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FDE Lab · 30 missions · 8 tracks · 3 free

Practice the job, on a box that answers back.

Every mission drops you on a customer's machine with a problem, logs, a contract and a terminal. You find the evidence, change the thing, and prove it. Objectives tick when the world changes, not when you type the expected command, and the debrief tells you what you can now say in the room. The same customers come back mission after mission, so what you learn compounds.

Start the first free missionno account · runs in this tab · nothing installed
Guided

The brief tells you where to start; objectives arrive in order. The free missions are guided.

Challenge

Objectives visible, method yours. Most premium missions; the interviewer-grade version of the guided one.

Incident

A clock, a broken system, and a diagnosis you commit to. The after-action shows the evidence path you took.

The simulator, no mission

free · no account · nothing scored

The two customer systems the missions are set in, running normally. Inject a fault from the sandbox, watch it land in the topology, the traces, the metrics and the logs at once, and fix it from the terminal. When it clicks, the missions are the same systems with a problem that has already happened.

Calder Equipment · support agent
Calder support agent

The Calder support agent system with no mission: gateway, runtime, router, providers, tools, queue and approvals. Inject faults, change config, let time pass.

Halvard Freight · carrier integration
Halvard carrier integration

Halvard's carrier integration with no mission: carrier webhooks, receiver, dedupe store, billing, pricing, rates API and the nightly sync.

Field basics

0/3 done

Orient on an unfamiliar box: logs, configs, the job that failed and the value that actually ran.

API debugging

0/3 done

Retries, idempotency, rate limits, pagination and the 200 that failed: the integration bugs that page you at night.

Building agents

0/4 done

Turn budgets, tool schemas, structured output and nested retries: what makes a model loop safe to ship.

Authority and safety

0/4 done

Injected instructions, stale approvals, cross-tenant retrieval and a red-team drill: where write authority lives.

Durable actions

0/4 done

Intent before send, unknown outcomes, reconciliation, backlogs: the actions a retry must never repeat.

Evaluation and release

0/4 done

Paired cases, judges, regression gates and drifting labels: the evidence that decides a release.

Serving and cost

0/4 done

Model routing, memory fit, cache keys and context budgets: the numbers behind a serving decision.

Incidents

0/4 done

The clock is running and the system is broken: contain, diagnose, commit, and defend it afterwards.

What is behind the terminal

The box is a deterministic simulation, and it says so on the prompt. Files, logs and payloads are written for the mission; services answer from a contract; the programs you run (a worker, an agent, a receiver) behave according to the config and policy you edit, so a wrong fix produces a wrong result you can read. That is the point: the commands are real, the evidence is consistent, and a fix is proven by replaying the failure, the way you would on a real engagement. It is also why a thousand readers can be on it at once without a server doing anything for any of them.

Missions link into the concepts, questions and courses that explain the mechanism, before and after. Read first if you want; most people learn it faster by breaking it.

Questions

Is anything actually running?

A simulated box runs in your browser tab: a filesystem, environment, logs, services that answer HTTP and programs that respond to what you change. Nothing you type is sent to a server. The world is a recording, so every reader sees the same evidence and every fix can be checked exactly.

Which missions are free?

3 missions are free without an account, 3 more open with a free account, and the remaining 24, including every incident and challenge, are part of premium. A locked mission still shows its brief and objectives so you know what you are practising before you pay.

How is a mission checked?

Objectives are checked against the state of the box, never against the exact command you typed. If you solve it another way, it still counts. Hints come in three levels, from a question to a near-answer, and the debrief says what you can now claim in an interview.

Does progress save?

Sessions persist in your browser, so you can leave and come back. A signed-in reader can save a completion to their account; the server replays the transcript before it counts, so a saved completion means the mission was solved.