diff --git a/src/zen/sync/ZenSpacesSync.sys.mjs b/src/zen/sync/ZenSpacesSync.sys.mjs
index 02f510ece..cdabe1a0d 100644
--- a/src/zen/sync/ZenSpacesSync.sys.mjs
+++ b/src/zen/sync/ZenSpacesSync.sys.mjs
@@ -141,12 +141,21 @@ export class ZenSpacesSyncEngine extends SyncEngine {
     return ZenSpacesSyncModel.computeChangedIDs();
   }
 
-  async _reconcile() {
+  async _reconcile(item) {
     // Incoming records always apply, and any local divergence re-uploads afterwards
     // (see ZenSpacesSyncModel.noteApplied). The base reconciliation would
     // instead drop incoming changes whenever the id is also in the local
     // changed set, because the snapshot diff stamps its changes with "now"
     // and the local side would always win the age comparison.
+    //
+    // An id we accept must also leave the outgoing set. Projections only
+    // reflect the applied state after the delayed session collection, so
+    // uploading the id later in this same sync would overwrite the record
+    // we just accepted with the stale pre-apply state (and the other
+    // device would then apply that, endlessly trading states back and
+    // forth). Divergence that survives the collection re-uploads on a
+    // later sync through the snapshot diff.
+    this._modified.delete(item.id);
     return true;
   }
 
diff --git a/src/zen/sync/ZenSpacesSyncApplier.sys.mjs b/src/zen/sync/ZenSpacesSyncApplier.sys.mjs
index b75e22a95..b2f5dfa9a 100644
--- a/src/zen/sync/ZenSpacesSyncApplier.sys.mjs
+++ b/src/zen/sync/ZenSpacesSyncApplier.sys.mjs
@@ -33,6 +33,25 @@ class nsZenSpacesSyncApplier {
     return id ? win.document.getElementById(id) : null;
   }
 
+  /**
+   * Maps a record's container guid to the local userContextId. A guid with
+   * no local mapping (its container record hasn't applied here yet) throws,
+   * failing the record so it redelivers once the container exists .
+   *
+   * @param {?string} containerGuid
+   * @returns {number}
+   */
+  #resolveContainerId(containerGuid) {
+    if (!containerGuid) {
+      return 0;
+    }
+    const mapped = ZenSpacesSyncModel.contextIdForGuid(containerGuid);
+    if (mapped === null) {
+      throw new Error(`unknown container guid ${containerGuid}`);
+    }
+    return mapped;
+  }
+
   /**
    * @param {Array} records - decrypted incoming records.
    * @returns {Array<string>} ids of records that failed to apply.
@@ -259,8 +278,7 @@ class nsZenSpacesSyncApplier {
           name: data.name ?? "",
           icon: data.icon ?? undefined,
           theme: data.theme ?? null,
-          containerTabId:
-            ZenSpacesSyncModel.contextIdForGuid(data.containerGuid) ?? 0,
+          containerTabId: this.#resolveContainerId(data.containerGuid),
         };
         // A space record re-syncs whenever its child ordering changes, so
         // only propagate (and especially repaint) when its own fields did.
@@ -482,8 +500,7 @@ class nsZenSpacesSyncApplier {
   }
 
   #createTab(win, tabId, data) {
-    const userContextId =
-      ZenSpacesSyncModel.contextIdForGuid(data.containerGuid) ?? 0;
+    const userContextId = this.#resolveContainerId(data.containerGuid);
     const tab = win.gBrowser.addTrustedTab(data.url, {
       createLazyBrowser: true,
       inBackground: true,