hwcontract

Your firmware is correct on paper and wrong on the wire.

Coding agents write WS2812 drivers, ESC bitstreams, and boot logs that pass review and then fail the moment the signal hits a real chip. hwcontract closes that loop. It captures what the hardware actually did and returns a verdict you can act on:

• pass: within spec
• marginal: in spec but too close to a rail. Works on your bench, dies on a cold board in the field. Treat it as a fail.
• fail: out of spec, with the measured value and how far off it is

No hardware in your hand? The demo below runs the whole thing on a real recorded signal, so you can see exactly what you get before wiring anything up.

See it work in 30 seconds

pip install hwcontract
python3 -m hwcontract.judge --demo

That judges a real 24-LED NeoPixel capture against two contracts. Same signal, two verdicts:

measured on the real WS2812B signal (300000 samples @24MHz):
  T0H 333 ns   T1H 833 ns   T1L 417 ns   T0L 917 ns   RESET 992250 ns

=== generic WS2812 contract -> FAIL ===
  T1L   600   417   FAIL   183ns short (typ 600)      # real WS2812B low times are shorter
  T0L   800   917   marginal
  T1H   700   833   marginal

=== matching WS2812B contract -> PASS ===
  (all pass)

Same hardware, two contracts: the generic one fails, the chip-specific one passes. A WS2812B isn't a WS2812. Measure the real signal, hold it to a spec, and match the contract to the actual chip.

What you get

• 27 bundled contracts for the parts people actually use: WS2812/WS2813/ SK6812 NeoPixels, DShot ESCs (150/300/600/1200), servos, I2C, NEC IR remotes, DS18B20, DHT11/DHT22, HC-SR04, A4988/DRV8825 stepper drivers, PWM fans, plus serial boot logs for ESP32, ESP8266, Zephyr, MicroPython, Raspberry Pi, U-Boot, and STM32 bootloaders. Each one has the datasheet's real min/typ/max numbers.
• Add a protocol by dropping in one YAML file. No code change.
• An MCP server your agent can call, or plain CLI commands you can run by hand.
• Reasonable by default: timing edges are all measured in nanoseconds, serial contracts are Python regex, verdicts come back with the measured value and the delta so an agent knows exactly what to fix.

Install

pip install hwcontract              # judge + logic-analyzer adapter
pip install "hwcontract[serial]"    # + live serial capture (pyserial)
pip install "hwcontract[untrusted]" # + google-re2 (ReDoS-immune, for untrusted contracts)
pip install "hwcontract[all]"       # everything

Live logic-analyzer capture (check_ws2812 / check_dshot) also needs sigrok-cli on PATH. Judge-only tools (judge_contract, judge_serial) need nothing extra.

Wire it into an agent

One stanza per client, add it once. After install, the hwcontract command is on your PATH.

Claude Code

claude mcp add hwcontract -- hwcontract

Codex CLI: ~/.codex/config.toml

[mcp_servers.hwcontract]
command = "hwcontract"

opencode / Cursor / Gemini / any stdio MCP client

{ "mcpServers": { "hwcontract": { "command": "hwcontract" } } }

Transport is stdio by default (local, no auth surface). For remote-only clients (e.g. ChatGPT connectors), run hwcontract --http 8791 and expose it via a tunnel with HWCONTRACT_TOKEN set for bearer auth.

Speaks MCP 2026-07-28, the stateless revision: per-request _meta, server/discover, no handshake. Clients that still open with initialize get the old shape back. Each request picks its own era, so nothing to configure.

If the client can't find hwcontract (PATH issues)

GUI apps and some agents don't inherit your shell PATH, so a bare hwcontract can fail with "command not found". Two robust fixes:

• Use the absolute path: which hwcontract → put that full path in command.
• Or invoke via Python (no PATH lookup for the script): command: "python3", args: ["-m", "hwcontract.server"]. Works from any directory once installed.

Contract paths: pass an absolute contract_path, or set HWCONTRACT_ROOT to your contracts folder. Relative paths resolve against it, defaulting to the process's working directory, which the client controls and may not be your project. Paths outside the root are rejected. Bundled examples install with the package under hwcontract/examples/.

The tools

How it fits together

  observers (capture)                judge (this repo)
  ─────────────────────              ─────────────────
  logic analyzer  ─ pulse widths ─┐
  serial port     ─ log text ─────┼─►  contract × observation  ─►  pass/marginal/fail
                                  ┘         (judge.py)

• judge.py. The pure judge for timing and serial. No hardware, no framework, cached.
• sigrok_adapter.py. Turns a logic-analyzer capture into pulse-width observations for WS2812 and DShot.
• serial_adapter.py. Captures a serial log, or replays a saved one.
• server.py. The MCP server, stdio and HTTP JSON-RPC, stdlib only.
• *.contract.yaml. What "correct" looks like. Human-editable, and they double as regression tests.

The contract format

Timing (ws2812.contract.yaml, dshot.contract.yaml): pulse widths in ns

contract: ws2812
headroom_pct: 20         # in-spec but within 20% of a rail => "marginal"
edges:
  - {name: T0H, min: 200, typ: 350, max: 500}   # '0' bit high time

Serial (boot.contract.yaml) uses Python regex:

contract: boot
kind: serial
expect: ["IMU init OK", "boot v\\d+"]
forbid: ["panic", "Guru Meditation", "\\bnan\\b"]

Add a protocol = drop a new YAML. No code change for another timing signal.

Kill switch

Instantly disable every hardware-touching tool (captures) while leaving the pure judge tools working:

export HWCONTRACT_SAFE=1          # env, or:
touch /home/tsd/projects/hardware/KILLSWITCH   # file next to server.py

Security

Every tool argument is treated as hostile, since the caller is an LLM that can be prompt-injected. Contract paths are confined to the server dir, HWCONTRACT_ROOT overrides that. driver/channel/port are charset-validated, samples/seconds/samplerate are clamped, sigrok-cli runs with a timeout, YAML is safe_load. Do not expose this server over the network without adding authentication.

Self-tests: no hardware, run from anywhere

hwcontract --selftest                       # full MCP round-trip
python3 -m hwcontract.judge --demo
python3 -m hwcontract.sigrok_adapter --demo
python3 -m hwcontract.serial_adapter --demo