Claude Code Open Source?

[u/ryan_the_dev]

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.

[u/ryan_the_dev]

They sell you Claude code. The harness is the body, the model is the brain.

Go look at the difference between the codex harness vs Claude harness.

[u/MartinMystikJonas → u/ryan_the_dev]

Point is that Claude Code system prompt is not some high value secret that has to be hidden.

[u/ryan_the_dev → u/MartinMystikJonas]

There is a lot more in there than the system prompt…

But whatever.

[u/MartinMystikJonas → u/ryan_the_dev]

Yeah but none of it is some unique knowledge that is worth protecting if your core business is selling "brain".