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Show HN: Plimsoll, a self-hosted sandbox for code your AI agent writes

github.com13 points2 comments
Screenshot of Show HN: Plimsoll, a self-hosted sandbox for code your AI agent writes

Plimsoll is a self-hosted sandbox designed to run untrusted code written by AI agents while always reporting how strong the isolation around each run was. Each result carries an isolation tier (process, container, kernel, vm) and a client can demand a minimum tier; runs that cannot meet the requested floor are refused before any code executes. Runs produce cryptographic evidence: SHA-256 fingerprints of the request and the returned trajectory or result, and an external tool, plimsoll-attest, signs and replays checked runs and refuses incomplete chains. A built-in trial runner enables exam-style verification: agent-supplied programs are run against sealed scenarios (deleted files) and every run’s trajectory is hashed so identical executions match exactly. Example: a cart-pole controller (simulator compiled to WebAssembly) was run and accepted/rejected drafts produced distinct fingerprints.

Security and operability details matter: refusals that ran nothing are marked with a reason and distinct error behavior (ErrInsufficientIsolation), while errors without that mark may indicate code started and are unsafe to retry. Capability grants let sandboxed code call specified API routes without ever holding credentials, network access, or endpoint addresses; a broker outside the sandbox mints short-lived per-run credentials and enforces exact route permissions. Integrations exist for agent frameworks (Vercel AI SDK, Trigger.dev, Google ADK, Agno, CrewAI, Mastra), and a QuickJS WebAssembly provider lets users run examples locally without Docker or a daemon.

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