“A complex system that works is invariably found to have evolved from a simple system that worked.”John Gall · Systemantics: How Systems Work and Especially How They Fail · 1975 · Systemantics: How Systems Work and Especially How They Fail (1975) — «legea lui Gall»
Start from the hundred-line raw loop; adopt a framework when you need durable state, human pauses or routing.
The 2026 open-source landscape, as examples: graph orchestrators with state and checkpoints (LangGraph), agents with typed inputs and outputs (PydanticAI), role-based crews (CrewAI), agents that write code instead of calling tools (smolagents), the model labs' SDKs (OpenAI Agents SDK, Google ADK, Microsoft Agent Framework — the autumn-2025 merger of AutoGen and Semantic Kernel — Claude Agent SDK). The criteria: can you see every model call and tool call? can you pause, resume, replay? does it tie you to one model provider? how much context does it add on its own? Start from the raw loop over the API (a hundred lines); adopt a framework when you need durable state, pauses for a human, or multi-agent routing. Beware "magic" that hides prompts you will have to debug.
Why it matters The wrong framework shows in month three, when you cannot explain why the agent did what it did.