diff --git a/packages/client-runtime/src/state/shellReducer.test.ts b/packages/client-runtime/src/state/shellReducer.test.ts
index 4689c1408f7..a069460e63c 100644
--- a/packages/client-runtime/src/state/shellReducer.test.ts
+++ b/packages/client-runtime/src/state/shellReducer.test.ts
@@ -44,6 +44,26 @@ const stubThread = {
 } as const;
 
 describe("applyShellStreamEvent", () => {
+  it("ignores stale project upserts without mutating the snapshot", () => {
+    const snapshotWithProject: OrchestrationShellSnapshot = {
+      ...baseSnapshot,
+      snapshotSequence: 4,
+      projects: [stubProject],
+    };
+
+    for (const sequence of [3, 4]) {
+      const next = applyShellStreamEvent(snapshotWithProject, {
+        kind: "project-upserted",
+        sequence,
+        project: { ...stubProject, title: "Stale Title" },
+      });
+
+      expect(next).toBe(snapshotWithProject);
+      expect(next.snapshotSequence).toBe(4);
+      expect(next.projects[0]?.title).toBe("Test Project");
+    }
+  });
+
   describe("project-upserted", () => {
     it("adds a new project", () => {
       const event: OrchestrationShellStreamEvent = {
diff --git a/packages/client-runtime/src/state/shellReducer.ts b/packages/client-runtime/src/state/shellReducer.ts
index 71c8a6b0eb3..3d3b22a1289 100644
--- a/packages/client-runtime/src/state/shellReducer.ts
+++ b/packages/client-runtime/src/state/shellReducer.ts
@@ -13,6 +13,8 @@ export function applyShellStreamEvent(
   snapshot: OrchestrationShellSnapshot,
   event: OrchestrationShellStreamEvent,
 ): OrchestrationShellSnapshot {
+  if (event.sequence <= snapshot.snapshotSequence) return snapshot;
+
   switch (event.kind) {
     case "project-upserted": {
       const projects = snapshot.projects.some((p) => p.id === event.project.id)