diff --git a/internal/server/handlers_items.go b/internal/server/handlers_items.go
index e43d917b..6d0b1105 100644
--- a/internal/server/handlers_items.go
+++ b/internal/server/handlers_items.go
@@ -76,9 +76,14 @@ func (s *Server) handleListItems(w http.ResponseWriter, r *http.Request) {
 // itemsIndexResponse wraps the skinny-projection items list with bookkeeping
 // the local-first read model (PLAN-1343) needs:
 //   - total: the row count so the client can size its in-memory array.
-//   - cursor: a placeholder for the Phase 2 monotonic seq cursor — for now
-//     we return the maximum updated_at (RFC3339), or "0" when the workspace
-//     is empty. Clients should treat its value as opaque.
+//   - cursor: the workspace-scoped monotonic `seq` cursor (TASK-1353).
+//     Holds MAX(seq) across the requested scope as a decimal-encoded
+//     string so it forward-compatibly tolerates future encoding changes
+//     (opaque on the wire — clients MUST NOT parse it as an integer).
+//     When the requested scope returns zero rows, the cursor falls back
+//     to the workspace's current MAX(seq) so /items-changes?since=cursor
+//     starts from the right floor on the next poll instead of replaying
+//     every prior mutation. Empty workspaces return "0".
 type itemsIndexResponse struct {
 	Items  []models.Item `json:"items"`
 	Total  int           `json:"total"`
@@ -127,6 +132,32 @@ func (s *Server) handleListItemsIndex(w http.ResponseWriter, r *http.Request) {
 		params.ItemIDs = grantedItemIDs
 	}
 
+	// Snapshot the workspace's MAX(seq) BEFORE the list query so the
+	// fallback cursor cannot leapfrog a concurrent insert that
+	// committed between the list query and a post-list MaxItemSeq
+	// read. Per Codex review of TASK-1353 round 1 [P1]:
+	//
+	//   List → empty.
+	//   Concurrent INSERT lands with seq = M+1 (visible to a future
+	//     /items-changes call).
+	//   MaxItemSeq → M+1.
+	//   Cursor = M+1, response = [].
+	//   Client polls /items-changes?since=M+1 → seq > M+1 returns
+	//     nothing. The new row is lost.
+	//
+	// Capturing M BEFORE the list eliminates that window: any insert
+	// after the snapshot has seq > M (workspace counter is strictly
+	// monotonic per TASK-1352), so /items-changes?since=M will return
+	// it. Rows the list DOES see may have seq > M (a concurrent insert
+	// the list query happened to observe); MAX(rows.seq) bumps the
+	// cursor for that case so the client never re-fetches what was
+	// already in the response.
+	wsMaxBefore, err := s.store.MaxItemSeq(workspaceID)
+	if err != nil {
+		writeInternalError(w, err)
+		return
+	}
+
 	result, err := s.store.ListItemsIndex(workspaceID, params)
 	if err != nil {
 		writeInternalError(w, err)
@@ -137,13 +168,16 @@ func (s *Server) handleListItemsIndex(w http.ResponseWriter, r *http.Request) {
 	}
 	s.enrichItemsWithParent(workspaceID, result, visibleIDs)
 
-	// Cursor placeholder: max(updated_at) across the returned set. ListItemsIndex
-	// already sorts by updated_at DESC, so the first row holds it. Phase 2 will
-	// replace this with a monotonic `seq` cursor (see PLAN-1343).
-	cursor := "0"
-	if len(result) > 0 {
-		cursor = result[0].UpdatedAt.UTC().Format(time.RFC3339Nano)
+	// Cursor: max(pre-list workspace floor, MAX(returned-rows.seq)).
+	// See the long comment above MaxItemSeq for why the floor is read
+	// BEFORE ListItemsIndex. Empty workspace + empty result → "0".
+	cursorSeq := wsMaxBefore
+	for _, it := range result {
+		if it.Seq > cursorSeq {
+			cursorSeq = it.Seq
+		}
 	}
+	cursor := strconv.FormatInt(cursorSeq, 10)
 
 	writeJSON(w, http.StatusOK, itemsIndexResponse{
 		Items:  result,
diff --git a/internal/server/handlers_items_test.go b/internal/server/handlers_items_test.go
index c773faf9..200697b1 100644
--- a/internal/server/handlers_items_test.go
+++ b/internal/server/handlers_items_test.go
@@ -6,6 +6,7 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"strconv"
 	"testing"
 	"time"
 
@@ -1255,7 +1256,8 @@ type itemsIndexBody struct {
 // behavior of the local-first read model bootstrap endpoint (TASK-1344):
 //   - response shape: {items, total, cursor}
 //   - content body excluded (skinny projection)
-//   - cursor placeholder reflects the newest updated_at
+//   - cursor reflects MAX(seq) across the result set (TASK-1353)
+//   - per-row seq is populated and non-zero
 //   - core projected fields populate (ref, collection_slug, fields, …)
 //
 // Sort order is exercised separately by TestListItemsIndex_SortByUpdatedAt,
@@ -1295,11 +1297,23 @@ func TestListItemsIndex_SkinnyProjectionAndShape(t *testing.T) {
 		t.Fatalf("expected cursor to be populated for non-empty workspace, got %q", resp.Cursor)
 	}
 
-	// Cursor must match the newest updated_at — and because the sort is
-	// updated_at DESC, that's the row at index 0.
-	want := resp.Items[0].UpdatedAt.UTC().Format(time.RFC3339Nano)
-	if resp.Cursor != want {
-		t.Fatalf("cursor mismatch: got %q, want %q", resp.Cursor, want)
+	// Cursor must be the decimal-encoded MAX(seq) across the returned
+	// rows (TASK-1353). Per-row seq is populated by the skinny scan.
+	cursorSeq, err := strconv.ParseInt(resp.Cursor, 10, 64)
+	if err != nil {
+		t.Fatalf("cursor not decimal-encoded int: %q (err=%v)", resp.Cursor, err)
+	}
+	var maxSeq int64
+	for _, it := range resp.Items {
+		if it.Seq == 0 {
+			t.Errorf("items[%s]: seq must be non-zero", it.Ref)
+		}
+		if it.Seq > maxSeq {
+			maxSeq = it.Seq
+		}
+	}
+	if cursorSeq != maxSeq {
+		t.Fatalf("cursor mismatch: got %d, want MAX(seq)=%d", cursorSeq, maxSeq)
 	}
 
 	// Skinny projection: content body excluded for every row.
@@ -1360,7 +1374,7 @@ func TestListItemsIndex_SortByUpdatedAt(t *testing.T) {
 }
 
 // TestListItemsIndex_EmptyWorkspace covers the edge case the cursor
-// placeholder is documented to handle: no items → cursor "0".
+// is documented to handle: no items → cursor "0".
 func TestListItemsIndex_EmptyWorkspace(t *testing.T) {
 	srv := testServer(t)
 	slug := createWSWithCollections(t, srv)
@@ -1389,6 +1403,93 @@ func TestListItemsIndex_EmptyWorkspace(t *testing.T) {
 	}
 }
 
+// TestListItemsIndex_EmptyResultFallsBackToWorkspaceMax verifies the
+// cursor-fallback path (TASK-1353 acceptance): when a filtered query
+// returns zero rows but the workspace itself is non-empty, the cursor
+// must hold the workspace's MAX(seq) so the client's next
+// /items-changes?since=cursor poll starts at the right floor rather
+// than replaying every prior mutation from 0.
+func TestListItemsIndex_EmptyResultFallsBackToWorkspaceMax(t *testing.T) {
+	srv := testServer(t)
+	slug := createWSWithCollections(t, srv)
+
+	// Populate the workspace so MAX(seq) is non-zero.
+	createItem(t, srv, slug, "tasks", map[string]interface{}{
+		"title":  "T1",
+		"fields": `{"status":"open"}`,
+	})
+	createItem(t, srv, slug, "ideas", map[string]interface{}{
+		"title":  "I1",
+		"fields": `{"status":"new"}`,
+	})
+
+	// Establish the baseline MAX(seq) via the unfiltered call.
+	rrAll := doRequest(srv, "GET", "/api/v1/workspaces/"+slug+"/items-index", nil)
+	if rrAll.Code != http.StatusOK {
+		t.Fatalf("items-index all: expected 200, got %d: %s", rrAll.Code, rrAll.Body.String())
+	}
+	var all itemsIndexBody
+	parseJSON(t, rrAll, &all)
+	if all.Cursor == "" || all.Cursor == "0" {
+		t.Fatalf("baseline cursor should be non-zero, got %q", all.Cursor)
+	}
+
+	// Now request a collection that exists but has no items — the response
+	// slice is empty, but the workspace MAX(seq) fallback should still
+	// produce the same cursor.
+	rrFiltered := doRequest(srv, "GET", "/api/v1/workspaces/"+slug+"/items-index?collection=docs", nil)
+	if rrFiltered.Code != http.StatusOK {
+		t.Fatalf("items-index docs: expected 200, got %d: %s", rrFiltered.Code, rrFiltered.Body.String())
+	}
+	var filtered itemsIndexBody
+	parseJSON(t, rrFiltered, &filtered)
+
+	if filtered.Total != 0 {
+		t.Fatalf("expected total=0 for empty collection filter, got %d", filtered.Total)
+	}
+	if filtered.Cursor != all.Cursor {
+		t.Fatalf("empty-result cursor should equal workspace MAX(seq) baseline %q, got %q", all.Cursor, filtered.Cursor)
+	}
+	if _, err := strconv.ParseInt(filtered.Cursor, 10, 64); err != nil {
+		t.Fatalf("fallback cursor not decimal-encoded int: %q", filtered.Cursor)
+	}
+}
+
+// TestListItemsIndex_CursorMonotonicAcrossMutations confirms that the
+// cursor advances after every items mutation and can be re-used as the
+// `since` parameter on a follow-up /items-changes call (cursor contract
+// for PLAN-1343 Phase 2).
+func TestListItemsIndex_CursorMonotonicAcrossMutations(t *testing.T) {
+	srv := testServer(t)
+	slug := createWSWithCollections(t, srv)
+
+	createItem(t, srv, slug, "tasks", map[string]interface{}{
+		"title":  "C1",
+		"fields": `{"status":"open"}`,
+	})
+
+	rr1 := doRequest(srv, "GET", "/api/v1/workspaces/"+slug+"/items-index", nil)
+	var resp1 itemsIndexBody
+	parseJSON(t, rr1, &resp1)
+	c1, _ := strconv.ParseInt(resp1.Cursor, 10, 64)
+	if c1 == 0 {
+		t.Fatalf("expected non-zero cursor after first create, got %q", resp1.Cursor)
+	}
+
+	createItem(t, srv, slug, "ideas", map[string]interface{}{
+		"title":  "C2",
+		"fields": `{"status":"new"}`,
+	})
+
+	rr2 := doRequest(srv, "GET", "/api/v1/workspaces/"+slug+"/items-index", nil)
+	var resp2 itemsIndexBody
+	parseJSON(t, rr2, &resp2)
+	c2, _ := strconv.ParseInt(resp2.Cursor, 10, 64)
+	if c2 <= c1 {
+		t.Fatalf("cursor should advance after second create: c1=%d, c2=%d", c1, c2)
+	}
+}
+
 // TestListItemsIndex_CollectionFilter covers ?collection=<slug>.
 func TestListItemsIndex_CollectionFilter(t *testing.T) {
 	srv := testServer(t)
diff --git a/internal/store/items.go b/internal/store/items.go
index 8c529fe2..4bafe40e 100644
--- a/internal/store/items.go
+++ b/internal/store/items.go
@@ -816,6 +816,27 @@ func (s *Store) ListItemsIndex(workspaceID string, params ItemIndexParams) ([]mo
 	return scanItemsIndex(rows)
 }
 
+// MaxItemSeq returns the largest items.seq across the workspace, or 0
+// if the workspace has no items. This is the cursor floor for the
+// local-first read model (PLAN-1343 / TASK-1353): /items-index hands
+// it back when its filtered result set is empty so the client can
+// poll /items-changes?since=<cursor> against the workspace's true
+// current position instead of restarting from 0.
+//
+// Soft-deleted items DO contribute to MAX(seq) — the seq column
+// bumps on tombstone writes (DeleteItem) so a client's cursor must
+// move past those events for the next /items-changes scan to skip
+// them. Filtering by `deleted_at IS NULL` here would silently regress
+// the cursor whenever the most recent mutation was a delete.
+func (s *Store) MaxItemSeq(workspaceID string) (int64, error) {
+	var seq int64
+	err := s.db.QueryRow(s.q(`SELECT COALESCE(MAX(seq), 0) FROM items WHERE workspace_id = ?`), workspaceID).Scan(&seq)
+	if err != nil {
+		return 0, fmt.Errorf("max item seq: %w", err)
+	}
+	return seq, nil
+}
+
 // ItemCheckboxProgress is the per-item count of markdown checkboxes
 // (`- [ ]` / `- [x]`) extracted from item content. Used by the
 // collection page to render checklist progress badges without
diff --git a/web/src/lib/api/client.ts b/web/src/lib/api/client.ts
index dbe4877d..ea4466dd 100644
--- a/web/src/lib/api/client.ts
+++ b/web/src/lib/api/client.ts
@@ -328,7 +328,14 @@ export const api = {
 		 * Skinny-projection cross-collection listing for the local-first
 		 * read model (PLAN-1343 / TASK-1344). Returns every item in a
 		 * workspace MINUS the rich-text `content` body, plus a `total`
-		 * count and a forward-looking `cursor` placeholder.
+		 * count and a real workspace-scoped `seq` cursor (TASK-1353).
+		 *
+		 * The response `cursor` is the decimal-encoded `MAX(seq)` across
+		 * the requested scope (or the workspace's true `MAX(seq)` when
+		 * the filtered set is empty, so /items-changes?since=cursor
+		 * starts at the right floor). Each row carries its own `seq`
+		 * field so the client can reason about ordering without parsing
+		 * the cursor.
 		 *
 		 * Optional filters mirror the server: `collection` narrows to one
 		 * collection slug, `include_archived` flips the soft-delete gate.
diff --git a/web/src/lib/types/index.ts b/web/src/lib/types/index.ts
index 72290512..6b3acf76 100644
--- a/web/src/lib/types/index.ts
+++ b/web/src/lib/types/index.ts
@@ -429,6 +429,12 @@ export interface Item {
 	collection_icon?: string;
 	collection_prefix?: string;
 	item_number?: number;
+	// `seq` is the workspace-scoped monotonic mutation cursor (PLAN-1343 /
+	// DOC-1342 decision #1). Stamped server-side on every create / update /
+	// soft-delete / restore. Clients track the max `seq` they have seen
+	// and request `?since=<seq>` deltas to resume. Optional because old
+	// snapshots may predate the column; new responses always populate it.
+	seq?: number;
 	parent_link_id?: string;
 	parent_ref?: string;
 	parent_title?: string;
@@ -455,10 +461,14 @@ export type ItemIndexRow = Omit<Item, 'content'>;
 export interface ItemIndexResponse {
 	items: ItemIndexRow[];
 	total: number;
-	// `cursor` is a placeholder until Phase 2 lands the monotonic `seq`
-	// column — today the server returns the maximum `updated_at` across
-	// the result set (RFC3339Nano), or `"0"` when the workspace is empty.
-	// Clients should treat the value as opaque.
+	// `cursor` is the workspace-scoped monotonic `seq` cursor (TASK-1353).
+	// Holds MAX(seq) across the requested scope as a decimal-encoded
+	// string. When the result set is empty but the workspace has items,
+	// it falls back to the workspace's current MAX(seq) so the next
+	// /items-changes?since=<cursor> poll starts at the right floor.
+	// Empty workspaces return `"0"`. Treat the value as opaque — the
+	// encoding may change in future, and clients should not parse it
+	// as an integer beyond passing it back as `since`.
 	cursor: string;
 }