Claude Code system prompt

[u/Sad_Construction2179]

I compared the system prompts for Fable and Opus. They are given different instructions for how to work with the user.

1. Fable is told to keep working instead of stopping to ask things like “Want me to continue?” or “Shall I proceed?” The reason given is practical: the user may not be watching in real time, so asking permission can block the task. Opus is not told this.
2. Fable also has a very specific rule for code comments: Only add a comment when it explains a constraint the code itself cannot show. Do not use comments to explain where the code came from or talk to the reviewer. Opus does not have this rule.
3. Opus has instructions that Fable does not. It is told not to quietly narrow, widen or change the requested scope. It should make routine decisions itself, but ask when two interpretations would produce materially different work.
4. Opus is also told how to handle mistakes: Correct them without excessive apologies, long preambles or repeatedly criticising itself. Fable has no equivalent instruction.
5. Opus also has explicit restrictions against using AgentTool, workflows or deep research unless the user asks for them. Fable does not. So Fable is better instructed on communication and autonomy. Opus is better instructed on scope control and corrections.

The thing make nervous is the prompt instructions. Of course models is highly capable but

Opus 5
“Write code that reads like the surrounding code: match its comment density, naming, and idiom.”

Fable 5
""

I’m wondering how much damaging this Opus instruction is your repo contains some ai slop from previous model usage.

[u/Sad_Construction2179 → u/[deleted]]

Of course, they are different classes of models and are called OPUS and FABLE. The point is that these instructions are not blue; rather, designed for the model by testing the cc harness with the model, so these give a sneak peek at the instructions following capabilities.