diff --git a/.serena/memories/frontend/composable-component-tests.md b/.serena/memories/frontend/composable-component-tests.md
index a4fb3aba7d..82c9ab9327 100644
--- a/.serena/memories/frontend/composable-component-tests.md
+++ b/.serena/memories/frontend/composable-component-tests.md
@@ -117,7 +117,7 @@ Two pure runners (no store, JVM + CLJS):
   `common-tests.logic.composable-runner-test`.
 - Undo/redo round trip (`comp/undo_check.cljc`, alias `uc`), on by default in the pure runner
   (`:undo-check?`): after each `:user` step that pushed a group, undo must give back the
-  normalized file before it and redo the file after it (the variant continues from the redone
+  normalized files before it and redo the files after it (the variant continues from the redone
   file); a user step that pushed nothing must change nothing (`:no-undo-entry`); at the end,
   undo back to the last assembly step (the baseline; a case's own undo below it moves it) and
   redo all. Files compare through `app.common.test-helpers.normalize` (`thn/normalize-file`:
@@ -137,6 +137,11 @@ Two pure runners (no store, JVM + CLJS):
   workspace undo index (`:undo-index`/`:redo-index` phases): a step must add exactly one undo
   entry or group. File installs (assembly ops) are its baselines; a skipped optional and an inline `Test` are not. The interpreter
   empties `:workspace-undo` per variant. Frontend snapshots drop `:position-data` under WASM.
+  Both runners use `uc/snapshot-files` for the current file and every
+  auxiliary/library file. File differences prefix signatures with
+  `[:file <label> ...]`. Normalization also drops library `:synced-at`,
+  a server status stamp rather than design data. `ftm/snapshot` exposes
+  the same snapshot to cross-file undo regressions.
   Undo groups get lost in derived work: `initialize-shape-layout` buffers `:layout/update`
   without its `:undo-group` (F43), so a swap's reflow lands in entries of its own. Match
   steps by node identity, never by record type or rendered name: `n/MoveChild` does not resolve
diff --git a/.serena/memories/frontend/workspace-state-persistence-subtleties.md b/.serena/memories/frontend/workspace-state-persistence-subtleties.md
index 594df35e2d..952b5da9da 100644
--- a/.serena/memories/frontend/workspace-state-persistence-subtleties.md
+++ b/.serena/memories/frontend/workspace-state-persistence-subtleties.md
@@ -24,6 +24,11 @@
   with its prior file data. The component watcher syncs each transition
   before the next user commit, keeping undo groups adjacent. Its barrier
   remains open through the event turn. Foreign file cascades stay in common.
+- Undo changes carry `:app.main.data.workspace.undo/file-id`, separate from
+  change payload fields named `:file-id` (e.g. `:set-tokens-source`). Undo
+  and redo replay consecutive runs per destination in order; old entries
+  without the routing key use the current file. Stacking and transactions
+  keep these keys on each change.
 - Layout batches cross an `:asap` boundary before reflow so synchronous
   component sync cannot emit into the buffer being closed and lose a task.
 - Persistence buffers local commits: status becomes pending after about 200ms, commits are flushed after about 3s or `::force-persist`, and buffered commits are merged per file before `:update-file`.
@@ -49,4 +54,4 @@
 
 - `refs/libraries` is explicitly deprecated for performance; prefer derefing `refs/files` and memoizing `select-libraries` in components.
 - `refs/workspace-page-objects` uses `identical?` equality, so preserving object map identity matters for avoiding derived-ref churn.
-- Selected-shapes refs use a small `{objects selected}` wrapper with custom equality before running `process-selected`; avoid bypassing that pattern in hot UI paths.
\ No newline at end of file
+- Selected-shapes refs use a small `{objects selected}` wrapper with custom equality before running `process-selected`; avoid bypassing that pattern in hot UI paths.
diff --git a/common/src/app/common/test_helpers/composable/comp/runner.cljc b/common/src/app/common/test_helpers/composable/comp/runner.cljc
index 73bc074b17..6e4711be73 100644
--- a/common/src/app/common/test_helpers/composable/comp/runner.cljc
+++ b/common/src/app/common/test_helpers/composable/comp/runner.cljc
@@ -213,7 +213,8 @@
 
 (defn- snapshot
   [situation]
-  (uc/snapshot (tm/file situation)))
+  (let [file (tm/file situation)]
+    (uc/snapshot-files (assoc (tm/aux-files situation) (:id file) file))))
 
 (defn- attempt
   "`[(f) nil]`, or `[nil error]` when `f` throws."
diff --git a/common/src/app/common/test_helpers/composable/comp/undo_check.cljc b/common/src/app/common/test_helpers/composable/comp/undo_check.cljc
index 807c69eeed..ce9fe0bef6 100644
--- a/common/src/app/common/test_helpers/composable/comp/undo_check.cljc
+++ b/common/src/app/common/test_helpers/composable/comp/undo_check.cljc
@@ -56,6 +56,12 @@
   ([file] (snapshot file {}))
   ([file opts] (thn/normalize-file file opts)))
 
+(defn snapshot-files
+  "The normalized files affected by a step, including linked libraries."
+  ([files] (snapshot-files files {}))
+  ([files opts]
+   {:files (into {} (map (fn [[id file]] [id (snapshot file opts)])) files)}))
+
 (defn mark-baseline
   "Record the state the end-of-variant round trip undoes back to: the
    normalized file `snap` with `depth` undo groups on the stack."
@@ -141,17 +147,24 @@
    `[:data key]` / `[:file key]` for anything else. Shapes without a test
    label are `:unlabelled`."
   [expected actual]
-  (let [ed (:data expected)
-        ad (:data actual)]
-    (-> #{}
-        (into (map #(vector :file %)) (differing-keys expected actual #{:data}))
-        (into (map #(vector :data %)) (differing-keys ed ad #{:pages-index :components}))
-        (into (entities-signature :page (:pages-index ed) (:pages-index ad) #{:objects}
-                                  (fn [e a]
-                                    (entities-signature :object (:objects e) (:objects a)
-                                                        #{} (constantly nil)))))
-        (into (entities-signature :component (:components ed) (:components ad)
-                                  #{} (constantly nil))))))
+  (if (or (contains? expected :files) (contains? actual :files))
+    (into #{}
+          (mapcat (fn [id]
+                    (map #(into [:file (label-of id)] %)
+                         (signature (get-in expected [:files id])
+                                    (get-in actual [:files id])))))
+          (into (set (keys (:files expected))) (keys (:files actual))))
+    (let [ed (:data expected)
+          ad (:data actual)]
+      (-> #{}
+          (into (map #(vector :file %)) (differing-keys expected actual #{:data}))
+          (into (map #(vector :data %)) (differing-keys ed ad #{:pages-index :components}))
+          (into (entities-signature :page (:pages-index ed) (:pages-index ad) #{:objects}
+                                    (fn [e a]
+                                      (entities-signature :object (:objects e) (:objects a)
+                                                          #{} (constantly nil)))))
+          (into (entities-signature :component (:components ed) (:components ad)
+                                    #{} (constantly nil)))))))
 
 (defn compare-states
   "Record a failure of `phase` at `step` unless the normalized files
diff --git a/common/src/app/common/test_helpers/normalize.cljc b/common/src/app/common/test_helpers/normalize.cljc
index 0c40ed284b..580299d3e1 100644
--- a/common/src/app/common/test_helpers/normalize.cljc
+++ b/common/src/app/common/test_helpers/normalize.cljc
@@ -12,6 +12,8 @@
    are the same for the user:
      - `:modified-at` of the file and of each component, bumped by every
        change, undo and redo included;
+     - a linked file's `:synced-at`, the server sync status rather than
+       authored design data;
      - with `{:position-data? false}`, the shapes' `:position-data`, the text
        layout that WASM measurement regenerates.
    Shapes and the maps that hold them become plain maps, so a shape record
@@ -54,7 +56,7 @@
   ([file opts]
    (let [opts (merge {:position-data? true} opts)]
      (-> file
-         (dissoc :modified-at)
+         (dissoc :modified-at :synced-at)
          (update :data
                  (fn [data]
                    (-> data
diff --git a/common/test/common_tests/logic/composable_runner_test.cljc b/common/test/common_tests/logic/composable_runner_test.cljc
index eb457933c7..b031fa5c08 100644
--- a/common/test/common_tests/logic/composable_runner_test.cljc
+++ b/common/test/common_tests/logic/composable_runner_test.cljc
@@ -363,10 +363,10 @@
                                   :operation (->WrongUndoEdit)})]
     ;; the redo leaves the wrong name in place, and so does the variant undo
     (t/is (= [:undo :redo :variant-undo] (phases situation)))
-    (t/is (= (repeat 3 #{[:object :copy-root :name]})
+    (t/is (= (repeat 3 #{[:file :file1 :object :copy-root :name]})
              (map :signature (uc/failures situation))))
     (t/is (= {:name "Wrong"}
-             (-> (uc/failures situation) first :diff second :data :pages-index
+             (-> (uc/failures situation) first :diff second :files :file1 :data :pages-index
                  vals first :objects (get :copy-root))))))
 
 (t/deftest round-trip-reports-a-change-with-no-undo-entry
@@ -410,7 +410,7 @@
   (let [edit      (tm/assign-id (->WrongUndoEdit))
         situation (r/run-variant {:setup     setup/simple-component-with-copy
                                   :operation edit})
-        pin       #{[:object :copy-root :name]}
+        pin       #{[:file :file1 :object :copy-root :name]}
         known     {:bug "F0" :fails {:undo pin :redo pin :variant-undo pin} :op edit}
         verdict   #(uc/verdict situation {:undo-check {:known-failures %}})]
     (t/is (= {:unexpected [] :resolved []} (verdict [known])))
@@ -457,7 +457,7 @@
   (let [edit      (tm/assign-id (->WrongUndoEdit))
         situation (r/run-variant {:setup     setup/simple-component-with-copy
                                   :operation edit})
-        pin       #{[:object :copy-root :name]}
+        pin       #{[:file :file1 :object :copy-root :name]}
         known     {:bug "F0" :fails {:undo pin :redo pin :variant-undo pin} :op edit}
         check     #(report-types (fn [] (uc/check! situation {:undo-check %})))]
     (t/is (= [:fail :fail :fail] (check {})))
@@ -470,7 +470,7 @@
         right     (n/change-property :main-child :fills red)
         situation (r/run-variant {:setup     setup/simple-component-with-copy
                                   :operation (tm/in-sequence [wrong right])})
-        pin       #{[:object :copy-root :name]}
+        pin       #{[:file :file1 :object :copy-root :name]}
         known     {:bug "F0" :fails {:undo pin :redo pin :variant-undo pin} :op wrong}
         verdict   #(uc/verdict situation {:undo-check {:known-failures [%]}})
         phases    (fn [k mark] (mapv :phase (k (verdict mark))))]
@@ -488,7 +488,7 @@
 
 (t/deftest a-mark-that-applies-to-no-variant-fails
   (let [edit      (tm/assign-id (->WrongUndoEdit))
-        pin       #{[:object :copy-root :name]}
+        pin       #{[:file :file1 :object :copy-root :name]}
         mark      {:bug "F0" :fails {:undo pin} :op edit}
         case-map  {:undo-check {:known-failures [mark]}}]
     (t/is (= [:fail] (report-types #(uc/check-marks-applied! case-map :pure []))))
diff --git a/frontend/src/app/main/data/workspace.cljs b/frontend/src/app/main/data/workspace.cljs
index a3b685cf25..ea18725eb7 100644
--- a/frontend/src/app/main/data/workspace.cljs
+++ b/frontend/src/app/main/data/workspace.cljs
@@ -497,10 +497,10 @@
                     (rx/filter dch/commit?)
                     (rx/map deref)
                     (rx/mapcat
-                     (fn [{:keys [save-undo? undo-changes redo-changes undo-group tags stack-undo? selected-before]}]
+                     (fn [{:keys [file-id save-undo? undo-changes redo-changes undo-group tags stack-undo? selected-before]}]
                        (if (and save-undo? (seq undo-changes))
-                         (let [entry {:undo-changes undo-changes
-                                      :redo-changes redo-changes
+                         (let [entry {:undo-changes (mapv #(assoc % ::dwu/file-id file-id) undo-changes)
+                                      :redo-changes (mapv #(assoc % ::dwu/file-id file-id) redo-changes)
                                       :undo-group undo-group
                                       :tags tags
                                       :selected-before selected-before}]
diff --git a/frontend/src/app/main/data/workspace/undo.cljs b/frontend/src/app/main/data/workspace/undo.cljs
index 156bc5b7cb..26c267561e 100644
--- a/frontend/src/app/main/data/workspace/undo.cljs
+++ b/frontend/src/app/main/data/workspace/undo.cljs
@@ -283,6 +283,19 @@
         (assoc-in state [:workspace-local :selected] selected-ids)
         state))))
 
+(defn- commit-history-changes
+  "Replay changes in order, keeping each run in its destination file.
+   Older entries without a destination use the current file."
+  [origin changes]
+  (->> (partition-by ::file-id changes)
+       (map (fn [changes]
+              (dch/commit-changes {:file-id (::file-id (first changes))
+                                   :redo-changes (mapv #(dissoc % ::file-id) changes)
+                                   :undo-changes []
+                                   :origin origin
+                                   :save-undo? false})))
+       (rx/from)))
+
 (defn undo-to-index
   "Repeat undoing or redoing until dest-index is reached."
   [dest-index]
@@ -310,11 +323,9 @@
                                                (map :redo-changes))
                                           :else [])))]
                 (when (seq changes)
-                  (rx/of (materialize-undo changes dest-index)
-                         (dch/commit-changes {:redo-changes changes
-                                              :undo-changes []
-                                              :origin it
-                                              :save-undo? false})))))))))))
+                  (rx/concat
+                   (rx/of (materialize-undo changes dest-index))
+                   (commit-history-changes it changes)))))))))))
 
 (declare ^:private assure-valid-current-page)
 
@@ -354,13 +365,11 @@
                   (let [first-item (get items undo-group-index)]
                     (rx/of (undo-to-index (dec undo-group-index))
                            (restore-selection (:selected-before first-item))))
-                  (rx/of (materialize-undo changes (dec index))
-                         (dch/commit-changes {:redo-changes changes
-                                              :undo-changes []
-                                              :save-undo? false
-                                              :origin it})
-                         (restore-selection (:selected-before item))
-                         (assure-valid-current-page)))))))))))
+                  (rx/concat
+                   (rx/of (materialize-undo changes (dec index)))
+                   (commit-history-changes it changes)
+                   (rx/of (restore-selection (:selected-before item))
+                          (assure-valid-current-page))))))))))))
 
 (def redo
   (ptk/reify ::redo
@@ -392,12 +401,10 @@
                   (let [last-item (get items redo-group-index)]
                     (rx/of (undo-to-index redo-group-index)
                            (restore-selection (:selected-after last-item))))
-                  (rx/of (materialize-undo changes (inc index))
-                         (dch/commit-changes {:redo-changes changes
-                                              :undo-changes []
-                                              :origin it
-                                              :save-undo? false})
-                         (restore-selection (:selected-after item))))))))))))
+                  (rx/concat
+                   (rx/of (materialize-undo changes (inc index)))
+                   (commit-history-changes it changes)
+                   (rx/of (restore-selection (:selected-after item)))))))))))))
 
 (defn- assure-valid-current-page
   []
diff --git a/frontend/test/frontend_tests/composable_tests/golden_master.edn b/frontend/test/frontend_tests/composable_tests/golden_master.edn
index cae54ad46f..28410fcbda 100644
--- a/frontend/test/frontend_tests/composable_tests/golden_master.edn
+++ b/frontend/test/frontend_tests/composable_tests/golden_master.edn
@@ -46,7 +46,7 @@
  :case-ao-copy-dropped-into-a-grid-cell-reaches-copies
  [{:transcript " {:copy :loose, :into :main-root, :cell [1 3]}", :states ["abc6e41a291516a2" "571d314ba8917a08"]}]
  :case-ap-library-main-child-removal-reaches-copy
- [{:transcript "", :states ["f53d220c42298128" "7c2dfde6478d399c"]}]
+ [{:transcript "", :states ["dd877acf0d7f120c" "0a472792b8a73e33"]}]
  :case-aq-main-change-reaches-a-copy-nested-in-a-sub-board
  [{:transcript " {:target #fn, :property :fills, :value \"#00ff00\"}\n   {:target :main-instance, :property :fills, :value \"#ff0000\"}", :states ["d458024081bee6ec" "df29aaf3b63bece6" "0aef0de70c1e2e98"]}
   {:transcript "one-of -> \n   {:target :main-instance, :property :fills, :value \"#ff0000\"}", :states ["c9ecea6283cef90c" "be12b667808778f3"]}]
@@ -109,7 +109,7 @@
  :case-f-move-shape-in-main-auto-propagates-order-to-clean-copy
  [{:transcript " {:target :main-child1, :parent :main-root, :to-index 2}", :states ["1994876f6bbe7d2e" "de144b280bc4fc82"]}]
  :case-h-library-change-propagates-across-file-boundary-on-sync
- [{:transcript "", :states ["d8c5e8bda3728127" "6ee281995d7d8f65"]}]
+ [{:transcript "", :states ["46c4b8337914be2d" "d95e20909221223c"]}]
  :case-i-undo-reverts-edit-and-its-auto-propagation
  [{:transcript " {:target :main-child, :property :fills, :value \"#ff0000\"}\n  \n   {:times 1}", :states ["386e940ac63af4b1" "734ec524d88eb6bb" "ff1fa863b88027e5"]}]
  :case-k-synchronisation-scenarios
diff --git a/frontend/test/frontend_tests/composable_tests/interpreter.cljs b/frontend/test/frontend_tests/composable_tests/interpreter.cljs
index c7fca7d5af..1e29b6618d 100644
--- a/frontend/test/frontend_tests/composable_tests/interpreter.cljs
+++ b/frontend/test/frontend_tests/composable_tests/interpreter.cljs
@@ -480,12 +480,12 @@
   (let [{:keys [index items]} (:workspace-undo @st/state)]
     (or index (dec (count items)))))
 
-(defn- snapshot
-  "The normalized live file the round trip compares. WASM regenerates
+(defn snapshot
+  "The normalized live files the round trip compares. WASM regenerates
    `:position-data` by measuring, so it is left out there."
   []
-  (uc/snapshot (current-file)
-               {:position-data? (not (features/active-feature? @st/state "render-wasm/v1"))}))
+  (uc/snapshot-files (:files @st/state)
+                     {:position-data? (not (features/active-feature? @st/state "render-wasm/v1"))}))
 
 (defn- mark-baseline
   [situation]
diff --git a/frontend/test/frontend_tests/data/workspace_remote_cascade_test.cljs b/frontend/test/frontend_tests/data/workspace_remote_cascade_test.cljs
index 90f6687c18..f709b65410 100644
--- a/frontend/test/frontend_tests/data/workspace_remote_cascade_test.cljs
+++ b/frontend/test/frontend_tests/data/workspace_remote_cascade_test.cljs
@@ -16,6 +16,7 @@
    [app.common.test-helpers.ids-map :as cthi]
    [app.common.test-helpers.shapes :as cths]
    [app.main.data.workspace.libraries :as dwl]
+   [app.main.data.workspace.undo :as dwu]
    [app.main.repo :as rp]
    [app.main.store :as st]
    [beicon.v2.core :as rx]
@@ -72,3 +73,29 @@
          (t/is (= new-color (nested-child-fill library'))))
        (t/is (= new-color (fill-of (ftm/current-file) :copy-a-child)))
        (t/is (= new-color (fill-of (ftm/current-file) :copy-b-nested-child)))))))
+
+(t/deftest ^:async undo-and-redo-update-main-restore-both-files
+  (await
+   (mock/with-mocks*
+     {rp/cmd! (mock/stub (fn [& _] (rx/of nil)))
+      rx/timer mock/timer-mock}
+     (let [library (install!)]
+       (await (ftm/await-step [(ftm/fill-commit :copy-a-child new-color {})]))
+       (let [before (ftm/snapshot)]
+         (await (ftm/await-step [(dwl/update-component-sync (cthi/id :copy-a) (:id library))]))
+         (let [after (ftm/snapshot)]
+           (await (ftm/await-step [dwu/undo]))
+           (t/is (= before (ftm/snapshot)))
+           (await (ftm/await-step [dwu/redo]))
+           (t/is (= after (ftm/snapshot)))))
+       (await (ftm/await-step [dwu/undo]))
+       (let [restored (get-in @st/state [:files (:id library)])]
+         (t/is (= old-color (fill-of restored :main-a-child)))
+         (t/is (= old-color (nested-child-fill restored))))
+       (t/is (= old-color (fill-of (ftm/current-file) :copy-b-nested-child)))
+       (t/is (= new-color (fill-of (ftm/current-file) :copy-a-child)))
+       (await (ftm/await-step [dwu/redo]))
+       (let [redone (get-in @st/state [:files (:id library)])]
+         (t/is (= new-color (fill-of redone :main-a-child)))
+         (t/is (= new-color (nested-child-fill redone))))
+       (t/is (= new-color (fill-of (ftm/current-file) :copy-b-nested-child)))))))