Onyx is a MCP server that securely executes code inside Docker sandboxes, enabling safe execution of user-supplied code across multiple languages. It supports Python 3.11, Java 17 (OpenJDK), C/C++ toolchains (gcc 12), Node.js 20, and Rust 1.72, running each job in isolated containers. The server exposes a run_code tool through the MCP protocol, allowing clients to send code and receive results while logs are emitted to stderr to avoid interfering with MCP client IO. Sandboxing enforces strict limits: network is disabled, filesystems are read-only with tmpfs for writable areas, CPU, memory, and process limits are applied, and execution runs as a non-root user (uid 1000, gid 1000). The project includes automated tests (CI-ready) for executors and comprehensive setup instructions. Onyx is designed to integrate with Claude Desktop or other MCP clients, simplifying the workflow for power users and developers building AI applications that execute arbitrary code as part of the pipeline. The repository includes a Go-based server and language-specific executors, and provides clear steps to extend to new languages. Setup also suggests pre-pulling language runtimes to minimize first-run latency.