Copperhead

Why it's ranked here

The safety story is enforced in code, not in the prompt. Editing tools are withheld from the model until a change proposal validates, every mutation is followed by a rules check, and the separate check command is contractually free of model calls and network requests, so it can run in CI and a pre-commit hook. The experimental agent-tool server exposes only five whole-pipeline actions, so a host agent cannot reach around the gates. Against that: the project calls itself early, and its own roadmap says the live end-to-end acceptance suite has not been observed passing. Apache 2.0, published on npm.

What's good

The deterministic surface is real and useful without any model: checking, a schematic drafter that produces the same bytes from the same intent file, a 0 to 100 legibility score that never changes the exit code, and a bill-of-materials export for JLCPCB, DigiKey and Mouser that refuses to run while the parts list disagrees with the schematic. Model selection refuses to guess when two API keys are set rather than silently picking one. Error messages are specific: a bad KiCad path override fails loudly instead of falling back to a different binary. Saved logins for Claude Code, Cursor and Codex mean no API key is needed.

Tradeoffs

It is a 0.x release whose README warns the interface will move before 1.0, and the agent-tool server is labelled unstable. The roadmap file still describes an older version, so its status claims may lag the code. Requirements disagree in one spot: the install docs ask for KiCad 8 or newer while the agent-tool server's error tells users to install KiCad 9. With the Codex backend, the README admits Codex can still read local files such as secrets in a dotenv file, and its session logs sit outside the tool's redaction. Long turns are allowed up to an hour each by default. The project states it is not an autorouter.

How to use it well

Best fit: a hardware team already running KiCad projects in git that wants doc-to-schematic drift caught in CI. Start with the init and check commands, which need no model, and wire check into the pre-commit hook it offers to install. Run the doctor command first on a new machine. Use the one-shot change command with its dry-run flag before trusting it with commits, and keep a human sign-off, which the docs themselves require. If you only want an autorouter or a GUI, look elsewhere: both are listed non-goals.

Observed

License

Apache-2.0 (LICENSE, package.json)

Language

TypeScript, ES modules, Node.js 20 or newer (package.json)

Install

npm global package with a copperhead binary; optional macOS/Linux bootstrap script

Interfaces

CLI with interactive shell and --json output; experimental MCP server over stdio (src/mcp/server.ts)

External requirement

KiCad 8 or newer with kicad-cli available (src/kicad/cli.ts)

Model backends

OpenAI, Anthropic API, Claude Code, Cursor Agent CLI, Codex CLI, any OpenAI-compatible endpoint (src/config.ts)

Maturity

0.x, README status says early; MCP surface declared unstable

Tests

Vitest test suite declared in package.json