diff --git a/web/app/components/evaluation/__tests__/store.spec.ts b/web/app/components/evaluation/__tests__/store.spec.ts
index 843a0107e8..18bd31a24c 100644
--- a/web/app/components/evaluation/__tests__/store.spec.ts
+++ b/web/app/components/evaluation/__tests__/store.spec.ts
@@ -41,6 +41,24 @@ describe('evaluation store', () => {
     expect(configuredMetric!.customConfig!.workflowName).toBe(config.workflowOptions[0].label)
   })
 
+  it('should only add one custom metric', () => {
+    const resourceType = 'apps'
+    const resourceId = 'app-custom-limit'
+    const store = useEvaluationStore.getState()
+
+    store.ensureResource(resourceType, resourceId)
+    store.addCustomMetric(resourceType, resourceId)
+    store.addCustomMetric(resourceType, resourceId)
+
+    expect(
+      useEvaluationStore
+        .getState()
+        .resources['apps:app-custom-limit']
+        .metrics
+        .filter(metric => metric.kind === 'custom-workflow'),
+    ).toHaveLength(1)
+  })
+
   it('should add and remove builtin metrics', () => {
     const resourceType = 'apps'
     const resourceId = 'app-2'
diff --git a/web/app/components/evaluation/components/metric-section/__tests__/index.spec.tsx b/web/app/components/evaluation/components/metric-section/__tests__/index.spec.tsx
index 03343d2a1c..0b76e4387b 100644
--- a/web/app/components/evaluation/components/metric-section/__tests__/index.spec.tsx
+++ b/web/app/components/evaluation/components/metric-section/__tests__/index.spec.tsx
@@ -210,5 +210,20 @@ describe('MetricSection', () => {
       expect(screen.getByText('evaluation.metrics.custom.workflowPlaceholder')).toBeInTheDocument()
       expect(screen.getByRole('button', { name: 'evaluation.metrics.custom.addMapping' })).toBeInTheDocument()
     })
+
+    it('should disable adding another custom metric when one already exists', () => {
+      // Arrange
+      act(() => {
+        useEvaluationStore.getState().addCustomMetric(resourceType, resourceId)
+      })
+
+      // Act
+      renderMetricSection()
+      fireEvent.click(screen.getByRole('button', { name: 'evaluation.metrics.add' }))
+
+      // Assert
+      expect(screen.getByRole('button', { name: /evaluation.metrics.custom.footerTitle/i })).toBeDisabled()
+      expect(screen.getByText('evaluation.metrics.custom.limitDescription')).toBeInTheDocument()
+    })
   })
 })
diff --git a/web/app/components/evaluation/components/metric-selector/index.tsx b/web/app/components/evaluation/components/metric-selector/index.tsx
index a01256bb8d..4870952b9b 100644
--- a/web/app/components/evaluation/components/metric-selector/index.tsx
+++ b/web/app/components/evaluation/components/metric-selector/index.tsx
@@ -12,7 +12,7 @@ import {
   PopoverTrigger,
 } from '@/app/components/base/ui/popover'
 import { cn } from '@/utils/classnames'
-import { useEvaluationStore } from '../../store'
+import { useEvaluationResource, useEvaluationStore } from '../../store'
 import SelectorEmptyState from './selector-empty-state'
 import SelectorFooter from './selector-footer'
 import SelectorMetricSection from './selector-metric-section'
@@ -26,12 +26,14 @@ const MetricSelector = ({
   triggerStyle = 'button',
 }: MetricSelectorProps) => {
   const { t } = useTranslation('evaluation')
+  const resource = useEvaluationResource(resourceType, resourceId)
   const addCustomMetric = useEvaluationStore(state => state.addCustomMetric)
   const [open, setOpen] = useState(false)
   const [query, setQuery] = useState('')
   const [nodeInfoMap, setNodeInfoMap] = useState<Record<string, Array<{ node_id: string, title: string, type: string }>>>({})
   const [collapsedMetricMap, setCollapsedMetricMap] = useState<Record<string, boolean>>({})
   const [expandedMetricNodesMap, setExpandedMetricNodesMap] = useState<Record<string, boolean>>({})
+  const hasCustomMetric = resource.metrics.some(metric => metric.kind === 'custom-workflow')
 
   const {
     builtinMetricMap,
@@ -134,7 +136,8 @@ const MetricSelector = ({
 
           <SelectorFooter
             title={t('metrics.custom.footerTitle')}
-            description={t('metrics.custom.footerDescription')}
+            description={hasCustomMetric ? t('metrics.custom.limitDescription') : t('metrics.custom.footerDescription')}
+            disabled={hasCustomMetric}
             onClick={() => {
               addCustomMetric(resourceType, resourceId)
               setOpen(false)
diff --git a/web/app/components/evaluation/components/metric-selector/selector-footer.tsx b/web/app/components/evaluation/components/metric-selector/selector-footer.tsx
index 3b8d04474a..74163c275d 100644
--- a/web/app/components/evaluation/components/metric-selector/selector-footer.tsx
+++ b/web/app/components/evaluation/components/metric-selector/selector-footer.tsx
@@ -1,18 +1,21 @@
 type SelectorFooterProps = {
   title: string
   description: string
+  disabled?: boolean
   onClick: () => void
 }
 
 const SelectorFooter = ({
   title,
   description,
+  disabled = false,
   onClick,
 }: SelectorFooterProps) => {
   return (
     <button
       type="button"
-      className="relative flex items-center gap-3 overflow-hidden border-t border-divider-subtle bg-background-default-subtle px-4 py-5 text-left hover:bg-state-base-hover-alt"
+      disabled={disabled}
+      className="relative flex items-center gap-3 overflow-hidden border-t border-divider-subtle bg-background-default-subtle px-4 py-5 text-left enabled:hover:bg-state-base-hover-alt disabled:cursor-not-allowed disabled:opacity-60"
       onClick={onClick}
     >
       <div className="absolute -left-6 -top-6 h-28 w-28 rounded-full bg-util-colors-indigo-indigo-100 opacity-50 blur-2xl" />
diff --git a/web/app/components/evaluation/store.ts b/web/app/components/evaluation/store.ts
index 5774dd5934..8890694ca4 100644
--- a/web/app/components/evaluation/store.ts
+++ b/web/app/components/evaluation/store.ts
@@ -142,7 +142,9 @@ export const useEvaluationStore = create<EvaluationStore>((set, get) => ({
     set(state => ({
       resources: updateResourceState(state.resources, resourceType, resourceId, resource => ({
         ...resource,
-        metrics: [...resource.metrics, createCustomMetric()],
+        metrics: resource.metrics.some(metric => metric.kind === 'custom-workflow')
+          ? resource.metrics
+          : [...resource.metrics, createCustomMetric()],
       })),
     }))
   },
diff --git a/web/i18n/en-US/evaluation.json b/web/i18n/en-US/evaluation.json
index 0ba7fe44cd..21939d9d15 100644
--- a/web/i18n/en-US/evaluation.json
+++ b/web/i18n/en-US/evaluation.json
@@ -67,6 +67,7 @@
   "metrics.custom.description": "Select an evaluation workflow and map your variables before running tests.",
   "metrics.custom.footerDescription": "Connect your published evaluation workflows",
   "metrics.custom.footerTitle": "Custom metrics",
+  "metrics.custom.limitDescription": "Only one custom metric can be added.",
   "metrics.custom.mappingTitle": "Variable Mapping",
   "metrics.custom.mappingWarning": "Complete the workflow selection and each variable mapping to enable batch tests.",
   "metrics.custom.sourcePlaceholder": "Source variable",