diff --git a/Sources/MapConductorCore/MapViewContent.swift b/Sources/MapConductorCore/MapViewContent.swift
index 82696f2..57353e7 100644
--- a/Sources/MapConductorCore/MapViewContent.swift
+++ b/Sources/MapConductorCore/MapViewContent.swift
@@ -24,7 +24,22 @@ public struct MapViewContent {
     public var groundImages: [GroundImage] = []
     public var rasterLayers: [RasterLayer] = []
     public var views: [AnyView] = []
-    public var markerTilingOptions: MarkerTilingOptions = .Default
+    /// Marker tiling for this content. Reads ``MarkerTilingOptions/Default`` until something sets it.
+    ///
+    /// Whether it was set is tracked separately from its value, so that merging the pieces a
+    /// ``MapViewContentBuilder`` closure produces keeps an explicit choice — including
+    /// ``MarkerTilingOptions/Disabled`` — while pieces that never touched it leave it alone.
+    public var markerTilingOptions: MarkerTilingOptions {
+        get { explicitMarkerTilingOptions ?? .Default }
+        set { explicitMarkerTilingOptions = newValue }
+    }
+    /// `nil` until ``markerTilingOptions`` is assigned.
+    ///
+    /// `merge` used to carry the option over only when it was enabled, because a fresh
+    /// content's `.Default` and a deliberate `.Disabled` could not be told apart otherwise.
+    /// A single-expression builder closure always goes through `buildBlock`, which merges
+    /// into a fresh content, so `.Disabled` was silently replaced by the default (tiling on).
+    var explicitMarkerTilingOptions: MarkerTilingOptions?
     /// Handlers that manage a subset of polygons imperatively (e.g. cluster hull polygons).
     /// Map view coordinators call ``PolygonSyncHandler/bindPolygonSync(_:)`` on each handler
     /// to provide a direct polygon sync function, ensuring those polygons are committed before
@@ -47,8 +62,10 @@ public struct MapViewContent {
         rasterLayers.append(contentsOf: other.rasterLayers)
         views.append(contentsOf: other.views)
         polygonSyncHandlers.append(contentsOf: other.polygonSyncHandlers)
-        if other.markerTilingOptions.enabled {
-            markerTilingOptions = other.markerTilingOptions
+        // An explicit setting wins over an unset one, whether it enables tiling or not;
+        // between two explicit settings the later one wins.
+        if let tiling = other.explicitMarkerTilingOptions {
+            explicitMarkerTilingOptions = tiling
         }
     }
 }
diff --git a/Tests/MapConductorCoreTests/MapViewContentMergeTests.swift b/Tests/MapConductorCoreTests/MapViewContentMergeTests.swift
new file mode 100644
index 0000000..060dde5
--- /dev/null
+++ b/Tests/MapConductorCoreTests/MapViewContentMergeTests.swift
@@ -0,0 +1,107 @@
+import XCTest
+
+@testable import MapConductorCore
+
+/// `MapViewContentBuilder` が組み立てた content で、明示した設定が残ることを確かめる。
+///
+/// 以前の `merge` は有効なタイル設定しか引き継がず、ビルダーの中で
+/// `markerTilingOptions = .Disabled` を設定しても既定（タイル有効）に戻っていた。
+/// 1 式だけのビルダーも `buildBlock` で新しい content にマージされるので必ず踏む。
+final class MapViewContentMergeTests: XCTestCase {
+    private func build(@MapViewContentBuilder _ body: () -> MapViewContent) -> MapViewContent {
+        body()
+    }
+
+    private func content(tiling: MarkerTilingOptions?) -> MapViewContent {
+        var content = MapViewContent()
+        if let tiling { content.markerTilingOptions = tiling }
+        return content
+    }
+
+    func testUnsetTilingReadsDefault() {
+        let content = MapViewContent()
+        XCTAssertTrue(content.markerTilingOptions.enabled)
+        XCTAssertEqual(content.markerTilingOptions.minMarkerCount, MarkerTilingOptions.Default.minMarkerCount)
+        XCTAssertNil(content.explicitMarkerTilingOptions)
+    }
+
+    /// 報告された形そのもの: ビルダーの中で即時実行クロージャが `.Disabled` を設定する。
+    func testDisabledSetInsideBuilderSurvives() {
+        let built = build {
+            { () -> MapViewContent in
+                var content = MapViewContent()
+                content.markerTilingOptions = .Disabled
+                return content
+            }()
+        }
+        XCTAssertFalse(built.markerTilingOptions.enabled)
+    }
+
+    func testDisabledSurvivesNextToItemsThatDoNotSetTiling() {
+        let built = build {
+            content(tiling: .Disabled)
+            content(tiling: nil)
+            Marker(state: MarkerState(position: GeoPoint(latitude: 35, longitude: 139)))
+        }
+        XCTAssertFalse(built.markerTilingOptions.enabled)
+        XCTAssertEqual(built.markers.count, 1)
+    }
+
+    func testEnabledCustomOptionsSurvive() {
+        let built = build {
+            content(tiling: MarkerTilingOptions(minMarkerCount: 10, declutterPx: 4))
+            content(tiling: nil)
+        }
+        XCTAssertTrue(built.markerTilingOptions.enabled)
+        XCTAssertEqual(built.markerTilingOptions.minMarkerCount, 10)
+        XCTAssertEqual(built.markerTilingOptions.declutterPx, 4)
+    }
+
+    func testUnsetEverywhereFallsBackToDefault() {
+        let built = build {
+            content(tiling: nil)
+            Marker(state: MarkerState(position: GeoPoint(latitude: 35, longitude: 139)))
+        }
+        XCTAssertTrue(built.markerTilingOptions.enabled)
+        XCTAssertNil(built.explicitMarkerTilingOptions)
+    }
+
+    func testLaterExplicitSettingWins() {
+        let disabledLast = build {
+            content(tiling: MarkerTilingOptions(minMarkerCount: 10))
+            content(tiling: .Disabled)
+        }
+        XCTAssertFalse(disabledLast.markerTilingOptions.enabled)
+
+        let enabledLast = build {
+            content(tiling: .Disabled)
+            content(tiling: MarkerTilingOptions(minMarkerCount: 10))
+        }
+        XCTAssertTrue(enabledLast.markerTilingOptions.enabled)
+        XCTAssertEqual(enabledLast.markerTilingOptions.minMarkerCount, 10)
+    }
+
+    func testDisabledSurvivesConditionalsAndLoops() {
+        let flag = true
+        let built = build {
+            if flag {
+                content(tiling: .Disabled)
+            }
+            for _ in 0..<2 {
+                content(tiling: nil)
+            }
+        }
+        XCTAssertFalse(built.markerTilingOptions.enabled)
+
+        let nested = build {
+            for i in 0..<3 {
+                if i == 1 {
+                    content(tiling: .Disabled)
+                } else {
+                    content(tiling: nil)
+                }
+            }
+        }
+        XCTAssertFalse(nested.markerTilingOptions.enabled)
+    }
+}
diff --git a/api/MapConductorCore.api.swift b/api/MapConductorCore.api.swift
index ecc15d9..54c7308 100644
--- a/api/MapConductorCore.api.swift
+++ b/api/MapConductorCore.api.swift
@@ -249,7 +249,10 @@ public struct MapViewContent {
   public var groundImages: [MapConductorCore::GroundImage]
   public var rasterLayers: [MapConductorCore::RasterLayer]
   public var views: [SwiftUICore::AnyView]
-  public var markerTilingOptions: MapConductorCore::MarkerTilingOptions
+  public var markerTilingOptions: MapConductorCore::MarkerTilingOptions {
+    get
+    set
+  }
   public var polygonSyncHandlers: [any MapConductorCore::PolygonSyncHandler]
   public init()
 }