landstrip

landstrip runs a tool in an OS-level sandbox using Landlock LSM on Linux, Seatbelt on macOS, and LPAC AppContainer on Windows. It accepts the Anthropic Sandbox Runtime JSON subset as the policy, in JSON or YAML syntax.

Installation

npm

npm install --save-dev @jarkkojs/landstrip

npx landstrip -p policy.json cargo test

The npm package installs a small Node.js wrapper and a platform-specific native binary package.

Platforms

Windows uses an AppContainer. The platform grants the generated AppContainer SID access to the lowered read and write roots, so Windows policies must use explicit read allowlists. Fine-grained TCP and Unix socket policies are rejected until Windows enforcement exists.

Policy Format

JSON is the default policy format. Use --format yaml for YAML policy files or YAML read from standard input.

landstrip --format yaml -p policy.yaml cargo test

YAML path fields can use normal lists or one statement per line:

filesystem:
  allowWrite: |
    .
    ~/.cargo
  denyRead: |
    ~/.ssh
  allowRead: |
    ~/.ssh/config
network:
  allowNetwork: true

Network Policy

Sandbox mode denies direct network access by default. Proxy ports, local binding, and Unix sockets can be allowed with the Anthropic Sandbox Runtime network fields.

For a filesystem-only sandbox with unrestricted direct network access, set:

{
  "network": {
    "allowNetwork": true
  }
}

On Linux and macOS, allowNetwork disables landstrip network enforcement while leaving filesystem policy enforcement in place. Windows rejects unrestricted network policies until Windows network support exists.

Error Output

Failures reported by landstrip are printed as JSON objects on standard error, one object per line. Fields with no value are omitted. This covers policy, tool launch, platform, and system errors. Usage errors are not formatted responses; they remain on standard error and exit with status 2.

{"reason":"Internal","file":"policy.json","source":"expected value at line 1 column 1"}

{"reason":"LaunchFailed","program":"cargo","type":"launch","source":"No such file or directory"}

The reason field describes the error kind (Internal, AccessDenied, LaunchFailed, Usage). The file field is present when a trap is tied to a policy file or filesystem access denial. The program field is present when landstrip could not start or encode a tool. The type field is present for policy or tool errors and is either filesystem, network, or platform for policy errors, or launch (failed to start the tool) or encoding (failed to encode the command line) for tool errors. Filesystem access denials may include operation set to read or write.

Logs and sandboxed tool output are not part of the response. Normal successful tool execution does not print a landstrip response unless a write denial was observed, because standard error belongs to landstrip; standard output belongs to the sandboxed tool.

Trap FD

Use --trap-fd FD to write landstrip trap denial blocks to an already-open file descriptor as JSON objects, one per line followed by a newline.

landstrip --trap-fd 3 -p policy.json cargo test 3>landstrip-traps.txt

Linux filesystem denials observed by the seccomp broker are emitted as:

{"reason":"AccessDenied","type":"filesystem","file":"/repo/out","operation":"write","mechanism":"seccomp"}

The mechanism field records the kernel enforcement layer that detected the denial (e.g. seccomp or landlock).

This stream is separate from the sandboxed tool's output. If the option is omitted, landstrip is quiet unless it has to report a policy, launch, or platform error. These long-lived error messages remain on standard error and are not duplicated in the trap stream.

Trap responses are informational. The configured sandbox policy always applies. However, writing trap responses requires an already-open file descriptor and a readable file path. If the sandbox blocks writing to the descriptor, or if writing fails, the denial is quietly dropped and the policy remains in effect. On backends without per-denial callbacks the option is best-effort.

The descriptor must be 3 or greater (standard I/O descriptors 0-2 are reserved).

Development

Commit messages

• <subsystem>: <message>
• Long description for non-trivial changes.
• Kernel style commit messages.
• Signed-off-by

Documenting errors

The following snippet demonstrates the recommended pattern for documenting the return values on error:

/// # Errors
///
/// Returns [`<variant's unqualified name>`](<variant's unqualified name>)
/// Returns ...

Licensing

The JavaScript npm wrapper is licensed under Apache-2.0. The Rust source and native binaries are licensed under LGPL-2.1-or-later. Corresponding source for each published native binary is available from the GitHub repository tag that matches the package version.