# Submission format > What goes in the archive, how each solver is run, and the environment it runs in. Source: A Main-track submission is one archive with a solver for each problem of the open round. We run each solver in a sandbox, once for every private instance of its problem, and check the solution file it writes. ## The archive Upload a **zip**, **tar** or **tar.gz** file of at most 4 MB. The format is read from the file's contents, so its name does not matter. `layout` ```text submission.zip ├── barnhart2000/ # one folder per problem, named by its slug │ ├── solve.py # the entry point │ ├── model.py # any other files the solver needs │ └── params.json └── / └── solve.py ``` - **One folder per problem of the open round**, named exactly by the problem's slug. The slugs are on the dashboard and in [`GET /problems`](https://frontieror-challenge.com/docs/main/api#list-problems). - **One entry point per folder**, directly inside it: `solve.py`, which we run with `python`. Solvers are Python only; for another language, use the [Self-hosted track](https://frontieror-challenge.com/docs/self-hosted/submission-flow). - **Each folder stands alone.** A solver sees only its own folder, so code shared between problems has to be copied into every folder that uses it. - **A problem without a folder scores 0** on all of its instances. The upload form warns you and asks you to confirm; over the API, add `acknowledge_missing=1`. - **One wrapper folder is fine.** If the whole layout sits inside a single folder, for example because you zipped the folder itself, that folder is skipped. - **Ignored:** `__MACOSX/`, `.DS_Store` files, and anything at the top level that is not a folder of an open-round problem. - **Refused:** symbolic and hard links, absolute paths, paths with `..` or backslashes, more than 10,000 entries, or more than 256 MB unpacked. ## How your solver is run For every instance, the solver's folder is copied into a fresh sandbox and the entry point is started from inside it: ```sh python solve.py --problem barnhart2000 \ --instance /work/in/instance.json \ --output /work/out/solution.json \ --time-limit 60 ``` | Argument | Value | | -------------- | ------------------------------------------------------------------------------------------ | | `--problem` | The problem's slug. | | `--instance` | Path to the instance file: JSON in the problem's instance schema. Read-only. | | `--output` | Where to write the solution file. See [The solution file](#the-solution-file). | | `--time-limit` | The instance's time limit in whole seconds. The sandbox stops the solver when it runs out. | The working directory is `/work/solver`, which holds your problem folder and is read-only. Standard input is empty. Standard output and error are captured but not shown to you, since they could reveal the private instances. ### Environment variables | Variable | Value | | --------------------------------------------- | ----------------------------------------------------------- | | `SANDBOX_PROBLEM` | The problem's slug. | | `SANDBOX_TIMEOUT_S` | The time limit in seconds, the same as `--time-limit`. | | `OMP_NUM_THREADS` | `2`, matching the CPU limit. | | `HOME` | `/tmp` | | `PYTHONUNBUFFERED`, `PYTHONDONTWRITEBYTECODE` | `1` | | `PATH`, `VIRTUAL_ENV` | Point at `/opt/env` when the submission has extra packages. | ## Sandbox limits Every instance runs alone on its own cores, so no other run shares its time. | Resource | Limit | | --------- | ------------------------------------------------------------------------------------------------------------------------------------------ | | CPU | 2 vCPU (x86-64) | | Memory | 4 GB, no swap. Going over ends the run. | | Time | The instance's time limit: 10 s, 60 s or 5 min, by the problem's [time class](https://frontieror-challenge.com/docs/problems#time-limits). | | Disk | 1 GB for `/work`. Only `/work/out` and `/tmp` (64 MB) are writable. | | Network | None. | | Processes | 256 processes and threads, 1,024 open files. | | User | An unprivileged user, uid 1000, with no capabilities. | > **Finish before the limit.** A solver still running at its time limit is stopped and the instance scores 0, even if it has already written a solution. Give your solver a time budget a few seconds under `--time-limit`, write the solution file and exit. ## The solution file Write one JSON object to the `--output` path, `/work/out/solution.json`, and exit with code 0. - It follows the problem's **solution schema**, shown on each problem page. See [Reading the schemas](https://frontieror-challenge.com/docs/problems#reading-the-schemas). - It has a numeric `objective_value`. The checker recomputes the objective from your decisions and rejects a solution whose claimed value does not match. - It is at most 16 MB. `solution.json · barnhart2000` ```json { "objective_value": 111, "commodities": [ { "commodity_id": 0, "rejected": true, "path_arcs": [] } ] } ``` How a run turns into a score, and every message an instance can get, is on [Evaluation and scoring](https://frontieror-challenge.com/docs/main/scoring). ## Environment Each submission runs in one environment, chosen when you submit. It is the same for every problem in the submission. ### Our images Python 3.12 (`python-3.12`, the default) or Python 3.14 (`python-3.14`), on Debian bookworm, with numpy, scipy, pandas, networkx, ortools, pulp, highspy and pyomo installed. **Every installed package** | Package | Python 3.12 | Python 3.14 | | ----------------- | ----------- | ----------- | | absl-py | 2.5.0 | 2.5.0 | | annotated-types | 0.8.0 | 0.8.0 | | cpsat-logutils | 1.0.0 | 1.0.0 | | highspy | 1.15.1 | 1.15.1 | | immutabledict | 4.3.1 | 4.3.1 | | networkx | 3.7 | 3.7 | | numpy | 2.5.3 | 2.5.3 | | orloge | 1.0.0 | 1.0.0 | | ortools | 9.15.6755 | 9.15.6755 | | pandas | 3.0.6 | 3.0.6 | | protobuf | 6.33.6 | 6.33.6 | | pulp | 4.0.0 | 4.0.0 | | pydantic | 2.13.5 | 2.13.5 | | pydantic-core | 2.46.5 | 2.46.5 | | pyomo | 6.10.1 | 6.10.1 | | python-dateutil | 2.9.0.post0 | 2.9.0.post0 | | scipy | 1.18.1 | 1.18.1 | | six | 1.17.0 | 1.17.0 | | typing-extensions | 4.16.0 | 4.16.0 | | typing-inspection | 0.4.4 | 0.4.4 | The lock files themselves are in [`GET /images`](https://frontieror-challenge.com/docs/main/api#list-images). ### Extra packages With our images you can add open-source packages from PyPI, one per line as in a `requirements.txt`, within the limits below: ```text pyscipopt==5.5.0 numba>=0.61 # a package you list replaces the image's version of it numpy==2.4.1 ``` - Up to 50 packages and 10 KB. Names, extras, version specifiers and environment markers are accepted. - Packages come from PyPI by name only: no options such as `-i`, `-e` or `-r`, and no URLs, paths, VCS links or wheel files. - They are installed once, before the first instance and outside every time limit, and mounted read-only. A later submission with the same image and list reuses them. - An install that fails or takes longer than 10 min fails the submission, and its build log is shown on the dashboard. **What is refused, and the message you get** | Line | Message | | -------------------------------- | ------------------------------------------------------------------------------- | | Ends with `\` | Line continuations aren't accepted. Put each package on one line. | | Starts with `-` | Options such as -i, -e or -r aren't accepted. Packages come from PyPI. | | A URL, path, VCS link or archive | Links, paths and archive files aren't accepted. Use the package's name on PyPI. | | Anything else unreadable | Not a package name with optional extras and versions, such as pyscipopt==5.5.0. | | A bad marker after `;` | Not a valid environment marker after ";", such as sys\_platform == "linux". | | More than 50 packages | *n* packages; the limit is 50. | | Longer than 10 KB | The list is longer than 10 KB. | Each message names its line. > The Main track allows open-source dependencies only. This is not checked automatically, but prize-eligible submissions are audited. ## Checking before you submit The [starter repository](https://frontieror-challenge.com/docs/main/example-code) runs your solvers the same way, in the same image and with the same limits: ```sh uv run frontieror test --docker ``` It checks the run and the solution's format on the public instances, then feasibility with the problem's [checker](https://frontieror-challenge.com/docs/problems#checkers) when it is published. Scores come only from a submission.