diff --git a/apps/web/src/components/Sidebar.tsx b/apps/web/src/components/Sidebar.tsx
index 445ae533c323..c6bc0aa4a08a 100644
--- a/apps/web/src/components/Sidebar.tsx
+++ b/apps/web/src/components/Sidebar.tsx
@@ -3992,10 +3992,24 @@ export default function Sidebar() {
         const isPinned = thread.pinnedAt != null;
         // Presets resolve at menu-open time (same as the popover).
         const snoozePresets = resolveSnoozePresets(new Date(), timestampFormat);
+        const threadProjectGroup =
+          projectGroupsRef.current.find((project) =>
+            project.memberProjectRefs.some(
+              (projectRef) =>
+                projectRef.environmentId === thread.environmentId &&
+                projectRef.projectId === thread.projectId,
+            ),
+          ) ?? null;
         const clicked = await settlePromise(() =>
           api.contextMenu.show(
             buildThreadActionMenuItems({
               branch: thread.branch ?? null,
+              projectFilter: threadProjectGroup
+                ? {
+                    label: threadProjectGroup.displayName,
+                    isActive: projectScopeKey === threadProjectGroup.projectKey,
+                  }
+                : null,
               isPinned,
               isSettled,
               isSnoozed,
@@ -4023,17 +4037,20 @@ export default function Sidebar() {
           return;
         }
         switch (clicked.value) {
-          case "project-settings": {
-            const projectGroup = projectGroupsRef.current.find((group) =>
-              group.memberProjectRefs.some(
-                (projectRef) =>
-                  projectRef.environmentId === thread.environmentId &&
-                  projectRef.projectId === thread.projectId,
-              ),
-            );
-            if (projectGroup) openProjectSettings(projectGroup);
+          case "filter-by-project":
+            // This item is the only scope control here, so picking the
+            // already-scoped project again is the way back to all projects.
+            if (threadProjectGroup) {
+              setProjectScopeKey(
+                projectScopeKey === threadProjectGroup.projectKey
+                  ? null
+                  : threadProjectGroup.projectKey,
+              );
+            }
+            return;
+          case "project-settings":
+            if (threadProjectGroup) openProjectSettings(threadProjectGroup);
             return;
-          }
           case "new-thread-on-branch": {
             // Explicit branch carry-over: reuse the thread's worktree when it
             // has one, otherwise its branch on the local checkout.
@@ -4194,8 +4211,10 @@ export default function Sidebar() {
       handleMultiSelectContextMenu,
       markThreadUnread,
       openProjectSettings,
+      projectScopeKey,
       projectByKey,
       serverConfigs,
+      setProjectScopeKey,
       startThreadRename,
       updateThreadMetadata,
       timestampFormat,
diff --git a/apps/web/src/components/threadActionMenu.logic.test.ts b/apps/web/src/components/threadActionMenu.logic.test.ts
index 1bdd04693759..9915a954943c 100644
--- a/apps/web/src/components/threadActionMenu.logic.test.ts
+++ b/apps/web/src/components/threadActionMenu.logic.test.ts
@@ -4,6 +4,7 @@ import { buildThreadActionMenuItems, type ThreadActionMenuState } from "./thread
 
 const baseState: ThreadActionMenuState = {
   branch: null,
+  projectFilter: null,
   isPinned: false,
   isSettled: false,
   isSnoozed: false,
@@ -47,6 +48,27 @@ describe("buildThreadActionMenuItems", () => {
     expect(items[copyIndex + 2]?.id).toBe("archive");
   });
 
+  it("offers project filtering only for surfaces with a scoped thread list", () => {
+    expect(ids(baseState)).not.toContain("filter-by-project");
+    expect(
+      buildThreadActionMenuItems({
+        ...baseState,
+        projectFilter: { label: "Beta Project", isActive: false },
+      }).find((item) => item.id === "filter-by-project"),
+    ).toMatchObject({ label: "Filter by Beta Project", icon: "folder-tree" });
+  });
+
+  it("offers the way back to all projects once the list is scoped", () => {
+    const items = buildThreadActionMenuItems({
+      ...baseState,
+      projectFilter: { label: "Beta Project", isActive: true },
+    });
+    const filterIndex = items.findIndex((candidate) => candidate.id === "filter-by-project");
+    expect(items[filterIndex]).toMatchObject({ label: "Show all projects", icon: "folder-tree" });
+    expect(items[filterIndex - 1]?.id).toBe("mark-unread");
+    expect(items[filterIndex + 1]?.id).toBe("copy");
+  });
+
   it("includes branch items only for threads with a branch", () => {
     const withBranch = allIds({ ...baseState, branch: "feat/menu" });
     expect(withBranch).toContain("new-thread-on-branch");
diff --git a/apps/web/src/components/threadActionMenu.logic.ts b/apps/web/src/components/threadActionMenu.logic.ts
index 5ba266f7709d..4d9243a2092a 100644
--- a/apps/web/src/components/threadActionMenu.logic.ts
+++ b/apps/web/src/components/threadActionMenu.logic.ts
@@ -8,6 +8,7 @@ import type { SnoozePreset } from "@t3tools/client-runtime/state/thread-settled"
  */
 export type ThreadActionMenuId =
   | "new-thread-on-branch"
+  | "filter-by-project"
   | "project-settings"
   | "pin"
   | "unpin"
@@ -28,6 +29,15 @@ export type ThreadActionMenuId =
 
 export interface ThreadActionMenuState {
   readonly branch: string | null;
+  /**
+   * Project scoping for the thread list. Null on surfaces with no scoped
+   * list behind the menu (the chat header), where the item must not show.
+   */
+  readonly projectFilter: {
+    readonly label: string;
+    /** True when the list is already scoped to this thread's project. */
+    readonly isActive: boolean;
+  } | null;
   readonly isPinned: boolean;
   readonly isSettled: boolean;
   readonly isSnoozed: boolean;
@@ -46,8 +56,8 @@ export interface ThreadActionMenuState {
 
 /**
  * Single source for the per-thread action menu: the sidebar row's right-click
- * menu and the chat header menu both render exactly this list, so labels,
- * ordering, and capability gating cannot drift between the two surfaces.
+ * menu and the chat header menu share labels, ordering, and capability gating.
+ * Each surface supplies state for the actions it supports.
  */
 export function buildThreadActionMenuItems(
   state: ThreadActionMenuState,
@@ -107,6 +117,17 @@ export function buildThreadActionMenuItems(
         ]
       : []),
     { id: "mark-unread", label: "Mark unread", icon: "mail-open" },
+    ...(state.projectFilter
+      ? [
+          {
+            id: "filter-by-project" as const,
+            label: state.projectFilter.isActive
+              ? "Show all projects"
+              : `Filter by ${state.projectFilter.label}`,
+            icon: "folder-tree",
+          },
+        ]
+      : []),
     {
       id: "copy",
       label: "Copy",
diff --git a/apps/web/src/hooks/useThreadActionMenu.ts b/apps/web/src/hooks/useThreadActionMenu.ts
index a66ea21b9891..13641810e105 100644
--- a/apps/web/src/hooks/useThreadActionMenu.ts
+++ b/apps/web/src/hooks/useThreadActionMenu.ts
@@ -139,6 +139,9 @@ export function useThreadActionMenu(input: {
         const snoozePresets = resolveSnoozePresets(now, timestampFormat);
         const items = buildThreadActionMenuItems({
           branch: thread.branch ?? null,
+          // The chat header has no project-scoped thread list behind the
+          // menu, so the "Filter by project" affordance is sidebar-only.
+          projectFilter: null,
           isPinned: thread.pinnedAt != null,
           isSettled: supports.settlement && thread.settledOverride === "settled",
           isSnoozed: supports.snooze && effectiveSnoozed(thread, { now: now.toISOString() }),