diff --git a/src/lib/components/chat/FileNav/FilePreview.svelte b/src/lib/components/chat/FileNav/FilePreview.svelte
index 0bfa4fe50..b5a3a30cc 100644
--- a/src/lib/components/chat/FileNav/FilePreview.svelte
+++ b/src/lib/components/chat/FileNav/FilePreview.svelte
@@ -103,6 +103,13 @@
 	$: isNotebook = getExt(selectedFile) === 'ipynb';
 	$: isCode = isCodeFile(selectedFile);
 	$: csvDelimiter = getExt(selectedFile) === 'tsv' ? '\t' : ',';
+
+	// For HTML files on system terminals (proxy URL), use path-based serving
+	// so the iframe can resolve relative CSS/JS/image references via cookie auth.
+	$: serveUrl =
+		isHtml && selectedFile && baseUrl && baseUrl.includes('/api/v1/terminals/')
+			? `${baseUrl}/files/serve/${selectedFile.replace(/^\//, '')}`
+			: null;
 	$: renderedHtml =
 		isMarkdown && fileContent
 			? DOMPurify.sanitize(marked.parse(fileContent, { async: false }) as string)
@@ -386,7 +393,19 @@
 			{/if}
 		</div>
 	{:else if fileContent !== null}
-		{#if isHtml && !showRaw}
+		{#if isHtml && !showRaw && serveUrl}
+			{#if overlay}
+				<div class="absolute top-0 left-0 right-0 bottom-0 z-10"></div>
+			{/if}
+			<iframe
+				src={serveUrl}
+				sandbox="allow-scripts allow-same-origin allow-downloads{($settings?.iframeSandboxAllowForms ?? false)
+					? ' allow-forms'
+					: ''}"
+				class="w-full h-full border-none bg-white"
+				title="HTML Preview"
+			/>
+		{:else if isHtml && !showRaw}
 			{#if overlay}
 				<div class="absolute top-0 left-0 right-0 bottom-0 z-10"></div>
 			{/if}