diff --git a/apps/web/app/api/internal/notes/[taskId]/read/route.ts b/apps/web/app/api/internal/notes/[taskId]/read/route.ts
index e03f5c0..77ab0d4 100644
--- a/apps/web/app/api/internal/notes/[taskId]/read/route.ts
+++ b/apps/web/app/api/internal/notes/[taskId]/read/route.ts
@@ -25,6 +25,9 @@ export async function POST(_request: Request, { params }: RouteContext) {
     },
     select: {
       id: true,
+      note: true,
+      summaryUpdatedAt: true,
+      taskUpdatedAt: true,
       project: { select: { companyId: true } },
     },
   })
@@ -39,20 +42,32 @@ export async function POST(_request: Request, { params }: RouteContext) {
     return notFound("Review reader agent not found.")
   }
 
-  await prisma.taskReadMarker.upsert({
-    where: {
-      taskId_agentId_status: {
+  const readAt = new Date()
+
+  await prisma.$transaction(async (tx) => {
+    if (!task.summaryUpdatedAt) {
+      await tx.task.update({
+        where: { id: task.id },
+        data: { summaryUpdatedAt: task.taskUpdatedAt },
+      })
+    }
+
+    await tx.taskReadMarker.upsert({
+      where: {
+        taskId_agentId_status: {
+          taskId: task.id,
+          agentId: reviewReader.id,
+          status: Status.done,
+        },
+      },
+      create: {
         taskId: task.id,
         agentId: reviewReader.id,
         status: Status.done,
+        readAt,
       },
-    },
-    create: {
-      taskId: task.id,
-      agentId: reviewReader.id,
-      status: Status.done,
-    },
-    update: { readAt: new Date() },
+      update: { readAt },
+    })
   })
 
   return NextResponse.json({
diff --git a/apps/web/app/api/internal/notes/route.ts b/apps/web/app/api/internal/notes/route.ts
index 6948d8a..2d687c6 100644
--- a/apps/web/app/api/internal/notes/route.ts
+++ b/apps/web/app/api/internal/notes/route.ts
@@ -49,6 +49,7 @@ export async function GET(request: NextRequest) {
       status: true,
       note: true,
       summaryUpdatedAt: true,
+      taskUpdatedAt: true,
       assigned: {
         select: {
           id: true,
@@ -74,7 +75,9 @@ export async function GET(request: NextRequest) {
   const unreadNotes = notes.filter((task) => {
     const readAt = task.readMarkers[0]?.readAt
 
-    return !readAt || !task.summaryUpdatedAt || readAt < task.summaryUpdatedAt
+    const summaryUpdatedAt = task.summaryUpdatedAt ?? task.taskUpdatedAt
+
+    return !readAt || readAt < summaryUpdatedAt
   })
 
   return NextResponse.json({
@@ -85,7 +88,7 @@ export async function GET(request: NextRequest) {
       name: task.name,
       status: task.status,
       note: task.note ?? "",
-      summaryUpdatedAt: task.summaryUpdatedAt?.toISOString() ?? null,
+      summaryUpdatedAt: (task.summaryUpdatedAt ?? task.taskUpdatedAt).toISOString(),
       assigned: task.assigned,
       project: task.project,
     })),
diff --git a/apps/web/app/dashboard/notes/page.tsx b/apps/web/app/dashboard/notes/page.tsx
index d777820..608f6a3 100644
--- a/apps/web/app/dashboard/notes/page.tsx
+++ b/apps/web/app/dashboard/notes/page.tsx
@@ -43,6 +43,7 @@ export default async function NotesPage({ searchParams }: NotesPageProps) {
       status: true,
       note: true,
       summaryUpdatedAt: true,
+      taskUpdatedAt: true,
       assigned: {
         select: {
           id: true,
@@ -69,7 +70,9 @@ export default async function NotesPage({ searchParams }: NotesPageProps) {
   const unreadNotes = notes.filter((task) => {
     const readAt = task.readMarkers[0]?.readAt
 
-    return !readAt || !task.summaryUpdatedAt || readAt < task.summaryUpdatedAt
+    const summaryUpdatedAt = task.summaryUpdatedAt ?? task.taskUpdatedAt
+
+    return !readAt || readAt < summaryUpdatedAt
   })
 
   return (
@@ -96,7 +99,7 @@ export default async function NotesPage({ searchParams }: NotesPageProps) {
               name: task.name,
               status: task.status,
               note: task.note ?? "",
-              summaryUpdatedAt: task.summaryUpdatedAt?.toISOString() ?? null,
+              summaryUpdatedAt: (task.summaryUpdatedAt ?? task.taskUpdatedAt).toISOString(),
               assigned: task.assigned,
               project: task.project,
             }))}
diff --git a/prisma/migrations/20260511062000_add_task_dependencies_and_summary_updated_at/migration.sql b/prisma/migrations/20260511062000_add_task_dependencies_and_summary_updated_at/migration.sql
index 5508da9..990f53a 100644
--- a/prisma/migrations/20260511062000_add_task_dependencies_and_summary_updated_at/migration.sql
+++ b/prisma/migrations/20260511062000_add_task_dependencies_and_summary_updated_at/migration.sql
@@ -16,6 +16,5 @@ CREATE INDEX "TaskDependency_dependencyTaskId_idx" ON "TaskDependency"("dependen
 ALTER TABLE "TaskDependency" ADD CONSTRAINT "TaskDependency_blockedTaskId_fkey" FOREIGN KEY ("blockedTaskId") REFERENCES "Task"("id") ON DELETE CASCADE ON UPDATE CASCADE;
 ALTER TABLE "TaskDependency" ADD CONSTRAINT "TaskDependency_dependencyTaskId_fkey" FOREIGN KEY ("dependencyTaskId") REFERENCES "Task"("id") ON DELETE CASCADE ON UPDATE CASCADE;
 
-UPDATE "Task"
-SET "summaryUpdatedAt" = CURRENT_TIMESTAMP
-WHERE "note" IS NOT NULL;
+-- summaryUpdatedAt is intentionally left null here. A later migration backfills
+-- it from taskUpdatedAt after that deterministic freshness column exists.
diff --git a/prisma/migrations/20260512210500_backfill_summary_updated_at/migration.sql b/prisma/migrations/20260512210500_backfill_summary_updated_at/migration.sql
new file mode 100644
index 0000000..4acd834
--- /dev/null
+++ b/prisma/migrations/20260512210500_backfill_summary_updated_at/migration.sql
@@ -0,0 +1,8 @@
+-- Backfill deterministic summary timestamps for legacy completed task notes.
+-- Some done tasks were created with note text before summaryUpdatedAt existed or was
+-- consistently written. Use the task's last update timestamp instead of CURRENT_TIMESTAMP
+-- so existing read markers newer than the legacy note stay read.
+UPDATE "Task"
+SET "summaryUpdatedAt" = "taskUpdatedAt"
+WHERE "note" IS NOT NULL
+  AND "summaryUpdatedAt" IS NULL;