SysCodeLab

AI writes code. Engineers solve problems.

LeetCode is over.
Can you solve what actually breaks?

Here you get a broken system with the data it left behind: code, logs and metrics. You investigate, write what is happening and what you would do, and at the end we show what it actually was, so you can see where you got it right. Challenges run from junior to staff.

See the challengesHow it works

A new account starts with 3 credits. No credit card.

checkout-api · pool.go21:04:12
  1. 21:04:09infopool: acquire wait=2ms in_use=18/24
  2. 21:04:11warnpool: acquire wait=1.9s in_use=24/24 queued=61
  3. 21:04:12errorpool: context deadline exceeded after 5s
  4. 21:04:12errorcheckout: 503 upstream_timeout order=8f21c4

p99 latency

340ms→8.2s

The problem

Nobody gets paged at 3am to invert a binary tree.

Worth knowing, genuinely: data structures and complexity show up the day you choose between an index and a cache. And a model inverts the tree faster than you can type the signature. Reading a latency graph and deciding where to start, it does not.

LeetCode

Invert a binary tree.

You study it, you recognise the pattern, you pass. A model does it in two seconds.

SysCodeLab

Thursday, 9pm: payments started going out twice. Here are the logs.

No pattern to recognise. You investigate, decide, and defend the call.

How it works

What counts is how you got there.

Every challenge hands you the system as you would find it: architecture, the code that matters, logs, metrics and config. Then it asks what you would do about it.

LeetCode

Input → Algorithm → Output

SysCodeLab

Each step is a question in the challenge. You answer in writing, and that is what the solution compares against.

  1. 01Symptom
  2. 02Investigation
  3. 03Hypothesis
  4. 04Root cause
  5. 05Trade-offs
  6. 06Fix
  7. 07Validation

Coverage

Where production usually breaks.

Writing a good one takes a while: the logs have to match the story, and the solution has to survive an argument. The next ones land in these categories.

  • 01

    Debugging

    Follow the symptom back to the cause through logs, traces and code.

  • 02

    Performance

    Find what got slow, why it got slow, and what it costs to fix.

  • 03

    Database

    Query plans, indexes, locks, connections and the surprises in between.

  • 04

    Architecture

    Decide what to build and defend the trade-offs you accepted.

  • 05

    Distributed Systems

    Retries, idempotency, consistency and partial failure.

  • 06

    API Design

    Contracts, versioning, timeouts and resilient boundaries.

The library

Broken systems, the way you would find them.

Each one takes 30 to 60 minutes: you read the material, write your diagnosis, and only then see the written solution.

DebuggingJunior

The search that ignores the filter

The cache hands the person who filtered by one city the list somebody else asked for.

CachingCache keysTTL5 min
DebuggingSenior

The Connection Storm

Under load the service exhausts its database connections. The pool config looks right.

Connection poolingResource lifecycleCapacity planning20 min
Open the library

FAQ

Questions

Do I write and run real code?

Not yet. Today you get the system (context, architecture, code, logs, metrics) and you answer in writing: what is causing this, what you would change, what that choice costs, and how you would know it worked. Running the broken system yourself is on the roadmap.

Is there one right answer?

Almost never. A solution different from ours can be right when the trade-off holds up. Every challenge opens with the root cause, the fix we would pick, the alternatives and how to validate it. It is there to compare reasoning, not to mark you against a key.

What does it cost?

A new account gets three credits and a challenge costs one, so you can start without paying. Buying more credits does not exist yet.

Who is this for?

People walking into a senior or staff interview, and people who want the production scars without waiting for the next outage.

I hire engineers. Can I use this to evaluate candidates?

Not yet. That is the next step for the product, and there is a page about it, with a waiting list, linked in the footer.

Is it available in Portuguese?

Yes. The interface and the challenges are published in English and Brazilian Portuguese. Code, logs, commands and technology names stay as they are.

None of this works by reading it.

Create an account, pick one, and write down what you would do. It takes 30 to 60 minutes, and nobody is timing you.

Create an accountLook at the challenges first

3 credits on a new account. No card.