diff --git a/frontend/src/components/ai-elements/reasoning.tsx b/frontend/src/components/ai-elements/reasoning.tsx
index 8c9d5b206..5f1d2321e 100644
--- a/frontend/src/components/ai-elements/reasoning.tsx
+++ b/frontend/src/components/ai-elements/reasoning.tsx
@@ -11,6 +11,7 @@ import { BrainIcon, ChevronDownIcon } from "lucide-react";
 import type { ComponentProps, ReactNode } from "react";
 import { createContext, memo, useContext, useEffect, useState } from "react";
 import { Streamdown } from "streamdown";
+import { reasoningPlugins } from "@/core/streamdown/plugins";
 import { Shimmer } from "./shimmer";
 
 type ReasoningContextValue = {
@@ -177,7 +178,7 @@ export const ReasoningContent = memo(
       )}
       {...props}
     >
-      <Streamdown {...props}>{children}</Streamdown>
+      <Streamdown {...reasoningPlugins}>{children}</Streamdown>
     </CollapsibleContent>
   ),
 );
diff --git a/frontend/src/core/streamdown/plugins.ts b/frontend/src/core/streamdown/plugins.ts
index e9214031b..d576252c5 100644
--- a/frontend/src/core/streamdown/plugins.ts
+++ b/frontend/src/core/streamdown/plugins.ts
@@ -28,6 +28,15 @@ export const streamdownPluginsWithWordAnimation = {
   ] as StreamdownProps["rehypePlugins"],
 };
 
+// Plugins for reasoning/thinking content — derived from streamdownPlugins but without rehypeRaw,
+// to prevent LLM-hallucinated HTML tags (e.g. <simd>) from being rendered as DOM elements.
+export const reasoningPlugins = {
+  remarkPlugins: streamdownPlugins.remarkPlugins,
+  rehypePlugins: streamdownPlugins.rehypePlugins?.filter(
+    (p) => p !== rehypeRaw,
+  ) as StreamdownProps["rehypePlugins"],
+};
+
 // Plugins for human messages - no autolink to prevent URL bleeding into adjacent text
 export const humanMessagePlugins = {
   remarkPlugins: [
diff --git a/frontend/tests/unit/core/streamdown/plugins.test.ts b/frontend/tests/unit/core/streamdown/plugins.test.ts
new file mode 100644
index 000000000..efe0ab719
--- /dev/null
+++ b/frontend/tests/unit/core/streamdown/plugins.test.ts
@@ -0,0 +1,13 @@
+import rehypeRaw from "rehype-raw";
+import { expect, test } from "vitest";
+
+import { reasoningPlugins, streamdownPlugins } from "@/core/streamdown/plugins";
+
+test("streamdownPlugins includes rehypeRaw", () => {
+  expect(streamdownPlugins.rehypePlugins).toContain(rehypeRaw);
+});
+
+test("reasoningPlugins does not include rehypeRaw", () => {
+  const flat = reasoningPlugins.rehypePlugins?.flat();
+  expect(flat).not.toContain(rehypeRaw);
+});