diff --git a/changelog/1152.fixed.md b/changelog/1152.fixed.md
new file mode 100644
index 000000000..39ec8da21
--- /dev/null
+++ b/changelog/1152.fixed.md
@@ -0,0 +1 @@
+Assigning to a cardinality-many relationship (e.g. `node.members = [peer]`) now populates the relationship manager, and assigning a non-list value raises a clear error at assignment time instead of being silently accepted and failing later in `save()` with a confusing `'InfrahubNode' object has no attribute 'initialized'` message.
diff --git a/infrahub_sdk/node/node.py b/infrahub_sdk/node/node.py
index 9bdaa20cc..713066b84 100644
--- a/infrahub_sdk/node/node.py
+++ b/infrahub_sdk/node/node.py
@@ -978,6 +978,29 @@ def __setattr__(self, name: str, value: Any) -> None:
             self._relationship_cardinality_one_data[name] = new_rel
             return
 
+        if "_relationship_cardinality_many_data" in self.__dict__ and name in self._relationship_cardinality_many_data:
+            rel_schemas = [rel_schema for rel_schema in self._schema.relationships if rel_schema.name == name]
+            if not rel_schemas:
+                raise SchemaNotFoundError(
+                    identifier=self._schema.kind,
+                    message=f"Unable to find relationship schema for '{name}' on {self._schema.kind}",
+                )
+            rel_schema = rel_schemas[0]
+            # RelationshipManager validates that a cardinality-many value is a list and raises a
+            # helpful error otherwise, so assigning a single node fails here instead of silently
+            # corrupting the node and blowing up later in save().
+            new_many_rel = RelationshipManager(
+                name=rel_schema.name,
+                client=self._client,
+                node=self,
+                branch=self._branch,
+                schema=rel_schema,
+                data=value,
+            )
+            new_many_rel._has_update = True
+            self._relationship_cardinality_many_data[name] = new_many_rel
+            return
+
         super().__setattr__(name, value)
 
     async def generate(self, nodes: list[str] | None = None) -> None:
@@ -2169,6 +2192,29 @@ def __setattr__(self, name: str, value: Any) -> None:
             self._relationship_cardinality_one_data[name] = new_rel
             return
 
+        if "_relationship_cardinality_many_data" in self.__dict__ and name in self._relationship_cardinality_many_data:
+            rel_schemas = [rel_schema for rel_schema in self._schema.relationships if rel_schema.name == name]
+            if not rel_schemas:
+                raise SchemaNotFoundError(
+                    identifier=self._schema.kind,
+                    message=f"Unable to find relationship schema for '{name}' on {self._schema.kind}",
+                )
+            rel_schema = rel_schemas[0]
+            # RelationshipManagerSync validates that a cardinality-many value is a list and raises a
+            # helpful error otherwise, so assigning a single node fails here instead of silently
+            # corrupting the node and blowing up later in save().
+            new_many_rel = RelationshipManagerSync(
+                name=rel_schema.name,
+                client=self._client,
+                node=self,
+                branch=self._branch,
+                schema=rel_schema,
+                data=value,
+            )
+            new_many_rel._has_update = True
+            self._relationship_cardinality_many_data[name] = new_many_rel
+            return
+
         super().__setattr__(name, value)
 
     def generate(self, nodes: list[str] | None = None) -> None:
diff --git a/tests/unit/sdk/test_node.py b/tests/unit/sdk/test_node.py
index ab0d72ec0..8a4954ae5 100644
--- a/tests/unit/sdk/test_node.py
+++ b/tests/unit/sdk/test_node.py
@@ -342,6 +342,89 @@ async def test_cardinality_many_accepts_list(
     assert len(node.tags.peers) == 2
 
 
+@pytest.mark.parametrize("client_type", client_types)
+async def test_cardinality_many_assignment_requires_list(
+    client: InfrahubClient, location_schema: NodeSchemaAPI, client_type: str
+) -> None:
+    """Assigning a single node to a cardinality-many relationship must fail at assignment time.
+
+    Previously this was silently accepted and only blew up later in save() with a confusing
+    "'InfrahubNode' object has no attribute 'initialized'" error.
+    """
+    if client_type == "standard":
+        node = InfrahubNode(client=client, schema=location_schema, data={"name": {"value": "JFK1"}})
+    else:
+        node = InfrahubNodeSync(client=client, schema=location_schema, data={"name": {"value": "JFK1"}})
+
+    with pytest.raises(ValueError, match=r"expects a list of nodes"):
+        node.tags = {"id": "pppppppp"}
+
+
+@pytest.mark.parametrize("client_type", client_types)
+async def test_cardinality_many_assignment_accepts_list(
+    client: InfrahubClient, location_schema: NodeSchemaAPI, client_type: str
+) -> None:
+    """Assigning a list to a cardinality-many relationship populates the manager and marks it updated."""
+    if client_type == "standard":
+        node = InfrahubNode(client=client, schema=location_schema, data={"name": {"value": "JFK1"}})
+    else:
+        node = InfrahubNodeSync(client=client, schema=location_schema, data={"name": {"value": "JFK1"}})
+
+    node.tags = [{"id": "aaaaaa"}, {"id": "bbbb"}]
+
+    assert isinstance(node.tags, RelationshipManagerBase)
+    assert node.tags.peer_ids == ["aaaaaa", "bbbb"]
+    # has_update must be set so save() actually persists the assignment instead of stripping it.
+    assert node.tags.has_update is True
+
+
+@pytest.mark.parametrize("client_type", client_types)
+async def test_cardinality_many_assignment_included_in_input_data(
+    client: InfrahubClient, location_schema: NodeSchemaAPI, client_type: str
+) -> None:
+    """A list assigned after construction is carried into the mutation payload built by save().
+
+    This is the exact path that used to raise 'InfrahubNode' object has no attribute 'initialized',
+    and it also guards that _strip_unmodified keeps the assignment (thanks to has_update).
+    """
+    data = {"name": {"value": "JFK1"}, "type": {"value": "SITE"}}
+    if client_type == "standard":
+        node = InfrahubNode(client=client, schema=location_schema, data=data)
+    else:
+        node = InfrahubNodeSync(client=client, schema=location_schema, data=data)
+
+    node.tags = [{"id": "aaaaaa"}, {"id": "bbbb"}]
+
+    input_data = node._generate_input_data()["data"]["data"]
+    assert input_data["tags"] == [{"id": "aaaaaa"}, {"id": "bbbb"}]
+
+
+@pytest.mark.parametrize("client_type", client_types)
+async def test_cardinality_many_assignment_saved(
+    httpx_mock: HTTPXMock, clients: BothClients, location_schema: NodeSchemaAPI, client_type: str
+) -> None:
+    """Assigning a list then calling save() succeeds and sends the peers in the create mutation."""
+    httpx_mock.add_response(
+        method="POST",
+        json={"data": {"BuiltinLocationCreate": {"ok": True, "object": {"id": "location-123"}}}},
+    )
+    data = {"name": {"value": "JFK1"}, "type": {"value": "SITE"}}
+    client = getattr(clients, client_type)
+    if client_type == "standard":
+        node = InfrahubNode(client=client, schema=location_schema, data=data)
+        node.tags = [{"id": "aaaaaa"}, {"id": "bbbb"}]
+        await node.save()
+    else:
+        node = InfrahubNodeSync(client=client, schema=location_schema, data=data)
+        node.tags = [{"id": "aaaaaa"}, {"id": "bbbb"}]
+        node.save()
+
+    assert node.id == "location-123"
+    body = httpx_mock.get_requests()[-1].content.decode()
+    assert "aaaaaa" in body
+    assert "bbbb" in body
+
+
 @pytest.mark.parametrize("client_type", client_types)
 async def test_query_data_no_filters_property(
     clients: BothClients, location_schema: NodeSchemaAPI, client_type: str