diff --git a/apps/web/src/components/chat/ChangedFilesTree.test.tsx b/apps/web/src/components/chat/ChangedFilesTree.test.tsx
index c371acdb362..120827b34b2 100644
--- a/apps/web/src/components/chat/ChangedFilesTree.test.tsx
+++ b/apps/web/src/components/chat/ChangedFilesTree.test.tsx
@@ -2,7 +2,32 @@ import { TurnId } from "@t3tools/contracts";
 import { renderToStaticMarkup } from "react-dom/server";
 import { describe, expect, it } from "vite-plus/test";
 
-import { ChangedFilesTree } from "./ChangedFilesTree";
+import { ChangedFilesCard, ChangedFilesTree } from "./ChangedFilesTree";
+
+describe("ChangedFilesCard", () => {
+  it("keeps its compact header sticky while preserving singular labels", () => {
+    const markup = renderToStaticMarkup(
+      <ChangedFilesCard
+        turnId={TurnId.make("turn-1")}
+        files={[{ path: "README.md", kind: "modified", additions: 2, deletions: 1 }]}
+        allDirectoriesExpanded
+        resolvedTheme="light"
+        onToggleAllDirectories={() => {}}
+        onOpenTurnDiff={() => {}}
+      />,
+    );
+
+    expect(markup).toContain('class="sticky top-0 z-10');
+    expect(markup).not.toContain("self-start");
+    expect(markup).toContain("whitespace-nowrap");
+    expect(markup).toContain("!size-[22px]");
+    expect(markup).toContain("size-3");
+    expect(markup).toContain('aria-label="Collapse all"');
+    expect(markup).toContain('aria-label="View diff"');
+    expect(markup).toContain("1 changed file");
+    expect(markup).not.toContain("1 changed files");
+  });
+});
 
 describe("ChangedFilesTree", () => {
   it.each([
diff --git a/apps/web/src/components/chat/ChangedFilesTree.tsx b/apps/web/src/components/chat/ChangedFilesTree.tsx
index 3ea0e6315fd..6fc1462c5e6 100644
--- a/apps/web/src/components/chat/ChangedFilesTree.tsx
+++ b/apps/web/src/components/chat/ChangedFilesTree.tsx
@@ -6,11 +6,19 @@ import {
   summarizeTurnDiffStats,
   type TurnDiffTreeNode,
 } from "../../lib/turnDiffTree";
-import { ChevronRightIcon, FolderIcon, FolderClosedIcon } from "lucide-react";
+import {
+  ChevronsDownUpIcon,
+  ChevronsUpDownIcon,
+  ChevronRightIcon,
+  FileDiffIcon,
+  FolderIcon,
+  FolderClosedIcon,
+} from "lucide-react";
 import { cn } from "~/lib/utils";
 import { DiffStatLabel, hasNonZeroStat } from "./DiffStatLabel";
 import { PierreEntryIcon } from "./PierreEntryIcon";
 import { Button } from "../ui/button";
+import { Tooltip, TooltipPopup, TooltipTrigger } from "../ui/tooltip";
 
 const EMPTY_DIRECTORY_OVERRIDES: Record<string, boolean> = {};
 
@@ -33,10 +41,12 @@ export const ChangedFilesCard = memo(function ChangedFilesCard(props: {
   const summaryStat = useMemo(() => summarizeTurnDiffStats(files), [files]);
 
   return (
-    <div className="relative mt-4 rounded-2xl bg-card/40 shadow-xs/5 not-dark:bg-clip-padding after:pointer-events-none after:absolute after:inset-0 after:z-20 after:rounded-2xl after:border after:border-input">
-      <div className="sticky top-0 z-10 mb-3 flex items-center justify-between gap-2 rounded-t-2xl bg-card/72 p-3 backdrop-blur-md">
-        <p className="flex items-center gap-1 font-medium text-foreground text-xs leading-4">
-          <span>{files.length} changed files</span>
+    <div className="mt-4 rounded-2xl border border-input bg-background p-2 pt-4 shadow-xs/5 not-dark:bg-clip-padding dark:bg-input/32">
+      <div className="sticky top-0 z-10 mb-3 flex items-center justify-between gap-2 bg-background px-2 before:absolute before:inset-x-0 before:-top-4 before:h-4 before:bg-background before:content-[''] dark:bg-[color-mix(in_srgb,var(--foreground)_2.5%,var(--background))] dark:before:bg-[color-mix(in_srgb,var(--foreground)_2.5%,var(--background))]">
+        <p className="flex items-center gap-1 whitespace-nowrap font-medium text-foreground text-xs leading-4">
+          <span>
+            {files.length} changed file{files.length === 1 ? "" : "s"}
+          </span>
           {hasNonZeroStat(summaryStat) && (
             <DiffStatLabel
               additions={summaryStat.additions}
@@ -47,35 +57,57 @@ export const ChangedFilesCard = memo(function ChangedFilesCard(props: {
           )}
         </p>
         <div className="flex items-center gap-1.5">
-          <Button
-            type="button"
-            size="xs"
-            variant="outline"
-            data-scroll-anchor-ignore
-            onClick={onToggleAllDirectories}
-          >
-            {allDirectoriesExpanded ? "Collapse all" : "Expand all"}
-          </Button>
-          <Button
-            type="button"
-            size="xs"
-            variant="outline"
-            onClick={() => onOpenTurnDiff(turnId, files[0]?.path)}
-          >
-            View diff
-          </Button>
+          <Tooltip>
+            <TooltipTrigger
+              render={
+                <Button
+                  type="button"
+                  size="icon-xs"
+                  variant="outline"
+                  className="!size-[22px]"
+                  aria-label={allDirectoriesExpanded ? "Collapse all" : "Expand all"}
+                  data-scroll-anchor-ignore
+                  onClick={onToggleAllDirectories}
+                />
+              }
+            >
+              {allDirectoriesExpanded ? (
+                <ChevronsDownUpIcon className="size-3" />
+              ) : (
+                <ChevronsUpDownIcon className="size-3" />
+              )}
+            </TooltipTrigger>
+            <TooltipPopup side="top">
+              {allDirectoriesExpanded ? "Collapse all" : "Expand all"}
+            </TooltipPopup>
+          </Tooltip>
+          <Tooltip>
+            <TooltipTrigger
+              render={
+                <Button
+                  type="button"
+                  size="icon-xs"
+                  variant="outline"
+                  className="!size-[22px]"
+                  aria-label="View diff"
+                  onClick={() => onOpenTurnDiff(turnId, files[0]?.path)}
+                />
+              }
+            >
+              <FileDiffIcon className="size-3" />
+            </TooltipTrigger>
+            <TooltipPopup side="top">View diff</TooltipPopup>
+          </Tooltip>
         </div>
       </div>
-      <div className="px-2 pb-2">
-        <ChangedFilesTree
-          key={`changed-files-tree:${turnId}`}
-          turnId={turnId}
-          files={files}
-          allDirectoriesExpanded={allDirectoriesExpanded}
-          resolvedTheme={resolvedTheme}
-          onOpenTurnDiff={onOpenTurnDiff}
-        />
-      </div>
+      <ChangedFilesTree
+        key={`changed-files-tree:${turnId}`}
+        turnId={turnId}
+        files={files}
+        allDirectoriesExpanded={allDirectoriesExpanded}
+        resolvedTheme={resolvedTheme}
+        onOpenTurnDiff={onOpenTurnDiff}
+      />
     </div>
   );
 });
diff --git a/apps/web/src/components/chat/MessagesTimeline.test.tsx b/apps/web/src/components/chat/MessagesTimeline.test.tsx
index a58724f3308..b340a248fbe 100644
--- a/apps/web/src/components/chat/MessagesTimeline.test.tsx
+++ b/apps/web/src/components/chat/MessagesTimeline.test.tsx
@@ -1,4 +1,4 @@
-import { EnvironmentId, MessageId } from "@t3tools/contracts";
+import { CheckpointRef, EnvironmentId, MessageId, TurnId } from "@t3tools/contracts";
 import { createRef, type ReactNode, type Ref } from "react";
 import { renderToStaticMarkup } from "react-dom/server";
 import { beforeAll, describe, expect, it, vi } from "vite-plus/test";
@@ -219,6 +219,58 @@ function buildUserTimelineEntry(text: string) {
 }
 
 describe("MessagesTimeline", () => {
+  it("keeps assistant changed-files headers sticky below the thread header", async () => {
+    const { MessagesTimeline } = await import("./MessagesTimeline");
+    const assistantMessageId = MessageId.make("message-assistant-with-files");
+    const turnId = TurnId.make("turn-with-files");
+    const markup = renderToStaticMarkup(
+      <MessagesTimeline
+        {...buildProps()}
+        timelineEntries={[
+          {
+            id: "entry-assistant-with-files",
+            kind: "message",
+            createdAt: MESSAGE_CREATED_AT,
+            message: {
+              id: assistantMessageId,
+              role: "assistant",
+              text: "Updated the fixture.",
+              turnId,
+              createdAt: MESSAGE_CREATED_AT,
+              updatedAt: MESSAGE_CREATED_AT,
+              streaming: false,
+            },
+          },
+        ]}
+        turnDiffSummaryByAssistantMessageId={
+          new Map([
+            [
+              assistantMessageId,
+              {
+                turnId,
+                checkpointTurnCount: 1,
+                checkpointRef: CheckpointRef.make("checkpoint-with-files"),
+                status: "ready",
+                files: [{ path: "README.md", kind: "modified", additions: 2, deletions: 1 }],
+                assistantMessageId,
+                completedAt: MESSAGE_CREATED_AT,
+              },
+            ],
+          ])
+        }
+      />,
+    );
+
+    expect(markup).toContain('class="sticky top-2 z-10');
+    expect(markup).not.toContain("self-start");
+    expect(markup).toContain("whitespace-nowrap");
+    expect(markup).toContain("!size-[22px]");
+    expect(markup).toContain("size-3");
+    expect(markup).toContain('aria-label="Collapse all"');
+    expect(markup).toContain('aria-label="View diff"');
+    expect(markup).toContain("1 changed file");
+  });
+
   it("uses LegendList isNearEnd when deciding whether the live edge is visible", async () => {
     const {
       resolveTimelineIsAtEnd,
diff --git a/apps/web/src/components/chat/MessagesTimeline.tsx b/apps/web/src/components/chat/MessagesTimeline.tsx
index 73fc86312e5..f759aa150be 100644
--- a/apps/web/src/components/chat/MessagesTimeline.tsx
+++ b/apps/web/src/components/chat/MessagesTimeline.tsx
@@ -44,8 +44,11 @@ import {
   CheckIcon,
   ChevronDownIcon,
   ChevronRightIcon,
+  ChevronsDownUpIcon,
+  ChevronsUpDownIcon,
   CircleAlertIcon,
   EyeIcon,
+  FileDiffIcon,
   GlobeIcon,
   HammerIcon,
   MessageCircleIcon,
@@ -1274,38 +1277,67 @@ function AssistantChangedFilesSectionInner({
   );
   const setExpanded = useUiStateStore((store) => store.setThreadChangedFilesExpanded);
   const summaryStat = summarizeTurnDiffStats(checkpointFiles);
-  const changedFileCountLabel = String(checkpointFiles.length);
 
   return (
-    <div className="mt-2 rounded-lg border border-border/80 bg-card/45 p-2.5">
-      <div className="sticky top-2 z-10 mb-1.5 flex items-center justify-between gap-2 bg-[color-mix(in_srgb,var(--card)_45%,var(--background))] before:absolute before:inset-x-0 before:-top-2 before:h-2 before:bg-[color-mix(in_srgb,var(--card)_45%,var(--background))] before:content-['']">
-        <p className="text-[10px] uppercase tracking-[0.12em] text-muted-foreground/65">
-          <span>Changed files ({changedFileCountLabel})</span>
+    <div className="mt-4 rounded-2xl border border-input bg-background p-2 pt-4 shadow-xs/5 not-dark:bg-clip-padding dark:bg-input/32">
+      <div className="sticky top-2 z-10 mb-3 flex items-center justify-between gap-2 bg-background px-2 before:absolute before:inset-x-0 before:-top-4 before:h-4 before:bg-background before:content-[''] dark:bg-[color-mix(in_srgb,var(--foreground)_2.5%,var(--background))] dark:before:bg-[color-mix(in_srgb,var(--foreground)_2.5%,var(--background))]">
+        <p className="flex items-center gap-1 whitespace-nowrap font-medium text-foreground text-xs leading-4">
+          <span>
+            {checkpointFiles.length} changed file{checkpointFiles.length === 1 ? "" : "s"}
+          </span>
           {hasNonZeroStat(summaryStat) && (
-            <>
-              <span className="mx-1">•</span>
-              <DiffStatLabel additions={summaryStat.additions} deletions={summaryStat.deletions} />
-            </>
+            <DiffStatLabel
+              additions={summaryStat.additions}
+              className="text-xs leading-4"
+              deletions={summaryStat.deletions}
+              layout="inline"
+            />
           )}
         </p>
         <div className="flex items-center gap-1.5">
-          <Button
-            type="button"
-            size="xs"
-            variant="outline"
-            data-scroll-anchor-ignore
-            onClick={() => setExpanded(routeThreadKey, turnSummary.turnId, !allDirectoriesExpanded)}
-          >
-            {allDirectoriesExpanded ? "Collapse all" : "Expand all"}
-          </Button>
-          <Button
-            type="button"
-            size="xs"
-            variant="outline"
-            onClick={() => onOpenTurnDiff(turnSummary.turnId, checkpointFiles[0]?.path)}
-          >
-            View diff
-          </Button>
+          <Tooltip>
+            <TooltipTrigger
+              render={
+                <Button
+                  type="button"
+                  size="icon-xs"
+                  variant="outline"
+                  className="!size-[22px]"
+                  aria-label={allDirectoriesExpanded ? "Collapse all" : "Expand all"}
+                  data-scroll-anchor-ignore
+                  onClick={() =>
+                    setExpanded(routeThreadKey, turnSummary.turnId, !allDirectoriesExpanded)
+                  }
+                />
+              }
+            >
+              {allDirectoriesExpanded ? (
+                <ChevronsDownUpIcon className="size-3" />
+              ) : (
+                <ChevronsUpDownIcon className="size-3" />
+              )}
+            </TooltipTrigger>
+            <TooltipPopup side="top">
+              {allDirectoriesExpanded ? "Collapse all" : "Expand all"}
+            </TooltipPopup>
+          </Tooltip>
+          <Tooltip>
+            <TooltipTrigger
+              render={
+                <Button
+                  type="button"
+                  size="icon-xs"
+                  variant="outline"
+                  className="!size-[22px]"
+                  aria-label="View diff"
+                  onClick={() => onOpenTurnDiff(turnSummary.turnId, checkpointFiles[0]?.path)}
+                />
+              }
+            >
+              <FileDiffIcon className="size-3" />
+            </TooltipTrigger>
+            <TooltipPopup side="top">View diff</TooltipPopup>
+          </Tooltip>
         </div>
       </div>
       <ChangedFilesTree