diff --git a/web/pcbjam-shared b/web/pcbjam-shared
index 26d2d79..cc9c73d 160000
--- a/web/pcbjam-shared
+++ b/web/pcbjam-shared
@@ -1 +1 @@
-Subproject commit 26d2d7951ba6a9574ff5149fb1a29a2a69d9176c
+Subproject commit cc9c73dba95188ad4a9d9d406c4868ac4c9fae14
diff --git a/web/standalone/src/wasm/collab/debug.ts b/web/standalone/src/wasm/collab/debug.ts
index aeb68d3..c5b3494 100644
--- a/web/standalone/src/wasm/collab/debug.ts
+++ b/web/standalone/src/wasm/collab/debug.ts
@@ -1,7 +1,9 @@
 // Lightweight collab debug logging to the BROWSER DEVTOOLS console (not the in-app
 // log panel). On by default while we bring the bridge up; silence with
 // `window.__COLLAB_DEBUG = false` (or `localStorage.collabDebug = "0"`).
+// Quiet (and crash-free) outside a DOM context — e.g. the vitest node env.
 function on(): boolean {
+  if (typeof window === "undefined") return false;
   const w = window as unknown as { __COLLAB_DEBUG?: boolean };
   if (typeof w.__COLLAB_DEBUG === "boolean") return w.__COLLAB_DEBUG;
   try {
diff --git a/web/standalone/src/wasm/collab/index.ts b/web/standalone/src/wasm/collab/index.ts
index 325921c..44d8d9a 100644
--- a/web/standalone/src/wasm/collab/index.ts
+++ b/web/standalone/src/wasm/collab/index.ts
@@ -7,6 +7,13 @@ import {
 } from "./provider";
 import { createReconciler, type Reconciler } from "./reconciler";
 import type { CollabBridge } from "./types";
+import {
+  bindKicadCollab,
+  moduleItemsBridge,
+  type KicadBinding,
+  type KicadItemsModule,
+  type KicadItemsWindow,
+} from "./kicad-binding";
 
 export type { CollabBridge, CollabDelta, CollabItem } from "./types";
 export { createReconciler } from "./reconciler";
@@ -17,6 +24,9 @@ export {
   type ProviderKind,
   type YjsProvider,
 } from "./provider";
+export { bindKicadCollab, moduleItemsBridge };
+export type { KicadBinding, KicadItemsModule, KicadItemsWindow };
+export type { KicadItemsBridge } from "./kicad-binding";
 
 /** The subset of the Emscripten Module the collab bridge needs (embind functions). */
 export interface CollabModule {
@@ -90,3 +100,44 @@ export async function startCollab(
     },
   };
 }
+
+export interface KicadCollabHandle {
+  doc: Y.Doc;
+  binding: KicadBinding;
+  provider: YjsProvider;
+  destroy(): void;
+}
+
+/**
+ * The Slot-model counterpart of `startCollab` (ysync 0008): wires the v2 items
+ * bridge (kicadCollabSnapshotItems / ApplyItems / onItems — Stage C exports) into
+ * a Y.Doc holding the canonical `KicadDoc` representation. Same provider +
+ * seed-once flow as the legacy path; supersedes it once the wasm speaks the
+ * items wire (Stage D).
+ */
+export async function startKicadCollab(
+  mod: KicadItemsModule,
+  win: KicadItemsWindow,
+  opts: StartCollabOptions,
+): Promise<KicadCollabHandle> {
+  clog("startKicadCollab:", opts.provider.kind, "room =", opts.room);
+  const doc = new Y.Doc();
+  const bridge = moduleItemsBridge(mod, win);
+  const binding = bindKicadCollab(doc, bridge);
+  const provider = await connectProvider(doc, opts.provider, { room: opts.room });
+
+  await provider.whenSynced();
+  binding.seed();
+  clog("startKicadCollab: ready; doc items =", binding.items.size);
+
+  return {
+    doc,
+    binding,
+    provider,
+    destroy() {
+      binding.destroy();
+      provider.destroy();
+      doc.destroy();
+    },
+  };
+}
diff --git a/web/standalone/src/wasm/collab/kicad-binding.test.ts b/web/standalone/src/wasm/collab/kicad-binding.test.ts
new file mode 100644
index 0000000..82d888e
--- /dev/null
+++ b/web/standalone/src/wasm/collab/kicad-binding.test.ts
@@ -0,0 +1,167 @@
+import { describe, expect, it } from "vitest";
+import * as Y from "yjs";
+import {
+  itemsWireToDelta,
+  parseItemsWireDelta,
+  renderItem,
+  sexprToItems,
+  type KicadItem,
+} from "@pcbjam/shared";
+import { bindKicadCollab, type KicadItemsBridge } from "./kicad-binding";
+
+/**
+ * A fake editor implementing the v2 items bridge over an in-memory flattened
+ * item store — the same semantics the Stage C C++ side will have: apply() is an
+ * idempotent per-item upsert/remove that does NOT re-emit (s_applyingRemote
+ * analogue); local edits mutate the store AND emit the items wire.
+ */
+class FakeEditor implements KicadItemsBridge {
+  store: Record<string, KicadItem> = {};
+  applied: string[] = []; // raw JSON of every applyItems call (echo assertions)
+  private emit: ((json: string) => void) | null = null;
+
+  snapshotItems(): string {
+    const roots = Object.entries(this.store)
+      .filter(([, it]) => it.parent === null)
+      .map(([uuid]) => ({
+        sexpr: renderItem({ items: this.store }, uuid),
+        parent: null,
+      }));
+    return JSON.stringify({ added: roots, changed: [], removed: [] });
+  }
+
+  applyItems(json: string): void {
+    this.applied.push(json);
+    this.applyToStore(json); // no emit — remote applies must not echo
+  }
+
+  onItems(cb: (json: string) => void): void {
+    this.emit = cb;
+  }
+
+  /** A local user edit: mutate the store, then emit (like OnModify → Format). */
+  localUpsert(sexpr: string, parent: string | null = null, kind: "added" | "changed" = "changed"): void {
+    const json = JSON.stringify({ [kind]: [{ sexpr, parent }] });
+    this.applyToStore(json);
+    this.emit?.(json);
+  }
+
+  localRemove(uuid: string): void {
+    const json = JSON.stringify({ removed: [uuid] });
+    this.applyToStore(json);
+    this.emit?.(json);
+  }
+
+  private applyToStore(json: string): void {
+    const delta = itemsWireToDelta(parseItemsWireDelta(json), this.store);
+    for (const it of [...delta.added, ...delta.updated]) {
+      const { uuid, ...item } = it;
+      this.store[uuid] = item;
+    }
+    for (const uuid of delta.removed) delete this.store[uuid];
+  }
+}
+
+/** Two Y.Docs joined by relaying updates (stand-in for any provider). */
+function pair(): { a: Y.Doc; b: Y.Doc } {
+  const a = new Y.Doc();
+  const b = new Y.Doc();
+  a.on("update", (u: Uint8Array) => Y.applyUpdate(b, u, "relay"));
+  b.on("update", (u: Uint8Array) => Y.applyUpdate(a, u, "relay"));
+  return { a, b };
+}
+
+const FP = `(footprint "lib:R" (layer "F.Cu") (uuid "fp-1") (at 10 10)
+  (property "Reference" "R1" (at 0 -2) (uuid "fld-1"))
+  (pad "1" smd (at 0 0) (uuid "pad-1")))`;
+
+function seedEditor(ed: FakeEditor, sexpr: string): void {
+  const { uuid, items } = sexprToItems(sexpr);
+  void uuid;
+  Object.assign(ed.store, items);
+}
+
+describe("bindKicadCollab — two editors over relayed Y.Docs", () => {
+  function setup() {
+    const { a, b } = pair();
+    const edA = new FakeEditor();
+    const edB = new FakeEditor();
+    const bindA = bindKicadCollab(a, edA);
+    const bindB = bindKicadCollab(b, edB);
+    return { a, b, edA, edB, bindA, bindB };
+  }
+
+  it("seed → add → edit → remove propagates both ways; no self-echo", () => {
+    const { edA, edB, bindA, bindB } = setup();
+    seedEditor(edA, FP);
+    bindA.seed(); // A is first: seeds the doc
+    bindB.seed(); // B joins: adopts the doc
+
+    // B's editor received the footprint subtree via adopt.
+    expect(Object.keys(edB.store).sort()).toEqual(["fld-1", "fp-1", "pad-1"]);
+
+    // B edits the pad locally → A's editor sees it.
+    edB.localUpsert(`(pad "1" smd (at 7 7) (uuid "pad-1"))`, "fp-1");
+    expect(edA.store["pad-1"]!.body).toEqual(
+      sexprToItems(`(pad "1" smd (at 7 7) (uuid "pad-1"))`, "fp-1").items["pad-1"]!.body,
+    );
+
+    // A adds a free segment → B gets it.
+    edA.localUpsert(`(segment (start 0 0) (end 1 1) (uuid "seg-1"))`, null, "added");
+    expect(edB.store["seg-1"]).toBeDefined();
+
+    // A removes the footprint → cascades to B's whole subtree.
+    edA.localRemove("fp-1");
+    expect(Object.keys(edB.store).sort()).toEqual(["seg-1"]);
+
+    // Echo suppression: every applyItems an editor received came from the PEER's
+    // edits (adopt + peer changes), never from its own emits bouncing back.
+    for (const json of edA.applied) {
+      const wire = parseItemsWireDelta(json);
+      // A's own edits were seg-1 add + fp-1 remove; they must not appear.
+      expect(wire.added.map((w) => w.sexpr).join()).not.toContain("seg-1");
+      expect(wire.removed).not.toContain("fp-1");
+    }
+  });
+
+  it("a remote apply does not bounce back to the originator", () => {
+    const { edA, edB, bindA, bindB } = setup();
+    seedEditor(edA, FP);
+    bindA.seed();
+    bindB.seed();
+    const appliedOnB = edB.applied.length;
+
+    edB.localUpsert(`(pad "1" smd (at 3 3) (uuid "pad-1"))`, "fp-1");
+    // B's own edit: nothing new applied on B (only A receives an apply).
+    expect(edB.applied.length).toBe(appliedOnB);
+    expect(edA.applied.length).toBeGreaterThan(0);
+  });
+
+  it("adopt removes divergent local-only roots (doc authority)", () => {
+    const { edA, edB, bindA, bindB } = setup();
+    seedEditor(edA, FP);
+    bindA.seed();
+
+    // B cold-opened the same file unsaved → its local model has a DIFFERENT uuid.
+    seedEditor(edB, `(footprint "lib:R" (layer "F.Cu") (uuid "fp-DIVERGENT") (at 10 10))`);
+    bindB.seed();
+
+    expect(edB.store["fp-DIVERGENT"]).toBeUndefined(); // dropped
+    expect(edB.store["fp-1"]).toBeDefined(); // adopted
+  });
+
+  it("destroy() detaches the editor from further remote changes", () => {
+    const { edA, edB, bindA, bindB } = setup();
+    seedEditor(edA, FP);
+    bindA.seed();
+    bindB.seed();
+
+    bindB.destroy();
+    edA.localUpsert(`(pad "1" smd (at 9 9) (uuid "pad-1"))`, "fp-1");
+
+    // B's Y.Doc still received the update (provider-level), but its editor didn't.
+    expect(edB.store["pad-1"]!.body).toEqual(
+      sexprToItems(`(pad "1" smd (at 0 0) (uuid "pad-1"))`, "fp-1").items["pad-1"]!.body,
+    );
+  });
+});
diff --git a/web/standalone/src/wasm/collab/kicad-binding.ts b/web/standalone/src/wasm/collab/kicad-binding.ts
new file mode 100644
index 0000000..4ecdae6
--- /dev/null
+++ b/web/standalone/src/wasm/collab/kicad-binding.ts
@@ -0,0 +1,177 @@
+import * as Y from "yjs";
+import {
+  applyDeltaToY,
+  deltaFromYEvents,
+  deltaToItemsWire,
+  isEmptyItemsWireDelta,
+  isEmptyKicadDelta,
+  itemsWireToDelta,
+  kicadItemsMap,
+  parseItemsWireDelta,
+  renderItem,
+  yToItem,
+  type ItemsWireDelta,
+  type KicadItem,
+  type KicadYItems,
+} from "@pcbjam/shared";
+import { clog, cwarn } from "./debug";
+
+/**
+ * The Slot-model collab binding (ysync 0008 Stage B) — the THIN RUNTIME over the
+ * shared, transport-unaware building blocks. This module owns exactly what
+ * `@pcbjam/shared` must not: the `observeDeep` subscription, the local-origin
+ * echo policy, and seed-once authority. Everything data-shaped — wire schemas,
+ * wire⇄delta conversion, Y reads/writes — is the shared lib.
+ *
+ *   DOWN (editor → Y): bridge.onItems(json) → itemsWireToDelta(wire, Y items)
+ *                      → applyDeltaToY (transaction tagged with our origin).
+ *   UP   (Y → editor): items.observeDeep → skip own origin → deltaFromYEvents
+ *                      → deltaToItemsWire (full subtree sexprs) → bridge.applyItems.
+ *
+ * The bridge speaks the v2 "items" wire: per-item s-expr + parent uuid — the C++
+ * exports kicadCollabSnapshotItems / kicadCollabApplyItems / onItems (Stage C).
+ * Until those land in the wasm, the binding is exercised by unit tests with a
+ * fake editor bridge (kicad-binding.test.ts).
+ */
+
+/** The v2 per-item s-expr bridge (Stage C C++ contract), runtime-adapted. */
+export interface KicadItemsBridge {
+  /** Full current model as an all-`added` ItemsWireDelta JSON. */
+  snapshotItems(): string;
+  /** Apply a remote ItemsWireDelta JSON (per-item Parse + splice by uuid). */
+  applyItems(json: string): void;
+  /** Register the local-edit emit hook (Format changed items → JSON). */
+  onItems(cb: (json: string) => void): void;
+}
+
+export interface KicadBinding {
+  /**
+   * Seed-once join: if the shared doc holds no items this client seeds it from
+   * the editor snapshot; otherwise the editor adopts the doc (doc authority —
+   * local-only roots are removed, doc roots applied). Call once after the
+   * doc/provider are connected.
+   */
+  seed(): void;
+  destroy(): void;
+  /** The underlying kdoc items map (exposed for tests/inspection). */
+  readonly items: KicadYItems;
+}
+
+export function bindKicadCollab(doc: Y.Doc, bridge: KicadItemsBridge): KicadBinding {
+  const items = kicadItemsMap(doc);
+  // Opaque per-instance origin tag so we can distinguish our own writes from peers'.
+  const ORIGIN = { local: true };
+
+  /** Plain snapshot of the Y items (the `current`/`view` the conversions need). */
+  const itemsView = (): Record<string, KicadItem> => {
+    const view: Record<string, KicadItem> = {};
+    items.forEach((ym, uuid) => {
+      view[uuid] = yToItem(ym);
+    });
+    return view;
+  };
+
+  // DOWN: local editor change → Y.Doc
+  bridge.onItems((json: string) => {
+    let wire: ItemsWireDelta;
+    try {
+      wire = parseItemsWireDelta(json);
+    } catch (err) {
+      cwarn("⬇ onItems from wasm: UNPARSEABLE", err, json);
+      return;
+    }
+    const delta = itemsWireToDelta(wire, itemsView());
+    if (isEmptyKicadDelta(delta)) return;
+    clog("⬇ onItems (local edit):", {
+      added: delta.added.length,
+      updated: delta.updated.length,
+      removed: delta.removed.length,
+    });
+    applyDeltaToY(doc, delta, ORIGIN);
+  });
+
+  // UP: remote Y change → editor. The subscription + origin policy live HERE
+  // (the runtime); the event→delta computation is the shared default impl.
+  const observer = (events: Y.YEvent<Y.Map<unknown>>[], txn: Y.Transaction) => {
+    if (txn.origin === ORIGIN) return; // our own echo — ignore
+    const delta = deltaFromYEvents(items, events);
+    if (isEmptyKicadDelta(delta)) return;
+    const wire = deltaToItemsWire(delta, itemsView());
+    if (isEmptyItemsWireDelta(wire)) return;
+    clog("⬆ remote Y change → apply to editor:", {
+      added: wire.added.length,
+      changed: wire.changed.length,
+      removed: wire.removed.length,
+    });
+    bridge.applyItems(JSON.stringify(wire));
+  };
+  items.observeDeep(observer);
+
+  function seed(): void {
+    let wire: ItemsWireDelta;
+    try {
+      wire = parseItemsWireDelta(bridge.snapshotItems());
+    } catch (err) {
+      cwarn("seed: snapshotItems unparseable", err);
+      return;
+    }
+    const local = itemsWireToDelta(wire, {});
+
+    clog(
+      `seed: doc has ${items.size} item(s), editor has ${local.added.length} →`,
+      items.size === 0 ? "SEEDING doc (first tab)" : "ADOPTING doc (joining)",
+    );
+
+    if (items.size === 0) {
+      // First tab: seed the shared doc from the editor model.
+      applyDeltaToY(doc, local, ORIGIN);
+      return;
+    }
+
+    // Joining a populated doc: the editor adopts it (seed-once authority, same
+    // rationale as the scalar reconciler §2 — divergent local uuids from a
+    // never-saved cold open must yield to the doc's identity). Apply the doc's
+    // ROOT items (their sexprs embed all descendants) and remove local-only roots.
+    const view = itemsView();
+    const docRoots = Object.entries(view)
+      .filter(([, item]) => item.parent === null)
+      .map(([uuid]) => ({ sexpr: renderItem({ items: view }, uuid), parent: null }));
+    const removed = local.added
+      .filter((it) => it.parent === null && !(it.uuid in view))
+      .map((it) => it.uuid);
+    bridge.applyItems(JSON.stringify({ added: docRoots, changed: [], removed }));
+  }
+
+  return {
+    seed,
+    destroy: () => items.unobserveDeep(observer),
+    items,
+  };
+}
+
+// ── Live wasm adapter ─────────────────────────────────────────────────────────
+
+/** The Stage C Module exports + window hook, as the browser exposes them. */
+export interface KicadItemsModule {
+  kicadCollabSnapshotItems(): string;
+  kicadCollabApplyItems(json: string): void;
+}
+
+export interface KicadItemsWindow {
+  kicadCollab?: { onItems?: (json: string) => void };
+}
+
+/** Adapt a live wasm Module + window to the bridge interface. */
+export function moduleItemsBridge(
+  mod: KicadItemsModule,
+  win: KicadItemsWindow,
+): KicadItemsBridge {
+  return {
+    snapshotItems: () => mod.kicadCollabSnapshotItems(),
+    applyItems: (json) => mod.kicadCollabApplyItems(json),
+    onItems: (cb) => {
+      // Preserve any sibling hooks (e.g. the legacy onDelta) on the global.
+      win.kicadCollab = { ...win.kicadCollab, onItems: cb };
+    },
+  };
+}