fix: keep WebSocket binaryType + close handling stable across compat dates

Three related WebSocket fixes so PartyServer behaves identically on every
Worker compatibility date, plus the tooling to prove it. All changes are
no-ops on older dates and corrective on newer ones (backward compatible).

1. Pin binaryType to "arraybuffer" for non-hibernating connections.
   On compat dates >= 2026-03-17 the `websocket_standard_binary_type` flag
   flips the default server-side `binaryType` from "arraybuffer" to "blob",
   so binary frames arrived as `Blob` instead of `ArrayBuffer` on the
   in-memory accept path. Every PartyServer consumer (and frameworks built
   on it, e.g. Cloudflare Agents / @cloudflare/voice) has always received
   `ArrayBuffer`, so it is pinned back in `InMemoryConnectionManager.accept`.
   The Hibernation API is unaffected (it always delivers `ArrayBuffer`).

2. Accept non-hibernating connections in half-open mode.
   `accept({ allowHalfOpen: true })` (with a try/catch fallback to bare
   accept() for older runtimes) keeps PartyServer's manual close handshake
   in control on compat dates >= 2026-04-07, where `web_socket_auto_reply_to_close`
   would otherwise auto-tear-down a tunneled DO socket and surface a spurious
   retryable "Network connection lost." rejection.

3. Stop reporting transport-teardown errors via onError.
   A retryable "Network connection lost." / "WebSocket peer disconnected"
   error that fires on an already CLOSING/CLOSED connection is the socket
   going away during the close handshake, not an application error. New
   `transport-errors.ts` (`isBenignTeardownError`, structured `retryable`-first
   detection so it stays correct under `enhanced-error-serialization` >=
   2026-04-21) suppresses it in both the in-memory `handleErrorFromClient`
   and hibernating `webSocketError` paths. Genuine mid-connection (OPEN)
   errors still reach onError.

Verification:
- Bumped wrangler ^4.86.0 -> ^4.100.0 so local `wrangler dev` runs compat
  dates through 2026-06-11 (older workerd capped at 2026-05-03).
- New `BinaryTypeProbe` DO + `compat-binarytype.test.ts` + parametrized
  `vitest.compat.config.ts` lock the ArrayBuffer contract across dates; the
  `test:compat-matrix` script runs it at 2026-01-28 / 03-24 / 04-07 / 04-21
  and is wired into PR CI. The gate is non-vacuous (reports "blob" without
  the pin, "arraybuffer" with it).
- 6 new `transport-errors.test.ts` unit tests cover the suppression predicate.
- Full suite: 86 unit tests pass; typecheck, lint, format clean.

Changeset: patch -> 0.5.7.
Co-authored-by: Cursor <cursoragent@cursor.com>

Author: threepointone

.changeset/in-memory-half-open-accept.md

@@ -0,0 +1,12 @@
+---
+"partyserver": patch
+---
+
+Accept non-hibernating WebSocket connections in half-open mode (`accept({ allowHalfOpen: true })`).
+
+On compatibility dates `>= 2026-04-07` the `web_socket_auto_reply_to_close` flag makes the runtime send a reciprocal Close frame and tear the socket down automatically. For a non-hibernating PartyServer (`hibernate: false`), the Durable Object sits on the server end of a connection that the runtime tunnels back to the client, so that auto-teardown could fire through an already-severed tunnel — surfacing as a spurious retryable `Network connection lost.` rejection (for example when a Durable Object is reset while a connection is still open). Half-open mode keeps PartyServer's existing close handling in control; it already reciprocates the peer's Close frame on every compatibility date, so client behavior is unchanged.
+
+Also in this release, two related WebSocket fixes that keep behavior consistent across all compatibility dates:
+
+- **Pin `binaryType` to `"arraybuffer"` for non-hibernating connections.** On compatibility dates `>= 2026-03-17` the `websocket_standard_binary_type` flag flips the default server-side `binaryType` from `"arraybuffer"` to `"blob"`, so binary frames arrived as `Blob` instead of `ArrayBuffer` on the in-memory path. PartyServer (and frameworks built on it, e.g. Cloudflare Agents) have always received `ArrayBuffer`, so it is now pinned back in `accept()`. This is a no-op on older dates and corrective on newer ones; the Hibernation API is unaffected (it always delivers `ArrayBuffer`).
+- **Stop reporting transport-teardown errors as `onError`.** A retryable `Network connection lost.` / `WebSocket peer disconnected` error that fires on an already closing/closed connection is the socket going away during the close handshake, not an application error. It is now suppressed when the connection is `CLOSING`/`CLOSED` (detected via the structured `retryable` flag, with a message fallback), so it no longer spams logs on abrupt client disconnects. Genuine mid-connection (`OPEN`) errors still reach `onError`.

.github/workflows/pullrequest.yml

@@ -25,3 +25,6 @@ jobs:
       - run: npm ci
       - run: npm run build
       - run: npm run check
+      # Compat-matrix: lock the WebSocket binaryType contract across the
+      # compatibility-date flag transitions (websocket_standard_binary_type).
+      - run: npm run test:compat-matrix -w partyserver

.gitignore

@@ -135,3 +135,6 @@ dist
 .DS_Store
 .env
 .dev.vars
+
+# Temporary compat-verification harness (Layer A)
+.compat-harness/

fixtures/alarm-restart-e2e/package.json

@@ -16,6 +16,6 @@
     "@cloudflare/vite-plugin": "^1.34.0",
     "@cloudflare/workers-types": "^4.20260426.1",
     "vite": "^8.0.10",
-    "wrangler": "^4.86.0"
+    "wrangler": "^4.100.0"
   }
 }