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.