What is a prototype design document for?
Large studios keep long design documents that grow with the project. Before a first prototype you need something else: a few pages that everyone involved can read in one sitting and use to make decisions. The document exists to stop the prototype from turning into a small version of the full game. If a feature, a level or a piece of art is not in it, it does not get built yet.
Write it for the people who will make and test the build: the designer, the programmer, the artist preparing placeholder assets and whoever will run the play sessions. If you plan to hand the build to an outside team, for example through game development services, the same document becomes the brief they quote and work from. A vague brief produces a vague prototype and a long list of questions in the first week.
Keep it alive but small. Update it when a decision changes, note the date and the reason, and resist adding sections “for later”. Ideas for the full game go into a separate list, so they are not lost but do not compete with the current test.
What goes into a game design document?
Start with a single paragraph that describes the game as a player would experience it: what they see, what they do and why they would want to do it again. Then describe the core loop step by step (our guide on how to prototype a game explains why it comes first): the action, the game’s response, the reward or progress, and what pulls the player into the next round. If the loop cannot be described in a few short sentences, it is not ready to prototype.
Next come the controls. List every input the prototype needs on each target device: taps and swipes, mouse and keyboard, a controller. Note what happens when the player does something unexpected, such as tapping twice or holding a button down. Controls are where many early prototypes fail, and they are cheap to describe precisely on paper.
Then the platform limits: screen size and orientation, the length of a typical session, whether the game must work offline, the performance of the weakest device you want to support and any rules of the store or platform you are aiming for. These limits decide what the prototype can contain long before anyone opens an engine.
Finally, the scope. Name the levels, characters, mechanics and screens that are in the prototype and, just as importantly, the things that are deliberately left out: menus, progression systems, monetization, story, final art and sound. A short “not in this build” list saves more arguments than any other section.
What must the test answer?
A prototype is an experiment, so the document should say what the experiment is. Write the main question in one sentence: do players understand the controls without help, does the core action stay interesting after several rounds, is the difficulty in the right place? Add what you will watch for during sessions and what result would lead you to continue, change the design or stop.
Describe the play sessions themselves: who the players are, which devices they use, how long a session lasts and who takes notes. Agree how feedback is recorded, with the build version next to every note, so the team can tell which comment belongs to which change.
Last, add a short decision log. Each time the test answers a question, write down what was decided and why. This record is what turns a prototype into a basis for production planning, and it is far more useful to the next stage than a polished pitch. When the document is ready, it is also the clearest way to send a project brief if you want help building the prototype.
How detailed should it be?
Detailed enough that two people reading it would build the same prototype, and no more. Precise wording matters most for the core loop, the controls and the test question; everything else can stay brief. If a section keeps growing, it is usually a sign that the prototype is trying to answer more than one question.
Use sketches where they are faster than words: a rough drawing of the screen, the position of the controls, the path through a level. Photos of paper sketches are perfectly fine at this stage. What matters is that the team shares one picture of the build, not that the document looks finished.
Common mistakes
- Describing the whole game. A document that covers every planned feature invites the team to build them. For a prototype, describe only what the test needs and park the rest in a separate list of ideas.
- Leaving the test question vague. “See if it’s fun” cannot be answered. Name the specific thing you want to learn and what you will watch during play sessions.
- Skipping the controls. Teams often describe what happens in the game but not how the player makes it happen. Inputs on each device are where early builds most often feel wrong.
- Never updating it. A document written once and then ignored stops being a reference. Record decisions as they are made, with the build they relate to.
Game design document template for a prototype
A template to adapt to your own project.
| Section | What to write |
|---|---|
| One-paragraph pitch | What the player sees and does, and why they come back. |
| Core loop | Action, response, reward and the next round, in a few short sentences. |
| Controls | Every input on each device and what happens on unexpected input. |
| Platform limits | Screen, orientation, session length, offline play, weakest device, store rules. |
| Scope | What is in this build, plus a separate list of what is not. |
| Test question | One question, what you will observe and which result means continue, change or stop. |
| Decision log | Date, decision, reason and the build it was based on. |
Checklist: game design document
- Describe the game in one paragraph and the core loop in a few sentences.
- List the controls for every target device, including unexpected input.
- Write down the platform limits before choosing features.
- Keep an explicit list of what is not in the prototype.
- State the test question and which result means continue, change or stop.
- Record every decision together with the build it was based on.
Example: a document for a puzzle prototype
A team wants to test a puzzle game for phones. The first draft of the document describes a world map, a shop, daily rewards and several game modes. None of these helps answer the real question: is sliding pieces to clear a board satisfying on a small touch screen?
The revised document fits on a few pages. It describes the core loop, the swipe controls and what happens after a mistaken swipe, limits the build to a handful of boards with placeholder graphics and lists everything else under “not in this build”. The test question is whether new players clear the first boards without explanation. That is what the team builds and watches.
