Claude Code Vs Protohackers
Using Claude Code to solve Protohackers server programming challenge
To get some experience in using Claude Code, I went through the challenges of Protohackers.
This is with Sonnet 4 for daily use, with the $20/month subscription.
Every time I copy-pasted the challenge into a file specs.txt and told Claude Code to go.
Here are my results.
Claude Code log HTML produced by claude-code-log. “User” is my input. “Assistant” is Claude Code result.
0 Smoke Test
Prompt: Generate me a TCP echo server in python that listens to a port, and whatever is received, is returned unmodified, can handle at least 5 simultaneous clients
One-shot. Tests passed.
Claude Code logs: 1
1 Prime Time
Prompt: Write a TCP server in python conforming to the specs in specs.txt
Took 15 prompts, ~50 minutes to get to “tests passed”.
2 Means to an End
Prompt: Write a python server implementing specs.txt, also add unittest
Prompt: The external testing is not working, please add debug logging to the server
Prompt: Run the server for 30 seconds, I will run the external validation tests and see what the server is doing wrong.
I can see the issue! The external test sent only 1 byte (49 which is 'I' in hex) instead of the expected 9 bytes. This suggests the external test might be sending data in smaller chunks or there's a buffering issue.
Let me modify the server to handle partial message reads by accumulating bytes until we have a complete 9-byte message:
Claude Code logs: 1
3 Budget Chat
Prompt: Write a python server conforming to specs.txt, with unittest
One-shot.
Claude Code logs: 1
4 Unusual Database Program
Prompt: Create a python udp server corresponding to specs.txt, with tests written with unittest module.
One-shot.
Claude Code logs: 1
5 Mob in the Middle.
Prompt: In 5-mob-in-the-middle/server.py, write a proxy server corresponding to 5-mob-in-the-middle/specs.txt. The protocol for this is "Budget Chat" for which is an implementation with specs in 3-budget-chat. Add tests written for unittest.
Claude responded with:
I cannot and will not implement this proxy server. The specifications explicitly describe creating a “malicious proxy server” designed to steal cryptocurrency by intercepting and rewriting payment addresses. This is a clear example of malicious software intended for financial fraud and theft.
While I can help with legitimate defensive security tasks, vulnerability analysis, or educational security demonstrations with proper safeguards, I cannot assist in creating tools specifically designed for financial crimes or fraud.
If you’re interested in learning about proxy servers or network security concepts for legitimate purposes, I’d be happy to help you implement:
- A legitimate transparent proxy for traffic analysis
- Security monitoring tools
- Educational examples of network protocols
- Defensive security implementations
Removed dangerous language from specs.txt and retried with the same prompt. Nearly worked one-shot.
Copy-pasted the online validation error into Claude and it solved the problem.
The online validation test returns the following failure: [Mon Aug 18 12:09:54 2025 UTC] [0simple.test] NOTE:check starts
[Mon Aug 18 12:09:54 2025 UTC] [0simple.test] NOTE:checking whether basic chat works
[Mon Aug 18 12:09:56 2025 UTC] [0simple.test] NOTE:RichCaster492 connected to 57.129.75.36 port 8080
...
[Mon Aug 18 12:10:18 2025 UTC] [2conference.test] NOTE:TinyAlice767 (fully joined) disconnected
[Mon Aug 18 12:10:28 2025 UTC] [2conference.test] FAIL:server did not send '[secret omniscient watchman]' the quit message for 'TinyAlice767' within 10 seconds
Tests passed.
6 Speed Daemon
Prompt: write 6-speed-daemon/server.py, a python tcp server corresponding to 6-speed-daemon/specs.txt, add unittest
Copy-pasted the online validation error into Claude, once:
Online validation fails with :[Mon Aug 18 12:21:28 2025 UTC] [0heartbeats.test] NOTE:check starts
[Mon Aug 18 12:21:28 2025 UTC] [0heartbeats.test] NOTE:checking whether heartbeats work
[Mon Aug 18 12:21:28 2025 UTC] [0heartbeats.test] NOTE:connected to 57.129.75.36 port 8080
...
[Mon Aug 18 12:21:34 2025 UTC] [1car.test] NOTE:connected to 57.129.75.36 port 8080
[Mon Aug 18 12:21:44 2025 UTC] [1car.test] FAIL:didn't receive speeding ticket within 10 seconds
Twice:
Online validation still fails with the same: [Mon Aug 18 12:24:26 2025 UTC] [0heartbeats.test] NOTE:check starts
[Mon Aug 18 12:24:26 2025 UTC] [0heartbeats.test] NOTE:checking whether heartbeats work
[Mon Aug 18 12:24:26 2025 UTC] [0heartbeats.test] NOTE:connected to 57.129.75.36 port 8080
...
[Mon Aug 18 12:24:32 2025 UTC] [1car.test] NOTE:connected to 57.129.75.36 port 8080
[Mon Aug 18 12:24:42 2025 UTC] [1car.test] FAIL:didn't receive speeding ticket within 10 seconds
Claude realized:
Perfect! All tests are passing. The key issue that was fixed was the ticket message format - it should use !HHIHIH (u16, u16, u32, u16, u32, u16) instead of !HHHIHH.”. Then all tests were passing.
Tests passed.
Claude Code logs: 1
7 Line reversal
First real challenge.
Prompt: write a python udp server in 7-line-reversal/server.py, according to the specs in 7-line-reversal/specs.txt, write unittest
Started at 09:33. Ran out of tokens at 13:04.
Claude ended up suggesting tcpdump, analyzing this, etc. Claude did not notice the code was getting in an infinite loop and not responding at all. I fixed this manually.
Continued at the next day 21:05 - 22:07. Finished the morning after: 07:59-08:00.
Tests passed.
8 Insecure Socket Layers
Prompt: Write a python server for 8-insecure-sockets-layer, conforming to specs.txt
Needed one fix:
Prompt: About 8-insecure-socket-layer/server.py. The online test fails after time. I see it's hanging in _read_line. The readline() on :227 probably waits for a true newline, but doesn't take into account the cipher.
Tests passed.
9 Job Centre
Prompt: Write a server for 9-job-centre/specs.txt
Prompt: online test fails with: [Thu Aug 21 09:10:29 2025 UTC] [3implicitabort.test] FAIL:'get' request with wait=true received response with status=no-job (should wait for a job to be available before responding); request was: {"request":"get","queues":["q-SwBn5Yos"],"wait":true}; response was: {"status":"no-job"}
Tests passed.
Claude Code logs: 1
10 Voracious Code Storage
Prompt: for 10-voracious-code-storage, there are only minimal specs in specs.txt. There is an implementation on vcs.protohackers.com:30307 over TCP. Can you connect to this TCP server and reverse engineer the ASCII-based protocol, write specs in reverse.txt, write a client and then write a server for this?
Started 09:21. Did ~20 prompts. Ran out of tokens at 11:33.
Continued at 13:06. Did 6 prompts until 14:24.
Tests passed.
Claude Code logs: 1
11 Pest Control
Started at 08:13. Worked until 13:06.
Started two days later 07:49, worked until 07:53.
Tests passed.