A Soul in a File

An agent is more than its model. Two systems can use the same endpoint, have the same tools and receive the same question - and still answer completely differently. The difference sits in a file that exists in almost every agent setup and that hardly anyone takes seriously: the soul.md.

It describes who the agent is. Not what it knows, not what it is allowed to do, but how it shows up: tone, stance, how it handles uncertainty, what it offers unprompted and what it would rather ask about first. In practice that file turns into a dumping ground - factual knowledge, project rules, formatting requirements and character description all end up in the same text, and afterwards nobody can say which part produced which effect.

Same world, different soul

This course makes the difference visible instead of asserting it. In the playground you ask the same question to several agents that differ only in their soul.md: one has none at all, one a minimal file, one a carefully written one, one an overloaded one. The answers sit side by side, and the effect is sharper on the first run than most people expect - especially the overloaded case, which shows that more text does not mean more character.

Out of that comes a practical rule: what belongs in the soul.md and what belongs elsewhere? Facts belong in the context or in a retrieval step, hard rules belong in tool guards and checks, recurring formatting wishes belong in the prompt of the task at hand. What is left is short - and precisely that short part is what works.

What you need

Nothing to install, nothing to program. The playground runs in the browser, you can edit the soul files directly and ask your own question. Some familiarity with language models helps but isn't required; if you have ever given a chatbot a system instruction, you are already in the middle of the topic.

At the end you have a soul.md you can carry over to your own agent - and, more importantly, a feel for when a change to it actually does something and when it only makes the context longer.