vera-scheme-sandbox

Purpose

Provide platform-independent Chez Scheme evaluation for Vera through a WASM runtime, with bounded execution and explicit host-filesystem access modes.

Registered surface

Tools

• scheme_eval — evaluates R6RS/Chez Scheme code; output is captured from (display ...), (printf ...), stderr, and the tool footer.

Commands

• none

Hooks

• session_start — verifies the bundled WASM artifacts and warns if integrity checks fail.

Access modes

• readonly (default) mounts the current working directory at /work and blocks writes under that mount; process spawning is disabled.
• write mounts the current working directory at /work with read/write access; process spawning is disabled.
• fullaccess mounts the host root filesystem at /host, keeps the normal process environment, and enables Chez Scheme system().

/work and /host

/work is the session working directory projected into the WASM filesystem for readonly and write runs. Use it for project-local reads, and request write only when the Scheme code needs to create or modify files in that working tree.

/host is only present in fullaccess mode and maps to / on the host. Use it only for tasks that genuinely require whole-host paths or system().

Integrity checks

On session_start, the extension computes sha256 hashes for scheme.js, scheme.wasm, and scheme.data. A mismatch, missing file, or unreadable artifact triggers a warning that scheme_eval may not work correctly; the tool still registers before the check runs.

Smoke

• npm run smoke
• from package root: node ./scripts/smoke.mjs

The smoke script exercises arithmetic, recursion, macros, readonly reads, readonly write blocking, write-mode writes, fullaccess host reads, fullaccess system(), and blocked spawning outside fullaccess.

Intended use cases

• readonly: inspect project files, run pure calculations, test Scheme snippets, and do safe read-only analysis.
• write: generate or transform project-local files when the requested output should be written under the current working directory.
• fullaccess: perform trusted host-level inspection or shell-backed Scheme tasks that cannot be completed through /work.

Loaded through agent/settings.json as ./packages/vera-scheme-sandbox.