OpenShell/docs/reference/policy-prover.mdx

---
# SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
# SPDX-License-Identifier: Apache-2.0
title: "Standalone Policy Prover"
sidebar-title: "Policy Prover"
description: "Install and use the standalone OpenShell policy prover to check a local candidate policy against a managed boundary."
keywords: "Generative AI, Cybersecurity, Policy, Prover, Boundary, Containment, CI"
position: 4
---

`openshell-prover` checks whether the authority in a local candidate policy is
contained within a local boundary policy. The command reads files from the host
and does not connect to an OpenShell gateway.

The candidate must be the fully composed effective policy after the proposed
change, including any provider-contributed authority. The boundary is an
operator-owned ceiling. It does not grant authority by itself.

## Install the Prover

The standard OpenShell installer includes `openshell-prover`:

```shell
curl -LsSf https://raw.githubusercontent.com/NVIDIA/OpenShell/main/install.sh | sh
openshell-prover --version
```

The prover remains independent of the gateway at runtime. If you only need the
standalone binary, use the artifacts listed in the
[Support Matrix](/reference/support-matrix#standalone-policy-prover). These
artifacts and `openshell-prover-checksums-sha256.txt` are attached to
[OpenShell releases](https://github.com/NVIDIA/OpenShell/releases).

The release archive includes the solver linkage required by the executable.
It does not require the main `openshell` command, a gateway configuration, or
a separate Z3 installation.

## Check a Policy Boundary

Create `boundary.yaml` with this boundary policy:

```yaml
version: 1
filesystem_policy:
  read_only:
    - /usr
    - /etc
```

Create `candidate.yaml` with this contained candidate:

```yaml
version: 1
filesystem_policy:
  read_only:
    - /usr
```

Run the check:

```shell
openshell-prover check candidate.yaml --boundary boundary.yaml
```

A contained candidate prints `result: within_boundary` and exits with status `0`.
To see an exceeding result and its counterexample, replace `candidate.yaml`
with a policy that grants writes to `/tmp`. The boundary grants no write
access:

```yaml
version: 1
filesystem_policy:
  read_only:
    - /usr
  read_write:
    - /tmp
```

```shell
openshell-prover check candidate.yaml --boundary boundary.yaml
```

To check the current effective policy of an existing sandbox, export it without
display metadata:

```shell
openshell sandbox get my-sandbox --policy-only > candidate.yaml
openshell-prover check candidate.yaml --boundary boundary.yaml
```

This export includes provider-contributed rules. For a proposed change that is
not active, supply the complete post-change effective policy. The standalone
prover does not compose a base policy with provider rules.

Use JSON when a script consumes the result:

```shell
openshell-prover check candidate.yaml \
  --boundary boundary.yaml \
  --output json
```

The JSON object includes the result, exit code, input paths, prover version,
model version, checked policy version, and modeled domains. The documented
semantics and environmental conditions for that scope are tied to its model
version. An exceeding result also includes a filesystem or network
counterexample. Automation should use `result` and `reason_code` instead of
parsing the human-readable explanation.

| Field | When populated |
|---|---|
| `counterexample` | An object for `exceeds_boundary`; otherwise `null`. |
| `reason_code` | A stable identifier for an emitted `error`, `unsupported`, or `inconclusive` result; otherwise `null`. |
| `reason` | A human-readable explanation paired with `reason_code`; otherwise `null`. |

The stable reason codes are `invalid_input`, `unsupported_policy_shape`,
`unresolved_workdir`, `unresolved_binary_path`,
`unresolved_filesystem_path`, `solver_timeout`, `solver_unknown`,
`resource_limit`, `invalid_witness`, and `cancelled`.

The solver has a finite 10-second default budget. Set a different positive
budget with an integer followed by `ms`, `s`, or `m`:

```shell
openshell-prover check candidate.yaml \
  --boundary boundary.yaml \
  --timeout 30s \
  --output json
```

## Exit Codes

| Exit | Result | Meaning |
|---|---|---|
| `0` | `within_boundary` | Containment was established for the reported model scope. |
| `1` | `exceeds_boundary` | The candidate exceeds the boundary; inspect the counterexample. |
| `2` | `error` | Arguments, input files, policy syntax, or command execution prevented a valid check. |
| `3` | `unsupported` or `inconclusive` | The model cannot soundly cover the policy shape, or the solver did not reach a determination. |
| `130` | `inconclusive` when graceful handling completes | The user interrupted the command with Ctrl-C on Unix; a second or very early interruption may prevent output. |

Only exit `0` means the verification succeeded. Treat unsupported and
inconclusive results as failures in CI.

## Interpretation and Limits

The initial containment model covers filesystem paths, L4 network authority,
and enforced REST method and path authority, including explicit REST denies.
The result object reports the domains used for each check. Policies that use
recognized authority outside that model return `unsupported` rather than
silently ignoring it.

Both inputs use the same bounded YAML/JSON parser and authored policy schema as
OpenShell. Unknown fields, duplicate keys, malformed field types, and unsupported
managed `metadata` or `review` annotations return `error` with
`reason_code: invalid_input` and exit `2`. Schema-valid controls outside the
containment model return `unsupported` and exit `3`.

The prover applies aggregate limits across the candidate and boundary before
semantic shape validation: 1,024 network rules, 4,096 endpoints, 4,096 binary
selectors, 65,536 authored port entries, 16,384 REST rules, 4 KiB per modeled
pattern, and 1 MiB of modeled pattern text. Exceeding any limit returns
`inconclusive` with `reason_code: resource_limit`. A cancellation already
requested at preflight takes precedence over that result; otherwise a resource
limit takes precedence over unsupported policy-shape diagnostics. This ordering
keeps validation work bounded for checked-in CI inputs.

Network binary selectors, endpoint host and path selectors, and REST allow and
deny method and path selectors must use ASCII literals in both the candidate and
boundary. A non-ASCII literal in one of these fields returns `unsupported` with
`reason_code: unsupported_policy_shape`, including when it appears only in a
deny. Embedded NUL bytes in these fields are also unsupported. This is a
prover-model limitation, not a general policy validation rule: filesystem paths
and unrelated policy text retain their existing Unicode behavior. ASCII
wildcard selectors still cover non-ASCII runtime values matched by the policy
engine.

Containment means `Allowed(candidate)` is a subset of `Allowed(boundary)` under
the reported model. It does not establish least privilege, automatic approval
eligibility, semantic safety, or equivalence to a running sandbox's kernel
state. In particular:

- The command does not fetch the current sandbox policy or compose provider
  rules. The caller must supply the effective candidate.
- The command does not apply, approve, or persist a policy.
- Environment-dependent authority, such as an unresolved image workdir,
  returns `unsupported` when the result depends on that missing context.
- Binary comparisons that can change when an image resolves an exact selector
  through a symlink return `unsupported`. This covers exact candidate grants
  under boundary globs and exact boundary denies replaced by candidate globs.
  Use matching exact canonical paths when possible.
- Policies with overlapping L4 and enforced REST endpoints return
  `unsupported` because inspection selection depends on the complete set of
  matching runtime endpoint configurations.
- Network containment checks both supported runtime configurations: binary
  identity enforcement enabled and disabled. When enabled, grants and denies
  match the executable or an ancestor identity. Network counterexamples report
  the configuration and identities that expose the additional authority.
- REST containment witnesses use canonical request methods and paths.
- If a solver string cannot be decoded and validated faithfully, the command
  returns `inconclusive` with `reason_code: invalid_witness` instead of emitting
  a counterexample.
- Both files are interpreted in the same sandbox filesystem namespace and
  mount model, with stable path resolution when enforcement rules are created.
  The CLI does not resolve sandbox paths against the host.
- Filesystem containment supports removing grants and reducing write grants to
  read-only grants at matching paths. A boundary grant for `/` also covers other
  paths for the same access. Comparisons between different paths otherwise
  return `unsupported` with `reason_code: unresolved_filesystem_path`: a lexical
  child can resolve outside its parent through a symlink, and unrelated paths
  can resolve to the same object. This includes narrowing `/tmp` to `/tmp/cache`.
  If the boundary grants no access of the requested kind, adding that access
  returns `exceeds_boundary`.

Use the [Policy Schema Reference](/reference/policy-schema) for the full policy
language. A successful prover result covers only the scope reported in its
evidence.