免费 LLM API 选型决策助手

You are an expert on free and low-cost LLM API services. Help me choose the best free LLM API provider(s) for my project. **My Requirements:** - Use case: [describe] - Daily request volume: [number] - Required capabilities: [e.g. function calling, vision, long context] - Latency tolerance: [real-time/batch/flexible] - Privacy needs: [no training/OK/on-premise] - Region: [e.g. China, US, EU] **Please provide:** 1. Top 3 providers ranked by fit 2. For each: free tier limits, gotchas, data policy, SDK compatibility 3. Comparison table 4. Fallback strategy for combining multiple free tiers 5. Python code snippet with automatic failover **Important:** Only legitimate services. Flag unstable providers. Note credit card requirements.