diff --git a/packages/core/src/task-detail/cloudToolChanges.test.ts b/packages/core/src/task-detail/cloudToolChanges.test.ts
index 3b6aeeee2..2cbd13789 100644
--- a/packages/core/src/task-detail/cloudToolChanges.test.ts
+++ b/packages/core/src/task-detail/cloudToolChanges.test.ts
@@ -1,11 +1,19 @@
 import { describe, expect, it } from "vitest";
 
 import {
+  cachedDiffStats,
   extractCloudFileContent,
   extractCloudToolChangedFiles,
   type ParsedToolCall,
 } from "./cloudToolChanges";
 
+function diffObj(
+  newText: string,
+  oldText: string,
+): NonNullable<Parameters<typeof cachedDiffStats>[0]> {
+  return { type: "diff", path: "src/f.ts", newText, oldText };
+}
+
 function toolCall(overrides: Partial<ParsedToolCall>): ParsedToolCall {
   return {
     toolCallId: overrides.toolCallId ?? "tc-1",
@@ -62,6 +70,56 @@ describe("extractCloudToolChangedFiles", () => {
     expect(result).toHaveLength(1);
     expect(result[0].path).toBe("src/app.ts");
   });
+
+  it.each([
+    {
+      name: "new file counts all lines as added",
+      kind: "write" as const,
+      oldText: undefined,
+      newText: "a\nb\nc",
+      added: 3,
+      removed: 0,
+    },
+    {
+      name: "modified file counts added and removed",
+      kind: "edit" as const,
+      oldText: "a\nb\nc",
+      newText: "a\nB\nc\nd",
+      added: 2,
+      removed: 1,
+    },
+    {
+      name: "pure removal counts removed only",
+      kind: "edit" as const,
+      oldText: "a\nb\nc",
+      newText: "a",
+      added: 0,
+      removed: 2,
+    },
+  ])("diff stats: $name", ({ kind, oldText, newText, added, removed }) => {
+    const calls = makeToolCalls(
+      toolCall({
+        toolCallId: "tc",
+        kind,
+        locations: [{ path: "src/f.ts" }],
+        content: diffContent("src/f.ts", newText, oldText),
+      }),
+    );
+    const [file] = extractCloudToolChangedFiles(calls);
+    expect(file.linesAdded).toBe(added);
+    expect(file.linesRemoved).toBe(removed);
+  });
+
+  it("memoizes diff stats by diff-object identity", () => {
+    const diff = diffObj("a\nB\nc", "a\nb\nc");
+    const first = cachedDiffStats(diff);
+    expect(cachedDiffStats(diff)).toBe(first);
+
+    const distinctButEqual = diffObj("a\nB\nc", "a\nb\nc");
+    const recomputed = cachedDiffStats(distinctButEqual);
+    expect(recomputed).not.toBe(first);
+    expect(recomputed).toEqual(first);
+  });
 });
 
 describe("extractCloudFileContent", () => {
diff --git a/packages/core/src/task-detail/cloudToolChanges.ts b/packages/core/src/task-detail/cloudToolChanges.ts
index 9030cfcd2..6759aa6b8 100644
--- a/packages/core/src/task-detail/cloudToolChanges.ts
+++ b/packages/core/src/task-detail/cloudToolChanges.ts
@@ -138,6 +138,22 @@ function getDiffStats(
   return { added, removed };
 }
 
+const diffStatsCache = new WeakMap<
+  Extract<ToolCallContent, { type: "diff" }>,
+  { added?: number; removed?: number }
+>();
+
+export function cachedDiffStats(
+  diff: Extract<ToolCallContent, { type: "diff" }> | undefined,
+): { added?: number; removed?: number } {
+  if (!diff) return {};
+  const cached = diffStatsCache.get(diff);
+  if (cached) return cached;
+  const stats = getDiffStats(diff.oldText, diff.newText);
+  diffStatsCache.set(diff, stats);
+  return stats;
+}
+
 export interface CloudEventSummary {
   toolCalls: Map<string, ParsedToolCall>;
 }
@@ -266,7 +282,7 @@ export function extractCloudToolChangedFiles(
         status: "deleted",
       };
     } else {
-      const diffStats = getDiffStats(diff?.oldText, diff?.newText);
+      const diffStats = cachedDiffStats(diff);
       file = {
         path,
         status: kind === "write" && !diff?.oldText ? "added" : "modified",