diff --git a/apps/web/src/components/DiffPanel.tsx b/apps/web/src/components/DiffPanel.tsx
index e6dbb57cc7..b625ee85c0 100644
--- a/apps/web/src/components/DiffPanel.tsx
+++ b/apps/web/src/components/DiffPanel.tsx
@@ -27,8 +27,7 @@ import { readLocalApi } from "../localApi";
 import { resolvePathLinkTarget } from "../terminal-links";
 import { parseDiffRouteSearch, stripDiffSearchParams } from "../diffRouteSearch";
 import { useTheme } from "../hooks/useTheme";
-import { buildPatchCacheKey } from "../lib/diffRendering";
-import { resolveDiffThemeName } from "../lib/diffRendering";
+import { buildPatchCacheKey, canRenderFileDiff, resolveDiffThemeName } from "../lib/diffRendering";
 import { useTurnDiffSummaries } from "../hooks/useTurnDiffSummaries";
 import { selectProjectByRef, useStore } from "../store";
 import { createThreadSelectorByRef } from "../storeSelectors";
@@ -306,12 +305,22 @@ export default function DiffPanel({ mode = "inline" }: DiffPanelProps) {
     if (!renderablePatch || renderablePatch.kind !== "files") {
       return [];
     }
-    return renderablePatch.files.toSorted((left, right) =>
-      resolveFileDiffPath(left).localeCompare(resolveFileDiffPath(right), undefined, {
-        numeric: true,
-        sensitivity: "base",
-      }),
-    );
+    return renderablePatch.files
+      .map((fileDiff) => ({
+        canRender: canRenderFileDiff(fileDiff),
+        fileDiff,
+        filePath: resolveFileDiffPath(fileDiff),
+      }))
+      .toSorted((left, right) => {
+        const renderOrder = Number(!left.canRender) - Number(!right.canRender);
+        if (renderOrder !== 0) {
+          return renderOrder;
+        }
+        return left.filePath.localeCompare(right.filePath, undefined, {
+          numeric: true,
+          sensitivity: "base",
+        });
+      });
   }, [renderablePatch]);
 
   useEffect(() => {
@@ -600,8 +609,7 @@ export default function DiffPanel({ mode = "inline" }: DiffPanelProps) {
                   intersectionObserverMargin: 1200,
                 }}
               >
-                {renderableFiles.map((fileDiff) => {
-                  const filePath = resolveFileDiffPath(fileDiff);
+                {renderableFiles.map(({ canRender, fileDiff, filePath }) => {
                   const fileKey = buildFileDiffRenderKey(fileDiff);
                   const themedFileKey = `${fileKey}:${resolvedTheme}`;
                   return (
@@ -629,8 +637,17 @@ export default function DiffPanel({ mode = "inline" }: DiffPanelProps) {
                           theme: resolveDiffThemeName(resolvedTheme),
                           themeType: resolvedTheme as DiffThemeType,
                           unsafeCSS: DIFF_PANEL_UNSAFE_CSS,
+                          collapsed: !canRender,
                         }}
                       />
+
+                      {!canRender && (
+                        <div className="px-3 py-3 text-[11px] leading-relaxed text-muted-foreground">
+                          <p>
+                            This file is too large to display. Open the file to inspect the change.
+                          </p>
+                        </div>
+                      )}
                     </div>
                   );
                 })}
diff --git a/apps/web/src/lib/diffRendering.test.ts b/apps/web/src/lib/diffRendering.test.ts
index 9fd7045ff4..ac619a36be 100644
--- a/apps/web/src/lib/diffRendering.test.ts
+++ b/apps/web/src/lib/diffRendering.test.ts
@@ -1,5 +1,9 @@
 import { describe, expect, it } from "vitest";
-import { buildPatchCacheKey } from "./diffRendering";
+import {
+  buildPatchCacheKey,
+  canRenderFileDiff,
+  MAX_RENDERABLE_DIFF_LINE_LENGTH,
+} from "./diffRendering";
 
 describe("buildPatchCacheKey", () => {
   it("returns a stable cache key for identical content", () => {
@@ -29,3 +33,23 @@ describe("buildPatchCacheKey", () => {
     );
   });
 });
+
+describe("diff render line limits", () => {
+  it("rejects file diffs with pathological line lengths", () => {
+    expect(
+      canRenderFileDiff({
+        additionLines: ["small"],
+        deletionLines: ["x".repeat(MAX_RENDERABLE_DIFF_LINE_LENGTH + 1)],
+      }),
+    ).toBe(false);
+  });
+
+  it("allows file diffs within the line length limit", () => {
+    expect(
+      canRenderFileDiff({
+        additionLines: ["x".repeat(MAX_RENDERABLE_DIFF_LINE_LENGTH)],
+        deletionLines: ["small"],
+      }),
+    ).toBe(true);
+  });
+});
diff --git a/apps/web/src/lib/diffRendering.ts b/apps/web/src/lib/diffRendering.ts
index 7218f72978..b67be8909e 100644
--- a/apps/web/src/lib/diffRendering.ts
+++ b/apps/web/src/lib/diffRendering.ts
@@ -1,3 +1,5 @@
+import type { FileDiffMetadata } from "@pierre/diffs";
+
 export const DIFF_THEME_NAMES = {
   light: "pierre-light",
   dark: "pierre-dark",
@@ -5,6 +7,8 @@ export const DIFF_THEME_NAMES = {
 
 export type DiffThemeName = (typeof DIFF_THEME_NAMES)[keyof typeof DIFF_THEME_NAMES];
 
+export const MAX_RENDERABLE_DIFF_LINE_LENGTH = 500_000;
+
 export function resolveDiffThemeName(theme: "light" | "dark"): DiffThemeName {
   return theme === "dark" ? DIFF_THEME_NAMES.dark : DIFF_THEME_NAMES.light;
 }
@@ -37,3 +41,12 @@ export function buildPatchCacheKey(patch: string, scope = "diff-panel"): string
   ).toString(36);
   return `${scope}:${normalizedPatch.length}:${primary}:${secondary}`;
 }
+
+export function canRenderFileDiff(
+  fileDiff: Pick<FileDiffMetadata, "additionLines" | "deletionLines">,
+): boolean {
+  return (
+    fileDiff.additionLines.every((line) => line.length <= MAX_RENDERABLE_DIFF_LINE_LENGTH) &&
+    fileDiff.deletionLines.every((line) => line.length <= MAX_RENDERABLE_DIFF_LINE_LENGTH)
+  );
+}