---
type: plan
status: active
updated: 2026-09-05
---
# Initial plan

## What is defined

The envelope, the topic namespace, and the standard OBD-II signals — the ones
whose names, units and ranges are fixed by the standard rather than by the car.

## What is not

**Z27AG-specific signals.** They do not exist until Phase 2 identifies them from
observed traffic. They arrive as DBC entries plus registry rows with
`source: "z27ag"`, one at a time as each is confirmed.

**Composite payload shapes.** `gnss/fix`, `nav/guidance`, `nav/route` and
`system/<service>` have their fields named in `topics.json` but no JSON Schema
yet. They get one when a publisher exists to constrain.

## Order

1. Schema validation available to all three languages — the Rust services, the
   Python fixtures and the Dart client must reject a malformed message
   identically, or the contract is decorative.
2. `gnss/fix` schema, driven by what `tool-mock` replays.
3. Z27AG signals, as Phase 2 confirms them.

## Consumers

| Repo | Reads |
|---|---|
| `svc-core` | signals, envelope, DBC |
| `svc-gw` | topics, envelope |
| `svc-nav` | topics, envelope |
| `app-flutter` | topics, envelope, signals (for display units) |
| `tool-mock` | all of it — fixtures publish the real schema |