diff --git a/frontend/src/lib/components/copilot/MetadataGen.svelte b/frontend/src/lib/components/copilot/MetadataGen.svelte
index bb18b3dc1e..1ef4b2ff08 100644
--- a/frontend/src/lib/components/copilot/MetadataGen.svelte
+++ b/frontend/src/lib/components/copilot/MetadataGen.svelte
@@ -12,7 +12,7 @@
 	import { yamlStringifyExceptKeys } from './utils'
 	import type { ChatCompletionMessageParam } from 'openai/resources/index.mjs'
 	import { triggerableByAI } from '$lib/actions/triggerableByAI.svelte'
-	import { validateToolName } from '$lib/components/graph/renderers/nodes/AIToolNode.svelte'
+	import { getToolNameError } from '$lib/components/graph/renderers/nodes/AIToolNode.svelte'
 	import {
 		inputBaseClass,
 		inputBorderClass,
@@ -117,6 +117,7 @@ Generate a tool name for the script below:
 		elementProps?: Record<string, any>
 		class?: string
 		onChange?: (content: string) => void
+		siblingToolNames?: string[]
 	}
 
 	let {
@@ -130,9 +131,16 @@ Generate a tool name for the script below:
 		elementType = 'input',
 		elementProps = {},
 		class: clazz = '',
-		onChange = undefined
+		onChange = undefined,
+		siblingToolNames = undefined
 	}: Props = $props()
 
+	let toolNameError = $derived(
+		promptConfigName === 'agentToolFunctionName'
+			? getToolNameError(content ?? '', undefined, siblingToolNames)
+			: undefined
+	)
+
 	let el: HTMLElement | undefined = $state()
 	let generatedContent = $state('')
 	let active = $state(false)
@@ -347,16 +355,16 @@ Generate a tool name for the script below:
 				inputBaseClass,
 				inputSizeClasses.md,
 				inputBorderClass({
-					error: promptConfigName === 'agentToolFunctionName' && !validateToolName(content ?? '')
+					error: !!toolNameError
 				}),
 				'w-full'
 			)}
 			onfocus={() => (focused = true)}
 			onblur={() => (focused = false)}
 		/>
-		{#if promptConfigName === 'agentToolFunctionName' && !validateToolName(content ?? '')}
+		{#if toolNameError}
 			<p class="text-3xs text-red-400 leading-tight mt-0.5">
-				Invalid tool name, should only contain letters, numbers and underscores
+				{toolNameError}
 			</p>
 		{/if}
 	{/if}
diff --git a/frontend/src/lib/components/flows/common/FlowCard.svelte b/frontend/src/lib/components/flows/common/FlowCard.svelte
index f780331a82..43cabc7aa0 100644
--- a/frontend/src/lib/components/flows/common/FlowCard.svelte
+++ b/frontend/src/lib/components/flows/common/FlowCard.svelte
@@ -12,6 +12,7 @@
 		action?: import('svelte').Snippet
 		children?: import('svelte').Snippet
 		isAgentTool?: boolean
+		siblingToolNames?: string[]
 	}
 
 	let {
@@ -23,7 +24,8 @@
 		header,
 		action,
 		children,
-		isAgentTool = false
+		isAgentTool = false,
+		siblingToolNames = undefined
 	}: Props = $props()
 </script>
 
@@ -38,6 +40,7 @@
 				{flowModuleValue}
 				{action}
 				{isAgentTool}
+				{siblingToolNames}
 			>
 				{@render header?.()}
 			</FlowCardHeader>
diff --git a/frontend/src/lib/components/flows/common/FlowCardHeader.svelte b/frontend/src/lib/components/flows/common/FlowCardHeader.svelte
index 401bf232b1..a5ff48053e 100644
--- a/frontend/src/lib/components/flows/common/FlowCardHeader.svelte
+++ b/frontend/src/lib/components/flows/common/FlowCardHeader.svelte
@@ -18,7 +18,7 @@
 	import { Flag, Lock, RefreshCw, Unlock } from 'lucide-svelte'
 	import { createEventDispatcher, untrack } from 'svelte'
 	import { twMerge } from 'tailwind-merge'
-	import { validateToolName } from '$lib/components/graph/renderers/nodes/AIToolNode.svelte'
+	import { getToolNameError } from '$lib/components/graph/renderers/nodes/AIToolNode.svelte'
 	import { DEFAULT_HUB_BASE_URL, PRIVATE_HUB_MIN_VERSION } from '$lib/hub'
 
 	interface Props {
@@ -28,6 +28,7 @@
 		children?: import('svelte').Snippet
 		action?: import('svelte').Snippet
 		isAgentTool?: boolean
+		siblingToolNames?: string[]
 	}
 
 	let {
@@ -36,9 +37,14 @@
 		summary = $bindable(undefined),
 		children,
 		action,
-		isAgentTool = false
+		isAgentTool = false,
+		siblingToolNames = undefined
 	}: Props = $props()
 
+	let toolNameError = $derived(
+		isAgentTool ? getToolNameError(summary ?? '', undefined, siblingToolNames) : undefined
+	)
+
 	let latestHash: string | undefined = $state(undefined)
 
 	// Extract version_id from hub path (format: hub/{version_id}/{app}/{summary})
@@ -103,6 +109,7 @@
 						elementProps={{
 							placeholder: isAgentTool ? 'Tool name' : 'Summary'
 						}}
+						{siblingToolNames}
 					/>
 				{:else if flowModuleValue.type === 'script' && 'path' in flowModuleValue && flowModuleValue.path}
 					<IconedPath path={flowModuleValue.path} hash={flowModuleValue.hash} class="grow" />
@@ -173,14 +180,16 @@
 							/>
 						</div>
 					{/if}
-					<input
-						bind:value={summary}
-						placeholder={isAgentTool ? 'Tool name' : 'Summary'}
-						class={twMerge(
-							'w-full grow',
-							isAgentTool && !validateToolName(summary ?? '') && '!border-red-400'
-						)}
-					/>
+					<div class="flex flex-col w-full grow">
+						<input
+							bind:value={summary}
+							placeholder={isAgentTool ? 'Tool name' : 'Summary'}
+							class={twMerge('w-full grow', toolNameError && '!border-red-400')}
+						/>
+						{#if toolNameError}
+							<p class="text-3xs text-red-400 leading-tight mt-0.5">{toolNameError}</p>
+						{/if}
+					</div>
 				{:else if flowModuleValue.type === 'flow'}
 					<Badge color="indigo" capitalize>flow</Badge>
 					<input bind:value={summary} placeholder="Summary" class="w-full grow" />
diff --git a/frontend/src/lib/components/flows/content/AgentToolWrapper.svelte b/frontend/src/lib/components/flows/content/AgentToolWrapper.svelte
index d624c2132f..9807e96603 100644
--- a/frontend/src/lib/components/flows/content/AgentToolWrapper.svelte
+++ b/frontend/src/lib/components/flows/content/AgentToolWrapper.svelte
@@ -14,6 +14,7 @@
 		previousModule?: FlowModule | undefined
 		forceTestTab?: Record<string, boolean>
 		highlightArg?: Record<string, string | undefined>
+		siblingToolNames?: string[]
 	}
 
 	let {
@@ -23,7 +24,8 @@
 		parentModule = undefined,
 		previousModule = undefined,
 		forceTestTab,
-		highlightArg
+		highlightArg,
+		siblingToolNames = undefined
 	}: Props = $props()
 </script>
 
@@ -43,6 +45,7 @@
 		forceTestTab={forceTestTab?.[tool.id]}
 		highlightArg={highlightArg?.[tool.id]}
 		isAgentTool={true}
+		{siblingToolNames}
 	/>
 {:else if isMcpTool(tool)}
 	<!-- MCP tool - use McpToolEditor -->
diff --git a/frontend/src/lib/components/flows/content/FlowEditorPanel.svelte b/frontend/src/lib/components/flows/content/FlowEditorPanel.svelte
index c8418e5183..3d9a533f48 100644
--- a/frontend/src/lib/components/flows/content/FlowEditorPanel.svelte
+++ b/frontend/src/lib/components/flows/content/FlowEditorPanel.svelte
@@ -101,9 +101,7 @@
 	)
 	let canMoveSelected = $derived(
 		resolvedModuleIds.length > 0 &&
-			areContiguousSiblings(
-				locateModules(resolvedModuleIds, flowStore.val.value.modules ?? [])
-			)
+			areContiguousSiblings(locateModules(resolvedModuleIds, flowStore.val.value.modules ?? []))
 	)
 </script>
 
diff --git a/frontend/src/lib/components/flows/content/FlowModuleComponent.svelte b/frontend/src/lib/components/flows/content/FlowModuleComponent.svelte
index 4a43c2c38d..d9da29f01e 100644
--- a/frontend/src/lib/components/flows/content/FlowModuleComponent.svelte
+++ b/frontend/src/lib/components/flows/content/FlowModuleComponent.svelte
@@ -110,6 +110,7 @@
 		forceTestTab?: boolean
 		highlightArg?: string
 		isAgentTool?: boolean
+		siblingToolNames?: string[]
 	}
 
 	let {
@@ -125,7 +126,8 @@
 		savedModule = undefined,
 		forceTestTab = false,
 		highlightArg = undefined,
-		isAgentTool = false
+		isAgentTool = false,
+		siblingToolNames = undefined
 	}: Props = $props()
 
 	let workspaceScriptTag: string | undefined = $state(undefined)
@@ -237,7 +239,9 @@
 	}
 
 	let forceReload = $state(0)
-	let editorPanelSize = $state(untrack(() => noEditor) ? 0 : flowModule.value.type == 'script' ? 30 : 50)
+	let editorPanelSize = $state(
+		untrack(() => noEditor) ? 0 : flowModule.value.type == 'script' ? 30 : 50
+	)
 	let editorSettingsPanelSize = $state(100 - untrack(() => editorPanelSize))
 	let stepHistoryLoader = getStepHistoryLoaderContext()
 
@@ -726,6 +730,7 @@
 			}}
 			bind:summary={flowModule.summary}
 			{isAgentTool}
+			{siblingToolNames}
 		>
 			{#snippet header()}
 				<FlowModuleHeader
@@ -1062,8 +1067,8 @@
 														{enableAi}
 														{isAgentTool}
 														allowedAiTransforms={isAgentTool && flowModule.value.type === 'aiagent'
-														? ['user_message']
-														: undefined}
+															? ['user_message']
+															: undefined}
 														helperScript={retrieveDynCodeAndLang(flowModule.value)}
 														chatInputEnabled={flowStore.val.value?.chat_input_enabled ?? false}
 													/>
diff --git a/frontend/src/lib/components/flows/content/FlowModuleWrapper.svelte b/frontend/src/lib/components/flows/content/FlowModuleWrapper.svelte
index cb50e8a327..c58819d59c 100644
--- a/frontend/src/lib/components/flows/content/FlowModuleWrapper.svelte
+++ b/frontend/src/lib/components/flows/content/FlowModuleWrapper.svelte
@@ -305,6 +305,7 @@
 				{enableAi}
 				{forceTestTab}
 				{highlightArg}
+				siblingToolNames={flowModule.value.tools.map((t) => t.summary ?? '')}
 			/>
 		{/if}
 	{/each}
diff --git a/frontend/src/lib/components/graph/graphBuilder.svelte.ts b/frontend/src/lib/components/graph/graphBuilder.svelte.ts
index 1d01b2e252..192b5c0c34 100644
--- a/frontend/src/lib/components/graph/graphBuilder.svelte.ts
+++ b/frontend/src/lib/components/graph/graphBuilder.svelte.ts
@@ -300,6 +300,7 @@ export type AiToolN = {
 	data: {
 		tool: string
 		type?: string
+		nameError?: string
 		eventHandlers: GraphEventHandlers
 		moduleId: string
 		insertable: boolean
diff --git a/frontend/src/lib/components/graph/renderers/nodes/AIToolNode.svelte b/frontend/src/lib/components/graph/renderers/nodes/AIToolNode.svelte
index 28e9d58fa4..1913d927c8 100644
--- a/frontend/src/lib/components/graph/renderers/nodes/AIToolNode.svelte
+++ b/frontend/src/lib/components/graph/renderers/nodes/AIToolNode.svelte
@@ -1,10 +1,29 @@
 <script module lang="ts">
-	export function validateToolName(name: string, type?: string) {
-		if (type === 'websearch') return true
+	import { forbiddenIds } from '$lib/components/flows/idUtils'
+
+	export function getToolNameError(
+		name: string,
+		type?: string,
+		siblingNames?: string[]
+	): string | undefined {
+		if (type === 'websearch') return undefined
 		if (type === 'mcp') {
-			return name.length > 0
+			return name.length > 0 ? undefined : 'Tool name must not be empty'
 		}
-		return /^[a-zA-Z0-9_]+$/.test(name)
+		if (!/^[a-zA-Z0-9_]+$/.test(name)) {
+			return 'Tool name must only contain letters, numbers and underscores'
+		}
+		if (forbiddenIds.includes(name)) {
+			return `'${name}' is a reserved name`
+		}
+		if (siblingNames && siblingNames.filter((n) => n === name).length > 1) {
+			return 'Duplicate tool name'
+		}
+		return undefined
+	}
+
+	export function validateToolName(name: string, type?: string) {
+		return getToolNameError(name, type) === undefined
 	}
 
 	export const AI_TOOL_BASE_OFFSET = 5
@@ -147,6 +166,7 @@
 				}
 			}
 
+			const siblingNames = tools.map((t) => t.name)
 			const toolNodes: (Node & AiToolN)[] = tools.map((tool, i) => {
 				let inputToolXGap = 12
 				let inputToolWidth = (ROW_WIDTH - inputToolXGap) / 2
@@ -160,6 +180,7 @@
 					data: {
 						tool: tool.name,
 						type: tool.type,
+						nameError: getToolNameError(tool.name, tool.type, siblingNames),
 						eventHandlers,
 						moduleId: tool.id,
 						insertable,
@@ -169,13 +190,13 @@
 					width: inputToolWidth,
 					position: {
 						x:
-							(tools.length === 1
+							tools.length === 1
 								? (ROW_WIDTH - inputToolWidth) / 2
 								: (i + 1) % 2 === 0
 									? inputToolWidth + inputToolXGap
 									: isLastRow && tools.length % 2 === 1
 										? (ROW_WIDTH - inputToolWidth) / 2
-										: 0),
+										: 0,
 						y:
 							baseOffset +
 							rowOffset *
@@ -287,7 +308,7 @@
 	const flowModuleState = $derived(data.flowModuleStates?.[data.moduleId])
 	let colorClasses = $derived(
 		getNodeColorClasses(
-			!validateToolName(data.tool, data.type) ? 'Failure' : flowModuleState?.type,
+			data.nameError ? 'Failure' : flowModuleState?.type,
 			selectionManager?.getSelectedId() === data.moduleId
 		)
 	)
@@ -324,12 +345,7 @@
 					<Wrench size={16} class="ml-1 shrink-0" />
 				{/if}
 
-				<span
-					class={twMerge(
-						'text-3xs truncate flex-1',
-						!validateToolName(data.tool, data.type) && 'text-red-400'
-					)}
-				>
+				<span class={twMerge('text-3xs truncate flex-1', data.nameError && 'text-red-400')}>
 					{data.tool || 'Missing name'}
 				</span>
 			</button>