---
title: "XCSP3Output"
description: "Answers in the format of the XCSP3 competition, and optionally checked by its official checker."
---

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

# XCSP3Output

## What it does

Writes what a solver in the [XCSP3 competition](https://xcsp.org/competitions) prints, for each run on
an XCSP3 instance, with the solution as an XML `<instantiation>` of the instance's variables:

```
o 12
o 9
s OPTIMUM FOUND
v <instantiation type="optimum" cost="9">
v   <list>x[0] x[1] x[2] y</list>
v   <values>2 1 0 3</values>
v </instantiation>
```

Each competition specifies what a solver prints:
- `o` lines with each better objective value, while it solves
- an `s` line with its answer: `OPTIMUM FOUND`, `SATISFIABLE`, `UNSATISFIABLE` or `UNKNOWN`
- `v` lines with the solution

The output goes to each run's `logs/<run_id>.xcsp3.out`, written inside the measured run, as a
competition solver would print it, so it costs the run the time it would cost in the competition. A run
that is killed keeps its `o` lines. Under [`cpbenchy solve`](/library/solve/), the output goes to stdout,
where competitions read it.

Given the competition's solution checker, it also checks every solution with it, after the run and
outside its measured time.

## Use it

```python
import cpbenchy
from cpbenchy import observers

cpbenchy.run("xcsp3/", solvers=["ortools"], time_limit=60, plugins=[observers.XCSP3Output()])
cpbenchy.run("xcsp3/", solvers=["ortools"], time_limit=60, plugins=[observers.XCSP3Output(checker="checker.jar")])
```

```sh
cpbenchy run xcsp3/ -s ortools -t 60 -p cpbenchy.observers:XCSP3Output
cpbenchy run xcsp3/ -s ortools -t 60 -p "cpbenchy.observers:XCSP3Output(checker='checker.jar')"
```

The [XCSP3 rules](/library/xcsp3-2025/) enable it, with the competition's limits.

## Options

| Argument | |
|---|---|
| `checker` | path of the [XCSP3 solution checker](https://xcsp.org/tools/) jar; needs `java`. Default: no check |

## What it records

| | |
|---|---|
| `logs/<run_id>.xcsp3.out` | the competition output |
| `extra["xcsp3_check"]` | with a checker: `"OK"`, or what the checker says is wrong |

## Implementation

Source: https://docs.cpbenchy.com/library/xcsp3-output/index.mdx