@@ -6,7 +6,6 @@ import { describe, expect, it, vi } from "vite-plus/test";
 
 import { getSyntaxHighlighterPromise } from "../lib/syntaxHighlighting";
 import * as mermaidRendering from "../lib/mermaidRendering";
-import { MarkdownDiffBlock } from "./MarkdownDiffBlock";
 import { MarkdownMermaidBlock } from "./MarkdownMermaidBlock";
 import { GitHubIcon } from "./Icons";
 import { Button } from "./ui/button";
@@ -75,91 +74,52 @@ function codeButton(renderer: ReactTestRenderer, label: string) {
   return button.props as ComponentProps<typeof Button>;
 }
 
-describe("ChatMarkdown code previews", () => {
-  it.each(["diff", "PATCH"])(
-    "switches %s snippets between preview and source without changing copied code",
-    async (language) => {
-      const source = " unchanged\n-old\n+new\n";
-      const writeText = vi.fn(async (_text: string) => {});
-      vi.stubGlobal("navigator", { clipboard: { writeText } });
-      vi.stubGlobal("IS_REACT_ACT_ENVIRONMENT", true);
-      let renderer: ReactTestRenderer | undefined;
-      try {
-        await act(async () => {
-          renderer = create(
-            <ChatMarkdown
-              cwd="/tmp/project"
-              text={`\`\`\`${language}\n${source}\`\`\``}
-              parseRawHtml={false}
-            />,
-          );
-        });
-        const preview = renderer!.root.findByType(MarkdownDiffBlock);
-        expect(
-          preview
-            .findAllByType("span")
-            .flatMap((line) => line.children)
-            .join(""),
-        ).toBe(source);
-        const showSource = codeButton(renderer!, "Show source");
-        await act(async () => {
-          showSource.onClick?.({} as Parameters<NonNullable<typeof showSource.onClick>>[0]);
-        });
-        expect(renderer!.root.findAllByType(MarkdownDiffBlock)).toHaveLength(0);
-        const copy = codeButton(renderer!, "Copy code");
-        await act(async () => {
-          copy.onClick?.({} as Parameters<NonNullable<typeof copy.onClick>>[0]);
-        });
-        expect(writeText).toHaveBeenCalledWith(source);
-        const showPreview = codeButton(renderer!, "Show preview");
-        await act(async () => {
-          showPreview.onClick?.({} as Parameters<NonNullable<typeof showPreview.onClick>>[0]);
-        });
-        expect(
-          renderer!.root
-            .findByType(MarkdownDiffBlock)
-            .findAllByType("span")
-            .flatMap((line) => line.children)
-            .join(""),
-        ).toBe(source);
-      } finally {
-        await act(async () => renderer?.unmount());
-        vi.unstubAllGlobals();
-        vi.restoreAllMocks();
-      }
-    },
-  );
-
+describe("ChatMarkdown Mermaid previews", () => {
   it("renders Mermaid only after streaming ends and lets the user return to its source", async () => {
     vi.stubGlobal("IS_REACT_ACT_ENVIRONMENT", true);
+    const writeText = vi.fn(async (_text: string) => {});
+    vi.stubGlobal("navigator", { clipboard: { writeText } });
     const renderDiagram = vi
       .spyOn(mermaidRendering, "renderMermaidDiagram")
       .mockResolvedValue("<svg><text>Draft</text></svg>");
     const text = "```mermaid\nflowchart LR\nDraft-->Review\n```";
     let renderer: ReactTestRenderer | undefined;
     try {
       await act(async () => {
-        renderer = create(<ChatMarkdown cwd="/tmp/project" text={text} isStreaming />, {
-          createNodeMock: (element) =>
-            element.type === "div" &&
-            (element.props as ComponentProps<"div">).className === "chat-markdown-mermaid"
-              ? {}
-              : null,
-        });
+        renderer = create(
+          <ChatMarkdown cwd="/tmp/project" text={text} isStreaming parseRawHtml={false} />,
+          {
+            createNodeMock: (element) =>
+              element.type === "div" &&
+              (element.props as ComponentProps<"div">).className === "chat-markdown-mermaid"
+                ? {}
+                : null,
+          },
+        );
       });
       expect(renderDiagram).not.toHaveBeenCalled();
       expect(renderer!.root.findAllByType(MarkdownMermaidBlock)).toHaveLength(0);
       await act(async () => {
-        renderer!.update(<ChatMarkdown cwd="/tmp/project" text={text} />);
+        renderer!.update(<ChatMarkdown cwd="/tmp/project" text={text} parseRawHtml={false} />);
       });
       expect(
         renderer!.root.findByProps({ role: "img" }).props.dangerouslySetInnerHTML.__html,
       ).toContain("Draft");
+      const copyPreview = codeButton(renderer!, "Copy code");
+      await act(async () => {
+        copyPreview.onClick?.({} as Parameters<NonNullable<typeof copyPreview.onClick>>[0]);
+      });
+      expect(writeText).toHaveBeenLastCalledWith("flowchart LR\nDraft-->Review\n");
       const showSource = codeButton(renderer!, "Show source");
       await act(async () => {
         showSource.onClick?.({} as Parameters<NonNullable<typeof showSource.onClick>>[0]);
       });
       expect(renderer!.root.findAllByProps({ role: "img" })).toHaveLength(0);
+      const copySource = codeButton(renderer!, "Copied");
+      await act(async () => {
+        copySource.onClick?.({} as Parameters<NonNullable<typeof copySource.onClick>>[0]);
+      });
+      expect(writeText).toHaveBeenLastCalledWith("flowchart LR\nDraft-->Review\n");
       const showPreview = codeButton(renderer!, "Show preview");
       await act(async () => {
         showPreview.onClick?.({} as Parameters<NonNullable<typeof showPreview.onClick>>[0]);
 
@@ -122,7 +122,6 @@ import { LRUCache } from "../lib/lruCache";
 import { getSyntaxHighlighterPromise } from "../lib/syntaxHighlighting";
 import { GitHubIcon } from "./Icons";
 import { RenderErrorBoundary } from "./RenderErrorBoundary";
-import { MarkdownDiffBlock } from "./MarkdownDiffBlock";
 import { MarkdownMermaidBlock } from "./MarkdownMermaidBlock";
 import { useTheme } from "../hooks/useTheme";
 import { getClientSettings, useClientSettings } from "../hooks/useSettings";
@@ -523,7 +522,6 @@ const GITHUB_ALERT_PRESENTATIONS: Record<
 function extractFenceLanguage(className: string | undefined): string {
   const match = className?.match(CODE_FENCE_LANGUAGE_REGEX);
   const raw = match?.[1] ?? "text";
-  if (raw.toLowerCase() === "patch") return "diff";
   // Shiki doesn't bundle a gitignore grammar; ini is a close match (#685)
   return raw === "gitignore" ? "ini" : raw;
 }
@@ -3113,9 +3111,7 @@ const CHAT_MARKDOWN_COMPONENTS = {
         fenceTitle={fenceTitle}
         theme={resolvedTheme}
         preview={
-          language.toLowerCase() === "diff" || language.toLowerCase() === "patch" ? (
-            <MarkdownDiffBlock code={codeBlock.code} />
-          ) : language.toLowerCase() === "mermaid" && !isStreaming ? (
+          language.toLowerCase() === "mermaid" && !isStreaming ? (
             <MarkdownMermaidBlock code={codeBlock.code} theme={resolvedTheme} />
           ) : undefined
         }
 
@@ -1875,32 +1875,6 @@ code {
   background: transparent !important;
 }
 
-.chat-markdown-diff code {
-  display: block;
-  min-width: fit-content;
-}
-
-.chat-markdown-diff [data-diff-line] {
-  display: block;
-  min-height: 1lh;
-}
-
-.chat-markdown-diff [data-diff-line="added"] {
-  background: color-mix(in srgb, var(--color-green-500) 15%, transparent);
-  color: light-dark(var(--color-green-800), var(--color-green-300));
-}
-
-.chat-markdown-diff [data-diff-line="removed"] {
-  background: color-mix(in srgb, var(--color-red-500) 15%, transparent);
-  color: light-dark(var(--color-red-800), var(--color-red-300));
-}
-
-.chat-markdown-diff [data-diff-line="header"],
-.chat-markdown-diff [data-diff-line="hunk"] {
-  color: var(--muted-foreground);
-  background: color-mix(in srgb, var(--foreground) 5%, transparent);
-}
-
 /* Diagnostics-style tables: row separators only, uppercase headers, and a
    scroll-fade container for horizontal overflow. The root chat-markdown
    wrapping rules (overflow-wrap: anywhere) would let columns shrink to single
 
@@ -6,19 +6,9 @@ include a skill when the task needs more context.
 Messages can contain up to 120,000 characters. Longer drafts stay in the composer
 so you can shorten them or split them into several messages.
 
-## Diffs and diagrams
+## Mermaid diagrams
 
-On web and desktop, use a fenced `diff` or `patch` block to display additions and
-removals. Short snippets work without file headers:
-
-````markdown
-```diff
--const retries = 1;
-+const retries = 3;
-```
-````
-
-Use a `mermaid` fence to display a diagram:
+On web and desktop, use a `mermaid` fence to display a diagram:
 
 ````markdown
 ```mermaid