diff --git a/backend/internal/app/cloud_agent_canvas_events.go b/backend/internal/app/cloud_agent_canvas_events.go
index 1458c4ab3..7b5b4e267 100644
--- a/backend/internal/app/cloud_agent_canvas_events.go
+++ b/backend/internal/app/cloud_agent_canvas_events.go
@@ -26,12 +26,65 @@ func cloudAgentObjectChanges(before, after any) []map[string]any {
 	for _, item := range creationMaps(after) {
 		old := previous[stringValue(item["id"])]
 		if !reflect.DeepEqual(old, item) {
-			changes = append(changes, map[string]any{"before": old, "after": item})
+			oldFields, newFields := cloudAgentShrinkChange(old, item)
+			changes = append(changes, map[string]any{"before": oldFields, "after": newFields})
 		}
 	}
 	return changes
 }
 
+// cloudAgentGeometryFields 是"只改位置/尺寸"的字段集合。
+var cloudAgentGeometryFields = []string{"position", "width", "height", "zIndex"}
+
+// cloudAgentShrinkChange 把"只动了几何"的节点变更缩成几何增量。
+//
+// 画布增量在前端是**三方合并**（按字段合并，未出现的字段保持本地值），所以只发发生变化的
+// 几何字段既安全又必要：一次整理 50 个节点时，若每个节点都带完整正文（分镜行、Markdown
+// 正文可达十几 KB），单条 canvas_updated 事件就会超过 validateCloudAgentRuntime 的
+// 单事件 128KiB 上限（"Agent runtime event payload is too large"），整轮直接终止。
+// 只有"除几何外完全一致"时才缩：带内容/metadata 变化的节点仍发完整 before/after，
+// 以免影响前端对生成任务状态的保护逻辑。
+func cloudAgentShrinkChange(before, after map[string]any) (map[string]any, map[string]any) {
+	if before == nil || after == nil {
+		return before, after
+	}
+	for key, value := range after {
+		if cloudAgentContainsString(cloudAgentGeometryFields, key) {
+			continue
+		}
+		if !reflect.DeepEqual(before[key], value) {
+			return before, after
+		}
+	}
+	for key, value := range before {
+		if cloudAgentContainsString(cloudAgentGeometryFields, key) {
+			continue
+		}
+		if _, exists := after[key]; !exists && value != nil {
+			return before, after
+		}
+	}
+	shrunkBefore := map[string]any{"id": after["id"]}
+	shrunkAfter := map[string]any{"id": after["id"]}
+	for _, key := range cloudAgentGeometryFields {
+		final, hasFinal := after[key]
+		initial, hasInitial := before[key]
+		if !hasFinal && !hasInitial {
+			continue
+		}
+		if reflect.DeepEqual(initial, final) {
+			continue
+		}
+		if hasInitial {
+			shrunkBefore[key] = initial
+		}
+		if hasFinal {
+			shrunkAfter[key] = final
+		}
+	}
+	return shrunkBefore, shrunkAfter
+}
+
 func emitCloudAgentCanvasChange(repo *repository.Repository, runID string, state *cloudAgentRuntime, input cloudAgentMutationInput) error {
 	before, err := creationDocument(input.BeforeJSON)
 	if err != nil {
diff --git a/backend/internal/app/cloud_agent_layout_patch_test.go b/backend/internal/app/cloud_agent_layout_patch_test.go
new file mode 100644
index 000000000..69f8fbc73
--- /dev/null
+++ b/backend/internal/app/cloud_agent_layout_patch_test.go
@@ -0,0 +1,71 @@
+package app
+
+import (
+	"encoding/json"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+// geometryPatchFixture 造一对"只差位置"的重节点：正文很大，几何变化很小。
+func geometryPatchFixture() (map[string]any, map[string]any) {
+	heavy := strings.Repeat("分镜行正文", 2000)
+	node := func(x, y float64, rows []any) map[string]any {
+		return map[string]any{
+			"id": "sb-1", "type": "script", "title": "第一场",
+			"position": map[string]any{"x": x, "y": y}, "width": 920.0, "height": 360.0,
+			"metadata": map[string]any{"storyboard": map[string]any{"rows": rows}, "status": "idle"},
+		}
+	}
+	return node(0, 0, []any{heavy}), node(120, 480, []any{heavy})
+}
+
+// 位置/尺寸变化只发几何增量：一次整理几十个分镜节点时，完整 before/after 会让单条
+// canvas_updated 事件超过 validateCloudAgentRuntime 的 128KiB 单事件上限，整轮直接终止。
+func TestCloudAgentGeometryOnlyChangeShrinksPatch(t *testing.T) {
+	before, after := geometryPatchFixture()
+	shrunkBefore, shrunkAfter := cloudAgentShrinkChange(before, after)
+	raw, _ := json.Marshal([]map[string]any{{"before": shrunkBefore, "after": shrunkAfter}})
+	if len(raw) > 512 {
+		t.Fatalf("几何增量仍然过大（%d 字节）：%s", len(raw), raw)
+	}
+	if shrunkBefore["id"] != "sb-1" || shrunkAfter["id"] != "sb-1" {
+		t.Fatalf("增量必须带 id：%+v %+v", shrunkBefore, shrunkAfter)
+	}
+	if _, ok := shrunkAfter["position"]; !ok {
+		t.Fatalf("几何增量缺少 position：%+v", shrunkAfter)
+	}
+	if _, ok := shrunkAfter["metadata"]; ok {
+		t.Fatalf("几何增量不应携带正文字段：%+v", shrunkAfter)
+	}
+
+	// 新增节点（before 为空）没有可比对的旧值，不缩。
+	if _, kept := cloudAgentShrinkChange(nil, after); kept["metadata"] == nil {
+		t.Fatalf("新增节点不应被缩减：%+v", kept)
+	}
+}
+
+// 内容变化（正文 / metadata）仍发完整 before/after：前端按字段三方合并，缩成几何增量会
+// 丢掉内容变化，也会影响前端对生成任务状态的保护逻辑。
+func TestCloudAgentContentChangeKeepsFullPatch(t *testing.T) {
+	before, after := geometryPatchFixture()
+	contentAfter := map[string]any{
+		"id": "sb-1", "type": "script", "title": "第一场",
+		"position": after["position"], "width": 920.0, "height": 360.0,
+		"metadata": map[string]any{"storyboard": map[string]any{"rows": []any{"改过的行"}}, "status": "idle"},
+	}
+	keptBefore, keptAfter := cloudAgentShrinkChange(before, contentAfter)
+	if _, ok := keptAfter["metadata"]; !ok {
+		t.Fatalf("内容变化被错误地缩成几何增量：%+v", keptAfter)
+	}
+	if keptBefore["id"] != before["id"] || keptBefore["metadata"] == nil {
+		t.Fatalf("内容变化应保留完整 before：%+v", keptBefore)
+	}
+
+	// 键被删掉时同样不缩：只发几何字段会让前端无法区分"消失的字段"与"保持本地值"。
+	keyRemoved := map[string]any{"id": "sb-1", "type": "script", "title": "第一场", "position": after["position"]}
+	deletedBefore, deletedAfter := cloudAgentShrinkChange(before, keyRemoved)
+	if !reflect.DeepEqual(deletedBefore, before) || !reflect.DeepEqual(deletedAfter, keyRemoved) {
+		t.Fatalf("缺键的变更不应被缩成几何增量：%+v %+v", deletedBefore, deletedAfter)
+	}
+}