🚗 MCP-CAN: Vehicle CAN Bus, OBD-II and J1939 Diagnostics for LLMs (Model Context Protocol)

🔌 Virtual CAN + MCP Server

MCP-CAN is a Model Context Protocol (MCP) server that exposes automotive CAN bus, OBD-II (SAE J1979), UDS, and SAE J1939 diagnostic data to LLMs and AI agents. It ships a built-in virtual CAN bus with an ECU simulator, decodes traffic via a DBC database (cantools), and serves MCP tools over SSE or streamable-HTTP. No CAN hardware, adapter, or vehicle is required by default; optional SocketCAN/vCAN on Linux.

Use it to let an LLM read live CAN frames, decode signals, run OBD-II PID and UDS diagnostic requests, inspect J1939 PGNs/SPNs and DM1 trouble codes, and drive fault-injection scenarios, all against a simulated vehicle.

Keywords: MCP server, Model Context Protocol, CAN bus, CANbus, OBD-II, OBD2, on-board diagnostics, SAE J1939, UDS, ECU simulator, vehicle diagnostics, automotive, DBC, python-can, cantools, SocketCAN, LLM tools, AI agents.

✨ Highlights

• MCP server for CAN/OBD/UDS-diagnostics/J1939 → LLM/SLM (tools + DBC metadata, SSE or streamable-HTTP).

• Virtual CAN backend (python-can) out of the box; optional SocketCAN/vCAN on Linux.

• DBC-driven encoding/decoding via cantools.

• ECU simulator that streams multiple messages, plus OBD-II, UDS-style, and SAE J1939 responders.

• SAE J1939 (heavy-duty, 29-bit extended IDs): ID decomposition (priority/PGN/source+destination address), a curated PGN/SPN catalog (EEC1, EEC2, ET1, CCVS1, LFE1, DD1), Request PGN (0xEA00) round trips, and DM1 active-DTC (SPN/FMI) broadcasts. Runs alongside the 11-bit bus; toggle with MCP_CAN_J1939_ENABLED.

• Correlated driving-dynamics signal generation, plus named fault-injection scenarios (overheat, abs_fault, low_fuel) with matching DTCs.

• Typer CLI: mcp-can (simulate, server, demo, frames, decode, monitor, dbc-info, obd-request, diag-request, fault, j1939-decode, j1939-pgns, j1939-request, j1939-dtcs).

• Structured tool output (typed Pydantic models), duration-capped tool calls, /healthz, colorized logging.

• Read-only live web dashboard (/dashboard): signal values and recent frames, updated over SSE.

• Dockerfile + docker compose for server + simulator.

• Unit tests, type hints, lint config (ruff, mypy); see CONTRIBUTING.md.

Related MCP server: connected-vehicle-health

📁 Repository Layout

• src/mcp_can/

  • cli.py – Typer commands

  • bus.py – python-can helpers

  • dbc.py – DBC loading/decoding

  • obd.py – OBD-II (SAE J1979) request/response helpers

  • diagnostics.py – UDS-style diagnostic service/response-code logic

  • j1939.py – SAE J1939: 29-bit ID decomposition, PGN/SPN catalog, DM1 DTCs, Request PGN

  • config.py – env settings (MCP_CAN_*) + logging setup

  • models.py – internal bus-layer dataclass (Frame)

  • simulator/runner.py – ECU simulator + OBD/diagnostic responders

  • simulator/j1939_runner.py – J1939 broadcasters (EEC1/ET1/CCVS1/…), Request PGN responder, DM1 emitter

  • simulator/state.py – correlated driving-dynamics state (RPM/speed/throttle/etc.)

  • simulator/faults.py – named fault-injection presets + activation protocol

  • server/fastmcp_server.py – MCP tools/resources + dashboard routes

  • server/schemas.py – Pydantic models for MCP tool structured output

  • server/live_state.py – background bus listener backing the dashboard

  • server/templates/dashboard.html – the dashboard page itself

• vehicle.dbc – sample CAN database (incl. a UDS-like diagnostic schema)

• simulate-ecus.py, can-mcp.py – standalone run-without-installing entrypoints

• docker/compose.yml, Dockerfile

• tests/ – unit tests

• CONTRIBUTING.md, CHANGELOG.md

✅ Prerequisites

• Python 3.10+

• (Optional) Docker / Docker Compose

• (Optional) Ollama if you want a local LLM backend

📦 Install (Python)

From repo root:

pip install -r requirements.txt
pip install -e .

🚀 Quickstart (Simulator + MCP Server)

Two terminals:

# Terminal A: start ECU simulator on virtual bus0
mcp-can simulate

# Terminal B: start MCP server (SSE on 6278)
mcp-can server --port 6278

Single-process (helps on Windows if virtual backend doesn't share across processes):

mcp-can demo --port 6278

Sample interactions:

mcp-can frames --seconds 2
mcp-can decode 0x100 "01 02 03 04 05 06 07 08"       # pretty table by default, --json for scripting
mcp-can dbc-info                                      # table of every message/signal in the DBC
mcp-can monitor ENGINE_SPEED --seconds 3
mcp-can obd-request --service 0x01 --pid 0x0D
mcp-can diag-request --service-id 0x22 --parameter-id 0x05   # READ_DATA_BY_ID
mcp-can j1939-pgns                                            # J1939 PGN/SPN catalog
mcp-can j1939-request 0xF004                                  # ask ECUs to send EEC1 (engine speed)
mcp-can j1939-dtcs                                            # read the latest DM1 active-DTC broadcast

📊 Live Dashboard

With mcp-can demo (or server) running, open http://localhost:6278/dashboard in a browser: live signal values grouped by ECU message, and a scrolling feed of recent frames, updating ~2x/second over Server-Sent Events. It's read-only (view only, no controls to send frames) and self-contained: no build step, no external assets, works offline. Like everything bus-related here, it only shows data when the simulator shares the same process as the server (mcp-can demo); pointed at a bare mcp-can server with no simulator, it just shows "waiting for CAN traffic."

🛠️ Available MCP Tools & Resources

read_can_frames/filter_frames/monitor_signal/read_j1939_dtcs are served from a single continuously-running history buffer (server/live_state.py) rather than each opening its own bus listener: they return immediately and won't miss frames sent between calls. send_obd_request/send_diagnostic_request/request_j1939_pgn are request/response and still wait live for a reply. In both cases, duration_s/timeout_s is capped by MCP_CAN_MAX_DURATION_S (default 30s; the history buffer retains at least that much, or 60s, whichever is larger). All tools return typed, structured content (see server/schemas.py) rather than ad-hoc JSON.

🩺 About the diagnostic responder

vehicle.dbc defines a UDS-like diagnostic schema: DIAGNOSTIC_REQUEST (one shared request frame) and four DIAGNOSTIC_RESPONSE_<ECU> messages, one per ECU, but the request has no per-ECU target field. The simulator treats every request as functionally addressed to all four ECUs, so send_diagnostic_request/diag-request may return more than one response. Supported services: START_DIAGNOSTIC_SESSION (0x10) and RESET_ECU (0x11) are acknowledged OK; READ_DATA_BY_ID (0x22) returns a deterministic canned value derived from the parameter ID; ROUTINE_CONTROL/READ_MEMORY/WRITE_MEMORY and anything unrecognized return SERVICE_NOT_SUPPORTED; see diagnostics.py::handle_service.

⚠️ Fault injection

Three named scenarios (simulator/faults.py::PRESETS) let you force the simulator into a specific fault state instead of waiting on random signal generation:

• overheat – ENGINE_TEMP pinned to its hottest reportable value, SYSTEM_STATUS set to FAULT_PRESENT, DTC P0217 (Engine Overtemp Condition).

• abs_fault – all four WHEEL_SPEED_* signals stuck at zero, SYSTEM_STATUS set to FAULT_PRESENT, DTC C0035 (Left Front Wheel Speed Sensor Circuit).

• low_fuel – FUEL_LEVEL pinned critically low; no DTC (a low-fuel light isn't a stored trouble code on a real vehicle either).

Activating a scenario sends a small control frame on the bus (like OBD/diagnostic requests, this is a round trip to whichever process is running the simulator, so it needs mcp-can demo/simulate already running) and overrides the named signals until cleared. Any DTCs the active scenario sets show up in send_obd_request/obd-request's Mode 03 (service=3) response. Use mcp-can fault list or the activate_fault_scenario tool's docstring to see the current preset descriptions; pass preset=None (CLI: clear) to deactivate.

🚛 SAE J1939 (heavy-duty)

Alongside the light-vehicle 11-bit bus, the simulator also speaks SAE J1939 — the protocol on trucks, buses and off-highway equipment. J1939 rides 29-bit extended CAN IDs whose arbitration field is itself structured data: a 3-bit priority, an 18-bit Parameter Group Number (PGN), and an 8-bit source address (plus, for peer-to-peer "PDU1" PGNs, a destination address). src/mcp_can/j1939.py is a self-contained implementation of that layer (not DBC-driven — vehicle.dbc models an 11-bit light-vehicle bus).

What's simulated (simulator/j1939_runner.py), driven by the same correlated driving-dynamics state as the 11-bit signals:

• Request PGN (0xEA00): request_j1939_pgn / mcp-can j1939-request <pgn> send a request; the simulator re-broadcasts the requested PGN once.

• DM1 / DTCs: read_j1939_dtcs / mcp-can j1939-dtcs read the latest DM1. The fault-injection presets map to J1939 DTCs too — overheat → SPN 110 FMI 0, abs_fault → SPN 84 FMI 5, low_fuel → SPN 96 FMI 18 — and the malfunction-indicator lamp turns on while a preset is active.

• J1939 signals also appear in get_vehicle_snapshot and the dashboard, grouped under J1939:<acronym>.

• Set MCP_CAN_J1939_ENABLED=false for an 11-bit-only bus.

🔍 MCP Inspector (GUI for your tools)

Use the official Inspector to explore and call your MCP tools without writing a host:

npx @modelcontextprotocol/inspector

When prompted, connect to your server:

• URL: http://localhost:6278/sse

You can then list tools/resources and call one (e.g. monitor ENGINE_SPEED for 5 seconds) and view structured output live.

🤖 Using with Ollama (local LLM)

1. Ensure Ollama is running: ollama serve and pull a model: ollama pull llama3

2. Run simulator + MCP server (see Quickstart).

3. Point your MCP-capable host at http://localhost:6278/sse and configure its model endpoint to http://localhost:11434 with your model name (e.g., llama3).

4. Prompt the host: "Monitor ENGINE_SPEED for 5 seconds", "List all DBC messages", or "Send a READ_DATA_BY_ID diagnostic request for parameter 5."

If you need a minimal host, pair @modelcontextprotocol/sdk with Ollama (see SDK docs) or use Inspector for manual tool calls.

Example host config (OpenAI-compatible endpoint to local Ollama):

{
  "model": {
    "type": "openai-compatible",
    "baseUrl": "http://localhost:11434/v1",
    "model": "llama3"
  },
  "mcpServers": {
    "can-mcp-server": {
      "serverUrl": "http://localhost:6278/sse"
    }
  }
}

⌨️ CLI Reference

• mcp-can simulate – start ECU simulator using vehicle.dbc.

• mcp-can server [--port 6278] [--transport sse|streamable-http|stdio] – run the MCP server.

• mcp-can demo [--port] [--transport] – simulator + server in one process.

• mcp-can frames --seconds 1.0 – capture raw frames as JSON.

• mcp-can decode <id> <data> [--json] – decode a single frame (table by default; id hex/decimal, data space/comma-separated bytes).

• mcp-can snapshot --seconds 1.0 [--json] – latest value of every signal seen while listening.

• mcp-can dbc-info [message] – table of every message/signal in the DBC, or just one message's.

• mcp-can monitor <signal> --seconds 2.0 [--json] – watch one signal (live output by default).

• mcp-can obd-request --service <hex|int> [--pid <hex|int>] – OBD-II request; response includes a decoded value for known PIDs.

• mcp-can diag-request --service-id <hex|int> [--parameter-id] [--data-field] – UDS-style diagnostic request; prints every ECU's response.

• mcp-can fault <preset|clear|list> – activate/clear a fault-injection scenario in a running simulator, or list available presets.

• mcp-can j1939-decode <id> <data> [--json] – decompose a 29-bit J1939 ID and decode known SPNs.

• mcp-can j1939-pgns – list the J1939 PGNs/SPNs this project can decode.

• mcp-can j1939-request <pgn> [--timeout 2.0] – send a J1939 Request PGN (0xEA00) and print decoded responses.

• mcp-can j1939-dtcs [--seconds 3.0] – listen for a J1939 DM1 broadcast and print its active trouble codes.

server/demo/simulate all print colorized logs (via rich) instead of raw text.

⚙️ Configuration

Env vars (prefix MCP_CAN_):

• CAN_INTERFACE (default virtual)

• CAN_CHANNEL (default bus0)

• DBC_PATH (default vehicle.dbc)

• MCP_PORT (default 6278)

• MCP_TRANSPORT (default sse; streamable-http requires a newer mcp SDK; the server logs a clear error and exits if the installed version doesn't support it, rather than crashing on an SDK traceback)

• MAX_DURATION_S (default 30.0) – caps every tool's duration_s/timeout_s

• J1939_ENABLED (default true) – run the SAE J1939 side of the simulator alongside the 11-bit signals

• LOG_LEVEL (default INFO)

• CORS_ALLOW_ORIGINS (default ["*"], JSON array e.g. ["https://your-host.example"]) – allowed browser origins for the SSE endpoint. Credentialed requests (allow_credentials) are only enabled once this is narrowed to specific origins; wildcard + credentials is a combination browsers reject outright, so it's never turned on for the default "*". Override before any real deployment.

You can set these in a .env file at repo root.

🐳 Docker

Build:

docker build -t mcp-can .

Run (combined server + simulator):

docker run -d --name mcp-can -p 6278:6278 -p 5000:5000 -p 8080:8080 mcp-can

Compose (from docker/):

docker compose up -d --build

The compose file currently runs server and simulator as separate containers; like running them as two separate local processes, they won't share the virtual CAN bus unless the host provides a real shared vcan0 interface. For a working combined setup today, use the single-container Dockerfile above (mcp-can demo).

🧪 Development & Testing

See CONTRIBUTING.md for the full guide. Quick version:

pip install -r requirements.txt
pip install -e .
pip install pytest ruff mypy

ruff check .
mypy src
pytest -q

🔧 Troubleshooting

• No frames? Ensure both simulator and server use the same interface/channel (virtual/bus0 by default), and, on Windows, that they're the same process (mcp-can demo) rather than two separate ones.

• DBC missing? Set MCP_CAN_DBC_PATH or place vehicle.dbc in repo root.

• Docker networking: expose 6278 so your MCP host can reach it.

• streamable-http transport fails immediately? Your installed mcp package predates its support; the log line tells you. Switch to sse or pip install -U mcp (staying below 2.0.0).

📄 License

MIT (see LICENSE). Educational/prototyping use only; use certified hardware for real automotive work.