vdm/contracts/README.md

# contracts/ — the wire contract

**This directory is the interface between every lane.** Owner: agent **PROTO**.
Nobody else commits here. Everybody else *generates from* here.

> ## Status: **v1.4.0** (frozen at v1.0.0 on 2026-09-09; minor bumps since)
>
> **v1.0.0** froze 38 methods, 9 events, 26 named types. **v1.1.0** widened `bufferBytes`
> bounds and added the segment-budget settings. **v1.2.0** added `download.provideAuth`
> (F2). **v1.3.0** widened when `error` is populated on a state change to cover a
> daemon-initiated `paused` (for `docs/adr/0013-...`). **v1.4.0** (current) is a
> **C++-binding-only** change: the generated `Dispatcher` gains a `HandlerError` /
> `HandlerResult<T>` error channel so a handler can return `-32010` / `-32011` /
> `-32013` with their `data` payloads instead of collapsing to `-32603`. The wire is
> byte-identical — no schema or fixture change — but any `Dispatcher` implementer
> must swap `Result` → `HandlerResult` on regen. Answered in
> `contracts/proto-answers-daemon-m1.md`. See also `docs/adr/0005-...` for the
> versioning rule and `docs/adr/0010-...` for the failure taxonomy and segment ranges.
>
> Lane requests are answered in writing: `contracts/proto-answers-m1.md` responds to
> `core/docs/proto-requests-m1.md` point by point.
>
> **What each lane can rely on, starting now:**
>
> | You need | It is here |
> |---|---|
> | C++ types, parsing, dispatch | `core/generated/velox_proto.{hpp,cpp}` (target `libveloxproto`) |
> | TypeScript types, typed client, runtime validators | `extension/src/shared/protocol/` |
> | The API document to read | `contracts/openrpc.json` |
> | A daemon to build against today | `tools/mockd` — both transports, 4 Hz progress, unhappy-path flags |
> | Proof you have not drifted | `./tests/conformance/run.sh` |
>
> **Changing this is a PR to `contracts/` alone.** Optional field or new method → minor.
> Rename, remove or retype → major, plus an ADR. File a request; do not add a field locally.

```
contracts/
├── VERSION                     # protocol semver — v1.4.0, minor-bumped from the v1.0.0 freeze
├── openrpc.json                # human-readable API doc (generated from schema/)
├── schema/
│   ├── envelope.schema.json    # JSON-RPC 2.0 envelope + our error codes
│   ├── types/                  # Task, Segment, Category, Queue, Settings, CaptureRules…
│   ├── methods/                # one file per method: params + result
│   └── events/                 # one file per server→client notification
├── fixtures/                   # golden request/response pairs, replayed by conformance
└── codegen/
    ├── schema_ir.py            # the one loader/IR all generators share
    ├── gen_cpp.py              # → core/generated/  (structs + to_json + parse + dispatch)
    ├── gen_ts.py               # → extension/src/shared/protocol/ (types, client, validators)
    ├── gen_openrpc.py          # → contracts/openrpc.json
    └── gen_cpp_conformance.py  # → tests/conformance/cpp/fixture_dispatcher.hpp
```

Each subdirectory has its own README: `codegen/` documents the supported JSON Schema
subset, `fixtures/` documents the fixture shape and the placeholder rules.

## Rules

1. **Generated code is committed.** No lane may be blocked because it can't run Python.
2. **Hand-editing generated files is a merge blocker.** Fix the schema and regenerate.
3. **Every method needs at least one fixture** — a success case and, where meaningful, an
   error case. A method with no fixture is not done.
4. **Versioning:** adding an optional field or a new method → minor bump. Removing,
   renaming, retyping, or changing a default → **major** bump and a written migration note
   in `docs/adr/`. `session.hello` rejects a major mismatch with error `-32001` and a
   message the GUI renders as "Velox needs updating".
   "Retype → major" is about the **wire** — a field a client parses off the socket. A
   change that leaves the wire byte-identical but breaks a *generated binding's* source
   API (a C++ virtual's return type, a struct name) is a **minor** bump plus a migration
   note — see `docs/adr/0015-generated-binding-changes-and-versioning.md`.
5. **Changes arrive as a PR to `contracts/` alone**, containing: schema edit + fixtures +
   regenerated code + `VERSION` bump. Lanes rebase onto it. This is the only synchronization
   point in the whole project — keep it cheap and frequent rather than big and rare.

## Per-method annotations

Every method schema carries these, and both generators emit them as data the code can act
on rather than as prose a reader has to honour:

| Key | Meaning |
|---|---|
| `x-privileged` | refused over the WebSocket transport with `-32003` |
| `x-transports` | which listeners serve it (`uds`, `ws`) |
| `x-deadlineMs` | how long a client waits before giving up |
| `x-errors` | the error codes this method is documented to return |
| `x-wsRestrictions` | extra limits when the call arrives from the extension |

20 of the 39 methods are privileged: everything that reconfigures the daemon, destroys user
data, or names an arbitrary destination path. The extension may *request* a download; it
may not choose where the bytes land.

## Transport framing

| Client | Transport | Framing |
|---|---|---|
| GUI, CLI | `$XDG_RUNTIME_DIR/velox/velox.sock` | newline-delimited JSON (NDJSON) |
| nmhost ← Firefox | stdio | 4-byte little-endian length prefix (Firefox's format) |
| nmhost → daemon | same Unix socket | NDJSON |
| Extension (fallback) | `ws://127.0.0.1:520xx` | one JSON message per WS text frame |

All four carry **the same JSON-RPC 2.0 payloads**. The framing differences stop at the
transport layer; no method behaves differently depending on how it arrived — except that
methods marked `"privileged": true` in the schema are refused over the WebSocket transport.

## Method surface (v1.4.0 — expand only via PR)

### Session
| Method | Params → Result |
|---|---|
| `session.hello` | `{clientType, clientName, protocolVersion, token?}` → `{daemonVersion, protocolVersion, capabilities[], sessionId}` |
| `session.pair` | `{clientName, extensionId}` → `{token, expiresAt}` *(WS only; triggers user prompt)* |
| `session.subscribe` | `{events[]}` → `{ok}` |

### Downloads
| Method | Params → Result |
|---|---|
| `download.probe` | `{url, headers?, cookies?, referrer?, userAgent?}` → `{filename, sizeBytes?, mime, resumable, effectiveUrl, suggestedCategoryId}` |
| `download.add` | `{url, headers?, cookies?, referrer?, userAgent?, filename?, saveDir?, categoryId?, segments?, bufferBytes?, startMode:"now"\|"later"\|"queue", queueId?, description?, checksum?}` → `{taskId, state}` |
| `download.addBatch` | `{items[], defaults}` → `{taskIds[]}` |
| `download.list` | `{filter?, sort?, offset?, limit?}` → `{total, items: TaskSummary[]}` |
| `download.get` | `{taskId}` → `TaskDetail` (includes `segments[]`) |
| `download.start` \| `.pause` \| `.resume` \| `.cancel` | `{taskIds[]}` → `{updated[]}` |
| `download.remove` | `{taskIds[], deleteFile:bool}` → `{removed[]}` |
| `download.update` | `{taskId, patch:{filename?, saveDir?, categoryId?, queueId?, description?, segments?, bufferBytes?}}` → `TaskSummary` |
| `download.refreshUrl` | `{taskId, url, headers?}` → `{ok}` *(IDM's "Refresh Download Address")* |
| `download.provideAuth` | `{taskId, username, password, save?}` → `{ok}` — answers `event.auth.required`. UDS only; privileged. Credentials go to the Secret Service, never SQLite, never logs |

### Organisation
`category.list` · `category.upsert` · `category.remove` · `queue.list` · `queue.upsert` ·
`queue.start` · `queue.stop` · `queue.reorder` · `rules.list` · `rules.upsert` ·
`schedule.get` · `schedule.set`

### Settings & limits
`settings.get {keys?}` · `settings.set {values}` · `limiter.get` · `limiter.set {globalBps?, enabled}`

### Browser integration
| Method | Notes |
|---|---|
| `capture.offer` | `{url, method, headers, cookies, contentType?, contentLength?, contentDisposition?, tabUrl, filename?}` → `{action:"take"\|"ignore", taskId?, reason?}` — **must answer within 750 ms**; the extension gives up and lets Firefox handle it otherwise |
| `capture.getRules` | Extension mirrors the daemon's monitored types so the two never disagree |
| `media.listVariants` | `{manifestUrl, headers}` → `{variants:[{id,resolution,bitrate,codec,sizeEstimate}]}` |
| `media.addVariant` | `{manifestUrl, variantId, ...addParams}` → `{taskId}` |

### Grabber
`grabber.start {startUrl, depth, includePatterns[], excludePatterns[], fileTypes[]}` →
`{jobId}`; `grabber.status {jobId}`; `grabber.harvest {jobId, select[]}` → `{taskIds[]}`

### Events (server → client notifications)
| Event | Payload |
|---|---|
| `event.task.added` / `.removed` | `{taskId, summary?}` |
| `event.task.state` | `{taskId, state, error?}` |
| `event.task.progress` | **Batched array**, emitted at ≤4 Hz: `[{taskId, downloaded, speedBps, etaSec, segments:[{i,completed,speedBps}]}]` |
| `event.speed.global` | `{downBps, activeCount}` |
| `event.auth.required` | `{taskId, host, realm, scheme}` |
| `event.notify` | `{level, title, body, taskId?}` |
| `event.settings.changed` | `{keys[]}` |
| `event.grabber.progress` | `{jobId, found, crawled, done}` |

## Two error spaces, and why they are not the same

This trips people up, so it is stated once, loudly:

| | `ErrorCode` | `TaskErrorCode` |
|---|---|---|
| Says | why a **call** failed | why a **download** failed |
| Space | JSON-RPC integers (`-32xxx`) | strings (`"server_file_changed"`) |
| Lives in | the JSON-RPC envelope's `error` | `TaskError.code`, on a task |
| Example | `-32602` — your params were malformed | `checksum_mismatch` — the bytes arrived and were wrong |

**A download fails while every RPC involved succeeds.** That is the normal case. Never put
a `-32xxx` into a `TaskError`, and never invent a JSON-RPC code for a transfer failure.

`TaskErrorCode`'s 27 values mirror `vdm::Error` in `core/include/vdm/util/error.hpp` by
name, so DAEMON's projection from the engine taxonomy is lossless and a new engine failure
that has no wire spelling is a visible hole rather than a silent collapse to `internal`.

## Segment ranges are inclusive

`Segment.startByte` and `Segment.endByte` describe a **closed** range `[startByte,
endByte]`: `endByte` is the last byte, not one past it, and the segment covers
`endByte - startByte + 1` bytes. The two fields are copied verbatim into
`Range: bytes=<startByte>-<endByte>`, which RFC 9110 defines as inclusive, so there is no
arithmetic between the wire and the socket and nowhere for an off-by-one to hide.
Conformance enforces contiguity and full coverage; a fixture written half-open fails.

## Requested is not effective

`DownloadSpec.segments` is what a client **asked for**. `TaskSummary.segments` is what is
**in use right now**, after the per-host cap and after the demotion to 1 for a
non-resumable source. They are routinely different and the GUI must render the effective
one.

## Error codes

| Code | Meaning |
|---|---|
| `-32600/-32601/-32602/-32603` | Standard JSON-RPC |
| `-32001` | Protocol major version mismatch |
| `-32002` | Not paired / invalid token |
| `-32003` | Method not permitted on this transport |
| `-32010` | Task not found |
| `-32011` | Invalid destination path (outside allowed roots, or not writable) |
| `-32012` | Disk full |
| `-32013` | Probe failed (with `data.httpStatus`) |
| `-32014` | Rate limited (pairing brute-force lockout) |