Codility Practice Guide: What to Expect & How to Prepare (2026)

Codility is the platform you may have already used without knowing its name. Microsoft has historically run intern and new-grad screens on it, along with a long tail of banks, telcos, and European tech companies. No leaderboard, no discussion forum, no company tags — just a calm editor, a visible clock, and a battery of hidden test cases waiting to disagree with your code. Here's what that environment rewards.

| What Codility gives you | What it quietly grades | | --- | --- | | A plain in-browser editor with one language switcher | Whether your code runs clean on inputs you can't see | | Sample tests you can run freely | Whether you bother testing beyond them | | A per-task timer plus a total timer | Whether you finish, not just whether you're clever | | A short task description, sometimes one paragraph | Whether you read carefully — the spec is the spec |

The format, in practice

A typical Codility screen is 60–90 minutes with two to four tasks. Task one is a warm-up — an easy array or string problem worth a modest share of the score. Tasks two and three are mediums with real edge cases: empty arrays, maximum-length inputs, duplicate values, integer boundary conditions. The scoring is heavily weighted toward the harder tasks, and partial credit flows per test case, so a correct-but-slow solution still earns real points.

Microsoft's screens on this platform are the best-known example; the Microsoft OA guide covers the company-specific version of the experience. The platform habits below transfer to any Codility-based screen regardless of company.

What separates a strong Codility score from a weak one

Three things, in order of how often they decide outcomes:

1. Boundary discipline. Codility's hidden tests hammer edges: n = 0, n = 200,000, negative numbers where you assumed positives, integer overflow in languages that allow it. Before submitting, write down the two or three nastiest inputs you can think of and run them mentally through your code. Candidates who do this consistently outscore candidates who are technically stronger but sloppier.
2. Complexity awareness. Every task page shows performance constraints ("expected O(n)"). A quadratic solution on an O(n)-task passes the small samples and fails the big ones — a classic trap. Estimate complexity before coding, and if you're above the target, rethink the approach before writing anything.
3. Submission discipline. You can submit multiple times and the best score counts. There is no reason to sit on a half-finished optimal solution when a working brute force is one submit button away. Bank the points, then climb.

A prep plan for Codility-style screens

Two focused weeks is the sweet spot.

• Days 1–4: arrays, strings, and hashing under a 20-minute per-problem clock, in any bare editor. Codility doesn't autocomplete your thinking; practice writing correct code straight through.
• Days 5–7: edge-case drills. Take problems you've already solved and break them — feed them empties, duplicates, and max-size inputs. Learn what your code does when the input is rude.
• Week 2: three full timed mocks (two to four tasks in 75 minutes), each followed by a review session as long as the mock itself. Identify whether your losses come from time, edge cases, or complexity misses — then drill that single weakness.

Practice with the feedback Codility doesn't give you

Here's the thing about Codility: it tells you that a test failed, not why. That silence is the platform's biggest teaching gap. Easy Code Interview fills exactly that hole during practice: get the problem on screen, work it timed, and receive a short answer plus a detailed explanation on your second device — the "why" while the attempt is still fresh. Two weeks of solve-then-understand reps on Codility-style tasks is the most direct prep there is for this platform.

Keep reading

• Microsoft online assessment guide
• Two Sum walkthrough: hashing fundamentals
• HackerRank practice guide

Frequently Asked Questions

Is Codility only used by Microsoft?

No. Microsoft is the most-discussed user, especially for intern screens, but Codility serves a broad set of enterprise and mid-size tech employers — particularly in Europe. Any company can white-label it, so the environment may not even be labeled.

Does Codility record my screen or webcam?

That depends on the hiring company's configuration, not the platform default. Some add tab-switch tracking or webcam snapshots; others run it with no monitoring at all. Either way, the winning strategy is identical: prepare until the problems feel routine.

Can I use my own IDE during a Codility test?

Sometimes — Codility has an optional local-IDE mode where you paste code back. Whether it's enabled is the company's choice. Practice both ways; the in-browser editor is the common denominator.

What languages does Codility support?

The mainstream set: Python, Java, C++, C#, JavaScript, and several others. Python remains the most popular choice for its speed of expression under a clock.

How is Codility different from HackerRank and CodeSignal?

Codility is the purest of the three: correctness and performance on hidden tests, minimal extra format. HackerRank adds question variety (SQL, multiple choice, multi-part), and CodeSignal wraps everything in a single composite score. The CodeSignal guide and the HackerRank guide cover those tradeoffs.