Event Schema

June 16, 2026 · View on GitHub

Every CANarchy command that produces structured output emits events using a canonical envelope. This document defines that envelope and every typed event subclass.

The event schema is the stable interface between CANarchy and downstream consumers: scripts, pipelines, analysis tools, and coding agents. If you are parsing CANarchy output programmatically, parse the event stream rather than the human-readable text output.

Canonical Envelope

All events share the same top-level shape:

{
  "event_type": "<string>",
  "source": "<string>",
  "timestamp": "<float | null>",
  "payload": { }
}

Field	Type	Description
`event_type`	string	Discriminator. See event types below.
`source`	string	Dotted path identifying which command or subsystem emitted the event. Examples: `transport.capture`, `dbc.decode`, `gateway.src->dst`.
`timestamp`	float \| null	Seconds since epoch (or relative to capture start). `null` when not available.
`payload`	object	Event-specific fields. Shape varies by `event_type`.

Structured Errors

Failures return the canonical envelope with ok: false and an errors array. Each entry has code, message, and hint:

{
  "ok": false,
  "command": "decode",
  "errors": [
    {
      "code": "DBC_CACHE_MISS",
      "message": "DBC 'opendbc:toyota_tnga_k_pt_generated' is not in the local cache.",
      "hint": "Run `canarchy dbc cache refresh --provider opendbc` to populate it, or set `auto_refresh = true` under `[dbc.providers.opendbc]` in `~/.canarchy/config.toml`."
    }
  ]
}

Hint convention

Every structured error carries a non-empty hint. Hints follow a consistent shape — DBC_CACHE_MISS above is the reference template:

Explain the problem in one sentence.
Provide a copy-pasteable remediation command where one exists.
Reference the relevant flag, config key, or doc page by name.
Avoid restating the message; the hint is the next action.

A regression test (tests/test_cli.py::ErrorHintConventionTests) scans every ErrorDetail(...) call site in src/canarchy/ and asserts that each construction includes a hint=. Drift fails CI. Treat the catalogue in Troubleshooting as the user-facing index of these hints.

Streaming

Use --jsonl on any command to receive one event per line on stdout, suitable for piping:

canarchy capture can0 --jsonl
canarchy j1939 decode --file trace.candump --jsonl
canarchy generate can0 --count 100 --gap 10 --jsonl

Use --json to receive the full command envelope with all events in a data.events array:

canarchy capture can0 --json

Event Types

`frame`

A raw CAN frame, as captured, generated, or forwarded.

Emitted by: capture, send, generate, gateway, filter, replay

{
  "event_type": "frame",
  "source": "transport.capture",
  "timestamp": 0.0,
  "payload": {
    "frame": {
      "arbitration_id": 419360305,
      "bitrate_switch": false,
      "data": "11223344",
      "dlc": 4,
      "error_state_indicator": false,
      "frame_format": "can",
      "interface": "can0",
      "is_error_frame": false,
      "is_extended_id": true,
      "is_remote_frame": false,
      "timestamp": 0.0
    }
  }
}

Frame fields:

Field	Type	Description
`arbitration_id`	int	11-bit standard (0–2047) or 29-bit extended (0–536870911) CAN identifier.
`data`	string	Hex-encoded payload bytes, lowercase, no separators.
`dlc`	int	Data length code: number of payload bytes (0–8 for classic CAN, 0–64 for CAN FD).
`frame_format`	`"can"` \| `"can_fd"`	Frame type.
`interface`	string \| null	Interface name such as `can0` or `239.0.0.1`.
`is_extended_id`	bool	`true` for 29-bit extended identifiers.
`is_remote_frame`	bool	`true` for RTR frames (no data bytes).
`is_error_frame`	bool	`true` for CAN error frames.
`bitrate_switch`	bool	CAN FD only — BRS flag.
`error_state_indicator`	bool	CAN FD only — ESI flag.
`timestamp`	float \| null	Frame timestamp, seconds.

Source values for frame events:

Source	Context
`transport.capture`	Capture via the selected transport backend (`python-can` or scaffold)
`transport.send`	Active frame transmission
`transport.generate`	Frame generation (`canarchy generate`)
`transport.filter`	File-backed filter result
`gateway.src->dst`	Forwarded frame, source to destination
`gateway.dst->src`	Forwarded frame, destination to source (bidirectional mode)
`replay.engine`	Replay plan frame

`decoded_message`

A CAN frame decoded against a DBC message definition. Includes the raw frame and all decoded signal values.

Emitted by: decode

{
  "event_type": "decoded_message",
  "source": "dbc.decode",
  "timestamp": 0.0,
  "payload": {
    "frame": { },
    "message_name": "EngineStatus1",
    "signals": {
      "CoolantTemp": -23,
      "LampState": 68,
      "Load": 20.4,
      "OilTemp": -6
    }
  }
}

Field	Type	Description
`frame`	object	Raw `CanFrame` payload (same shape as `frame` event).
`message_name`	string	DBC message name.
`signals`	object	Signal name → decoded physical value map.

`signal`

An individual decoded signal value. One signal event is emitted per signal per decoded frame, immediately following its parent decoded_message event in the stream.

Emitted by: decode

{
  "event_type": "signal",
  "source": "dbc.decode",
  "timestamp": null,
  "payload": {
    "message_name": "EngineStatus1",
    "signal_name": "CoolantTemp",
    "units": "degC",
    "value": -23
  }
}

Field	Type	Description
`message_name`	string \| null	Parent DBC message name.
`signal_name`	string	DBC signal name.
`units`	string \| null	Physical unit string from the DBC (empty string if not defined).
`value`	number	Decoded physical value after scaling and offset.

`dbc_database`

A DBC database summary emitted by dbc inspect before message and signal metadata.

Emitted by: dbc inspect

{
  "event_type": "dbc_database",
  "source": "dbc.inspect",
  "timestamp": null,
  "payload": {
    "format": "dbc",
    "message_count": 2,
    "node_count": 0,
    "path": "tests/fixtures/sample.dbc",
    "signal_count": 6
  }
}

`dbc_message`

DBC message metadata emitted by dbc inspect.

Emitted by: dbc inspect

When dbc inspect --layout is used, the payload also includes string fields layout, signal_tree, and signal_choices rendered by cantools.

{
  "event_type": "dbc_message",
  "source": "dbc.inspect",
  "timestamp": null,
  "payload": {
    "arbitration_id": 419360305,
    "arbitration_id_hex": "0x18FEEE31",
    "cycle_time_ms": null,
    "is_extended_id": true,
    "length": 4,
    "name": "EngineStatus1",
    "senders": [],
    "signal_count": 4
  }
}

`dbc_signal`

DBC signal metadata emitted by dbc inspect.

Emitted by: dbc inspect

{
  "event_type": "dbc_signal",
  "source": "dbc.inspect",
  "timestamp": null,
  "payload": {
    "byte_order": "little_endian",
    "choices": null,
    "is_multiplexer": false,
    "is_signed": false,
    "length": 8,
    "maximum": 210,
    "message_name": "EngineStatus1",
    "minimum": 0,
    "multiplexer_ids": null,
    "name": "CoolantTemp",
    "offset": -40,
    "scale": 1,
    "start_bit": 0,
    "unit": "degC"
  }
}

`j1939_pgn`

A J1939 frame observation with decomposed identifier fields.

Emitted by: j1939 monitor, j1939 decode, j1939 pgn

{
  "event_type": "j1939_pgn",
  "source": "transport.j1939.decode",
  "timestamp": 0.0,
  "payload": {
    "destination_address": null,
    "frame": { },
    "pgn": 65262,
    "priority": 6,
    "source_address": 49
  }
}

Field	Type	Description
`pgn`	int	Parameter Group Number (0–262143).
`source_address`	int	J1939 source address (0–255).
`destination_address`	int \| null	Peer-to-peer destination address, or `null` for broadcast PGNs.
`priority`	int \| null	J1939 frame priority (0–7).
`frame`	object	Raw `CanFrame` payload.

`j1587_parameter`

A decoded J1587 PID parameter extracted from a J1708 message.

Emitted by: j1587 decode

{
  "event_type": "j1587_parameter",
  "source": "j1587",
  "timestamp": 0.0,
  "payload": {
    "checksum_valid": true,
    "mid": 128,
    "name": "Engine Speed",
    "pid": 190,
    "raw": "7017",
    "units": "rpm",
    "value": 1500.0
  }
}

Field	Type	Description
`mid`	int	Source MID (0–255) from the J1708 message.
`pid`	int	J1587 Parameter ID (0–511; values above 253 use the extended-PID encoding).
`raw`	string	Raw parameter data bytes as hex.
`name`	string \| null	Parameter name from the bundled PID catalog, or `null` if unknown.
`value`	float \| null	Scaled value, or `null` if unknown or the all-ones "data not available" sentinel.
`units`	string \| null	Units from the bundled PID catalog, or `null` if unknown.
`checksum_valid`	bool	Whether the source J1708 message's byte-sum checksum was valid.

`j2497_message`

A decoded J2497 (PLC4TRUCKS trailer power-line) frame.

Emitted by: j2497 decode

{
  "event_type": "j2497_message",
  "source": "j2497",
  "timestamp": 0.0,
  "payload": {
    "checksum_valid": true,
    "data": "2c01",
    "mid": 137,
    "name": "Brakes - Trailer #1 (ABS)"
  }
}

Field	Type	Description
`mid`	int	Source MID (0–255) from the J2497 frame.
`data`	string	Message-data bytes (between the MID and the checksum) as hex.
`name`	string \| null	ECU name from the bundled MID catalog, or `null` if unknown.
`checksum_valid`	bool	Whether the frame's byte-sum checksum was valid.

`uds_transaction`

A UDS request/response pair observed during a scan or trace.

Emitted by: uds scan, uds trace

{
  "event_type": "uds_transaction",
  "source": "transport.uds.scan",
  "timestamp": 0.0,
  "payload": {
    "complete": true,
    "decoder": "built-in",
    "ecu_address": 2024,
    "negative_response_code": null,
    "negative_response_name": null,
    "request_data": "1001",
    "request_id": 2015,
    "request_summary": null,
    "response_data": "5001003201f4",
    "response_id": 2024,
    "response_summary": null,
    "service": 16,
    "service_name": "DiagnosticSessionControl"
  }
}

Field	Type	Description
`service`	int	UDS service ID (hex values: 0x10 = DiagnosticSessionControl, 0x22 = RDBI, etc.).
`service_name`	string	Human-readable service name.
`request_id`	int	CAN arbitration ID of the request frame.
`response_id`	int	CAN arbitration ID of the response frame.
`ecu_address`	int \| null	Responding ECU address (typically equals `response_id`).
`request_data`	string	Request payload bytes, hex-encoded.
`response_data`	string	Response payload bytes, hex-encoded.
`complete`	bool	`true` when the response payload was fully reassembled; `false` when the capture ended early or consecutive frames arrived out of order.
`decoder`	string	Protocol decoder path used for transaction enrichment, such as `built-in` or `scapy`.
`request_summary`	string \| null	Optional summary-level request interpretation when the Scapy-backed adapter is available.
`response_summary`	string \| null	Optional summary-level response interpretation when the Scapy-backed adapter is available.
`negative_response_code`	int \| null	Negative response code for UDS negative responses.
`negative_response_name`	string \| null	Human-readable negative response name for UDS negative responses.

`replay_event`

A scheduled frame transmission from a replay plan.

Emitted by: replay

{
  "event_type": "replay_event",
  "source": "replay.engine",
  "timestamp": 0.0,
  "payload": {
    "action": "send_frame",
    "frame": { },
    "rate": 1.0
  }
}

Field	Type	Description
`action`	string	`"send_frame"` for frame transmission events; `"plan_start"` / `"plan_end"` for lifecycle events.
`rate`	float \| null	Replay rate multiplier (1.0 = original timing, 2.0 = double speed).
`frame`	object \| null	Raw `CanFrame` payload (absent for lifecycle events).

`alert`

A diagnostic notice from a command: active-transmit warnings, backend availability notices, or informational messages.

Emitted by: send, generate, gateway, and any command that raises a notable condition.

{
  "event_type": "alert",
  "source": "transport.generate",
  "timestamp": null,
  "payload": {
    "code": "ACTIVE_TRANSMIT",
    "level": "warning",
    "message": "Active frame generation requested on the selected interface."
  }
}

Field	Type	Description
`level`	`"info"` \| `"warning"` \| `"error"`	Severity.
`code`	string \| null	Machine-readable alert code.
`message`	string	Human-readable description.

Common alert codes:

Code	Level	Context
`ACTIVE_TRANSMIT`	warning	Frame generation or active transmission requested.
`CANDUMP_LIVE_BACKEND_REQUIRED`	error	`--candump` mode requires `python-can` backend.
`GATEWAY_LIVE_BACKEND_REQUIRED`	error	`gateway` requires `python-can` backend.

Consuming Events in Scripts

Parse JSONL output line by line and filter on event_type:

# Extract all frame arbitration IDs from a live capture
canarchy capture can0 --jsonl \
  | jq 'select(.event_type == "frame") | .payload.frame.arbitration_id'

# Decode a trace and print each signal value
canarchy decode --file trace.candump --dbc vehicle.dbc --jsonl \
  | jq 'select(.event_type == "signal") | [.payload.signal_name, .payload.value, .payload.units]'

# Watch for J1939 coolant temperature observations
canarchy j1939 decode --file trace.candump --jsonl \
  | jq 'select(.event_type == "j1939_pgn" and .payload.pgn == 65262)'

Agent Integration

See Agent Guide for how to invoke CANarchy commands as a coding agent and parse structured results.