHENTICATED"
  }
}

Confusion Points

1. The Gemini CLI docs say Vertex AI supports API keys
2. The linked Vertex AI documentation mentions API keys (though it may be referring to service account keys)
3. The actual Vertex AI API endpoint rejects API keys entirely

Suggested Fix

Update documentation to clarify:

• Vertex AI only supports OAuth2/ADC/Service Account authentication
• API keys are only supported by Google AI Studio (generativelanguage.googleapis.com)
• Remove or clarify the GOOGLE_API_KEY environment variable section under Vertex AI

Impact

Users waste time trying to configure API keys that will never work with Vertex AI, leading to frustration and confusion about why their valid API key is being rejected.

[jerop]

Related issue: #3924

[srithreepo]

@sergenX1 we recently updated the documentation to hopefully clarify this.

#6193

Would you mind giving it a spin and providing us with feedback ?

[ElliotK-IB]

Hi all. I was just able to use a Vertex AI API Key to authenticate.

I'm an existing user at Google Cloud and I have a project set up with billing.

I followed the documentation to create an API key (https://cloud.google.com/vertex-ai/generative-ai/docs/start/api-keys).

This was my first time setting up Gemini CLI and the API key. Would be happy to elaborate further as needed, but Vertex AI authentication via API key seems like a relatively recent development (correct me if I'm wrong). I don't see much discussion or awareness of this method yet, but it worked for Gemini CLI authentication.