GPT OSS vs Premium Models for Agentic Applications

Summary:

I am evaluating open-source LLMs (OSS) versus premium models such as GPT-4.1 Mini and GPT-5 Mini for an agentic AI application involving business objects and RAG workflows.

From a practical and production perspective:

How do OSS models compare with GPT-4.1 Mini/GPT-5 Mini in reasoning, planning, and task completion?
Are there significant differences in tool-calling accuracy and hallucination rates?
What are the latency and cost trade-offs at scale?
How do they compare in handling long-context and multi-agent workflows?
What has been your experience regarding reliability, scalability, and production readiness?