diff --git a/apps/web/src/components/DiffPanel.logic.test.ts b/apps/web/src/components/DiffPanel.logic.test.ts
index f08360fe..8bcce22b 100644
--- a/apps/web/src/components/DiffPanel.logic.test.ts
+++ b/apps/web/src/components/DiffPanel.logic.test.ts
@@ -3,7 +3,7 @@ import { describe, expect, it } from "vitest";
 
 import type { DraftThreadState } from "../composerDraftStore";
 import type { Thread } from "../types";
-import { resolveDiffPanelThread } from "./DiffPanel.logic";
+import { resolveDiffPanelThread, resolveDiffSelectAllArmed } from "./DiffPanel.logic";
 
 const PROJECT_ID = ProjectId.makeUnsafe("project-1");
 const THREAD_ID = ThreadId.makeUnsafe("thread-1");
@@ -103,3 +103,50 @@ describe("resolveDiffPanelThread", () => {
     ).toBeUndefined();
   });
 });
+
+describe("resolveDiffSelectAllArmed", () => {
+  it("arms on Cmd/Ctrl+A inside the diff viewport", () => {
+    expect(
+      resolveDiffSelectAllArmed(false, { key: "a", metaKey: true, ctrlKey: false }, true),
+    ).toBe(true);
+    expect(
+      resolveDiffSelectAllArmed(false, { key: "A", metaKey: false, ctrlKey: true }, true),
+    ).toBe(true);
+  });
+
+  it("does not arm on Cmd/Ctrl+A outside the diff viewport", () => {
+    expect(
+      resolveDiffSelectAllArmed(false, { key: "a", metaKey: true, ctrlKey: false }, false),
+    ).toBe(false);
+    expect(
+      resolveDiffSelectAllArmed(true, { key: "a", metaKey: false, ctrlKey: true }, false),
+    ).toBe(false);
+  });
+
+  it("preserves the armed state through the copy half of the gesture", () => {
+    expect(
+      resolveDiffSelectAllArmed(true, { key: "c", metaKey: true, ctrlKey: false }, false),
+    ).toBe(true);
+    expect(
+      resolveDiffSelectAllArmed(false, { key: "c", metaKey: true, ctrlKey: false }, false),
+    ).toBe(false);
+  });
+
+  it("preserves the armed state through bare modifier keydowns", () => {
+    expect(
+      resolveDiffSelectAllArmed(true, { key: "Meta", metaKey: true, ctrlKey: false }, false),
+    ).toBe(true);
+    expect(
+      resolveDiffSelectAllArmed(true, { key: "Shift", metaKey: false, ctrlKey: false }, false),
+    ).toBe(true);
+  });
+
+  it("disarms on any other key that starts a fresh selection", () => {
+    expect(
+      resolveDiffSelectAllArmed(true, { key: "ArrowDown", metaKey: false, ctrlKey: false }, true),
+    ).toBe(false);
+    expect(
+      resolveDiffSelectAllArmed(true, { key: "x", metaKey: false, ctrlKey: false }, true),
+    ).toBe(false);
+  });
+});
diff --git a/apps/web/src/components/DiffPanel.logic.ts b/apps/web/src/components/DiffPanel.logic.ts
index f44245f9..6bd0391b 100644
--- a/apps/web/src/components/DiffPanel.logic.ts
+++ b/apps/web/src/components/DiffPanel.logic.ts
@@ -1,6 +1,6 @@
 // FILE: DiffPanel.logic.ts
 // Purpose: Resolve the thread context the diff panel should use across server-backed and local draft chats.
-// Exports: resolveDiffPanelThread
+// Exports: resolveDiffPanelThread, resolveDiffSelectAllArmed
 // Depends on: ChatView.logic draft-thread normalization.
 
 import { DEFAULT_MODEL_BY_PROVIDER, type ModelSelection, type ThreadId } from "@t3tools/contracts";
@@ -33,3 +33,25 @@ export function resolveDiffPanelThread(input: {
     null,
   );
 }
+
+// Tracks a select-all-then-copy gesture so the copy handler can swap in the full serialized diff; the diff renders into shadow DOM, so the native copy event never reaches the viewport.
+export function resolveDiffSelectAllArmed(
+  previous: boolean,
+  event: Pick<KeyboardEvent, "key" | "metaKey" | "ctrlKey">,
+  isWithinDiffViewport: boolean,
+): boolean {
+  const key = event.key.toLowerCase();
+  const hasShortcutModifier = event.metaKey || event.ctrlKey;
+
+  if (hasShortcutModifier && key === "a") {
+    return isWithinDiffViewport;
+  }
+  if (hasShortcutModifier && key === "c") {
+    return previous;
+  }
+  // Bare modifier keydowns precede the shortcut key, so they don't count as a new selection.
+  if (key === "meta" || key === "control" || key === "shift" || key === "alt") {
+    return previous;
+  }
+  return false;
+}
diff --git a/apps/web/src/components/DiffPanel.tsx b/apps/web/src/components/DiffPanel.tsx
index c1760141..864cc41b 100644
--- a/apps/web/src/components/DiffPanel.tsx
+++ b/apps/web/src/components/DiffPanel.tsx
@@ -40,6 +40,7 @@ import {
   getRenderablePatch,
   resolveDiffCopyText,
   resolveDiffThemeName,
+  serializeRenderablePatchText,
   summarizePatchStats,
 } from "../lib/diffRendering";
 import { resolveDiffEnvironmentState } from "../lib/threadEnvironment";
@@ -56,7 +57,7 @@ import { getProviderStartOptions, useAppSettings } from "../appSettings";
 import { useComposerDraftStore } from "../composerDraftStore";
 import { formatShortTimestamp } from "../timestampFormat";
 import ChatMarkdown from "./ChatMarkdown";
-import { resolveDiffPanelThread } from "./DiffPanel.logic";
+import { resolveDiffPanelThread, resolveDiffSelectAllArmed } from "./DiffPanel.logic";
 import { DiffPanelLoadingState, DiffPanelShell, type DiffPanelMode } from "./DiffPanelShell";
 import { Button } from "./ui/button";
 import { Menu, MenuPopup, MenuRadioGroup, MenuRadioItem, MenuTrigger } from "./ui/menu";
@@ -195,6 +196,7 @@ export default function DiffPanel({
   const setRepoDiffScope = useRepoDiffScopeStore((store) => store.setScope);
   const [collapsedFiles, setCollapsedFiles] = useState<Set<string>>(() => new Set());
   const patchViewportRef = useRef<HTMLDivElement>(null);
+  const diffSelectAllArmedRef = useRef(false);
   const turnStripRef = useRef<HTMLDivElement>(null);
   const previousDiffOpenRef = useRef(false);
   const [canScrollTurnStripLeft, setCanScrollTurnStripLeft] = useState(false);
@@ -403,11 +405,15 @@ export default function DiffPanel({
   const isSidebarMode = mode === "sidebar";
   const { copyToClipboard, isCopied: isSummaryCopied } = useCopyToClipboard();
   const { copyToClipboard: copyDiffToClipboard, isCopied: isDiffCopied } = useCopyToClipboard();
-  const diffCopyText = useMemo(() => resolveDiffCopyText(activeReviewPatch), [activeReviewPatch]);
   const renderablePatch = useMemo(
     () => getRenderablePatch(activeReviewPatch, `diff-panel:${resolvedTheme}`),
     [activeReviewPatch, resolvedTheme],
   );
+  // Serialize the whole diff from the parsed model so copy never depends on which virtualized rows are mounted.
+  const diffCopyText = useMemo(
+    () => serializeRenderablePatchText(renderablePatch) ?? resolveDiffCopyText(activeReviewPatch),
+    [renderablePatch, activeReviewPatch],
+  );
   const renderableFiles = useMemo(() => {
     if (!renderablePatch || renderablePatch.kind !== "files") {
       return [];
@@ -507,16 +513,10 @@ export default function DiffPanel({
         ? "Failed to generate diff summary."
         : null;
   const canShowSummary = Boolean(
-    !diffEnvironmentPending &&
-    activeCwd &&
-    (!hasResolvedRepoPatch || !hasNoRepoChanges),
+    !diffEnvironmentPending && activeCwd && (!hasResolvedRepoPatch || !hasNoRepoChanges),
   );
   const canPrefetchSummary = Boolean(
-    diffOpen &&
-    !diffEnvironmentPending &&
-    activeCwd &&
-    normalizedRepoPatch &&
-    !hasNoRepoChanges,
+    diffOpen && !diffEnvironmentPending && activeCwd && normalizedRepoPatch && !hasNoRepoChanges,
   );
   const canShowTotal = Boolean(!diffEnvironmentPending && activeCwd);
 
@@ -565,6 +565,43 @@ export default function DiffPanel({
     });
   }, []);
 
+  // Watch the document for select-all-then-copy: the Cmd/Ctrl+A keydown passes through the viewport but the copy event does not, so the native copy would only grab the mounted rows.
+  useEffect(() => {
+    const handleKeyDown = (event: KeyboardEvent) => {
+      const viewport = patchViewportRef.current;
+      const isWithinDiffViewport = viewport ? event.composedPath().includes(viewport) : false;
+      diffSelectAllArmedRef.current = resolveDiffSelectAllArmed(
+        diffSelectAllArmedRef.current,
+        event,
+        isWithinDiffViewport,
+      );
+    };
+    const handlePointerDown = () => {
+      diffSelectAllArmedRef.current = false;
+    };
+    const handleCopy = (event: ClipboardEvent) => {
+      if (!diffSelectAllArmedRef.current) {
+        return;
+      }
+      // One-shot: the next select-all must re-arm it.
+      diffSelectAllArmedRef.current = false;
+      if (!diffCopyText || !event.clipboardData) {
+        return;
+      }
+      event.preventDefault();
+      event.clipboardData.setData("text/plain", diffCopyText);
+    };
+
+    document.addEventListener("keydown", handleKeyDown, true);
+    document.addEventListener("pointerdown", handlePointerDown, true);
+    document.addEventListener("copy", handleCopy, true);
+    return () => {
+      document.removeEventListener("keydown", handleKeyDown, true);
+      document.removeEventListener("pointerdown", handlePointerDown, true);
+      document.removeEventListener("copy", handleCopy, true);
+    };
+  }, [diffCopyText]);
+
   const selectTurn = (turnId: TurnId) => {
     if (!activeThread) return;
     if (onUpdatePanelState) {
@@ -943,8 +980,8 @@ export default function DiffPanel({
                 <div className="min-w-0">
                   <p className="text-sm font-medium text-foreground">Repo summary</p>
                   <p className="text-[11px] text-muted-foreground">
-                    Generated from the current{" "}
-                    {REPO_DIFF_SCOPE_LABELS[repoDiffScope].toLowerCase()} diff.
+                    Generated from the current {REPO_DIFF_SCOPE_LABELS[repoDiffScope].toLowerCase()}{" "}
+                    diff.
                   </p>
                 </div>
                 {diffSummaryText ? (
diff --git a/apps/web/src/lib/diffRendering.test.ts b/apps/web/src/lib/diffRendering.test.ts
index 843d2a24..6ba0deea 100644
--- a/apps/web/src/lib/diffRendering.test.ts
+++ b/apps/web/src/lib/diffRendering.test.ts
@@ -4,7 +4,13 @@
 // Depends on: Vitest and diffRendering helpers
 
 import { describe, expect, it } from "vitest";
-import { buildPatchCacheKey, resolveDiffCopyText, summarizePatchStats } from "./diffRendering";
+import {
+  buildPatchCacheKey,
+  getRenderablePatch,
+  resolveDiffCopyText,
+  serializeRenderablePatchText,
+  summarizePatchStats,
+} from "./diffRendering";
 
 describe("buildPatchCacheKey", () => {
   it("returns a stable cache key for identical content", () => {
@@ -48,6 +54,88 @@ describe("resolveDiffCopyText", () => {
   });
 });
 
+describe("serializeRenderablePatchText", () => {
+  it("returns every line for a large diff that would be virtualized in the DOM", () => {
+    const LINE_COUNT = 6000;
+    const bodyLines = Array.from(
+      { length: LINE_COUNT },
+      (_, index) => `+line ${String(index + 1).padStart(4, "0")}`,
+    );
+    const patch = [
+      "diff --git a/big.txt b/big.txt",
+      "new file mode 100644",
+      "index 0000000..1111111",
+      "--- /dev/null",
+      "+++ b/big.txt",
+      `@@ -0,0 +1,${LINE_COUNT} @@`,
+      ...bodyLines,
+      "",
+    ].join("\n");
+
+    const renderable = getRenderablePatch(patch, "diff-panel:test");
+    expect(renderable?.kind).toBe("files");
+
+    const serialized = serializeRenderablePatchText(renderable);
+    expect(serialized).not.toBeNull();
+
+    const serializedAdditions = serialized!.split("\n").filter((line) => line.startsWith("+line "));
+    expect(serializedAdditions).toHaveLength(LINE_COUNT);
+    expect(serializedAdditions[0]).toBe("+line 0001");
+    expect(serializedAdditions[2999]).toBe("+line 3000");
+    expect(serializedAdditions.at(-1)).toBe(`+line ${String(LINE_COUNT).padStart(4, "0")}`);
+    for (const expected of bodyLines) {
+      expect(serialized).toContain(expected);
+    }
+    // The serializer must not inject blank lines between diff rows.
+    expect(serialized).not.toContain("\n\n");
+  });
+
+  it("reconstructs context and change lines in order for a mixed patch", () => {
+    const patch = [
+      "diff --git a/src/example.ts b/src/example.ts",
+      "index 1111111..2222222 100644",
+      "--- a/src/example.ts",
+      "+++ b/src/example.ts",
+      "@@ -1,3 +1,4 @@",
+      " const stable = true;",
+      "-const oldValue = 1;",
+      "+const newValue = 1;",
+      "+const addedValue = 2;",
+      " export { stable };",
+      "",
+    ].join("\n");
+
+    const serialized = serializeRenderablePatchText(getRenderablePatch(patch, "diff-panel:test"));
+
+    expect(serialized).not.toBeNull();
+    const serializedLines = serialized!.split("\n");
+    expect(serializedLines).toContain(" const stable = true;");
+    expect(serializedLines).toContain("-const oldValue = 1;");
+    expect(serializedLines).toContain("+const newValue = 1;");
+    expect(serializedLines).toContain("+const addedValue = 2;");
+    expect(serializedLines).toContain(" export { stable };");
+    // Deletions are emitted before additions within a change block.
+    expect(serializedLines.indexOf("-const oldValue = 1;")).toBeLessThan(
+      serializedLines.indexOf("+const newValue = 1;"),
+    );
+  });
+
+  it("passes raw patches through untouched", () => {
+    const serialized = serializeRenderablePatchText({
+      kind: "raw",
+      text: "not a parseable diff",
+      reason: "Showing raw patch.",
+    });
+
+    expect(serialized).toBe("not a parseable diff");
+  });
+
+  it("returns null when there is nothing to copy", () => {
+    expect(serializeRenderablePatchText(null)).toBeNull();
+    expect(serializeRenderablePatchText({ kind: "files", files: [] })).toBeNull();
+  });
+});
+
 describe("summarizePatchStats", () => {
   it("summarizes additions and deletions from a unified patch", () => {
     const patch = [
diff --git a/apps/web/src/lib/diffRendering.ts b/apps/web/src/lib/diffRendering.ts
index 078da017..22fe1ed2 100644
--- a/apps/web/src/lib/diffRendering.ts
+++ b/apps/web/src/lib/diffRendering.ts
@@ -4,6 +4,7 @@
 // Depends on: @pierre/diffs patch parsing
 
 import { parsePatchFiles } from "@pierre/diffs";
+import type { Hunk } from "@pierre/diffs";
 import type { FileDiffMetadata } from "@pierre/diffs/react";
 
 export const DIFF_THEME_NAMES = {
@@ -98,6 +99,83 @@ export function getRenderablePatch(
   }
 }
 
+// @pierre/diffs leaves trailing newlines on parsed lines; drop them so the `\n` join below doesn't add blank lines.
+function stripLineBreak(line: string): string {
+  return line.replace(/\r?\n$/, "");
+}
+
+function serializeHunkHeader(hunk: Hunk): string {
+  const specs = stripLineBreak(
+    hunk.hunkSpecs ??
+      `@@ -${hunk.deletionStart},${hunk.deletionCount} +${hunk.additionStart},${hunk.additionCount} @@`,
+  );
+  const context = hunk.hunkContext ? stripLineBreak(hunk.hunkContext) : "";
+  return context ? `${specs} ${context}` : specs;
+}
+
+// Rebuild the unified-diff text for one parsed file from the @pierre/diffs model.
+export function serializeFileDiffMetadata(file: FileDiffMetadata): string {
+  const newPath = file.name;
+  const oldPath = file.prevName ?? file.name;
+  const lines: string[] = [`diff --git a/${oldPath} b/${newPath}`];
+
+  if (file.type === "new") {
+    lines.push(`new file mode ${file.mode ?? "100644"}`);
+  } else if (file.type === "deleted") {
+    lines.push(`deleted file mode ${file.prevMode ?? file.mode ?? "100644"}`);
+  } else if (file.type === "rename-pure" || file.type === "rename-changed") {
+    lines.push(`rename from ${oldPath}`, `rename to ${newPath}`);
+  }
+
+  lines.push(
+    `--- ${file.type === "new" ? "/dev/null" : `a/${oldPath}`}`,
+    `+++ ${file.type === "deleted" ? "/dev/null" : `b/${newPath}`}`,
+  );
+
+  for (const hunk of file.hunks) {
+    lines.push(serializeHunkHeader(hunk));
+    for (const segment of hunk.hunkContent) {
+      if (segment.type === "context") {
+        for (let offset = 0; offset < segment.lines; offset += 1) {
+          const content =
+            file.additionLines[segment.additionLineIndex + offset] ??
+            file.deletionLines[segment.deletionLineIndex + offset] ??
+            "";
+          lines.push(` ${stripLineBreak(content)}`);
+        }
+        continue;
+      }
+      for (let offset = 0; offset < segment.deletions; offset += 1) {
+        lines.push(
+          `-${stripLineBreak(file.deletionLines[segment.deletionLineIndex + offset] ?? "")}`,
+        );
+      }
+      for (let offset = 0; offset < segment.additions; offset += 1) {
+        lines.push(
+          `+${stripLineBreak(file.additionLines[segment.additionLineIndex + offset] ?? "")}`,
+        );
+      }
+    }
+  }
+
+  return lines.join("\n");
+}
+
+// Serialize an entire renderable patch (every file, every line) for clipboard writes.
+export function serializeRenderablePatchText(renderable: RenderablePatch | null): string | null {
+  if (!renderable) {
+    return null;
+  }
+  if (renderable.kind === "raw") {
+    return renderable.text.length > 0 ? renderable.text : null;
+  }
+  if (renderable.files.length === 0) {
+    return null;
+  }
+  const serialized = renderable.files.map(serializeFileDiffMetadata).join("\n");
+  return serialized.length > 0 ? serialized : null;
+}
+
 // Summarize parsed hunks for compact, consistent diff stats across panel chrome.
 export function summarizeFileDiffStats(files: ReadonlyArray<FileDiffMetadata>): {
   additions: number;