tsunagi/AGENTS.md

# AGENTS.md

Rules for anyone — human or automated — changing this repository. Read this
before touching the code. Do not restate the full design here; follow the links.

## What this project is

A proof-of-concept agent library for small private mesh networks. A user
supplies a network name and one shared secret; agents derive the same network
space independently, find each other, prove membership and exchange control
messages. Scope and non-scope are in [README.md](README.md).

There is no central server, no owner, no registration and no majority vote.
Design accordingly: a majority is not a root of trust.

## Architectural boundaries

Keep these separate. Crossing them is the main thing to review for.

- **Control plane vs data plane.** The separation is **logical, not physical**.
  The control protocol in `crates/tsunagi/src/proto/` knows nothing about
  packets, and a protocol crate knows nothing about the control protocol;
  either can be replaced on its own. Both may ride on iroh — refusing to
  would throw away iroh's NAT traversal and force the data plane to
  reimplement it. They use different ALPNs and different connections, so a
  busy or broken data plane cannot disturb control traffic.
- **One agent, one interface.** It belongs to the system level, along with
  the addresses on it and the decision of whose packet is whose. Several
  protocols may be carrying traffic at once and none of them owns the thing
  they carry it for. A protocol is handed a routed packet and hands back a
  decrypted one; it never creates an interface and never picks an address.
- **One state directory, one identity, one live agent — many networks.** A
  network is added to the agent that is already running, never by starting a
  second one on the same directory. A second agent is a second identity and
  is isolated: its own directories, its own interface, its own runtime. Do
  not add anything that lets two agents share a directory, and do not make a
  network's lifetime depend on the process that happened to start it.
- **A protocol is a separate crate with its own version.** The version peers
  compare is the *wire* version, never the software version: two peers on
  different releases work together for as long as the bytes between them
  have not changed. Nothing negotiated may be derived from anything that
  moves with a release.
- **A protocol never sees the network secret.** It proves who is at the other
  end of a tunnel; proving membership of a network stays in the core, which
  is why the authenticated transport does too.
- **A relay carries bytes, never packets.** A datagram passed on for two
  other peers goes link in, link out: it is never written to the middle's
  interface or decrypted there. Multihop routing has a bounded hop limit;
  equal-cost next hops are chosen per flow. Reachability is first-hand and
  volatile: each authenticated member advertises its own protocol-specific
  links. Build routing tables on topology changes, never per packet. Transit
  must not acquire a routing mutex or wait for a protocol/TUN reader.
- **Plugins never learn reachability.** An `IpPlugin` is handed a `PacketLink`
  per peer and moves datagrams over it. Addresses, hole punching and relays
  belong to `crates/tsunagi/src/dataplane/transport/`. A plugin announcement says *who*, never
  *where*.
- **The core never parses a plugin payload.** See `crates/tsunagi/src/dataplane/mod.rs`. Only
  `crates/tsunagi-wg-quic/src/announcement.rs` interprets `wg-quic` payloads, and
  only after bounding every field. A data plane failure must never stop the
  control plane.
- **Derived, not claimed.** A peer's overlay address is derived from its public
  key. Outbound packets are routed to the owner of the destination address;
  inbound packets are dropped unless their source is the address derived for
  the peer that sent them. Never trust an address a peer announces.
- **Signed state is the only durable agreement.** A fact that must survive a
  participant being away goes in `crates/tsunagi/src/state/` as a record signed by its
  author, never in a session. Merging is deterministic, an older version never
  rolls back a newer one, and absence from a snapshot is not deletion. Never
  add a vote or a quorum: a majority is not a trust root here, and it would
  stall with one peer online.
- **A record and the author's own version counter commit together**, in one
  transaction, **before** the record is announced.
- **Plugins own their system objects.** A plugin creates and removes its own
  interface and nothing else. Never touch routing, DNS or firewall settings.
- **Device identity vs network identity.** The iroh endpoint id is the device's
  public key. `NetworkId` is derived from name + secret only. Never conflate
  them, and never let one change the other.
- **Discovery vs authentication.** Discovery returns *unverified candidates*.
  It never authenticates, never carries control messages and never mutates
  agent state. Membership is decided only by the handshake.
- **Mandatory state vs disposable cache.** `state.sqlite` is never silently
  reset: damage is a hard error. `cache.sqlite` may be deleted at any time and
  is recreated. A stale cache must never bypass identity or network
  authentication.
- **Candidate vs observed address vs verified path.** Report these as three
  different things. A missing value is `None`, never invented.

## Rules that are not negotiable

1. **Tests come first.** Every substantial change ships with tests for the
   normal path *and* the important failures. See [docs/testing.md](docs/testing.md).
   Do not add tests for getters or to move a coverage number.
2. **Never change `IDENTITY_SCHEME`, the derivation labels, or the transcript
   encoding** in `crates/tsunagi/src/identity/network.rs` and `crates/tsunagi/src/proto/handshake.rs` without
   treating it as an incompatible protocol change. Bumping the crate version or
   the control protocol version must not change an existing `NetworkId`.
3. **Secrets never leak.** Not into logs, not into `Debug`, not into status
   output, not into anything exported to the network. `NetworkSecret` and
   derived keys redact themselves and zeroize; keep it that way.
4. **No panics on untrusted input.** No `unwrap`, `expect` or `panic` on
   anything that came off the network. Clippy enforces this at warn level in
   the library; tests opt out explicitly at the top of each file.
5. **Bounds before allocation.** Frame lengths are checked against
   `Limits::max_frame_len` before a buffer is allocated. Every string, list and
   queue has a limit in `crates/tsunagi/src/config.rs`.
6. **Failure is contained.** A bad signature, wrong secret, malformed packet or
   unknown version rejects one message or one session. It never stops another
   network and never stops the agent. There is no irreversible global error
   flag. A data plane failure must never stop the control plane.
7. **The library owns no globals.** No global tokio runtime, no global tracing
   subscriber, no signal handlers, no `fork`, no `process::exit`, no global
   mutable state. Several agents must run in one test process.
8. **SQLite never blocks the executor.** All database work goes through
   `spawn_blocking`. Never hold a transaction or a database lock across a
   network `await`. When signed records land, writing the record and bumping
   the author's own counter must be one transaction, committed **before**
   publishing to the network.
9. **Shutdown is bounded.** A peer that stops reading must not be able to hold
   up shutdown. Wind tasks down with a grace period and then abort.
10. **Nothing from a remote announcement becomes a shell command, a filesystem
    path or an OS setting.** The agent never touches interfaces or OS settings
    it did not create.

## Where things live

| path                | responsibility |
|---------------------|----------------|
| `crates/tsunagi/src/identity/`     | device identity; deterministic network space identity and derived keys |
| `crates/tsunagi/src/storage/`      | `state.sqlite`, `cache.sqlite`, directory ownership lock |
| `crates/tsunagi/src/discovery.rs`  | candidate sources; test and static backends |
| `crates/tsunagi/src/proto/`        | framing, message formats, membership handshake |
| `crates/tsunagi/src/net.rs`        | iroh endpoint adapter and observability snapshots |
| `crates/tsunagi/src/agent/`        | agent lifecycle, per-network runtimes, sessions, events, status |
| `crates/tsunagi/src/state/`        | signed records that outlive a session, their merge rules and address allocation |
| `crates/tsunagi/src/overlay/`   | the one interface an agent owns: provisioning, the TUN, routing, source checks |
| `crates/tsunagi/src/dataplane/` | the protocol contract and the authenticated packet transport |
| `crates/tsunagi/src/dns/`       | the DNS view of a network: zone, server, resolver publication |
| `crates/tsunagi-wg-quic/`       | the `wg-quic` protocol: its keys, its announcement, its tunnels |
| `crates/tsunagi-cli/` | the command line agent; the only place that owns a runtime, a logger and signals |
| `tests/`            | integration tests; `tests/common/` is the shared harness |

Add abstractions only at real substitution or testing boundaries. Do not add a
trait per struct.

**The system level and its plugins are separate crates**, so that boundary is
checked by the compiler and not by discipline: a plugin can reach only what
`tsunagi` makes public, and carries its own version. Everything else stays
one crate with clear modules — do not split further without a reason of that
kind.

## Testing rules

- Use real iroh endpoints on loopback, real handshakes, real SQLite in
  temporary directories, and independent agent instances.
- Discovery may be substituted. **iroh, authentication, message passing and
  persistent storage may not be.**
- No arbitrary multi-second `sleep` as the primary synchronisation. Wait for a
  specific event or condition under one overall deadline (`wait_event`,
  `wait_until`). `settle()` exists only for asserting that something did *not*
  happen.
- The default suite must pass with no internet, no DHT, no public relay, no
  administrator rights and no changes to OS network settings. Anything needing
  the internet or root stays out of the default set.
- The WireGuard packet interface may be substituted (`MemoryTunFactory`). Its
  key handling, announcements, derived addressing, the WireGuard protocol
  itself and address-ownership enforcement may not — the default suite runs
  real handshakes and real encryption.
- Running several library instances in one process is not a test of several
  system processes; do not describe it as one.

## Before you open a change

```bash
cargo fmt --all -- --check
cargo clippy --locked --workspace --all-targets -- -D warnings
cargo test --locked --workspace --all-targets
```

CI runs these on Linux, macOS and Windows. A CI config existing is not evidence
that tests ran on every OS — say what you actually ran.

Check the real API of the iroh version in `Cargo.lock` before using it. Do not
invent methods from memory.