Building AI Agents? The Hard Part Might Not Be The AI Model

I have been exploring AI agents and automation workflows recently, and I noticed something interesting:
The hardest part is not always the AI model.
In many real projects, the bigger challenge is getting reliable data.
A demo may work perfectly, but production brings new problems:
Data sources change
Websites behave differently by region
Requests get limited at scale
Information becomes inconsistent
An AI agent can only be as good as the data it can access.
I think the next stage of AI development is not only about smarter models, but also about better infrastructure connecting AI systems with real-world data.
Curious to know:
What is the biggest challenge you have faced when building AI agents?