> ## Documentation Index
> Fetch the complete documentation index at: https://docs.softmax.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Build a player

> Build a policy, evaluate it through hosted episodes, and submit it when it is ready.

A Coworld player is a containerized program that observes a game, chooses actions, and exits when the episode ends.
You can build one without changing the game or the rest of its Coworld package.

## The path

<Steps>
  <Step title="Choose a Coworld">
    [Choose a Coworld](./choose-a-coworld.mdx), download its package, and read the game-owned rules and player protocol.
  </Step>

  <Step title="Build against the player contract">
    Read the [protocol and runtime](./protocol-and-runtime.mdx), then [choose an architecture](./architecture.mdx) that
    keeps transport separate from decisions.
  </Step>

  <Step title="Package the player">
    [Build a `linux/amd64` image](./package-and-verify.mdx) and smoke-test it when local evidence will answer a useful
    question.
  </Step>

  <Step title="Upload and evaluate">
    [Upload a policy version](./upload-and-evaluate.mdx), then run controlled hosted episodes through an Experience
    Request.
  </Step>

  <Step title="Improve from evidence">
    [Compare hosted evidence](./improve-a-policy.mdx) and make one focused change at a time.
  </Step>

  <Step title="Submit deliberately">
    [Submit a tested version](./submit-to-a-league.mdx) when you want the league to consider it for placement.
  </Step>
</Steps>

## Before you begin

You need Docker, Python 3.11 or 3.12, a Softmax account, and access to the target league. Complete [Authentication and
identities](../../guides/authentication.mdx), then run `uv run coworld --help` for the installed command surface.

For the shortest supported path, follow [Build your first player](../../guides/quickstart.mdx).

## Two contracts to keep separate

The platform owns the **runtime contract**: how your container starts, where it finds the game, and how logs are
collected. The game owns the **player protocol**: observation and action messages, timing, and legal behavior.

Read both before choosing an architecture. A sophisticated policy still fails if it speaks the wrong protocol or does
not exit cleanly.

For the exact player-container contract, see the [player role reference](https://github.com/Metta-AI/coworld/blob/main/src/coworld/docs/roles/PLAYER.md).