Microsoft Semantic Kernel

Semantic Kernel is a free, open-source developer kit that adds AI agents to applications written in C#, Python, or Java. Developers describe existing functions and APIs to it as 'plugins,' and it acts as middleware that lets an AI model call those functions and pass results back, so existing code does not need rewriting. It has reached a stable 1.0-plus release across all three languages, and Microsoft has committed to avoid breaking changes going forward. It includes hooks and filters that add safety and monitoring checks around what an agent does. Microsoft's documentation states that Microsoft and other large companies use it internally.

This fits a development team, especially one already working in C# or the Microsoft stack, that wants to add an agent on top of existing business logic rather than rewrite it as a new AI-first application. A good first project is an internal assistant that calls existing internal APIs to look up orders or update records. There is no license fee; the ongoing cost is developer time plus the API usage of whichever model it connects to, such as Azure OpenAI. The main risk is that being lightweight means fewer built-in guardrails than a packaged enterprise product, so plan to build testing and approval steps for anything that changes data, not just anything that answers questions.