diff --git a/__tests__/unit/merged-search-filter.test.ts b/__tests__/unit/merged-search-filter.test.ts
new file mode 100644
index 00000000..0d49b610
--- /dev/null
+++ b/__tests__/unit/merged-search-filter.test.ts
@@ -0,0 +1,71 @@
+/**
+ * Regression guard for #646: the hub's merged list must not re-filter results
+ * that came from /api/search.
+ *
+ * /api/search matches message bodies; the client only ever sees title and
+ * preview, so re-running the predicate on server results can only discard
+ * conversations the server correctly matched — exactly the "I know we
+ * discussed it, no idea where" case server-side search exists to serve.
+ *
+ * Sessions keep filtering client-side in both regimes, because /api/search
+ * does not cover them.
+ */
+import { mergedItemMatchesQuery, type MergedItem } from '@/app/index'
+import type { MultiConversation, MultiSession } from '@/types/api'
+
+const conversation = (over: Partial<MultiConversation>): MergedItem => ({
+  kind: 'conversation',
+  ms: 0,
+  item: {
+    id: 'conv-1',
+    title: 'Retry loop refactor',
+    projectPath: '/tmp/p',
+    messageCount: 12,
+    lastActivity: '2026-08-12T10:00:00Z',
+    serverId: 'srv-1',
+    ...over,
+  },
+})
+
+const session = (over: Partial<MultiSession>): MergedItem => ({
+  kind: 'session',
+  ms: 0,
+  item: {
+    id: 'sess-1',
+    status: 'running',
+    ptyAttached: true,
+    projectPath: '/tmp/p',
+    projectName: 'wombat-api',
+    lastOutput: 'compiling',
+    elapsedMs: 1000,
+    promptCount: 2,
+    startedAt: '2026-08-12T09:00:00Z',
+    serverId: 'srv-1',
+    ...over,
+  },
+})
+
+describe('mergedItemMatchesQuery', () => {
+  // The defect: body-only matches have nothing to match on client-side.
+  const bodyOnlyMatch = conversation({ title: 'Retry loop refactor', preview: 'Let us ship it' })
+
+  it('keeps a server-matched conversation whose query hits neither title nor preview', () => {
+    expect(mergedItemMatchesQuery(bodyOnlyMatch, 'wombat', true)).toBe(true)
+  })
+
+  it('still filters conversations on title and preview when they came from the paged set', () => {
+    expect(mergedItemMatchesQuery(bodyOnlyMatch, 'wombat', false)).toBe(false)
+    expect(mergedItemMatchesQuery(bodyOnlyMatch, 'retry', false)).toBe(true)
+    expect(
+      mergedItemMatchesQuery(conversation({ preview: 'the wombat timeout' }), 'wombat', false),
+    ).toBe(true)
+  })
+
+  it('filters session rows client-side even while conversations are server-backed', () => {
+    expect(mergedItemMatchesQuery(session({}), 'wombat', true)).toBe(true)
+    expect(mergedItemMatchesQuery(session({ projectName: 'billing' }), 'wombat', true)).toBe(false)
+    expect(
+      mergedItemMatchesQuery(session({ projectName: 'billing', lastOutput: 'wombat timed out' }), 'wombat', true),
+    ).toBe(true)
+  })
+})
diff --git a/app/index.tsx b/app/index.tsx
index 2f1c221e..153ee9a3 100644
--- a/app/index.tsx
+++ b/app/index.tsx
@@ -67,10 +67,28 @@ const EMPTY_CONVERSATIONS: MultiConversation[] = []
 
 type ClassicTab = 'sessions' | 'history'
 
-type MergedItem =
+export type MergedItem =
   | { kind: 'session'; ms: number; item: MultiSession }
   | { kind: 'conversation'; ms: number; item: MultiConversation }
 
+// Sessions always match client-side — /api/search does not cover them.
+// Conversations only do when they came from the paged set: server results are
+// already matched on message bodies, so re-checking title/preview here could
+// only drop rows the server correctly found.
+export function mergedItemMatchesQuery(
+  item: MergedItem,
+  q: string,
+  conversationsFromServer: boolean,
+): boolean {
+  if (item.kind === 'session') {
+    return Boolean(
+      item.item.projectName?.toLowerCase().includes(q) || item.item.lastOutput?.toLowerCase().includes(q),
+    )
+  }
+  if (conversationsFromServer) return true
+  return Boolean(item.item.title?.toLowerCase().includes(q) || item.item.preview?.toLowerCase().includes(q))
+}
+
 function lastActivityMs(s: MultiSession): number {
   if (s.completedAt) return Date.parse(s.completedAt)
   return Date.parse(s.startedAt) + (s.elapsedMs ?? 0)
@@ -473,6 +491,7 @@ export default function ProjectsHub() {
               onRefresh={handleRefresh}
               searchOpen={searchOpen}
               searchQuery={classicConvSearch}
+              conversationsFromServer={Boolean(debouncedConvSearch)}
               onSearchChange={setClassicConvSearch}
               isBackgroundRefreshing={isBackgroundRefreshing}
             />
@@ -610,6 +629,7 @@ const MergedClassicList = React.memo(function MergedClassicList({
   onRefresh,
   searchOpen,
   searchQuery,
+  conversationsFromServer,
   onSearchChange,
   isBackgroundRefreshing,
 }: {
@@ -618,6 +638,7 @@ const MergedClassicList = React.memo(function MergedClassicList({
   onRefresh: () => void
   searchOpen: boolean
   searchQuery: string
+  conversationsFromServer: boolean
   onSearchChange: (q: string) => void
   isBackgroundRefreshing?: boolean
 }) {
@@ -646,18 +667,8 @@ const MergedClassicList = React.memo(function MergedClassicList({
   const filteredItems = useMemo(() => {
     if (!searchQuery) return items
     const q = searchQuery.toLowerCase()
-    return items.filter((item) => {
-      if (item.kind === 'session') {
-        const s = item.item as MultiSession
-        return s.projectName?.toLowerCase().includes(q) || s.lastOutput?.toLowerCase().includes(q)
-      }
-      const c = item.item as MultiConversation
-      return (
-        c.title?.toLowerCase().includes(q) ||
-        c.preview?.toLowerCase().includes(q)
-      )
-    })
-  }, [searchQuery, items])
+    return items.filter((item) => mergedItemMatchesQuery(item, q, conversationsFromServer))
+  }, [searchQuery, items, conversationsFromServer])
 
   const visibleServerCount = useMemo(
     () => new Set(filteredItems.map((it) => it.item.serverId)).size,