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Root cause: load_dotenv() default override=False meant system-level ANTHROPIC_BASE_URL (https://api.deepseek.com/anthropic) took precedence over .env's OPENAI_BASE_URL (https://api.minimaxi.com/anthropic). All Anthropic API calls went to DeepSeek with a MiniMax key, causing 401. Changes: - backend/llm.py: load_dotenv(override=True) — .env always wins - .env.example: add explicit ANTHROPIC_API_KEY + ANTHROPIC_BASE_URL - CLAUDE.md: document env var priority pitfall