diff --git a/apps/frontend/src/plugins/debug-pojo.ts b/apps/frontend/src/plugins/debug-pojo.ts
deleted file mode 100644
index e9fe18c7d7..0000000000
--- a/apps/frontend/src/plugins/debug-pojo.ts
+++ /dev/null
@@ -1,24 +0,0 @@
-import { defineNuxtPlugin } from '#imports'
-
-export default defineNuxtPlugin((nuxt) => {
-	if (import.meta.server) {
-		nuxt.hooks.hook('app:rendered', (ctx) => {
-			if (ctx.ssrContext?.payload?.data) {
-				const check = (obj: any, path = 'payload') => {
-					if (!obj || typeof obj !== 'object') return
-					if (
-						obj.constructor &&
-						obj.constructor.name !== 'Object' &&
-						obj.constructor.name !== 'Array'
-					) {
-						console.error(`Non-POJO at ${path}:`, obj.constructor.name)
-					}
-					for (const [k, v] of Object.entries(obj)) {
-						check(v, `${path}.${k}`)
-					}
-				}
-				check(ctx.ssrContext.payload.data)
-			}
-		})
-	}
-})
diff --git a/apps/frontend/src/plugins/i18n.ts b/apps/frontend/src/plugins/i18n.ts
index b39ef67afb..fc62bea104 100644
--- a/apps/frontend/src/plugins/i18n.ts
+++ b/apps/frontend/src/plugins/i18n.ts
@@ -61,22 +61,26 @@ export default defineNuxtPlugin({
 	name: 'i18n',
 	enforce: 'pre',
 	async setup(nuxtApp) {
-		// ONLY locale needs request-scoping (what language this request uses)
 		const locale = useState<string>('i18n-locale', () => DEFAULT_LOCALE)
 
 		function t(key: string, values?: Record<string, unknown>): string {
-			const msg = messageCache.get(locale.value)?.[key] ?? messageCache.get(DEFAULT_LOCALE)?.[key]
+			const currentLocale = locale.value
+			const msg = messageCache.get(currentLocale)?.[key] ?? messageCache.get(DEFAULT_LOCALE)?.[key]
 			if (!msg) return key
 			if (!values || Object.keys(values).length === 0) return msg
 
-			const cacheKey = `${locale.value}:${msg}`
+			const cacheKey = `${currentLocale}:${msg}`
 			let formatter = formatterCache.get(cacheKey)
 			if (!formatter) {
-				formatter = new IntlMessageFormat(msg, locale.value)
+				formatter = new IntlMessageFormat(msg, currentLocale)
 				formatterCache.set(cacheKey, formatter)
 			}
 			try {
-				return formatter.format(values) as string
+				const result = formatter.format(values) as string
+				if (import.meta.dev && typeof result !== 'string') {
+					console.error('[i18n] t() returned non-string:', typeof result)
+				}
+				return result
 			} catch {
 				return msg
 			}
diff --git a/apps/frontend/src/plugins/intl-payload-safety.ts b/apps/frontend/src/plugins/intl-payload-safety.ts
new file mode 100644
index 0000000000..f8211932fa
--- /dev/null
+++ b/apps/frontend/src/plugins/intl-payload-safety.ts
@@ -0,0 +1,17 @@
+export default definePayloadPlugin(() => {
+	definePayloadReducer('IntlMessageFormat', (value) => {
+		if (value?.constructor?.name === 'IntlMessageFormat' || value?._ast !== undefined) {
+			if (import.meta.dev) {
+				console.warn('[i18n] IntlMessageFormat instance leaked into payload - returning null')
+				console.warn('[i18n] This indicates a bug that should be fixed upstream')
+				console.warn('[i18n] Leaked value:', value)
+			}
+
+			return null
+		}
+
+		return false
+	})
+
+	definePayloadReviver('IntlMessageFormat', () => null)
+})
diff --git a/apps/frontend/src/plugins/payload-debugger.ts b/apps/frontend/src/plugins/payload-debugger.ts
new file mode 100644
index 0000000000..dd579008d7
--- /dev/null
+++ b/apps/frontend/src/plugins/payload-debugger.ts
@@ -0,0 +1,37 @@
+function findNonPOJOs(
+	obj: unknown,
+	path: string,
+	found: Array<{ path: string; type: string }> = [],
+): Array<{ path: string; type: string }> {
+	if (obj === null || typeof obj !== 'object') return found
+
+	const proto = Object.getPrototypeOf(obj)
+	if (proto !== Object.prototype && proto !== null && !Array.isArray(obj)) {
+		found.push({ path, type: obj.constructor?.name ?? 'Unknown' })
+	}
+
+	for (const [key, value] of Object.entries(obj)) {
+		findNonPOJOs(value, `${path}.${key}`, found)
+	}
+
+	return found
+}
+
+export default defineNuxtPlugin((nuxtApp) => {
+	if (!import.meta.dev || !import.meta.server) return
+
+	nuxtApp.hooks.hook('app:rendered', () => {
+		try {
+			JSON.stringify(nuxtApp.payload)
+		} catch (e) {
+			console.error('[payload-debugger] Payload serialization would fail:', e)
+			const nonPOJOs = findNonPOJOs(nuxtApp.payload, 'payload')
+			if (nonPOJOs.length > 0) {
+				console.error('[payload-debugger] Non-POJO objects found in payload:')
+				for (const { path, type } of nonPOJOs) {
+					console.error(`  - ${path}: ${type}`)
+				}
+			}
+		}
+	})
+})
diff --git a/packages/ui/src/components/base/IntlFormatted.vue b/packages/ui/src/components/base/IntlFormatted.vue
index 2836087494..19dc0d7c59 100644
--- a/packages/ui/src/components/base/IntlFormatted.vue
+++ b/packages/ui/src/components/base/IntlFormatted.vue
@@ -1,6 +1,6 @@
 <script setup lang="ts">
 import IntlMessageFormat, { type FormatXMLElementFn, type PrimitiveType } from 'intl-messageformat'
-import { computed, useSlots, type VNode } from 'vue'
+import { computed, markRaw, useSlots, type VNode } from 'vue'
 
 import type { MessageDescriptor } from '../../composables/i18n'
 import { injectI18n } from '../../providers/i18n'
@@ -32,11 +32,13 @@ const formattedParts = computed(() => {
 		slotHandlers[normalizedName] = (chunks) => {
 			const slot = slots[slotName]
 			if (slot) {
-				return slot({
-					children: chunks,
-				})
+				return markRaw(
+					slot({
+						children: chunks,
+					}),
+				) as VNode[]
 			}
-			return chunks as VNode[]
+			return markRaw(chunks) as VNode[]
 		}
 
 		msg = msg.replace(
@@ -52,10 +54,14 @@ const formattedParts = computed(() => {
 			...slotHandlers,
 		})
 
+		// ensure result array items are marked as raw if they're VNodes
+		// prevents VNodes from entering the reactive system and SSR payload
 		if (Array.isArray(result)) {
-			return result
+			return result.map((part) =>
+				typeof part === 'object' && part !== null ? markRaw(part) : part,
+			)
 		}
-		return [result]
+		return [typeof result === 'object' && result !== null ? markRaw(result) : result]
 	} catch {
 		return [msg]
 	}