Claude Code system prompt

The Claude Code system prompt here is (as usual) fascinating.

There are a lot of instructions on things it got wrong out of the box, like these:

  1. "Do what has been asked; nothing more, nothing less."

  2. "NEVER create files unless they're absolutely necessary for achieving your goal."

  3. "ALWAYS prefer editing an existing file to creating a new one."

but mostly it's surprising how few instructions there really are.

40% of the prompt are tool use instructions.

They're pretty standard, so let's skip those. There are some interesting bits like

  1. "ALWAYS prefer editing an existing file to creating a new one".

20% are development workflows.

There's a lot of stuff about GIT workflows and testing.

  1. "VERY IMPORTANT: run the lint and typecheck commands if they were provided"

Another 25% are behavioral instructions. Clearly developed while using it. Things like

  1. "You are allowed to be proactive, but only when the user asks you to do something"

Then there are some I found particularly interesting:

  1. "You MUST answer concisely with fewer than 4 lines of text"

  2. "One word answers are best" <- HA!

  3. "Avoid introductions, conclusions, and explanations"

  4. "DO NOT ADD ***ANY*** COMMENTS unless asked"

  5. "You should NOT answer with unnecessary preamble or postamble"

  6. "Do not add additional code explanation summary unless requested"

And finally, a ton of examples.

What I'm thinking:

  1. Reading system prompts is a great way to develop intuition on how to better use these tools.

  2. It's also a great way to get better at writing prompts. (For example, add examples.)

  3. But mostly, I'd LOVE to learn how the evals for this are set up.

# Aug 12, 2025
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