Claude, Auto Updater.

When Claude runs a Skill, is the logic driving it baked into the model itself, some hidden capability, or is it written down somewhere a person could actually read? It's the latter. Every Claude Skill is a folder containing one file, SKILL.md, written in plain sentences rather than code, and that file is the whole program.

The file is the program

Claude reads SKILL.md and follows its steps in order, top to bottom, branching only where a step explicitly says to. This holds for the auto-updater skill just as it holds for any other Skill. Open the file and you'd find something like a short numbered list of steps. There's no separate hidden layer filling in gaps or making creative decisions the file doesn't describe.

What happens if a step is missing

Delete a step from SKILL.md and it simply doesn't happen. Nothing hidden fills the gap, because there's nothing hidden to begin with. This is a useful way to understand the mechanism: the file isn't a hint or a suggestion Claude interprets loosely, it's closer to a literal script. If the script doesn't say to do something, it doesn't get done.

Same input, same result, inside the spec

Run the same input through a Skill twice and you get the same steps executed and the same result both times. That reliability is real and it's valuable: you can predict what a Skill will do because the file tells you exactly what it will do. But the guarantee only holds for input that falls inside what the file actually describes.

Where the reliability ends

Step outside the scope the file was written for, and Claude has nothing written there to fall back on. This isn't a flaw specific to auto-updater; it's a structural property of how Skills work. A Skill is reliable exactly to the boundary of its own written instructions, and unsupported past that boundary. Knowing where that boundary sits, by reading the file, is how you know what you can actually trust the Skill to do.

Trying it yourself

A concrete way to see this is to ask Claude to write a SKILL.md with five short numbered steps for a repeatable task you do often, have it read the steps back to you before running anything, and then watch whether every action it takes maps to a specific line in that file. If it stops the moment the steps run out rather than improvising further, that's the mechanism working as designed.

Key takeaways

• Every Claude Skill's logic lives in one plain-language file, SKILL.md, not in hidden model behavior.
• Claude follows the file's steps in a fixed, linear order, branching only where a step says to.
• Deleting a step means it doesn't run; there's no hidden fallback filling gaps.
• The same input reliably produces the same result, but only within what the file describes.
• Stepping outside the file's described scope leaves Claude with nothing to fall back on.

Who this is for

Anyone trying to understand what a Claude Skill actually is before relying on one, especially people evaluating whether a Skill's behavior is predictable enough to trust for a repeatable task.