diff --git a/apps/web/src/components/Sidebar.logic.test.ts b/apps/web/src/components/Sidebar.logic.test.ts
index 1348e6561049..9d1eb6bcd196 100644
--- a/apps/web/src/components/Sidebar.logic.test.ts
+++ b/apps/web/src/components/Sidebar.logic.test.ts
@@ -6,9 +6,11 @@ import {
   buildBulkTitleRegenerationContextMenuItem,
   buildMultiSelectThreadContextMenuItems,
   createThreadJumpHintVisibilityController,
+  filterSidebarProjectScopeItems,
   getSidebarThreadIdsToPrewarm,
   getVisibleSidebarThreadIds,
   resolveAdjacentThreadId,
+  reduceSidebarProjectScopeMenuState,
   getFallbackThreadIdAfterDelete,
   getVisibleThreadsForProject,
   getProjectSortTimestamp,
@@ -783,6 +785,69 @@ describe("searchSidebarThreadsByTitle", () => {
   });
 });
 
+describe("filterSidebarProjectScopeItems", () => {
+  const items = [
+    { value: "all", label: "All projects" },
+    { value: "alpha", label: "Alpha workspace" },
+    { value: "beta", label: "Beta tools" },
+  ] as const;
+  const filter = (activeScopeKey: string | null, query: string) =>
+    filterSidebarProjectScopeItems({
+      items,
+      activeScopeKey,
+      query,
+      matches: (item, candidate) =>
+        item.label.toLocaleLowerCase().includes(candidate.toLocaleLowerCase()),
+    });
+
+  it("omits the reset row when the sidebar is already unscoped", () => {
+    expect(filter(null, "")).toEqual(items.slice(1));
+  });
+
+  it("shows the reset row first while a project scope is active", () => {
+    expect(filter("alpha", "")).toEqual(items);
+  });
+
+  it("hides the reset row while filtering an active scope", () => {
+    expect(filter("alpha", "all")).toEqual([]);
+  });
+
+  it("returns matching projects in source order and supports no-match results", () => {
+    expect(filter(null, "WORK")).toEqual([items[1]]);
+    expect(filter(null, "missing")).toEqual([]);
+  });
+});
+
+describe("reduceSidebarProjectScopeMenuState", () => {
+  const queriedOpenState = { open: true, query: "alpha" };
+
+  it("clears the query when the combobox closes through onOpenChange", () => {
+    expect(
+      reduceSidebarProjectScopeMenuState(queriedOpenState, {
+        type: "open-changed",
+        open: false,
+      }),
+    ).toEqual({ open: false, query: "" });
+  });
+
+  it("clears the query when project settings closes the combobox", () => {
+    expect(
+      reduceSidebarProjectScopeMenuState(queriedOpenState, {
+        type: "project-settings-opened",
+      }),
+    ).toEqual({ open: false, query: "" });
+  });
+
+  it("keeps the popup open while the query changes", () => {
+    expect(
+      reduceSidebarProjectScopeMenuState(
+        { open: true, query: "" },
+        { type: "query-changed", query: "beta" },
+      ),
+    ).toEqual({ open: true, query: "beta" });
+  });
+});
+
 describe("sortThreadsForSidebar", () => {
   const sortable = (input: { id: string; createdAt: string }) => ({
     id: input.id,
diff --git a/apps/web/src/components/Sidebar.logic.ts b/apps/web/src/components/Sidebar.logic.ts
index 07fb4bc8ab59..7e42a8f8e638 100644
--- a/apps/web/src/components/Sidebar.logic.ts
+++ b/apps/web/src/components/Sidebar.logic.ts
@@ -583,6 +583,44 @@ export function searchSidebarThreadsByTitle<T extends { readonly title: string }
   return threads.filter((thread) => thread.title.toLowerCase().includes(normalizedQuery));
 }
 
+export function filterSidebarProjectScopeItems<TItem extends { readonly value: string }>(input: {
+  items: readonly TItem[];
+  activeScopeKey: string | null;
+  query: string;
+  matches: (item: TItem, query: string) => boolean;
+}): readonly TItem[] {
+  const projectItems = input.items.filter((item) => item.value !== "all");
+  const query = input.query.trim();
+  if (query.length > 0) {
+    return projectItems.filter((item) => input.matches(item, query));
+  }
+  return input.activeScopeKey === null ? projectItems : input.items;
+}
+
+export interface SidebarProjectScopeMenuState {
+  readonly open: boolean;
+  readonly query: string;
+}
+
+export type SidebarProjectScopeMenuAction =
+  | { readonly type: "query-changed"; readonly query: string }
+  | { readonly type: "open-changed"; readonly open: boolean }
+  | { readonly type: "project-settings-opened" };
+
+export function reduceSidebarProjectScopeMenuState(
+  state: SidebarProjectScopeMenuState,
+  action: SidebarProjectScopeMenuAction,
+): SidebarProjectScopeMenuState {
+  switch (action.type) {
+    case "query-changed":
+      return { ...state, query: action.query };
+    case "open-changed":
+      return { open: action.open, query: "" };
+    case "project-settings-opened":
+      return { open: false, query: "" };
+  }
+}
+
 type SettledTimestampInput = Pick<
   SidebarThreadSummary,
   "settledAt" | "latestUserMessageAt" | "latestTurn" | "updatedAt"
diff --git a/apps/web/src/components/Sidebar.tsx b/apps/web/src/components/Sidebar.tsx
index 63681a54813a..ee96b407db75 100644
--- a/apps/web/src/components/Sidebar.tsx
+++ b/apps/web/src/components/Sidebar.tsx
@@ -60,6 +60,7 @@ import {
   useCallback,
   useEffect,
   useMemo,
+  useReducer,
   useRef,
   useState,
   type KeyboardEvent as ReactKeyboardEvent,
@@ -125,6 +126,7 @@ import { buildThreadActionMenuItems } from "./threadActionMenu.logic";
 import {
   animatePinnedLayoutChanges,
   buildBulkTitleRegenerationContextMenuItem,
+  filterSidebarProjectScopeItems,
   formatWorkingDurationLabel,
   firstValidTimestampMs,
   hasUnseenCompletion,
@@ -132,6 +134,7 @@ import {
   isTrailingDoubleClick,
   orderItemsByPreferredIds,
   planPinnedReorder,
+  reduceSidebarProjectScopeMenuState,
   resolveAdjacentThreadId,
   resolveSettledTimestamp,
   resolveSidebarThreadStatus,
@@ -177,7 +180,16 @@ import { useThreadRunningTerminalIds } from "../state/terminalSessions";
 import { stackedThreadToast, toastManager } from "./ui/toast";
 import { Button } from "./ui/button";
 import { Input } from "./ui/input";
-import { Menu, MenuPopup, MenuRadioGroup, MenuRadioItem, MenuTrigger } from "./ui/menu";
+import {
+  Combobox,
+  ComboboxEmpty,
+  ComboboxInput,
+  ComboboxItem,
+  ComboboxList,
+  ComboboxPopup,
+  ComboboxTrigger,
+  useComboboxFilter,
+} from "./ui/combobox";
 import { SidebarContent, SidebarGroup, SidebarMenuButton, useSidebar } from "./ui/sidebar";
 import { SidebarChromeFooter, SidebarChromeHeader } from "./sidebar/SidebarChrome";
 import { Popover, PopoverPopup, PopoverTrigger } from "./ui/popover";
@@ -1803,7 +1815,6 @@ export default function Sidebar() {
       );
     },
   });
-  const [projectScopeMenuOpen, setProjectScopeMenuOpen] = useState(false);
   const newThreadContext = useHandleNewThread();
   const openAddProjectCommandPalette = useCallback(
     () => openCommandPalette({ open: "add-project" }),
@@ -1941,6 +1952,51 @@ export default function Sidebar() {
   // Project scope: one menu above the list. Scoping filters the list without
   // making the header width depend on the number or length of project names.
   const [projectScopeKey, setProjectScopeKey] = useState<string | null>(null);
+  // {value, label} items let Base UI drive the combobox selection contract
+  // while the popup search filters the same collection.
+  const projectScopeItems = useMemo(
+    () => [
+      { value: "all", label: "All projects" },
+      ...projectGroups.map((project) => ({
+        value: project.projectKey,
+        label: project.displayName,
+      })),
+    ],
+    [projectGroups],
+  );
+  const projectGroupByScopeKey = useMemo(
+    () => new Map(projectGroups.map((project) => [project.projectKey, project] as const)),
+    [projectGroups],
+  );
+  const selectedProjectScopeItem = useMemo(
+    () =>
+      projectScopeItems.find((item) => item.value === (projectScopeKey ?? "all")) ??
+      projectScopeItems[0]!,
+    [projectScopeItems, projectScopeKey],
+  );
+  const [projectScopeMenuState, dispatchProjectScopeMenu] = useReducer(
+    reduceSidebarProjectScopeMenuState,
+    { open: false, query: "" },
+  );
+  const projectScopeFilter = useComboboxFilter();
+  // Filtering derives from the same React state that controls the input, so
+  // the visible query and the visible list can never desync — the peer wiring
+  // in DiffPanel and BranchToolbarBranchSelector. "All projects" is a scope
+  // reset, not a searchable entry: it only shows while a project scope is
+  // active (there is something to reset) and the query is empty, so it can't
+  // outrank a project match under autoHighlight and no-hit queries reach the
+  // empty state.
+  const filteredProjectScopeItems = useMemo(
+    () =>
+      filterSidebarProjectScopeItems({
+        items: projectScopeItems,
+        activeScopeKey: projectScopeKey,
+        query: projectScopeMenuState.query,
+        matches: (item, query) =>
+          projectScopeFilter.contains(item, query, (candidate) => candidate.label),
+      }),
+    [projectScopeFilter, projectScopeItems, projectScopeKey, projectScopeMenuState.query],
+  );
   const scopedProjectGroup = useMemo(
     () =>
       projectScopeKey === null
@@ -2000,7 +2056,7 @@ export default function Sidebar() {
     (event: ReactMouseEvent<HTMLButtonElement>, projectGroup: SidebarProjectSnapshot) => {
       event.preventDefault();
       event.stopPropagation();
-      setProjectScopeMenuOpen(false);
+      dispatchProjectScopeMenu({ type: "project-settings-opened" });
       if (isMobile) {
         setOpenMobile(false);
       }
@@ -3490,8 +3546,23 @@ export default function Sidebar() {
             </div>
             {projectGroups.length > 0 ? (
               <div className="flex items-center gap-1">
-                <Menu open={projectScopeMenuOpen} onOpenChange={setProjectScopeMenuOpen}>
-                  <MenuTrigger
+                <Combobox
+                  items={projectScopeItems}
+                  filteredItems={filteredProjectScopeItems}
+                  autoHighlight
+                  itemToStringLabel={(item) => item.label}
+                  isItemEqualToValue={(a, b) => a.value === b.value}
+                  open={projectScopeMenuState.open}
+                  onOpenChange={(open) => {
+                    dispatchProjectScopeMenu({ type: "open-changed", open });
+                  }}
+                  value={selectedProjectScopeItem}
+                  onValueChange={(item) => {
+                    if (!item) return;
+                    setProjectScopeKey(item.value === "all" ? null : item.value);
+                  }}
+                >
+                  <ComboboxTrigger
                     render={
                       <SidebarMenuButton
                         aria-label="Filter threads by project"
@@ -3513,57 +3584,76 @@ export default function Sidebar() {
                       {scopedProjectGroup?.displayName ?? "All projects"}
                     </span>
                     <ChevronDownIcon className="-mr-px size-4 shrink-0" />
-                  </MenuTrigger>
-                  <MenuPopup align="start" className="w-(--anchor-width)">
-                    <MenuRadioGroup
-                      value={projectScopeKey ?? "all"}
-                      onValueChange={(value) =>
-                        setProjectScopeKey(value === "all" ? null : (value as string))
-                      }
-                    >
-                      <MenuRadioItem
-                        value="all"
-                        closeOnClick
-                        className="h-8 min-h-8 py-0 text-sm font-medium [&>span:last-child]:flex [&>span:last-child]:min-w-0 [&>span:last-child]:items-center [&>span:last-child]:gap-2"
-                      >
-                        <FolderIcon className="size-4 shrink-0" />
-                        <span className="min-w-0 truncate text-sm">All projects</span>
-                      </MenuRadioItem>
-                      {projectGroups.map((project) => {
-                        const scopeKey = project.projectKey;
+                  </ComboboxTrigger>
+                  <ComboboxPopup align="start" className="w-(--anchor-width)">
+                    <div className="shrink-0 px-3 pt-2.5">
+                      <div className="relative -translate-y-px border-b border-border/70 pb-1.5 transition-colors focus-within:border-ring">
+                        <SearchIcon
+                          aria-hidden="true"
+                          className="pointer-events-none absolute top-1.5 left-0 size-4 shrink-0 text-muted-foreground/55"
+                        />
+                        <ComboboxInput
+                          aria-label="Search projects"
+                          className="[&_input]:h-6.5 [&_input]:ps-5 [&_input]:font-sans [&_input]:leading-6.5"
+                          inputClassName="rounded-none bg-transparent text-sm"
+                          placeholder="Search projects..."
+                          showTrigger={false}
+                          size="sm"
+                          unstyled
+                          value={projectScopeMenuState.query}
+                          onChange={(event) =>
+                            dispatchProjectScopeMenu({
+                              type: "query-changed",
+                              query: event.target.value,
+                            })
+                          }
+                        />
+                      </div>
+                    </div>
+                    <ComboboxEmpty>No matching projects.</ComboboxEmpty>
+                    <ComboboxList>
+                      {(item: (typeof projectScopeItems)[number]) => {
+                        const project = projectGroupByScopeKey.get(item.value) ?? null;
                         return (
-                          <MenuRadioItem
-                            key={scopeKey}
-                            value={scopeKey}
-                            closeOnClick
-                            className="h-8 min-h-8 py-0 text-sm font-medium [&>span:last-child]:flex [&>span:last-child]:min-w-0 [&>span:last-child]:items-center [&>span:last-child]:gap-2"
+                          <ComboboxItem
+                            key={item.value}
+                            hideIndicator
+                            value={item}
+                            className="h-8 min-h-8 py-0 font-medium"
+                            contentClassName="flex min-w-0 items-center gap-2"
                           >
-                            <ProjectFavicon
-                              environmentId={project.environmentId}
-                              cwd={project.workspaceRoot}
-                              faviconPath={project.faviconPath}
-                              className="size-4 shrink-0"
-                            />
-                            <span className="min-w-0 truncate text-sm">{project.displayName}</span>
-                            <Button
-                              size="icon-xs"
-                              variant="ghost-muted"
-                              aria-label={`Project settings for ${project.displayName}`}
-                              title={`Project settings for ${project.displayName}`}
-                              className="ml-auto size-6 [--control-icon-color:currentColor] text-icon-muted focus-visible:bg-accent focus-visible:text-foreground"
-                              onPointerDown={(event) => event.stopPropagation()}
-                              onClick={(event) => {
-                                void handleProjectSettings(event, project);
-                              }}
-                            >
-                              <SettingsIcon className="size-3.5" />
-                            </Button>
-                          </MenuRadioItem>
+                            {project ? (
+                              <ProjectFavicon
+                                environmentId={project.environmentId}
+                                cwd={project.workspaceRoot}
+                                faviconPath={project.faviconPath}
+                                className="size-4 shrink-0"
+                              />
+                            ) : (
+                              <FolderIcon className="size-4 shrink-0" />
+                            )}
+                            <span className="min-w-0 flex-1 truncate text-sm">{item.label}</span>
+                            {project ? (
+                              <Button
+                                size="icon-xs"
+                                variant="ghost-muted"
+                                aria-label={`Project settings for ${project.displayName}`}
+                                title={`Project settings for ${project.displayName}`}
+                                className="ml-auto size-6 [--control-icon-color:currentColor] text-icon-muted focus-visible:bg-accent focus-visible:text-foreground"
+                                onPointerDown={(event) => event.stopPropagation()}
+                                onClick={(event) => {
+                                  void handleProjectSettings(event, project);
+                                }}
+                              >
+                                <SettingsIcon className="size-3.5" />
+                              </Button>
+                            ) : null}
+                          </ComboboxItem>
                         );
-                      })}
-                    </MenuRadioGroup>
-                  </MenuPopup>
-                </Menu>
+                      }}
+                    </ComboboxList>
+                  </ComboboxPopup>
+                </Combobox>
                 <Tooltip>
                   <TooltipTrigger
                     render={