diff --git a/src/engines/SessionCore/core/atoms/actions.simulatorPreview.ts b/src/engines/SessionCore/core/atoms/actions.simulatorPreview.ts
index 0d4b3fff9d..df14ebd11e 100644
--- a/src/engines/SessionCore/core/atoms/actions.simulatorPreview.ts
+++ b/src/engines/SessionCore/core/atoms/actions.simulatorPreview.ts
@@ -6,6 +6,7 @@
  * the authoritative Rust snapshot arrives. Extracted from actions.ts.
  */
 import { isLiveRuntimeResourceEvent } from "../runningEventGate";
+import { getFallbackSimulatorEventFilterCategory } from "../simulatorEventFilterCategory";
 import type { SessionEvent, SimulatorEventPreview } from "../types";
 
 /**
@@ -34,33 +35,6 @@ export function isSimulatorVisibleApprox(event: SessionEvent): boolean {
   );
 }
 
-function getSimulatorFilterCategory(
-  event: SessionEvent
-): SimulatorEventPreview["filterCategory"] {
-  if (event.source === "user") return "key_interactions";
-  if (
-    event.uiCanonical === "edit_file" ||
-    event.uiCanonical === "delete_file"
-  ) {
-    return "file_changes";
-  }
-  if (event.command || event.uiCanonical === "run_shell") {
-    return "terminal_events";
-  }
-  if (
-    event.uiCanonical === "read_file" ||
-    event.uiCanonical === "list_dir" ||
-    event.uiCanonical === "code_search" ||
-    event.uiCanonical === "glob" ||
-    event.uiCanonical === "find_files" ||
-    event.uiCanonical === "search"
-  ) {
-    return "explore";
-  }
-  if (event.filePath) return "file_changes";
-  return "other";
-}
-
 function buildSimulatorPreview(event: SessionEvent): SimulatorEventPreview {
   return {
     id: event.id,
@@ -74,7 +48,7 @@ function buildSimulatorPreview(event: SessionEvent): SimulatorEventPreview {
     displayStatus: event.displayStatus,
     displayVariant: event.displayVariant,
     activityStatus: event.activityStatus,
-    filterCategory: getSimulatorFilterCategory(event),
+    filterCategory: getFallbackSimulatorEventFilterCategory(event),
     threadId: event.threadId,
     processId: event.processId,
     callId: event.callId,
diff --git a/src/engines/SessionCore/core/simulatorEventFilterCategory.ts b/src/engines/SessionCore/core/simulatorEventFilterCategory.ts
new file mode 100644
index 0000000000..e224681bd0
--- /dev/null
+++ b/src/engines/SessionCore/core/simulatorEventFilterCategory.ts
@@ -0,0 +1,42 @@
+import type { SessionEvent, SimulatorEventFilterValue } from "./types";
+
+export const SIMULATOR_EVENT_FILTER_VALUES = [
+  "key_interactions",
+  "file_changes",
+  "terminal_events",
+  "explore",
+  "other",
+] as const satisfies readonly SimulatorEventFilterValue[];
+
+/**
+ * Canonical fallback category used by every local simulator preview path.
+ * The backend-projected category remains authoritative once its snapshot
+ * arrives; this function keeps optimistic and materialized local previews in
+ * lockstep until then.
+ */
+export function getFallbackSimulatorEventFilterCategory(
+  event: SessionEvent
+): SimulatorEventFilterValue {
+  if (event.source === "user") return "key_interactions";
+  if (
+    event.uiCanonical === "edit_file" ||
+    event.uiCanonical === "delete_file"
+  ) {
+    return "file_changes";
+  }
+  if (event.command || event.uiCanonical === "run_shell") {
+    return "terminal_events";
+  }
+  if (
+    event.uiCanonical === "read_file" ||
+    event.uiCanonical === "list_dir" ||
+    event.uiCanonical === "code_search" ||
+    event.uiCanonical === "glob" ||
+    event.uiCanonical === "find_files" ||
+    event.uiCanonical === "search"
+  ) {
+    return "explore";
+  }
+  if (event.filePath) return "file_changes";
+  return "other";
+}
diff --git a/src/engines/SessionCore/core/store/snapshotMaterialization.simulatorPreview.ts b/src/engines/SessionCore/core/store/snapshotMaterialization.simulatorPreview.ts
index 6d9a3161f2..fe998c289d 100644
--- a/src/engines/SessionCore/core/store/snapshotMaterialization.simulatorPreview.ts
+++ b/src/engines/SessionCore/core/store/snapshotMaterialization.simulatorPreview.ts
@@ -7,40 +7,10 @@
  * objects are cached by event object identity so events untouched by a
  * delta reuse their preview across materializations.
  */
-import type {
-  SessionEvent,
-  SimulatorEventFilterValue,
-  SimulatorEventPreview,
-} from "../types";
+import { getFallbackSimulatorEventFilterCategory } from "../simulatorEventFilterCategory";
+import type { SessionEvent, SimulatorEventPreview } from "../types";
 import type { NormalizedSnapshotCache } from "./EventStoreProxyTypes";
 
-function getFallbackFilterCategory(
-  event: SessionEvent
-): SimulatorEventFilterValue {
-  if (event.source === "user") return "key_interactions";
-  if (
-    event.uiCanonical === "edit_file" ||
-    event.uiCanonical === "delete_file"
-  ) {
-    return "file_changes";
-  }
-  if (event.command || event.uiCanonical === "run_shell") {
-    return "terminal_events";
-  }
-  if (
-    event.uiCanonical === "read_file" ||
-    event.uiCanonical === "list_dir" ||
-    event.uiCanonical === "code_search" ||
-    event.uiCanonical === "glob" ||
-    event.uiCanonical === "find_files" ||
-    event.uiCanonical === "search"
-  ) {
-    return "explore";
-  }
-  if (event.filePath) return "file_changes";
-  return "other";
-}
-
 function buildSimulatorEventPreview(
   event: SessionEvent
 ): SimulatorEventPreview {
@@ -56,7 +26,7 @@ function buildSimulatorEventPreview(
     displayStatus: event.displayStatus,
     displayVariant: event.displayVariant,
     activityStatus: event.activityStatus,
-    filterCategory: getFallbackFilterCategory(event),
+    filterCategory: getFallbackSimulatorEventFilterCategory(event),
     threadId: event.threadId,
     processId: event.processId,
     callId: event.callId,
diff --git a/src/engines/SessionCore/derived/simulatorEventFilters.ts b/src/engines/SessionCore/derived/simulatorEventFilters.ts
index 3a96841ae9..4bca6c5c41 100644
--- a/src/engines/SessionCore/derived/simulatorEventFilters.ts
+++ b/src/engines/SessionCore/derived/simulatorEventFilters.ts
@@ -1,46 +1,15 @@
 import type {
-  SessionEvent,
   SimulatorEventFilterValue,
   SimulatorEventPreview,
 } from "../core/types";
 
-export const SIMULATOR_EVENT_FILTER_VALUES = [
-  "key_interactions",
-  "file_changes",
-  "terminal_events",
-  "explore",
-  "other",
-] as const satisfies readonly SimulatorEventFilterValue[];
+export {
+  SIMULATOR_EVENT_FILTER_VALUES,
+  getFallbackSimulatorEventFilterCategory,
+} from "../core/simulatorEventFilterCategory";
 
 export type { SimulatorEventFilterValue };
 
-export function getFallbackSimulatorEventFilterCategory(
-  event: SessionEvent
-): SimulatorEventFilterValue {
-  if (event.source === "user") return "key_interactions";
-  if (
-    event.uiCanonical === "edit_file" ||
-    event.uiCanonical === "delete_file"
-  ) {
-    return "file_changes";
-  }
-  if (event.command || event.uiCanonical === "run_shell") {
-    return "terminal_events";
-  }
-  if (
-    event.uiCanonical === "read_file" ||
-    event.uiCanonical === "list_dir" ||
-    event.uiCanonical === "code_search" ||
-    event.uiCanonical === "glob" ||
-    event.uiCanonical === "find_files" ||
-    event.uiCanonical === "search"
-  ) {
-    return "explore";
-  }
-  if (event.filePath) return "file_changes";
-  return "other";
-}
-
 export function isSimulatorEventVisibleForFilters(
   preview: SimulatorEventPreview,
   selectedFilters: readonly SimulatorEventFilterValue[]