diff --git a/app/services/namespaces/projects/assign_runtimes_service.rb b/app/services/namespaces/projects/assign_runtimes_service.rb
index 57697e90..d4dcf0ff 100644
--- a/app/services/namespaces/projects/assign_runtimes_service.rb
+++ b/app/services/namespaces/projects/assign_runtimes_service.rb
@@ -24,7 +24,7 @@ def execute
           end
 
           namespace_project.runtimes = runtimes
-          namespace_project.primary_runtime = runtimes.first if runtimes.size == 1
+          namespace_project.primary_runtime = primary_runtime_after_assignment
 
           unless namespace_project.save
             t.rollback_and_return! ServiceResponse.error(
@@ -50,6 +50,15 @@ def execute
           ServiceResponse.success(message: 'Assigned runtimes to a project', payload: namespace_project)
         end
       end
+
+      private
+
+      def primary_runtime_after_assignment
+        return if runtimes.empty?
+        return namespace_project.primary_runtime if runtimes.include?(namespace_project.primary_runtime)
+
+        runtimes.first
+      end
     end
   end
 end
diff --git a/app/services/velorum/generation_flow_serializer.rb b/app/services/velorum/generation_flow_serializer.rb
index 7ccab44b..0196f45c 100644
--- a/app/services/velorum/generation_flow_serializer.rb
+++ b/app/services/velorum/generation_flow_serializer.rb
@@ -34,7 +34,7 @@ def to_h
       {
         name: flow.name,
         type: flow_type,
-        starting_node_id: node_reference_id(flow.starting_node_id) || generated_starting_node_id,
+        starting_node_id: starting_node_id_for,
         settings: flow.settings.map.with_index { |setting, index| flow_setting_to_h(setting, index, flow_type) },
         nodes: serialized_nodes,
       }
@@ -92,7 +92,7 @@ def node_to_h(node, index)
       {
         id: generated_node_ids.fetch(node),
         function_definition: function_definition,
-        next_node_id: node_reference_id(node.next_node_id) || generated_next_node_id(index),
+        next_node_id: next_node_id_for(node, index),
         parameters: node.parameters.map.with_index do |parameter, parameter_index|
           parameter_to_h(parameter, parameter_index, function_definition)
         end,
@@ -224,19 +224,23 @@ def flow_setting_to_h(setting, index, flow_type)
 
     def node_reference_id(value)
       value = blank_zero(value)
-      return if value.blank?
+      return if value.blank? || value.to_s == 'None'
 
       node_id_by_source_id.fetch(value.to_s, value)
     end
 
-    def generated_starting_node_id
-      generated_node_ids.values.first
+    def starting_node_id_for
+      node_reference_id(flow.starting_node_id)
     end
 
     def generated_next_node_id(index)
       flow.node_functions[index + 1]&.then { |next_node| generated_node_ids.fetch(next_node) }
     end
 
+    def next_node_id_for(node, index)
+      node_reference_id(node.next_node_id) || (node.has_next_node_id? ? generated_next_node_id(index) : nil)
+    end
+
     def blank_zero(value)
       return if value.blank? || value.to_s == '0'
 
diff --git a/spec/services/namespaces/projects/assign_runtimes_service_spec.rb b/spec/services/namespaces/projects/assign_runtimes_service_spec.rb
index 0ec0b78d..3a53e96e 100644
--- a/spec/services/namespaces/projects/assign_runtimes_service_spec.rb
+++ b/spec/services/namespaces/projects/assign_runtimes_service_spec.rb
@@ -73,7 +73,18 @@
       context 'when adding multiple runtimes' do
         let(:runtimes) { 2.times.map { create(:runtime, namespace: project.namespace) } }
 
-        it { expect { service_response }.not_to change { project.reload.primary_runtime } }
+        it { expect { service_response }.to change { project.reload.primary_runtime }.to(runtimes.first) }
+
+        context 'when the current primary runtime remains assigned' do
+          let(:primary_runtime) { create(:runtime, namespace: project.namespace) }
+          let(:runtimes) { [create(:runtime, namespace: project.namespace), primary_runtime] }
+
+          before do
+            project.update!(primary_runtime: primary_runtime)
+          end
+
+          it { expect { service_response }.not_to change { project.reload.primary_runtime } }
+        end
       end
     end
 
@@ -85,6 +96,7 @@
       let(:runtimes) { [] }
 
       before do
+        project.update!(primary_runtime: runtime)
         stub_allowed_ability(NamespaceProjectPolicy, :assign_project_runtimes, user: current_user, subject: project)
       end
 
@@ -116,6 +128,8 @@
           { namespace_project_id: project.id }
         )
       end
+
+      it { expect { service_response }.to change { project.reload.primary_runtime }.to(nil) }
     end
 
     context 'when adding and removing a runtime' do
diff --git a/spec/services/velorum/generation_flow_serializer_spec.rb b/spec/services/velorum/generation_flow_serializer_spec.rb
index ec6e603c..ce7addb3 100644
--- a/spec/services/velorum/generation_flow_serializer_spec.rb
+++ b/spec/services/velorum/generation_flow_serializer_spec.rb
@@ -61,7 +61,7 @@
     expect(described_class.new(flow, project: project).to_h).to include(
       name: 'Generated flow',
       type: flow_type,
-      starting_node_id: 'generated-1',
+      starting_node_id: nil,
       settings: [
         a_hash_including(
           id: 1,
@@ -109,13 +109,14 @@
     )
   end
 
-  it 'maps generated node IDs into references and inferred next-node links' do
+  it 'maps generated node IDs into references and explicit fallback next-node links' do
     flow = Tucana::Shared::GenerationFlow.new(
       name: 'Generated flow',
       type: 'default',
       node_functions: [
         Tucana::Shared::NodeFunction.new(
           runtime_function_id: 'input',
+          next_node_id: 0,
           parameters: []
         ),
         Tucana::Shared::NodeFunction.new(
@@ -262,4 +263,73 @@
       function_definition: function_definition
     )
   end
+
+  it 'does not infer a next-node link when next_node_id is absent' do
+    flow = Tucana::Shared::GenerationFlow.new(
+      name: 'Generated flow',
+      type: 'default',
+      node_functions: [
+        Tucana::Shared::NodeFunction.new(
+          database_id: 6,
+          runtime_function_id: 'rest::control::respond'
+        ),
+        Tucana::Shared::NodeFunction.new(
+          database_id: 7,
+          runtime_function_id: 'std::control::value',
+          next_node_id: 6
+        )
+      ],
+      starting_node_id: '7'
+    )
+
+    serialized = described_class.new(flow).to_h
+
+    expect(serialized).to include(starting_node_id: '7')
+    expect(serialized[:nodes][0]).to include(id: '6', next_node_id: nil)
+    expect(serialized[:nodes][1]).to include(id: '7', next_node_id: '6')
+  end
+
+  it 'does not infer a starting node when starting_node_id is absent' do
+    flow_type = create(:flow_type, runtime: runtime, identifier: 'REST')
+    create(:flow_type_setting, flow_type: flow_type)
+    create(:flow_type_setting, flow_type: flow_type)
+    create(:flow_type_setting, flow_type: flow_type)
+    flow = Tucana::Shared::GenerationFlow.new(
+      name: 'Hello World Flow',
+      type: 'REST',
+      node_functions: [],
+      settings: [
+        Tucana::Shared::FlowSetting.new(value: Tucana::Shared::Value.from_ruby({})),
+        Tucana::Shared::FlowSetting.new(value: Tucana::Shared::Value.from_ruby('/hello')),
+        Tucana::Shared::FlowSetting.new(value: Tucana::Shared::Value.from_ruby('GET'))
+      ]
+    )
+
+    serialized = described_class.new(flow, project: project).to_h
+
+    expect(serialized).to include(
+      name: 'Hello World Flow',
+      type: flow_type,
+      starting_node_id: nil,
+      nodes: []
+    )
+  end
+
+  it 'does not serialize a None starting node reference' do
+    flow_type = create(:flow_type, runtime: runtime, identifier: 'REST')
+    flow = Tucana::Shared::GenerationFlow.new(
+      name: 'Hello World Flow',
+      type: 'REST',
+      node_functions: [],
+      starting_node_id: 'None'
+    )
+
+    serialized = described_class.new(flow, project: project).to_h
+
+    expect(serialized).to include(
+      type: flow_type,
+      starting_node_id: nil,
+      nodes: []
+    )
+  end
 end