Agent design is still hard
Armin Ronacher’s Thoughts and Writings blog archive projects travel talks about Agent Design Is Still Hard written on November 21, 2025 I felt like it might be a good time to write about some new things I’ve learned. Most of this is going to be about building agents, with a little bit about using agentic coding tools. TL;DR: Building agents is still messy. SDK abstractions break once you hit real tool use. Caching works better when you manage it yourself, but differs between models. Reinforcement ends up doing more heavy lifting than expected, and failures need strict isolation to avoid derailing the loop. Shared state via a file-system-like layer is an important building block. Output tooling is surprisingly tricky, and model choice still depends on the task. Which Agent SDK…
