The gyms
Thirteen free, hands-on gyms for the fundamentals: Review, Secure, Debug, Test, Concurrency, Regex, SQL, Shell, Net, Git, Vim, System Design and Schema. Each one runs a real engine in your browser and grades you on state. No account required.
A code review gym. Read a diff of the kind a model writes — clean, idiomatic, green test suite — find the one defect, name what class it is, and prove it with an input the correct build survives and this one does not. Free, local-first, no account required.
A security gym with a sandboxed application and an attacker console, entirely in your browser. Read a generated handler, find the defect, prove it with a request that succeeds on the vulnerable build and fails on the fixed one, then ship the fix. Free, local-first, no account required.
An incident-response gym. Get paged into a simulated production system, read its traces, logs and metrics, prove a diagnosis, and ship a fix that holds. Free, local-first, no account required.
Write tests, then watch an engine break the code underneath them one edit at a time and count how many of those breaks your tests notice. Mutation testing as a learning instrument. Free, local-first, no account required.
A deterministic scheduler and model checker in your browser. Write concurrent code, and the engine explores the interleavings rather than running one — it finds the schedule that breaks your assertion and hands it back to step through, forwards and backwards.
A real backtracking regex engine in your browser, with every step visible. Predict what a pattern matches, write patterns against a step budget, and find the input that makes a generated regex hang. Free, local-first, no account required.
A real relational engine in your browser — parser, cost-based planner, statistics, volcano executor, MVCC. Write a query, see the rows it returned and the pages it cost. Free, local-first, no account required.
Learn the shell by driving a real one in your browser. Genuine quoting rules, the full expansion pipeline in order, a virtual filesystem with inodes, and grading on the state of that filesystem plus stdout plus the exit code.
A packet-level network simulator in your browser: links with latency and loss, DNS, TCP, TLS and HTTP as they actually happen. Read the capture, meet the latency budget, and find the retry loop that takes the upstream down. Free, local-first, no account required.
Learn Git by driving a real commit graph in your browser. Type actual commands, watch the DAG, index, worktree and reflog change, and be graded on whether the repository truly reached the target state.
Learn Vim through interactive lessons, timed practice runs, and graded tests. Free, local-first, and no account required.
Interactive System Design education. Build architectures, generate traffic, break them, and watch what happens. Completely free, no account required.
A database migration simulator in your browser: real lock levels, rewrites versus metadata changes, a live workload, and old and new application code running side by side. Write the migration, watch what it blocks, and ship it with the table still answering. Free, local-first, no account required.
All free. All local. Pick whichever you'd fail today.
Or ask the terminal