feat: Add full Gemini OAuth support for subscriptions

- Update Google OAuth scopes to include generative-language.tuning and generative-language.retriever for Gemini API subscription features
- Create case study analysis for issue #66 in docs/case-studies/issue-66/

This enables OAuth authentication for Gemini subscriptions, similar to Claude Pro/Max support, allowing users to access subscription benefits through OAuth instead of only API keys.

Fixes #66

Author: konard

.changeset/fix-help-display.md

@@ -0,0 +1,53 @@
+# Case Study: Issue #66 - Full support for Gemini OAuth (subscriptions login)
+
+## Issue Summary
+
+The issue requests full support for Gemini OAuth authentication to enable subscriptions login, similar to the existing Claude Pro/Max OAuth support. Currently, the agent only supports API token authentication for Gemini.
+
+## Timeline of Events
+
+- **December 16, 2025**: Issue #66 opened requesting Gemini OAuth support
+- **Analysis**: The codebase already contains Google OAuth implementation in `src/auth/plugins.ts`, but the scopes were insufficient for Gemini API subscriptions
+
+## Root Cause Analysis
+
+1. **Insufficient OAuth Scopes**: The Google OAuth plugin only included basic scopes (`cloud-platform`, `userinfo.email`, `userinfo.profile`) but lacked the `generative-language.tuning` and `generative-language.retriever` scopes required for Gemini API subscription features.
+
+2. **Scope Mismatch**: While the implementation was present, the scopes didn't match those recommended in the official Gemini API OAuth documentation for advanced features and subscriptions.
+
+3. **Client Credentials**: The client ID and secret were sourced from the Gemini CLI reference, which may not be optimal for API usage, but appear to be compatible.
+
+## Proposed Solution
+
+Updated the `GOOGLE_OAUTH_SCOPES` array in `src/auth/plugins.ts` to include the additional generative-language scopes required for Gemini API subscriptions.
+
+### Code Changes
+
+```typescript
+const GOOGLE_OAUTH_SCOPES = [
+  'https://www.googleapis.com/auth/cloud-platform',
+  'https://www.googleapis.com/auth/userinfo.email',
+  'https://www.googleapis.com/auth/userinfo.profile',
+  'https://www.googleapis.com/auth/generative-language.tuning',
+  'https://www.googleapis.com/auth/generative-language.retriever',
+];
+```
+
+## Implementation Details
+
+- The Google OAuth plugin already supports the full OAuth flow with local server callback
+- The provider logic correctly detects OAuth credentials and uses Bearer token authentication
+- The implementation follows the same pattern as Anthropic OAuth support
+
+## Testing Recommendations
+
+1. Test OAuth login flow: `agent auth login` → select Google → complete OAuth flow
+2. Verify that Gemini models work with OAuth credentials
+3. Confirm that subscription benefits (higher limits, premium features) are accessible
+4. Test fallback to API key when OAuth is not configured
+
+## References
+
+- Reference implementation: `reference-gemini-cli/packages/core/src/code_assist/oauth2.ts`
+- Gemini API OAuth documentation: https://ai.google.dev/gemini-api/docs/oauth
+- Current agent OAuth: `src/auth/plugins.ts` GooglePlugin

docs/case-studies/issue-66/case-study.md

@@ -849,6 +849,8 @@ const GOOGLE_OAUTH_SCOPES = [
   'https://www.googleapis.com/auth/cloud-platform',
   'https://www.googleapis.com/auth/userinfo.email',
   'https://www.googleapis.com/auth/userinfo.profile',
+  'https://www.googleapis.com/auth/generative-language.tuning',
+  'https://www.googleapis.com/auth/generative-language.retriever',
 ];
 
 // Google OAuth endpoints