fix(llm): unify all providers to use VERTEX_API_KEY only

[ComBba]

Summary

• 모든 LLM provider가 VERTEX_API_KEY만 사용하도록 통합
• Grand Tasting (75-Technique) 모드의 401 UNAUTHENTICATED 에러 수정

Root Cause

GOOGLE_CLOUD_PROJECT 환경변수가 설정되면 LangChain SDK가 ADC (Application Default Credentials) lookup을 시도하여 실패함.

Changes

• backend/app/providers/llm.py: gemini/vertex provider 로직 통합, 모두 vertexai=True 사용
• docs/VERTEX_AI_CONFIGURATION.md: 설정 문서 추가

Configuration

# .env (only VERTEX_API_KEY required)
VERTEX_API_KEY=AQ.Ab8RN6KKSS-...

# Do NOT set:
# GOOGLE_CLOUD_PROJECT=xxx  (triggers ADC lookup failure)

Test Results

[1] provider='vertex' + gemini-3-flash-preview: SUCCESS
[2] provider='gemini' + gemini-3-flash-preview: SUCCESS
[3] provider='vertex' + gemini-3-pro-preview: SUCCESS

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Project	Deployment	Actions	Updated (UTC)
somm-dev	Ready	Preview, Comment	Feb 9, 2026 2:14pm

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[gemini-code-assist]

Summary of Changes

Hello @ComBba, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request streamlines the integration of Google-based Large Language Models (LLMs) by standardizing the API key usage across all providers to VERTEX_API_KEY. This change eliminates the need for separate GEMINI_API_KEY and resolves a critical 401 authentication error that occurred when the GOOGLE_CLOUD_PROJECT environment variable was set, causing an erroneous Application Default Credentials lookup. The update simplifies configuration, enhances reliability, and includes comprehensive documentation for the new setup.

Highlights

• API Key Unification: All LLM providers are now configured to exclusively use VERTEX_API_KEY, simplifying API key management and removing the need for a separate GEMINI_API_KEY.
• Authentication Fix: Resolves a 401 UNAUTHENTICATED error, particularly affecting the 'Grand Tasting (75-Technique)' mode, by preventing an erroneous Application Default Credentials (ADC) lookup.
• Root Cause Addressed: The issue was traced to the GOOGLE_CLOUD_PROJECT environment variable inadvertently triggering a failed ADC lookup, which is now explicitly advised against.

Changelog

• backend/app/providers/llm.py
  • Consolidated the logic for gemini and vertex providers into a single, unified block.
  • Ensured all LLM instantiations use VERTEX_API_KEY and vertexai=True for consistent behavior.
  • Removed the conditional branching for gemini and vertex specific kwargs dictionaries.
  • Updated error handling to check for VERTEX_API_KEY availability universally.
• docs/VERTEX_AI_CONFIGURATION.md
  • Introduced a new documentation file detailing the unified Vertex AI configuration.
  • Provided guidance on environment variables, explicitly stating GOOGLE_CLOUD_PROJECT should not be set.
  • Listed supported models and their thinking_level configurations.
  • Documented test results confirming the fix.
  • Explained the root cause of the previous 401 errors and the solution.
  • Summarized the code changes in llm.py and .env implications.

Activity

• The pull request was created by ComBba.
• It addresses a critical authentication issue with LLM providers.
• The changes involve refactoring LLM provider integration logic and adding new documentation.
• The author has provided detailed summary, root cause, changes, configuration, and test results in the PR description.

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[gemini-code-assist]

Code Review

이 PR은 'gemini'와 'vertex' LLM provider 로직을 VERTEX_API_KEY만 사용하도록 성공적으로 통합했습니다. 이를 통해 설정이 단순화되고 ADC 조회 실패로 인한 401 인증 오류가 해결되었습니다. 코드가 더 깔끔해지고 유지보수하기 쉬워졌습니다. 하드코딩된 기본 모델 이름을 제거하여 유지보수성을 더욱 향상시키기 위한 제안을 하나 추가했습니다. 새로 추가된 문서 파일도 매우 명확하고 유용합니다.

[gemini-code-assist]

기본 모델명인 gemini-3-flash-preview가 여기에 하드코딩되어 있습니다. 이 값은 이미 PROVIDER_DEFAULTS에 정의되어 있으므로, 딕셔너리를 직접 사용하면 향후 기본 모델 변경 시 한 곳만 수정하면 되어 유지보수성이 향상됩니다. provider_key가 유효한지 이미 앞에서 확인하고 있으므로, KeyError 발생 없이 PROVIDER_DEFAULTS[provider_key]를 안전하게 사용할 수 있습니다.

    resolved_model = model or PROVIDER_DEFAULTS[provider_key]