[Taure]

Dart's half of asobi#498 (v0.89.0), which stamps zone, frame_seq and kf on world.tick.

Why a typed SDK needs a change at all

WorldTick.fromJson read only tick and updates, so all three fields were being silently dropped before any game could see them. A typed model is a filter: an additive server change reaches userland only if the model is widened to let it through. That is the difference between this SDK and asobi-godot, which forwards the raw payload and got the fields for free.

The field that matters

zone. A player is subscribed to an interest ring of several zones at once, each an independent server process, and frames from two of them have no order relative to each other. A crossing emits op: "r" from the zone being left and op: "a" from the zone being entered, so a game merging every zone into one entity map is last-writer-wins - and when the remove lands last the entity is gone for good, because the server will not re-add something already in its own baseline.

Without zone exposed, a Dart game could not have kept per-zone state even if it knew to. The doc comment on the field says this outright, because it is the part a game gets wrong.

frameSeq makes loss detectable, and the doc explains why tick cannot serve: tick skips on the server's broadcast_interval and is suppressed on a tick that changed nothing, so a gap in it is ambiguous. kf marks a complete baseline to replace rather than merge, documented as safe to adopt even when frameSeq moves backwards, since a zone restart resets the sequence while zone identity is unchanged and rejecting a lower value rejects the repair.

resyncWorldZone(zone)

Asks for a fresh baseline for one zone. One zone per call rather than the ring, since the ring is nine zones by default and the caller already knows which one gapped.

This SDK deliberately does not detect the gap and resync for you, unlike asobi-godot which does. The asymmetry is intentional and worth naming so it is a decision rather than an oversight: Godot forwards a raw payload and holds no state, so it can auto-ask harmlessly. Here the whole shape is typed models handed to a game that owns its own state, and only that model knows whether a gap cost it anything. If we want parity later, the hook is one call in the world.tick case.

Defensive decoding, where it has to be

A malformed zone reads as absent rather than throwing, because this runs inside a stream handler - a raise there takes the subscription down and the game stops receiving ticks entirely. kf is true only for a real JSON true, not any truthy value. Coordinates coerce to int, since they index a zone grid and a JSON number may arrive as double.

Tests

7 new cases: all three fields present; all absent reading as null rather than zero (zero is a real frame_seq and a real coordinate); zone: [0, 0] as a real zone; six malformed zone shapes; kf's strictness; float coordinate coercion; and updates still decoding alongside.

They deliberately do not read the fixture corpus - the vendored copy still predates the server change and its sync PR (#45) is separate. Once that lands, dispatch_test exercises the same fields end to end.

Checks

dart analyze: 7 infos, identical count on origin/main, none in this code. dart test: 162/162.

Not reformatted. CI gates analyze and test but not format, and dart format on this Dart version (3.12.2) wants to touch 32 of 40 files, which would bury the change in unrelated churn. Worth deciding separately whether to do a one-off format sweep and add the gate.