TL;DR

• ChatGPT 5.1 (GPT-5 series) is OpenAI's 2025 flagship, built for productionization (Responses API) and multimodal (text/image/audio).
• Compared to GPT-4o/4.1: adjustable reasoning depth, more stable function/JSON output, better long context and code/table comprehension.
• Typical combo: ChatGPT 5.1 for complex planning/generation, GPT-4o-mini for batch/automation, o1/o1-mini for long chain-of-thought reasoning.

When to Use

• Production-grade Q&A/chat (stable format output, low hallucination)
• Code/architecture design (combined with tool calling and file context)
• Multimodal comprehension (screenshots/flowcharts/tables) and report generation
• Scenarios where system prompt needs "adjustable reasoning depth/verbosity"

Prompt & API Tips

• Responses API: set reasoning.effort (e.g., medium / high) to balance quality and cost.
• Structured output: use tool_choice/response_format to constrain JSON schema; add a simple self-check to have the model verify fields when needed.
• Long context: compress or add headings before concatenating; for traceability, have the model output "evidence list/sources."

Comparisons & Selection

• Cost/latency: 5.1 > 4.1 > 4o, quality scales accordingly; batch/automation should prefer 4o-mini.
• Reasoning: chain-of-thought or rigorous planning can use o1; when combining with 5.1, let 5.1 draft and o1 do strict verification.
• Multimodal: screenshot/table comprehension 5.1 is roughly equal to 4.1, still stronger than 4o-mini.

Common Gotchas

• Large prompts still need throttling: batch, cache retrieval results; don't fill the window with meaningless context.
• Structured output failures: increase temperature=0-0.3, and write "verify before output" clearly in system prompt.
• Tool calling loops: add "max N rounds" to function calls with failure fallback responses.

References

• Introducing ChatGPT 5.1
• Responses API
• Function calling guide