diff --git a/src/App.tsx b/src/App.tsx
index f5a5739..74d6ab6 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -68,6 +68,7 @@ export default function App() {
   const [edges, setEdges, onEdgesChange] = useEdgesState(initialEdges)
   const [rfInstance, setRfInstance] = React.useState<any | null>(null)
   const [renamingNodeId, setRenamingNodeId] = React.useState<string | null>(null)
+  const [connectionFrom, setConnectionFrom] = React.useState<{ nodeId: string; sourceHandle?: string } | null>(null)
 
   const wrapperRef = React.useRef<HTMLDivElement | null>(null)
   const lastClickRef = React.useRef<{ clientX: number; clientY: number } | null>(null)
@@ -85,6 +86,37 @@ export default function App() {
     [setEdges]
   )
 
+  const isValidConnection = React.useCallback(
+    (connection: Connection) => {
+      const sourceNode = nodes.find((n) => n.id === connection.source)
+      const targetNode = nodes.find((n) => n.id === connection.target)
+      const sourceType = sourceNode?.type
+      const targetType = targetNode?.type
+      if (!sourceType || !targetType) return false
+      if (connection.source === connection.target) return false
+      if (targetType === 'variable') return false
+      if (targetType === 'render' && (sourceType === 'variable' || sourceType === 'function')) return false
+      if (sourceType === 'render') return false
+      return true
+    },
+    [nodes]
+  )
+
+  const onConnectStart = React.useCallback(
+    (_: React.MouseEvent | React.TouchEvent, params: { nodeId?: string | null; handleId?: string | null; handleType?: string | null }) => {
+      if (params.handleType !== 'source' || !params.nodeId) {
+        setConnectionFrom(null)
+        return
+      }
+      setConnectionFrom({ nodeId: params.nodeId, sourceHandle: params.handleId ?? undefined })
+    },
+    []
+  )
+
+  const onConnectEnd = React.useCallback(() => {
+    setConnectionFrom(null)
+  }, [])
+
   const onInit = React.useCallback((instance: any) => {
     setRfInstance(instance)
   }, [])
@@ -204,7 +236,7 @@ export default function App() {
       >
         {theme === 'dark' ? <Sun className="h-4 w-4" /> : <Moon className="h-4 w-4" />}
       </Button>
-      <FlowContext.Provider value={{ nodes, setNodes, edges, setEdges, renamingNodeId, setRenamingNodeId }}>
+      <FlowContext.Provider value={{ nodes, setNodes, edges, setEdges, renamingNodeId, setRenamingNodeId, connectionFrom, setConnectionFrom, isValidConnection }}>
         <ContextMenu>
           <ContextMenuTrigger asChild>
             <div style={{ width: '100%', height: '100%' }}>
@@ -215,6 +247,9 @@ export default function App() {
                   onNodesChange={onNodesChange}
                   onEdgesChange={onEdgesChange}
                   onConnect={onConnect}
+                  onConnectStart={onConnectStart}
+                  onConnectEnd={onConnectEnd}
+                  isValidConnection={isValidConnection}
                   nodeTypes={nodeTypes}
                   edgeTypes={edgeTypes}
                   defaultEdgeOptions={{ type: 'animated' }}
diff --git a/src/components/graph/ConfigNode.tsx b/src/components/graph/ConfigNode.tsx
index 9dee7f5..321473c 100644
--- a/src/components/graph/ConfigNode.tsx
+++ b/src/components/graph/ConfigNode.tsx
@@ -161,7 +161,7 @@ export const ConfigNode = memo(function ConfigNode({ id, data, width, height }:
             : undefined
 
     return (
-        <BaseNode className="min-w-80 min-h-[280px]" dimensions={dimensions} resizable nodeId={id} handles={<><InputHandle id="ain" /><OutputHandle id="out" /></>}>
+        <BaseNode className="min-w-80 min-h-[280px]" dimensions={dimensions} resizable nodeId={id} handles={<><InputHandle id="ain" nodeId={id} /><OutputHandle id="out" /></>}>
             <BaseNodeHeaderRow icon={<ScrollText className="size-4" />} title={<NodeHeaderTitle nodeId={id} displayTitle={`${id}`} />} />
 
             <BaseNodeContent>
diff --git a/src/components/graph/FunctionNode.tsx b/src/components/graph/FunctionNode.tsx
index ff0cc60..f3f57b7 100644
--- a/src/components/graph/FunctionNode.tsx
+++ b/src/components/graph/FunctionNode.tsx
@@ -48,7 +48,7 @@ export const FunctionNode = memo(function FunctionNode({ id, data, width, height
             : undefined
 
     return (
-        <BaseNode className="min-w-72 min-h-[260px]" dimensions={dimensions} resizable nodeId={id} handles={<><InputHandle id="in" /><OutputHandle id="out" /></>}>
+        <BaseNode className="min-w-72 min-h-[260px]" dimensions={dimensions} resizable nodeId={id} handles={<><InputHandle id="in" nodeId={id} /><OutputHandle id="out" /></>}>
             <BaseNodeHeaderRow icon={<Code2 className="size-4" />} title={<NodeHeaderTitle nodeId={id} displayTitle={id} />} />
 
             <BaseNodeContent>
diff --git a/src/components/graph/NodeHandles.tsx b/src/components/graph/NodeHandles.tsx
index 24b5df2..ebf2a60 100644
--- a/src/components/graph/NodeHandles.tsx
+++ b/src/components/graph/NodeHandles.tsx
@@ -1,17 +1,35 @@
-import React from 'react'
+import React, { useContext } from 'react'
 import { Handle, Position } from '@xyflow/react'
 import { ArrowDownLeft, ArrowUpRight } from 'lucide-react'
+import FlowContext from '@/lib/flowContext'
+import { cn } from '@/lib/utils'
 
 type NodeHandleProps = {
     id: string
+    /** Pass when this handle is a connection target so valid highlight can show as soon as connection starts */
+    nodeId?: string
 }
 
-export function InputHandle({ id }: NodeHandleProps) {
+export function InputHandle({ id, nodeId }: NodeHandleProps) {
+    const ctx = useContext(FlowContext)
+    const connectionFrom = ctx?.connectionFrom ?? null
+    const isValidConnection = ctx?.isValidConnection
+    const isConnecting = Boolean(nodeId && connectionFrom && connectionFrom.nodeId !== nodeId)
+    const isValidTarget =
+        isConnecting &&
+        isValidConnection?.({
+            source: connectionFrom!.nodeId,
+            sourceHandle: connectionFrom!.sourceHandle ?? undefined,
+            target: nodeId!,
+            targetHandle: id,
+        })
+
     return (
         <Handle
             type="target"
             position={Position.Left}
             id={id}
+            className={cn(isValidTarget && 'connection-valid-target', isConnecting && !isValidTarget && 'connection-invalid-target')}
             style={{
                 top: 20,
                 width: 20,
diff --git a/src/components/graph/RenderingNode.tsx b/src/components/graph/RenderingNode.tsx
index 56c0ea3..393949f 100644
--- a/src/components/graph/RenderingNode.tsx
+++ b/src/components/graph/RenderingNode.tsx
@@ -16,7 +16,7 @@ import { NodeMenubar } from './NodeMenubar'
 import { NodeStatusIndicator } from './NodeStatusIndicator'
 import { MenubarItem, MenubarSub, MenubarSubContent, MenubarSubTrigger } from '../ui/menubar'
 import { Sparkles } from 'lucide-react'
-import { InputHandle, OutputHandle } from './NodeHandles'
+import { InputHandle } from './NodeHandles'
 
 // Use relative URL so Vite dev proxy (and optional prod proxy) avoids CORS
 const KROKI_PLANTUML_SVG = '/api/kroki/plantuml/svg'
@@ -414,7 +414,7 @@ export const RenderingNode = memo(function RenderingNode({ id, width, height }:
 
     return (
         <NodeStatusIndicator status={status} variant="border" width={dimensions?.width} height={dimensions?.height}>
-        <BaseNode className="min-w-96 min-h-[320px]" dimensions={dimensions} resizable nodeId={id} handles={<><InputHandle id="ain" /><OutputHandle id="out" /></>}>
+        <BaseNode className="min-w-96 min-h-[320px]" dimensions={dimensions} resizable nodeId={id} handles={<InputHandle id="ain" nodeId={id} />}>
             <BaseNodeHeaderRow icon={<Sparkles className="size-4" />} title={<NodeHeaderTitle nodeId={id} displayTitle={id} />} />
 
             <BaseNodeContent>
diff --git a/src/lib/flowContext.tsx b/src/lib/flowContext.tsx
index abd751f..9b5b0ee 100644
--- a/src/lib/flowContext.tsx
+++ b/src/lib/flowContext.tsx
@@ -1,4 +1,7 @@
 import React from 'react'
+import type { Connection } from '@xyflow/react'
+
+export type ConnectionFrom = { nodeId: string; sourceHandle?: string } | null
 
 export type FlowContextValue = {
     nodes: any[]
@@ -7,6 +10,10 @@ export type FlowContextValue = {
     setEdges: (updater: any) => void
     renamingNodeId: string | null
     setRenamingNodeId: (id: string | null) => void
+    /** Set when user starts dragging from an output handle; cleared on connect end. Used to highlight valid targets. */
+    connectionFrom: ConnectionFrom
+    setConnectionFrom: (v: ConnectionFrom) => void
+    isValidConnection: (connection: Connection) => boolean
 }
 
 const FlowContext = React.createContext<FlowContextValue | null>(null)
diff --git a/src/styles.css b/src/styles.css
index bcdda4e..a9275b8 100644
--- a/src/styles.css
+++ b/src/styles.css
@@ -33,6 +33,29 @@ body {
     box-shadow: 0 0 0 2px rgba(255, 255, 255, 0.15);
 }
 
+/* Connection validation: valid targets clearly highlighted (from drag start and when hovering), invalid dimmed while connecting */
+.react-flow__handle.react-flow__handle-connecting.react-flow__handle-valid,
+.react-flow__handle.connection-valid-target {
+    background: hsl(var(--primary)) !important;
+    color: hsl(var(--primary-foreground));
+    box-shadow: 0 0 0 3px hsl(var(--primary)), 0 0 12px 2px hsl(var(--primary) / 0.5) !important;
+    opacity: 1 !important;
+}
+.react-flow__handle.react-flow__handle-connecting.react-flow__handle-valid svg,
+.react-flow__handle.connection-valid-target svg {
+    color: inherit;
+}
+.dark .react-flow__handle.react-flow__handle-connecting.react-flow__handle-valid,
+.dark .react-flow__handle.connection-valid-target {
+    background: hsl(var(--primary)) !important;
+    box-shadow: 0 0 0 3px hsl(var(--primary)), 0 0 14px 4px hsl(var(--primary) / 0.6) !important;
+}
+.react-flow__handle.react-flow__handle-connecting:not(.react-flow__handle-valid):not(.connection-valid-target),
+.react-flow__handle.connection-invalid-target {
+    opacity: 0.25 !important;
+    pointer-events: none;
+}
+
 /* Animated React Flow edges: thicker stroke + path animation */
 .react-flow__edge path.animated-edge-path,
 .animated-edge-path {