diff --git a/frontend/src/components/ai-elements/streamdown.tsx b/frontend/src/components/ai-elements/streamdown.tsx
index be8dfc9fd..607d18eea 100644
--- a/frontend/src/components/ai-elements/streamdown.tsx
+++ b/frontend/src/components/ai-elements/streamdown.tsx
@@ -4,7 +4,7 @@ import { Component, useMemo, type ComponentProps, type ReactNode } from "react";
 import { Streamdown } from "streamdown";
 
 import { installClipboardFallback } from "@/core/clipboard";
-import { capBlockquoteNesting } from "@/core/streamdown/preprocess";
+import { capMarkdownNesting } from "@/core/streamdown/preprocess";
 
 export type ClipboardSafeStreamdownProps = ComponentProps<typeof Streamdown>;
 
@@ -58,11 +58,14 @@ export function ClipboardSafeStreamdown({
   children,
   ...props
 }: ClipboardSafeStreamdownProps) {
-  // Fast path for the dominant pathological input (pure ">" chains) so the
-  // error boundary below rarely has to absorb a full stack overflow.
+  // Fast path for the dominant pathological inputs (deep ">" chains and deeply
+  // nested lists both blow up marked's recursive tokenizers) so the error
+  // boundary below rarely has to absorb a stack overflow — and never has to
+  // face the heap exhaustion the same lists cause on larger stacks, which it
+  // cannot catch.
   const safeChildren = useMemo(
     () =>
-      typeof children === "string" ? capBlockquoteNesting(children) : children,
+      typeof children === "string" ? capMarkdownNesting(children) : children,
     [children],
   );
   return (
diff --git a/frontend/src/core/streamdown/preprocess.ts b/frontend/src/core/streamdown/preprocess.ts
index 47bd242e0..f0ef30d23 100644
--- a/frontend/src/core/streamdown/preprocess.ts
+++ b/frontend/src/core/streamdown/preprocess.ts
@@ -18,6 +18,18 @@ const BLOCKQUOTE_PREFIX_RE = /^ {0,3}(?:[ \t]*>)+/;
 const CODE_FENCE_RE = /^ {0,3}(?:```|~~~)/;
 const INDENTED_CODE_RE = /^(?: {4}|\t)/;
 
+// marked's list tokenizer recurses once per nesting level too (list ->
+// blockTokens -> list -> ...). In the browser's tighter stack a deeply nested
+// list overflows during render and throws "Maximum call stack size exceeded"
+// from inside Streamdown's lexing useMemo (see issue #3393); on larger stacks
+// the same input instead goes quadratic and exhausts the heap. Each list level
+// requires at least ~2 columns of indentation, so capping leading whitespace at
+// 200 columns bounds the effective nesting near 100 levels — far beyond any
+// legitimate content while keeping marked safe. Anything indented past this is
+// pathological nesting, not prose or code.
+const MAX_LIST_INDENT = 200;
+const DEEP_INDENT_HINT_RE = new RegExp(`^[ \\t]{${MAX_LIST_INDENT + 1},}`, "m");
+
 export function capBlockquoteNesting(markdown: string): string {
   if (!DEEP_BLOCKQUOTE_HINT_RE.test(markdown)) {
     return markdown;
@@ -55,6 +67,39 @@ export function capBlockquoteNesting(markdown: string): string {
     .join("\n");
 }
 
+export function capListNesting(markdown: string): string {
+  if (!DEEP_INDENT_HINT_RE.test(markdown)) {
+    return markdown;
+  }
+
+  let insideFence = false;
+  return markdown
+    .split("\n")
+    .map((line) => {
+      if (CODE_FENCE_RE.test(line)) {
+        insideFence = !insideFence;
+        return line;
+      }
+      // Indentation inside fenced code is literal layout (ASCII art, pasted
+      // source); collapsing it would corrupt the rendered block.
+      if (insideFence) {
+        return line;
+      }
+      const whitespace = /^[ \t]*/.exec(line)![0];
+      if (whitespace.length <= MAX_LIST_INDENT) {
+        return line;
+      }
+      return " ".repeat(MAX_LIST_INDENT) + line.slice(whitespace.length);
+    })
+    .join("\n");
+}
+
+// Cap every runaway nesting construct that can take down a message render
+// before marked sees the content.
+export function capMarkdownNesting(markdown: string): string {
+  return capListNesting(capBlockquoteNesting(markdown));
+}
+
 export function preprocessStreamdownMarkdown(markdown: string): string {
   if (!MERMAID_BLOCK_HINT_RE.test(markdown) || !markdown.includes("-.->")) {
     return markdown;
diff --git a/frontend/tests/unit/core/streamdown/preprocess.test.ts b/frontend/tests/unit/core/streamdown/preprocess.test.ts
index 71ca7f242..20adcefec 100644
--- a/frontend/tests/unit/core/streamdown/preprocess.test.ts
+++ b/frontend/tests/unit/core/streamdown/preprocess.test.ts
@@ -2,6 +2,8 @@ import { expect, test } from "vitest";
 
 import {
   capBlockquoteNesting,
+  capListNesting,
+  capMarkdownNesting,
   preprocessStreamdownMarkdown,
 } from "@/core/streamdown/preprocess";
 
@@ -51,6 +53,53 @@ test("capBlockquoteNesting only rewrites pathological lines", () => {
   expect(lines[2]).toBe("plain");
 });
 
+test("capListNesting returns normally indented content unchanged", () => {
+  const input = "- a\n  - b\n    - c\n\n      code continuation";
+  expect(capListNesting(input)).toBe(input);
+});
+
+test("capListNesting caps pathologically deep list indentation", () => {
+  const deep = "  ".repeat(2000) + "- x";
+  const result = capListNesting(deep);
+  const indent = /^[ \t]*/.exec(result)![0];
+  expect(indent.length).toBe(200);
+  expect(result.endsWith("- x")).toBe(true);
+});
+
+test("capListNesting leaves fenced code content untouched", () => {
+  const literal = " ".repeat(400) + "deeply indented ascii art";
+  const input = `\`\`\`text\n${literal}\n\`\`\``;
+  expect(capListNesting(input).split("\n")[1]).toBe(literal);
+});
+
+// Outside a fence, deep indentation is capped regardless of blank-line context:
+// we cannot tell an indented-code line from deeply nested list content (both can
+// follow a blank line), and exempting either reopens the crash — blank-separated
+// deep-indent lists otherwise blow up marked just like contiguous ones.
+test("capListNesting caps deep indentation even after a blank line", () => {
+  const input = `- a\n\n${" ".repeat(500)}- deep`;
+  const lines = capListNesting(input).split("\n");
+  expect(/^[ \t]*/.exec(lines[2]!)![0].length).toBe(200);
+});
+
+test("capListNesting only rewrites pathological lines", () => {
+  const normal = "    indented paragraph";
+  const deep = " ".repeat(500) + "- deep";
+  const result = capListNesting(`${normal}\n${deep}\nplain`);
+  const lines = result.split("\n");
+  expect(lines[0]).toBe(normal);
+  expect(/^[ \t]*/.exec(lines[1]!)![0].length).toBe(200);
+  expect(lines[2]).toBe("plain");
+});
+
+test("capMarkdownNesting caps both blockquote and list nesting", () => {
+  const input = `${"> ".repeat(3000)}quote\n${" ".repeat(500)}- item`;
+  const result = capMarkdownNesting(input);
+  const lines = result.split("\n");
+  expect((lines[0]?.match(/>/g) ?? []).length).toBe(100);
+  expect(/^[ \t]*/.exec(lines[1]!)![0].length).toBe(200);
+});
+
 test("preprocessStreamdownMarkdown leaves non-mermaid content unchanged", () => {
   const input = "just some text";
   expect(preprocessStreamdownMarkdown(input)).toBe(input);