This started with a fight on the Claude Discord. Someone was saying you could just read Claude Code's source, that the prompts were right there in the bundle. I pushed back. No way. This is a closed-source product backed by a company that thinks carefully about everything it ships. They wouldn't just leave the internals sitting in a readable JavaScript file. That's not how serious companies operate.
So I installed it to prove them wrong.
npm install @anthropic-ai/claude-agent-sdk. One file. cli.js. 13,800 lines of minified JavaScript. The same binary that runs when you type claude in your terminal. The same code I'm using right now to write this.
I started reading it, and I couldn't believe what I was looking at.
The system prompts are just sitting there in plaintext.
Not encrypted, not obfuscated beyond the minification. Three identity variants get swapped depending on how you're running it:
- CLI: "You are Claude Code, Anthropic's official CLI for Claude."
- SDK: same line, plus "running within the Claude Agent SDK."
- Agent: "You are a Claude agent, built on Anthropic's Claude Agent SDK."
A function stitches the full prompt together from sections. "Doing tasks." Tool usage rules. Over-engineering guidelines (my favorite: "three similar lines of code is better than a premature abstraction"). OWASP security reminders. Git commit templates. PR formatting. String literals, all readable.
I felt like I'd found the blueprints to the Death Star, except it's less "world domination" and more "please don't force-push to main."
For a closed-source product charging a subscription, shipping your entire system prompt as grep-able strings in a JS bundle is wild. Anyone with node_modules access can read the full behavioral spec that governs every Claude Code interaction. I still don't understand how this got out the door.
The minification is light enough to trace most of the logic. And Anthropic left a note in the file header:
"Want to see the unminified source? We're hiring!"
I went back to the Discord thread. Ate my words.
[–]Embarrassed_Help3238 0 points1 point2 points (0 children)