---
title: "Record and replay a game"
url: https://recursiveai.org/onegame/cookbook/record-and-replay/
description: "Record a run of a C# game to a file, replay it later and check every tick’s state hash, and find the first tick where a code change makes the game behave…"
version: "1.2.0"
---

# Record and replay a game

Record a run of a C# game to a file, replay it later and check every tick’s state hash, and find the first tick where a code change makes the game behave differently.

A recording holds the starting state and every command. Because the simulation is deterministic, replaying it rebuilds the same world tick by tick, and `onegame replay` compares the state hash at each one.

Record the [pick-apples scenario](https://recursiveai.org/onegame/cookbook/automated-game-testing/) while it runs:

```shell
onegame run --headless -- --scenario scenarios/pick-apples.yaml --record run.ogrp --artifacts .
```

```text
PASS pick-apples (39 ms, tick 302, hash 9a447aae324c764b)
1 passed, 0 failed
```

Replay it:

```shell
onegame replay run.ogrp
onegame replay run.ogrp --json
```

```text
replay identical: 302 ticks, 3 commands, final hash 9a447aae324c764b
{"ok":true,"identical":true,"ticks":302,"commands":3,"finalHash":"9a447aae324c764b","segments":1}
```

The three commands are the two clicks on Pick and the refused `Pick(99)`. Refused commands are recorded too.

## Find where a change diverges

Change the simulation, here apples growing every 20 ticks instead of every 30, and replay the same file:

```text
replay DIFFERS (segment 0 of 1): Checkpoint at tick 20 (last good 19): 2 part(s) of the state differ.
```

```text
{"ok":false,"identical":false,"ticks":302,"commands":3,"tick":20,"expected":"36918dc7c358bda6","actual":"7915daf0d5a403bb","finalHash":"7915daf0d5a403bb","segments":1,"segment":0}
```

The replay exits 1 and names the first tick that differs and both hashes. Put the change back and the replay is identical again. This makes a recording a regression test for the simulation: keep the file, replay it in CI, and any change that alters what a player saw fails at the exact tick.

## Good to know

- `--record` and `--scenario` are options of the game, so they go after `--`. `--artifacts .` puts output files in the current folder.
- A recording starts from a snapshot, so a bug caught in a long session can be replayed after the fix without replaying the whole session.
- A recording is a session of segments. Anything that replaces the world (a scenario `reset:`, `fixture:` or `snapshot:`, a `set:`) ends one segment and starts the next, and the replay reports `segments`.
- A recording with no ticks and no commands replays as `identical: false`, because nothing was compared.
- In C#: `game.StartSessionRecording()`, `RecordingFile.Save(session, path)` and `game.Replay(session)`. `Determinism.Replay` in [Test a game from C#](https://recursiveai.org/onegame/cookbook/csharp-game-tests/) records and replays a scripted run in one call.

More in [Driving a game](https://recursiveai.org/onegame/docs/driving-a-game/).