diff --git a/webui/src/components/thread/AgentActivityCluster.tsx b/webui/src/components/thread/AgentActivityCluster.tsx
index acde2f6a..fddd92e0 100644
--- a/webui/src/components/thread/AgentActivityCluster.tsx
+++ b/webui/src/components/thread/AgentActivityCluster.tsx
@@ -40,6 +40,7 @@ import {
   isAgentActivityMember,
   isReasoningOnlyAssistant,
 } from "@/lib/activity-timeline";
+import { useFileEditDisplayMode } from "@/hooks/useFileEditDisplayMode";
 import { useLogoFallback } from "@/hooks/useLogoFallback";
 import { logoFallbackUrls } from "@/lib/provider-brand";
 import { canonicalToolTrace, formatToolCallTrace } from "@/lib/tool-traces";
@@ -144,6 +145,7 @@ export function AgentActivityCluster({
   onOpenFilePreview,
 }: AgentActivityClusterProps) {
   const { t } = useTranslation();
+  const fileEditDisplayMode = useFileEditDisplayMode();
   const pageVisible = usePageVisibility();
   const activityMessages = useMemo(() => coalesceActivityMessages(messages), [messages]);
   const fileEdits = useMemo(
@@ -305,6 +307,7 @@ export function AgentActivityCluster({
       <div className={cn("w-full", hasBodyBelow && "mb-2")}>
         <FileEditGroup
           edits={fileEdits}
+          displayMode={fileEditDisplayMode}
           onOpenFilePreview={onOpenFilePreview}
         />
       </div>
@@ -331,6 +334,7 @@ export function AgentActivityCluster({
         {fileEdits.length ? (
           <FileEditGroup
             edits={fileEdits}
+            displayMode={fileEditDisplayMode}
             onOpenFilePreview={onOpenFilePreview}
           />
         ) : null}
@@ -1031,6 +1035,7 @@ function summarizeFileEdits(edits: UIFileEdit[], active: boolean): FileEditSumma
       operation: edit.operation,
       pending: !!edit.pending && !edit.path,
       error: edit.error,
+      diff: edit.diff,
     }];
   });
 }
diff --git a/webui/src/components/thread/activity/FileEditRow.tsx b/webui/src/components/thread/activity/FileEditRow.tsx
index 3d7ce287..f87e36b1 100644
--- a/webui/src/components/thread/activity/FileEditRow.tsx
+++ b/webui/src/components/thread/activity/FileEditRow.tsx
@@ -1,16 +1,45 @@
+import { useEffect, useMemo, useState } from "react";
 import {
   AlertCircle,
   CheckCircle2,
+  ChevronDown,
+  ChevronRight,
+  ChevronUp,
   CircleDashed,
+  ExternalLink,
 } from "lucide-react";
 import { useTranslation } from "react-i18next";
 
 import { FileReferenceChip } from "@/components/FileReferenceChip";
-import type { UIFileEdit } from "@/lib/types";
+import { codeLanguageFromPath } from "@/lib/code-language";
+import {
+  hasRenderableFileDiff,
+  parseRenderableFileDiff,
+  type RenderableFileDiff,
+  type RenderableFileDiffHunk,
+} from "@/lib/file-diff";
+import type { FileEditDisplayMode } from "@/lib/local-preferences";
+import type { UIFileDiff, UIFileEdit } from "@/lib/types";
 import { cn } from "@/lib/utils";
 
 import { ActivityStep } from "./ActivityStep";
 import { DiffPair } from "./DiffPair";
+import { DiffSyntaxHighlight } from "./DiffSyntaxHighlight";
+
+const INITIAL_VISIBLE_DIFF_LINES = 160;
+const AUTO_COLLAPSE_DIFF_LINES = INITIAL_VISIBLE_DIFF_LINES;
+
+interface VisibleDiffHunk {
+  hunk: RenderableFileDiffHunk;
+  skippedBefore: number;
+}
+
+interface VisibleDiff {
+  hunks: VisibleDiffHunk[];
+  hiddenLineCount: number;
+}
+
+const EMPTY_VISIBLE_DIFF: VisibleDiff = { hunks: [], hiddenLineCount: 0 };
 
 export interface FileEditSummary {
   key: string;
@@ -24,13 +53,16 @@ export interface FileEditSummary {
   operation?: UIFileEdit["operation"];
   pending: boolean;
   error?: string;
+  diff?: UIFileDiff;
 }
 
 export function FileEditGroup({
   edits,
+  displayMode,
   onOpenFilePreview,
 }: {
   edits: FileEditSummary[];
+  displayMode: FileEditDisplayMode;
   onOpenFilePreview?: (path: string) => void;
 }) {
   if (edits.length === 0) return null;
@@ -40,6 +72,7 @@ export function FileEditGroup({
         <FileEditRow
           key={edit.key}
           edit={edit}
+          displayMode={displayMode}
           onOpenFilePreview={onOpenFilePreview}
         />
       ))}
@@ -49,9 +82,11 @@ export function FileEditGroup({
 
 function FileEditRow({
   edit,
+  displayMode,
   onOpenFilePreview,
 }: {
   edit: FileEditSummary;
+  displayMode: FileEditDisplayMode;
   onOpenFilePreview?: (path: string) => void;
 }) {
   const { t } = useTranslation();
@@ -59,6 +94,7 @@ function FileEditRow({
   const failed = edit.status === "error";
   const action = fileEditAction(edit, editing, failed);
   const hasCountedDiff = !failed && !edit.binary && hasVisibleDiffStats(edit);
+  const showDiff = canRenderDiff(edit, displayMode);
   const statusIcon = failed ? (
     <AlertCircle className="h-3 w-3" aria-hidden />
   ) : editing ? (
@@ -68,42 +104,54 @@ function FileEditRow({
   );
 
   return (
-    <ActivityStep
-      marker={(
-        <span
-          className={cn(
-            "grid h-3.5 w-3.5 place-items-center rounded-full border bg-background transition-colors",
-            failed && "border-destructive/30 text-destructive/78",
-            editing && "border-muted-foreground/24 text-muted-foreground/65",
-            !failed && !editing && "border-emerald-500/28 text-emerald-500/78",
-          )}
-        >
-          {statusIcon}
-        </span>
-      )}
-      active={editing}
-      tone={failed ? "error" : editing ? "active" : "success"}
-      className="text-xs"
-      ariaLabel={edit.path ? `${action} ${edit.path}` : action}
-      label={edit.pending && !edit.path
-        ? t("message.fileEditPreparing", { defaultValue: "Preparing file edit…" })
-        : (
-          <span className="flex min-w-0 items-center gap-1.5 overflow-hidden whitespace-nowrap">
-            <span className="shrink-0">{action}</span>
-            <FileReferenceChip
-              path={edit.path}
-              previewPath={edit.absolute_path || edit.path}
-              onOpen={onOpenFilePreview}
-              display="path"
-              active={editing}
-              className="min-w-0"
-              textClassName="truncate text-[12px]"
-              testId="activity-file-reference"
-            />
-            {hasCountedDiff ? <DiffPair added={edit.added} deleted={edit.deleted} /> : null}
+    <div className="min-w-0">
+      <ActivityStep
+        marker={(
+          <span
+            className={cn(
+              "grid h-3.5 w-3.5 place-items-center rounded-full border bg-background transition-colors",
+              failed && "border-destructive/30 text-destructive/78",
+              editing && "border-muted-foreground/24 text-muted-foreground/65",
+              !failed && !editing && "border-emerald-500/28 text-emerald-500/78",
+            )}
+          >
+            {statusIcon}
           </span>
         )}
-    />
+        active={editing}
+        tone={failed ? "error" : editing ? "active" : "success"}
+        className="text-xs"
+        ariaLabel={edit.path ? `${action} ${edit.path}` : action}
+        label={edit.pending && !edit.path
+          ? t("message.fileEditPreparing", { defaultValue: "Preparing file edit…" })
+          : (
+            <span className="flex min-w-0 items-center gap-1.5 overflow-hidden whitespace-nowrap">
+              <span className="shrink-0">{action}</span>
+              <FileReferenceChip
+                path={edit.path}
+                previewPath={edit.absolute_path || edit.path}
+                onOpen={onOpenFilePreview}
+                display="path"
+                active={editing}
+                className="min-w-0"
+                textClassName="truncate text-[12px]"
+                testId="activity-file-reference"
+              />
+              {hasCountedDiff ? <DiffPair added={edit.added} deleted={edit.deleted} /> : null}
+            </span>
+          )}
+      />
+      {showDiff ? (
+        <div className="ml-[2.125rem] min-w-0">
+          <FileUnifiedDiff
+            diff={edit.diff!}
+            collapsed={displayMode === "collapsed_diff"}
+            previewPath={edit.absolute_path || edit.path}
+            onOpenFilePreview={onOpenFilePreview}
+          />
+        </div>
+      ) : null}
+    </div>
   );
 }
 
@@ -117,3 +165,239 @@ function fileEditAction(edit: FileEditSummary, editing: boolean, failed: boolean
   if (editing) return deleting ? "Deleting" : "Editing";
   return deleting ? "Deleted" : "Edited";
 }
+
+function canRenderDiff(edit: FileEditSummary, displayMode: FileEditDisplayMode): boolean {
+  return (
+    displayMode !== "summary"
+    && edit.status !== "editing"
+    && edit.status !== "error"
+    && hasRenderableFileDiff(edit.diff)
+  );
+}
+
+function FileUnifiedDiff({
+  diff,
+  collapsed,
+  previewPath,
+  onOpenFilePreview,
+}: {
+  diff: UIFileDiff;
+  collapsed: boolean;
+  previewPath?: string;
+  onOpenFilePreview?: (path: string) => void;
+}) {
+  const { t } = useTranslation();
+  const tx = (key: string, fallback: string) => t(key, { defaultValue: fallback });
+  const [open, setOpen] = useState(false);
+  const [expandedLines, setExpandedLines] = useState(false);
+  const renderableDiff = useMemo(() => parseRenderableFileDiff(diff), [diff]);
+  const language = useMemo(() => codeLanguageFromPath(previewPath), [previewPath]);
+  const totalLineCount = useMemo(() => countDiffLines(renderableDiff), [renderableDiff]);
+  const shouldAutoCollapse = totalLineCount > AUTO_COLLAPSE_DIFF_LINES || !!diff.truncated;
+  const startsCollapsed = collapsed || shouldAutoCollapse;
+  const shouldRenderBody = !startsCollapsed || open;
+  const shouldLimitLines = totalLineCount > INITIAL_VISIBLE_DIFF_LINES;
+  const lineLimit = expandedLines || !shouldLimitLines
+    ? totalLineCount
+    : INITIAL_VISIBLE_DIFF_LINES;
+  const visibleDiff = useMemo(
+    () => shouldRenderBody
+      ? selectVisibleDiffLines(renderableDiff, lineLimit, totalLineCount)
+      : EMPTY_VISIBLE_DIFF,
+    [lineLimit, renderableDiff, shouldRenderBody, totalLineCount],
+  );
+  const lineCountLabel = t("message.fileEditDiffLineCount", {
+    count: diff.truncated ? `${totalLineCount}+` : totalLineCount,
+    defaultValue: "{{count}} lines",
+  });
+  const viewDiffLabel = shouldAutoCollapse
+    ? tx("message.fileEditViewLargeDiff", "View large diff")
+    : tx("message.fileEditViewDiff", "View diff");
+
+  useEffect(() => {
+    setOpen(false);
+    setExpandedLines(false);
+  }, [diff]);
+
+  const handleToggleOpen = () => {
+    if (open) setExpandedLines(false);
+    setOpen(!open);
+  };
+
+  if (totalLineCount === 0) return null;
+
+  const renderBody = () => (
+    <div
+      className="mt-1 overflow-hidden rounded-md border border-border/55 bg-background/80 shadow-[0_1px_0_rgba(15,23,42,0.03)]"
+      data-testid="file-edit-diff"
+    >
+      {visibleDiff.hunks.map(({ hunk, skippedBefore }, index) => (
+        <div
+          key={`${hunk.old_start}-${hunk.new_start}-${index}`}
+          className={cn("min-w-0", index > 0 && "border-t border-border/45")}
+        >
+          {skippedBefore > 0 ? <DiffHunkGap lineCount={skippedBefore} /> : null}
+          <div className="overflow-x-auto">
+            <DiffSyntaxHighlight language={language} lines={hunk.lines} />
+          </div>
+        </div>
+      ))}
+      {visibleDiff.hiddenLineCount > 0 ? (
+        <div className="border-t border-border/45 bg-muted/30 px-2 py-1">
+          <button
+            type="button"
+            className={cn(
+              "inline-flex items-center gap-1 rounded px-1 py-0.5 text-[11px] font-medium",
+              "text-muted-foreground transition-colors hover:bg-muted/65 hover:text-foreground",
+            )}
+            data-testid="file-edit-diff-expand-lines"
+            onClick={() => setExpandedLines(true)}
+          >
+            <ChevronDown className="h-3 w-3" aria-hidden />
+            {t("message.fileEditShowMoreLines", {
+              count: visibleDiff.hiddenLineCount,
+              defaultValue: "Show {{count}} more lines",
+            })}
+          </button>
+        </div>
+      ) : expandedLines && shouldLimitLines ? (
+        <div className="border-t border-border/45 bg-muted/30 px-2 py-1">
+          <button
+            type="button"
+            className={cn(
+              "inline-flex items-center gap-1 rounded px-1 py-0.5 text-[11px] font-medium",
+              "text-muted-foreground transition-colors hover:bg-muted/65 hover:text-foreground",
+            )}
+            data-testid="file-edit-diff-collapse-lines"
+            onClick={() => setExpandedLines(false)}
+          >
+            <ChevronUp className="h-3 w-3" aria-hidden />
+            {tx("message.fileEditShowFewerLines", "Show fewer lines")}
+          </button>
+        </div>
+      ) : null}
+      {diff.truncated ? (
+        <div
+          className="flex flex-wrap items-center gap-x-2 gap-y-1 border-t border-border/45 bg-muted/35 px-2 py-1 text-[11px] text-muted-foreground"
+          data-testid="file-edit-diff-truncated"
+        >
+          <span>
+            {tx("message.fileEditDiffTruncated", "Diff truncated. Open the file for the full change.")}
+          </span>
+          {previewPath && onOpenFilePreview ? (
+            <button
+              type="button"
+              className={cn(
+                "inline-flex items-center gap-1 rounded px-1 py-0.5 font-medium",
+                "text-muted-foreground transition-colors hover:bg-muted/65 hover:text-foreground",
+              )}
+              data-testid="file-edit-diff-open-file"
+              onClick={() => onOpenFilePreview(previewPath)}
+            >
+              <ExternalLink className="h-3 w-3" aria-hidden />
+              {tx("message.fileEditOpenFile", "Open file")}
+            </button>
+          ) : null}
+        </div>
+      ) : null}
+    </div>
+  );
+
+  if (!startsCollapsed) return renderBody();
+
+  return (
+    <div className="mt-1">
+      <button
+        type="button"
+        aria-expanded={open}
+        data-testid="file-edit-diff-toggle"
+        onClick={handleToggleOpen}
+        className={cn(
+          "flex w-full cursor-pointer items-center gap-2 rounded-md border border-border/45 bg-muted/35 px-2 py-1 text-left",
+          "text-[11px] font-medium text-muted-foreground transition-colors hover:bg-muted/50",
+        )}
+      >
+        <ChevronRight
+          className={cn("h-3 w-3 shrink-0 transition-transform", open && "rotate-90")}
+          aria-hidden
+        />
+        <span className="min-w-0 flex-1">{viewDiffLabel}</span>
+        <span className="shrink-0 text-muted-foreground/65">{lineCountLabel}</span>
+      </button>
+      {open ? renderBody() : null}
+    </div>
+  );
+}
+
+function countDiffLines(diff: RenderableFileDiff): number {
+  return diff.hunks.reduce((total, hunk) => total + hunk.lines.length, 0);
+}
+
+function selectVisibleDiffLines(
+  diff: RenderableFileDiff,
+  lineLimit: number,
+  totalLineCount: number,
+): VisibleDiff {
+  if (lineLimit >= totalLineCount) {
+    return {
+      hunks: diff.hunks.map((hunk, index) => ({
+        hunk,
+        skippedBefore: index > 0 ? countSkippedUnchangedLines(diff.hunks[index - 1], hunk) : 0,
+      })),
+      hiddenLineCount: 0,
+    };
+  }
+
+  let remaining = Math.max(0, lineLimit);
+  const hunks: VisibleDiffHunk[] = [];
+  let previousHunk: RenderableFileDiffHunk | null = null;
+  for (const hunk of diff.hunks) {
+    if (remaining <= 0) break;
+    const skippedBefore = previousHunk ? countSkippedUnchangedLines(previousHunk, hunk) : 0;
+    if (hunk.lines.length <= remaining) {
+      hunks.push({ hunk, skippedBefore });
+      remaining -= hunk.lines.length;
+      previousHunk = hunk;
+      continue;
+    }
+    hunks.push({ hunk: { ...hunk, lines: hunk.lines.slice(0, remaining) }, skippedBefore });
+    remaining = 0;
+    previousHunk = hunk;
+  }
+  return {
+    hunks,
+    hiddenLineCount: Math.max(0, totalLineCount - lineLimit),
+  };
+}
+
+function countSkippedUnchangedLines(
+  previous: RenderableFileDiffHunk,
+  current: RenderableFileDiffHunk,
+): number {
+  const oldGap = current.old_start - (previous.old_start + previous.old_lines);
+  const newGap = current.new_start - (previous.new_start + previous.new_lines);
+  return Math.max(0, oldGap, newGap);
+}
+
+function DiffHunkGap({ lineCount }: { lineCount: number }) {
+  const { t } = useTranslation();
+  return (
+    <div
+      className="flex items-center gap-2 bg-muted/35 px-2 py-1 text-[11px] text-muted-foreground"
+      data-testid="file-edit-diff-hunk-gap"
+    >
+      <span
+        className="select-none rounded border border-border/45 bg-background/70 px-1 font-mono text-muted-foreground/70"
+        aria-hidden
+      >
+        ...
+      </span>
+      <span>
+        {t("message.fileEditUnchangedLinesHidden", {
+          count: lineCount,
+          defaultValue: "{{count}} unchanged lines hidden",
+        })}
+      </span>
+    </div>
+  );
+}
diff --git a/webui/src/tests/agent-activity-cluster.test.tsx b/webui/src/tests/agent-activity-cluster.test.tsx
index 1b0fee11..2ad1117d 100644
--- a/webui/src/tests/agent-activity-cluster.test.tsx
+++ b/webui/src/tests/agent-activity-cluster.test.tsx
@@ -2,6 +2,7 @@ import { act, fireEvent, render, screen, waitFor } from "@testing-library/react"
 import { describe, expect, it, vi } from "vitest";
 
 import { AgentActivityCluster } from "@/components/thread/AgentActivityCluster";
+import { DEFAULT_LOCAL_PREFS, writeLocalPreferences } from "@/lib/local-preferences";
 import type { CliAppInfo, McpPresetInfo, UIMessage } from "@/lib/types";
 
 const BLENDER_CLI_APP: CliAppInfo = {
@@ -448,7 +449,7 @@ describe("AgentActivityCluster", () => {
     }
   });
 
-  it("keeps file edits flat even when the legacy diff preference is enabled", () => {
+  it("renders file edit diffs and responds to preference changes", () => {
     localStorage.setItem(
       "nanobot-webui.settings-preferences",
       JSON.stringify({ fileEditDisplayMode: "diff" }),
@@ -488,17 +489,27 @@ describe("AgentActivityCluster", () => {
         />,
       );
 
-      expect(screen.queryByTestId("file-edit-diff")).not.toBeInTheDocument();
-      expect(screen.queryByText("return <Old />;")).not.toBeInTheDocument();
-      expect(screen.queryByText("return <New />;")).not.toBeInTheDocument();
+      expect(screen.getByTestId("file-edit-diff")).toBeInTheDocument();
+      expect(screen.getByText("return <Old />;")).toBeInTheDocument();
+      expect(screen.getByText("return <New />;")).toBeInTheDocument();
       expect(screen.getByTestId("activity-file-reference")).toHaveTextContent("src/app.tsx");
       expect(screen.getAllByTestId("activity-diff-pair")).toHaveLength(1);
+
+      act(() => {
+        writeLocalPreferences({ ...DEFAULT_LOCAL_PREFS, fileEditDisplayMode: "summary" });
+      });
+      expect(screen.queryByTestId("file-edit-diff")).not.toBeInTheDocument();
+
+      act(() => {
+        writeLocalPreferences({ ...DEFAULT_LOCAL_PREFS, fileEditDisplayMode: "diff" });
+      });
+      expect(screen.getByTestId("file-edit-diff")).toBeInTheDocument();
     } finally {
       localStorage.removeItem("nanobot-webui.settings-preferences");
     }
   });
 
-  it("does not render diff hunks inside the activity list", () => {
+  it("renders folded separators between separated file edit hunks", () => {
     localStorage.setItem(
       "nanobot-webui.settings-preferences",
       JSON.stringify({ fileEditDisplayMode: "diff" }),
@@ -544,16 +555,18 @@ describe("AgentActivityCluster", () => {
         />,
       );
 
-      expect(screen.queryByTestId("file-edit-diff-hunk-gap")).not.toBeInTheDocument();
+      expect(screen.getByTestId("file-edit-diff-hunk-gap")).toHaveTextContent(
+        "21 unchanged lines hidden",
+      );
       expect(screen.queryByText("@@ -25,3 +25,3 @@")).not.toBeInTheDocument();
-      expect(screen.queryByText("return newSecond;")).not.toBeInTheDocument();
+      expect(screen.getByText("return newSecond;")).toBeInTheDocument();
       expect(screen.getByTestId("activity-file-reference")).toHaveTextContent("src/app.tsx");
     } finally {
       localStorage.removeItem("nanobot-webui.settings-preferences");
     }
   });
 
-  it("summarizes long file edit diffs without an expansion control", () => {
+  it("keeps long file edit diffs collapsed until opened", () => {
     localStorage.setItem(
       "nanobot-webui.settings-preferences",
       JSON.stringify({ fileEditDisplayMode: "diff" }),
@@ -592,16 +605,46 @@ describe("AgentActivityCluster", () => {
         />,
       );
 
-      expect(screen.queryByTestId("file-edit-diff-toggle")).not.toBeInTheDocument();
+      const toggle = screen.getByTestId("file-edit-diff-toggle");
+      expect(toggle).toHaveAttribute("aria-expanded", "false");
+      expect(toggle).toHaveTextContent("View large diff");
+      expect(toggle).toHaveTextContent("165 lines");
       expect(screen.queryByTestId("file-edit-diff")).not.toBeInTheDocument();
       expect(screen.queryByText("line-1")).not.toBeInTheDocument();
-      expect(screen.getByText("+165")).toBeInTheDocument();
+
+      fireEvent.click(toggle);
+
+      expect(toggle).toHaveAttribute("aria-expanded", "true");
+      expect(screen.getByText("line-160")).toBeInTheDocument();
+      expect(screen.queryByText("line-161")).not.toBeInTheDocument();
+      expect(screen.getByTestId("file-edit-diff-expand-lines")).toHaveTextContent(
+        "Show 5 more lines",
+      );
+
+      fireEvent.click(screen.getByTestId("file-edit-diff-expand-lines"));
+
+      expect(screen.getByText("line-165")).toBeInTheDocument();
+      expect(screen.getByTestId("file-edit-diff-collapse-lines")).toHaveTextContent(
+        "Show fewer lines",
+      );
+
+      fireEvent.click(screen.getByTestId("file-edit-diff-collapse-lines"));
+
+      expect(screen.queryByText("line-165")).not.toBeInTheDocument();
+      expect(screen.getByTestId("file-edit-diff-expand-lines")).toHaveTextContent(
+        "Show 5 more lines",
+      );
+
+      fireEvent.click(toggle);
+
+      expect(toggle).toHaveAttribute("aria-expanded", "false");
+      expect(screen.queryByTestId("file-edit-diff")).not.toBeInTheDocument();
     } finally {
       localStorage.removeItem("nanobot-webui.settings-preferences");
     }
   });
 
-  it("ignores the legacy collapsed diff mode in the activity list", () => {
+  it("does not mount collapsed file edit diffs until opened", () => {
     localStorage.setItem(
       "nanobot-webui.settings-preferences",
       JSON.stringify({ fileEditDisplayMode: "collapsed_diff" }),
@@ -641,16 +684,24 @@ describe("AgentActivityCluster", () => {
         />,
       );
 
-      expect(screen.queryByTestId("file-edit-diff-toggle")).not.toBeInTheDocument();
+      const toggle = screen.getByTestId("file-edit-diff-toggle");
+      expect(toggle).toHaveAttribute("aria-expanded", "false");
+      expect(toggle).toHaveTextContent("View diff");
+      expect(toggle).toHaveTextContent("3 lines");
       expect(screen.queryByTestId("file-edit-diff")).not.toBeInTheDocument();
       expect(screen.queryByText("return <New />;")).not.toBeInTheDocument();
-      expect(screen.getByTestId("activity-file-reference")).toHaveTextContent("src/app.tsx");
+
+      fireEvent.click(toggle);
+
+      expect(toggle).toHaveAttribute("aria-expanded", "true");
+      expect(screen.getByTestId("file-edit-diff")).toBeInTheDocument();
+      expect(screen.getByText("return <New />;")).toBeInTheDocument();
     } finally {
       localStorage.removeItem("nanobot-webui.settings-preferences");
     }
   });
 
-  it("opens the edited file directly instead of expanding a truncated diff", () => {
+  it("offers the file preview entry point when a diff payload is truncated", () => {
     localStorage.setItem(
       "nanobot-webui.settings-preferences",
       JSON.stringify({ fileEditDisplayMode: "diff" }),
@@ -691,9 +742,15 @@ describe("AgentActivityCluster", () => {
         />,
       );
 
-      expect(screen.queryByTestId("file-edit-diff-toggle")).not.toBeInTheDocument();
+      const toggle = screen.getByTestId("file-edit-diff-toggle");
+      expect(toggle).toHaveAttribute("aria-expanded", "false");
+      expect(toggle).toHaveTextContent("View large diff");
       expect(screen.queryByTestId("file-edit-diff-truncated")).not.toBeInTheDocument();
-      fireEvent.click(screen.getByTestId("activity-file-reference"));
+
+      fireEvent.click(toggle);
+
+      expect(screen.getByTestId("file-edit-diff-truncated")).toHaveTextContent("Diff truncated");
+      fireEvent.click(screen.getByTestId("file-edit-diff-open-file"));
 
       expect(onOpenFilePreview).toHaveBeenCalledWith("/repo/src/app.tsx");
     } finally {
@@ -1601,7 +1658,7 @@ describe("AgentActivityCluster", () => {
     expect(screen.queryByText(/\[Errno 13\]/)).not.toBeInTheDocument();
   });
 
-  it("renders repeated edits for the same path as separate actions", () => {
+  it("keeps repeated edits for the same path as separate actions", () => {
     localStorage.setItem(
       "nanobot-webui.settings-preferences",
       JSON.stringify({ fileEditDisplayMode: "diff" }),
@@ -1679,9 +1736,9 @@ describe("AgentActivityCluster", () => {
       expect(failedRow).toBeInTheDocument();
       expect(failedRow).not.toHaveAttribute("title");
       expect(screen.queryByText("patch failed")).not.toBeInTheDocument();
-      expect(screen.queryByTestId("file-edit-diff")).not.toBeInTheDocument();
-      expect(screen.queryByText("<canvas />")).not.toBeInTheDocument();
-      expect(screen.queryByText("const fps = 60;")).not.toBeInTheDocument();
+      expect(screen.getAllByTestId("file-edit-diff")).toHaveLength(2);
+      expect(screen.getByText("<canvas />")).toBeInTheDocument();
+      expect(screen.getByText("const fps = 60;")).toBeInTheDocument();
       expect(screen.getAllByText("+2").length).toBeGreaterThan(0);
       expect(screen.getAllByText("-1").length).toBeGreaterThan(0);
       expect(screen.getAllByText("+6").length).toBeGreaterThan(0);
@@ -1691,6 +1748,50 @@ describe("AgentActivityCluster", () => {
     }
   });
 
+  it("keeps the latest failed attempt visible after an earlier edit succeeded", () => {
+    render(
+      <AgentActivityCluster
+        messages={activityMessages("", {
+          id: "t2",
+          role: "tool",
+          kind: "trace",
+          content: "edit_file()",
+          traces: ["edit_file()"],
+          fileEdits: [
+            {
+              call_id: "call-edit-1",
+              tool: "edit_file",
+              path: "src/app.tsx",
+              phase: "end",
+              added: 1,
+              deleted: 0,
+              approximate: false,
+              status: "done",
+            },
+            {
+              call_id: "call-edit-2",
+              tool: "edit_file",
+              path: "src/app.tsx",
+              phase: "error",
+              added: 0,
+              deleted: 0,
+              approximate: false,
+              status: "error",
+              error: "patch failed",
+            },
+          ],
+          createdAt: 3,
+        })}
+        isTurnStreaming={false}
+        hasBodyBelow={false}
+      />,
+    );
+
+    expect(screen.getByText("Edited")).toBeInTheDocument();
+    expect(screen.getByText("Could not edit")).toBeInTheDocument();
+    expect(screen.getAllByTestId("activity-file-reference")).toHaveLength(2);
+  });
+
   it("keeps tool event embeds out of the flat activity list", () => {
     render(
       <AgentActivityCluster