Claude
Loves XML tags. Long-context strong. Prefers explicit persona. Refuses more on unclear ethical requests. 'Think step by step' still helps.
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GPT-4/5
Prefers JSON schema for structured output. Function calling most mature. Strong at code. Watch for verbose output; ask for concise explicitly.
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Gemini
Multimodal-native. Long context (2M+). Handles video. Prompt caching + system instructions supported. Grounding built-in.
Open models (Llama, Qwen)
Follow instruction format precisely ([INST], <|im_start|>). System prompt handling varies. Test with vLLM/llama.cpp.