Modern language-model agents are built around the agent loop: the LLM is placed in an environment exposing a set of tools, and the LLM has full control over the workflow by alternating between tool calls and observing their output. However, certain capabilities such as long-term memory and self-improvement currently require specialized systems beyond the agent loop itself. We built an LLM agent framework, JAZ, to explore the extent to which a minimal harness that is little more than the agent loop itself can accomplish tasks these specialized systems are built for. JAZ exposes a single LLM-based primitive `invoke` and provides a set of built-in hooks that allow the programmer to apply constraints and perform monitoring. Generalizing existing code-mode agent loops, `invoke` is the simplest loop that satisfies two defining properties: (1) the LLM can write arbitrary executable code that can include recursive `invoke`; (2) everything visible to the LLM - all inputs to `invoke` as well as its interaction history with the code environment - are variables in the code environment. We motivate our design from first principles, viewing `invoke` as a language primitive representing a function whose implementation is provided at runtime by an LLM every time it is called. To validate the design of our core `invoke` primitive, we evaluate `invoke` - with only prompting, no manually designed tools, harness, or external systems (e.g., memory or the file system) - on workflows traditionally implemented through specialized harnesses. On long-horizon workflows requiring recall far beyond the context window, JAZ `invoke` outperforms Letta (MemGPT) by 8% at half its cost on the recall-heavy portion of StuLife. On continual self-improvement, JAZ `invoke` outperforms ACE by 4% at a lower cost on AppWorld.

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