diff --git a/execution.py b/execution.py
index 7ccdbf93e..a47d93dca 100644
--- a/execution.py
+++ b/execution.py
@@ -418,11 +418,12 @@ async def execute(server, dynprompt, caches, current_item, extra_data, executed,
     inputs = dynprompt.get_node(unique_id)['inputs']
     class_type = dynprompt.get_node(unique_id)['class_type']
     class_def = nodes.NODE_CLASS_MAPPINGS[class_type]
+    merge = inputs.get('accumulate') is True
     cached = caches.outputs.get(unique_id)
     if cached is not None:
         if server.client_id is not None:
             cached_ui = cached.ui or {}
-            server.send_sync("executed", { "node": unique_id, "display_node": display_node_id, "output": cached_ui.get("output",None), "prompt_id": prompt_id }, server.client_id)
+            server.send_sync("executed", { "node": unique_id, "display_node": display_node_id, "output": cached_ui.get("output",None), "prompt_id": prompt_id, "merge": merge }, server.client_id)
             if cached.ui is not None:
                 ui_outputs[unique_id] = cached.ui
         get_progress_state().finish_progress(unique_id)
@@ -549,7 +550,7 @@ async def execute(server, dynprompt, caches, current_item, extra_data, executed,
                 "output": output_ui
             }
             if server.client_id is not None:
-                server.send_sync("executed", { "node": unique_id, "display_node": display_node_id, "output": output_ui, "prompt_id": prompt_id }, server.client_id)
+                server.send_sync("executed", { "node": unique_id, "display_node": display_node_id, "output": output_ui, "prompt_id": prompt_id, "merge": merge }, server.client_id)
         if has_subgraph:
             cached_outputs = []
             new_node_ids = []
diff --git a/nodes.py b/nodes.py
index 5be9b16f9..fe9d23fe0 100644
--- a/nodes.py
+++ b/nodes.py
@@ -1640,6 +1640,9 @@ class SaveImage:
                 "images": ("IMAGE", {"tooltip": "The images to save."}),
                 "filename_prefix": ("STRING", {"default": "ComfyUI", "tooltip": "The prefix for the file to save. This may include formatting information such as %date:yyyy-MM-dd% or %Empty Latent Image.width% to include values from nodes."})
             },
+            "optional": {
+                "accumulate": ("BOOLEAN", {"default": False, "tooltip": "When enabled, outputs accumulate into a growing gallery across queue runs instead of being replaced.", "advanced": True}),
+            },
             "hidden": {
                 "prompt": "PROMPT", "extra_pnginfo": "EXTRA_PNGINFO"
             },
@@ -1655,7 +1658,7 @@ class SaveImage:
     DESCRIPTION = "Saves the input images to your ComfyUI output directory."
     SEARCH_ALIASES = ["save", "save image", "export image", "output image", "write image", "download"]
 
-    def save_images(self, images, filename_prefix="ComfyUI", prompt=None, extra_pnginfo=None):
+    def save_images(self, images, filename_prefix="ComfyUI", prompt=None, extra_pnginfo=None, accumulate=False):
         filename_prefix += self.prefix_append
         full_output_folder, filename, counter, subfolder, filename_prefix = folder_paths.get_save_image_path(filename_prefix, self.output_dir, images[0].shape[1], images[0].shape[0])
         results = list()
@@ -1696,6 +1699,9 @@ class PreviewImage(SaveImage):
     def INPUT_TYPES(s):
         return {"required":
                     {"images": ("IMAGE", ), },
+                "optional": {
+                    "accumulate": ("BOOLEAN", {"default": False, "tooltip": "When enabled, outputs accumulate into a growing gallery across queue runs instead of being replaced.", "advanced": True}),
+                },
                 "hidden": {"prompt": "PROMPT", "extra_pnginfo": "EXTRA_PNGINFO"},
                 }
 
diff --git a/tests-unit/execution_test/test_accumulate_merge.py b/tests-unit/execution_test/test_accumulate_merge.py
new file mode 100644
index 000000000..00910a2cd
--- /dev/null
+++ b/tests-unit/execution_test/test_accumulate_merge.py
@@ -0,0 +1,89 @@
+"""
+Unit tests for the accumulate toggle on SaveImage and PreviewImage nodes.
+
+Tests that the accumulate input is correctly defined and that the merge flag
+derivation logic in execution.py works for all input shapes.
+"""
+import inspect
+
+import pytest
+
+import nodes
+
+
+class TestSaveImageAccumulateInput:
+    """Test SaveImage node definition includes accumulate input."""
+
+    def test_accumulate_in_optional_inputs(self):
+        input_types = nodes.SaveImage.INPUT_TYPES()
+        assert "optional" in input_types
+        assert "accumulate" in input_types["optional"]
+
+    def test_accumulate_is_boolean_type(self):
+        input_types = nodes.SaveImage.INPUT_TYPES()
+        accumulate_def = input_types["optional"]["accumulate"]
+        assert accumulate_def[0] == "BOOLEAN"
+
+    def test_accumulate_defaults_to_false(self):
+        input_types = nodes.SaveImage.INPUT_TYPES()
+        accumulate_def = input_types["optional"]["accumulate"]
+        assert accumulate_def[1]["default"] is False
+
+    def test_accumulate_is_advanced(self):
+        input_types = nodes.SaveImage.INPUT_TYPES()
+        accumulate_def = input_types["optional"]["accumulate"]
+        assert accumulate_def[1].get("advanced") is True
+
+    def test_save_images_accepts_accumulate_parameter(self):
+        sig = inspect.signature(nodes.SaveImage.save_images)
+        assert "accumulate" in sig.parameters
+        assert sig.parameters["accumulate"].default is False
+
+
+class TestPreviewImageAccumulateInput:
+    """Test PreviewImage node definition includes accumulate input."""
+
+    def test_accumulate_in_optional_inputs(self):
+        input_types = nodes.PreviewImage.INPUT_TYPES()
+        assert "optional" in input_types
+        assert "accumulate" in input_types["optional"]
+
+    def test_accumulate_is_boolean_type(self):
+        input_types = nodes.PreviewImage.INPUT_TYPES()
+        accumulate_def = input_types["optional"]["accumulate"]
+        assert accumulate_def[0] == "BOOLEAN"
+
+    def test_accumulate_defaults_to_false(self):
+        input_types = nodes.PreviewImage.INPUT_TYPES()
+        accumulate_def = input_types["optional"]["accumulate"]
+        assert accumulate_def[1]["default"] is False
+
+
+class TestAccumulateMergeFlagDerivation:
+    """Test the merge flag logic used in execution.py.
+
+    In execution.py, the merge flag is derived as:
+        merge = inputs.get('accumulate') is True
+
+    This must return True only for literal True, not for truthy values
+    like lists (which represent node links in the prompt).
+    """
+
+    @pytest.mark.parametrize(
+        "inputs,expected",
+        [
+            ({"accumulate": True}, True),
+            ({"accumulate": False}, False),
+            ({}, False),
+            ({"accumulate": None}, False),
+            # Node link: accumulate connected to another node's output
+            ({"accumulate": ["other_node_id", 0]}, False),
+            # String "true" should not match
+            ({"accumulate": "true"}, False),
+            # Integer 1 should not match
+            ({"accumulate": 1}, False),
+        ],
+    )
+    def test_merge_flag(self, inputs, expected):
+        merge = inputs.get("accumulate") is True
+        assert merge is expected