System Prompt
System Prompt
The standing instructions the model reads before any user message
✓ Runtime & SessionWhat is a System Prompt
The system prompt is the standing set of instructions the model reads before it ever sees the user's message, set by the harness rather than typed in chat. It covers who the agent is, what rules it follows, what tools exist, and, critically for this glossary, the name and description of every installed skill. It is present on every single turn of the session, not just the first.
This is why skill descriptions are described as always resident: they are baked directly into the system prompt the harness sends with every request. A project's own instructions file, like a CLAUDE.md, typically gets folded into the system prompt as well, which is why those files apply automatically without the user having to paste them in each time.
The system prompt is not something the user edits mid-conversation the way they would send a message. It is fixed for the session by the harness's configuration, though the harness can update it between sessions, for example by adding a newly installed skill's description the next time one starts.
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