I'm surprised this post has so many upvotes. This is a gross oversimplification of what Claude Code (and other agents can do). On top of that, it's very poorly engineered.
How to code Claude Code in 200 lines of code
191–200 of 249 posts
Re: How to code Claude Code in 200 lines of code
#192Seems everyone is working on the same things these days. I built a persistent Python REPL subprocess as an MCP tool for CC, it worked so insanely well that I decided to go all the way. I already had an agentic framework built around tool calling (agentlib), so I adapted it for this new paradigm and code agent was born.
The agent "boots up" inside the REPL. Here's the beginning of the system prompt:
>>> help(assistant)
You are an interactive coding assistant operating within a Python REPL.
Your responses ARE Python code—no markdown blocks, no prose preamble.
The code you write is executed directly.
>>> how_this_works()
1. You write Python code as your response
2. The code executes in a persistent REPL environment
3. Output is shown back to you IN YOUR NEXT TURN
4. Call `respond(text)` ...
I also built a browser control plugin that puts Claude directly into the heart of a live browser session. It can inject element pickers so I can click around and show it what I'm talking about. It can render prototype code before committing to disk, killing the annoying build-fix loop. I can even SSH in from my phone and use TTS instead of typing, surprisingly great for frontend design work. Knocked out a website for my father-in-law's law firm (gresksingleton.com) in a few hours that would've taken 10X that a couple years ago, and it was super fun.
The big win: complexity. CC has been a disaster on my bookkeeping system, there's a threshold past which Claude loses the forest for the trees and makes the same mistakes over and over. Code agent pushes that bar out significantly. Claude can build new tools on the fly when it needs them. Gemini works great too (larger context).
Have fun out there! /end-rant
Re: How to code Claude Code in 200 lines of code
#193Something I would add is planning. A big "aha" for effective use of these tools is realizing they run on dynamic TODO lists. Ex: Plan mode is basically bootstrapping how that TODO list gets seeded and how todos ground themselves when they get reached, and user interactions are how you realign the todo lists. The todolist is subtle but was a big shift in coding tools, and many seem to be surprised when we discuss it -…
I've had a LOT of success keeping a "working memory" file for CLI agents. Currently testing out Codex now, and what I'll do is spend ~10mins hashing out the spec and splitting it into a list of changes, then telling the agent to save those changes to a file and keep that file updated as it works through them. The crucial part here is to tell it to review the plan and modify it if needed after every change. This keeps…
I use a folder for each feature I add. The LLM is only allowed to output markdown file in the output subfolder (of course it doesn't always obey, but it still limits pollution in the main folder)
The folder will contain a plan file and a changelog. The LLM is asked to continously update the changelog.
When I open a new chat, I attach the folder and say: onboard yourself on this feature then get back to me.
This way, it has context on what has been done, the attempts it did (and perhaps failed), the current status and the chronological order of the changes (with the recent ones being usually considered more authoritative)
Re: How to code Claude Code in 200 lines of code
#194It's a great point and everyone should know it: the core of a coding agent is really simple, it's a loop with tool calling. Having said that, I think if you're going to write an article like this and call it "The Emperor Has No Clothes: How to Code Claude Code in 200 Lines of Code", you should at least include a reference to Thorsten Ball's excellent article from wayyy back in April 2025 entitled "How to Build an Age…
I've been exploring the internals of Claude Code and Codex via the transcripts they generate locally (these serve as the only record of your interactions with the products)[1]. Given the stance of the article, just the transcript formats reveals what might be a surprisingly complex system once you dig in. For Claude Code, beyond the basic user/assistant loop, there's uuid/parentUuid threading for conversation chains,…
> So "200 lines" captures the concept but not the production reality of what is involved.
How many lines would you estimate it takes to capture that production reality of something like CC? I ask because I got downvoted for asking that question on a different story[1].
I asked because in that thread someone quoted the CC dev(s) as saying:
>> In the last thirty days, I landed 259 PRs -- 497 commits, 40k lines added, 38k lines removed.
My feeling is that a tool like this, while it won't be 200 lines, can't really be 40k lines either.
[1] If anyone is interested, https://news.ycombinator.com/item?id=46533132
Re: How to code Claude Code in 200 lines of code
#195Seems everyone is working on the same things these days. I built a persistent Python REPL subprocess as an MCP tool for CC, it worked so insanely well that I decided to go all the way. I already had an agentic framework built around tool calling (agentlib), so I adapted it for this new paradigm and code agent was born. The agent "boots up" inside the REPL. Here's the beginning of the system prompt: >>> help(assistant…
This sounds really cool! I love the idea behind it, the agent having persistent access to a repl session, as I like repl-based workflows in general. Do you have any code public from this by any chance?
Re: How to code Claude Code in 200 lines of code
#196here's my take, in 70 lines of code: https://github.com/kirjavascript/nanoagent/blob/master/nanoa...
I mean, if you take out the guard rails, here's codex in 46 lines of bash: #!/usr/bin/env bash set -euo pipefail # Fail fast if OPENAI_API_KEY is unset or empty : "${OPENAI_API_KEY:?set OPENAI_API_KEY}" MODEL="${MODEL:-gpt-5.2-chat-latest}" extract_text_joined() { # Collect all text fields from the Responses API output and join them jq -r '[.output[]?.content[]? | select(has("text")) | .text] | join("")' } apply_writ…
Impressive!
Here's an agent in 24 lines of PHP, written in 2023. But it relies on `llm` to do HTTP and JSON.
Re: How to code Claude Code in 200 lines of code
#197Earlier quoted context omitted.
I'm unsure of its accuracy/provenance/outdatedness, but this purportedly extracted system prompt for Claude Code provides a lot more detail about TODO iteration and how powerful it can be: https://gist.github.com/wong2/e0f34aac66caf890a332f7b6f9e2ba... https://gist.github.com/wong2/e0f34aac66caf890a332f7b6f9e2ba... I find it fascinating that while in theory one could just append these as reasoning tokens to the conte…
I find in coding + investigating there's a lot of mileage to being fancier on the todo list. Eg, we make sure timestamps, branches, outcomes, etc are represented. It's impressive how far they get with so little! For coding, I actually fully take over the todo list in codex + claude: https://github.com/graphistry/pygraphistry/blob/master/ai/pr... In Louie.ai, for investigations, we're experimenting with enabling more…
For humans org-mode is good at this
Re: How to code Claude Code in 200 lines of code
#198Seems everyone is working on the same things these days. I built a persistent Python REPL subprocess as an MCP tool for CC, it worked so insanely well that I decided to go all the way. I already had an agentic framework built around tool calling (agentlib), so I adapted it for this new paradigm and code agent was born. The agent "boots up" inside the REPL. Here's the beginning of the system prompt: >>> help(assistant…
This sounds really cool! I love the idea behind it, the agent having persistent access to a repl session, as I like repl-based workflows in general. Do you have any code public from this by any chance?
https://github.com/jacobsparts/agentlib
See CodeAgent or subrepl.py if you're just interested in the REPL orchestration. I also have a Python REPL MCP server that works with CC. It isn't published, but I could share it by request.
My favorite part of code agent is the /repl command. I can drop into the REPL mid session and load modules, poke around with APIs and data, or just point Claude in the right direction. Sometimes a snippet of code is worth 1000 words.
Re: How to code Claude Code in 200 lines of code
#199Earlier quoted context omitted.
This sounds really cool! I love the idea behind it, the agent having persistent access to a repl session, as I like repl-based workflows in general. Do you have any code public from this by any chance?
https://github.com/jacobsparts/agentlib See CodeAgent or subrepl.py if you're just interested in the REPL orchestration. I also have a Python REPL MCP server that works with CC. It isn't published, but I could share it by request. My favorite part of code agent is the /repl command. I can drop into the REPL mid session and load modules, poke around with APIs and data, or just point Claude in the right direction. Some…
Thanks!
Re: How to code Claude Code in 200 lines of code
#200Earlier quoted context omitted.
This site has gone full Tower of Babel. I've seen at least a thousand "AI comment" callouts on this site in the last month and at this point I'm pretty sure 99% of them are wrong. In fact, can someone link me to a disputed comment that the consensus ends up being it's actually AI? I don't think I've seen one.
I think this might be one of the first times I didnt notice it, but just look through the comment history of https://news.ycombinator.com/threads?id=jackfranklyn , they all look the same.
Yeah on a second look GP might actually be on to something here. Jackfranklyn only makes top level comments, never dialogs with anyone, and I count at least 3 instances of "as someone who does this for a living" that are too seperated in scope to be plausibly realistic.