diff --git a/.changeset/fix-live-query-default-getkey.md b/.changeset/fix-live-query-default-getkey.md
new file mode 100644
index 000000000..6ff4c2536
--- /dev/null
+++ b/.changeset/fix-live-query-default-getkey.md
@@ -0,0 +1,7 @@
+---
+'@tanstack/db': patch
+---
+
+fix: default getKey on live query collections fails when used as a source in chained collections
+
+The default WeakMap-based getKey breaks when enriched change values (with virtual props like $synced, $origin, $key) are passed through chained live query collections. The enriched objects are new references not found in the WeakMap, causing all items to resolve to key `undefined` and collapse into a single item. Falls back to `item.$key` when the WeakMap lookup misses.
diff --git a/packages/db/src/query/live/collection-config-builder.ts b/packages/db/src/query/live/collection-config-builder.ts
index 20fb26f79..3a4e948e1 100644
--- a/packages/db/src/query/live/collection-config-builder.ts
+++ b/packages/db/src/query/live/collection-config-builder.ts
@@ -227,7 +227,8 @@ export class CollectionConfigBuilder<
       id: this.id,
       getKey:
         this.config.getKey ||
-        ((item) => this.resultKeys.get(item) as string | number),
+        ((item: any) =>
+          (this.resultKeys.get(item) ?? item.$key) as string | number),
       sync: this.getSyncConfig(),
       compare: this.compare,
       defaultStringCollation: this.compareOptions,
diff --git a/packages/db/tests/query/live-query-collection.test.ts b/packages/db/tests/query/live-query-collection.test.ts
index d56e5a715..7179cd388 100644
--- a/packages/db/tests/query/live-query-collection.test.ts
+++ b/packages/db/tests/query/live-query-collection.test.ts
@@ -2585,4 +2585,121 @@ describe(`createLiveQueryCollection`, () => {
       await preloadPromise
     })
   })
+
+  describe(`chained live query collections without custom getKey`, () => {
+    it(`should return all items when a live query collection without getKey is used as a source`, async () => {
+      // Create a live query collection with the default (internal) getKey
+      const filteredUsers = createLiveQueryCollection({
+        id: `filtered-users`,
+        query: (q) =>
+          q
+            .from({ user: usersCollection })
+            .where(({ user }) => eq(user.active, true))
+            .select(({ user }) => ({
+              id: user.id,
+              name: user.name,
+            })),
+      })
+
+      // Use the live query collection as a source in another live query collection
+      const derived = createLiveQueryCollection({
+        id: `derived-from-live-query`,
+        query: (q) => q.from({ u: filteredUsers }),
+      })
+
+      await derived.preload()
+
+      // Should contain all active users (Alice and Bob), not just 1
+      expect(derived.size).toBe(2)
+    })
+
+    it(`should return all items when a live query collection with a join and no getKey is used as a source`, async () => {
+      type Team = {
+        id: number
+        name: string
+        lead_id: number
+      }
+
+      const teamsCollection = createCollection(
+        mockSyncCollectionOptions<Team>({
+          id: `test-teams`,
+          getKey: (team) => team.id,
+          initialData: [
+            { id: 1, name: `Alpha`, lead_id: 1 },
+            { id: 2, name: `Beta`, lead_id: 2 },
+            { id: 3, name: `Gamma`, lead_id: 1 },
+          ],
+        }),
+      )
+
+      // Join teams with users — no custom getKey
+      const teamsWithLeads = createLiveQueryCollection({
+        id: `teams-with-leads`,
+        query: (q) =>
+          q
+            .from({ team: teamsCollection })
+            .join({ user: usersCollection }, ({ team, user }) =>
+              eq(team.lead_id, user.id),
+            )
+            .select(({ team, user }) => ({
+              teamName: team.name,
+              leadName: user.name,
+            })),
+      })
+
+      // Use the joined live query collection as a source
+      const derived = createLiveQueryCollection({
+        id: `derived-from-join`,
+        query: (q) => q.from({ t: teamsWithLeads }),
+      })
+
+      await derived.preload()
+
+      // Should contain all 3 joined rows, not just 1
+      expect(derived.size).toBe(3)
+      expect(derived.toArray.map((row) => stripVirtualProps(row))).toEqual(
+        expect.arrayContaining([
+          expect.objectContaining({ teamName: `Alpha`, leadName: `Alice` }),
+          expect.objectContaining({ teamName: `Beta`, leadName: `Bob` }),
+          expect.objectContaining({ teamName: `Gamma`, leadName: `Alice` }),
+        ]),
+      )
+    })
+
+    it(`should propagate updates through chained live query collections without custom getKey`, async () => {
+      // Intermediate live query collection — no custom getKey
+      const intermediate = createLiveQueryCollection({
+        id: `update-intermediate`,
+        query: (q) =>
+          q.from({ user: usersCollection }).select(({ user }) => ({
+            id: user.id,
+            name: user.name,
+          })),
+      })
+
+      // Derived from the intermediate
+      const derived = createLiveQueryCollection({
+        id: `update-derived`,
+        query: (q) => q.from({ u: intermediate }),
+      })
+
+      await derived.preload()
+
+      // Should have all 3 users from sampleUsers, not just 1
+      expect(derived.size).toBe(3)
+
+      // Sync a new user into the source collection
+      usersCollection.utils.begin()
+      usersCollection.utils.write({
+        type: `insert`,
+        value: { id: 4, name: `Diana`, active: true },
+      })
+      usersCollection.utils.commit()
+
+      await flushPromises()
+
+      // The derived collection should see all 4 items
+      expect(derived.size).toBe(4)
+    })
+  })
 })