diff --git a/packages/dashboard/src/__tests__/routes-diff-done-tasks.test.ts b/packages/dashboard/src/__tests__/routes-diff-done-tasks.test.ts
index f3e939123..96724159c 100644
--- a/packages/dashboard/src/__tests__/routes-diff-done-tasks.test.ts
+++ b/packages/dashboard/src/__tests__/routes-diff-done-tasks.test.ts
@@ -353,6 +353,96 @@ describe("FN-4524 done-task diff stats", () => {
     }
   });
 
+  it("uses live shortstat for done tasks even when mergeDetails stats are stale", async () => {
+    const rootDir = mkdtempSync(join(tmpdir(), "fn-4527-done-stale-merge-details-"));
+
+    try {
+      git(rootDir, "init", "-b", "main");
+      git(rootDir, "config", "user.email", "fusion@example.com");
+      git(rootDir, "config", "user.name", "Fusion");
+
+      commitFile(rootDir, "base.ts", "export const base = 1;\n", "base");
+
+      git(rootDir, "checkout", "-b", "task-branch");
+      commitFile(rootDir, "one.ts", "export const one = 1;\n", "task one");
+      commitFile(rootDir, "two.ts", "export const two = 2;\n", "task two");
+
+      git(rootDir, "checkout", "main");
+      git(rootDir, "merge", "--squash", "task-branch");
+      git(rootDir, "commit", "-m", "squash merge task");
+      const squashCommit = git(rootDir, "rev-parse", "HEAD");
+      const expected = shortstatForShow(rootDir, squashCommit);
+      expect(expected).toEqual({ filesChanged: 2, additions: 2, deletions: 0 });
+
+      const store = new RealGitStore(rootDir);
+      store.addTask({
+        id: "FN-4524",
+        title: "stale mergeDetails test",
+        description: "stale mergeDetails test",
+        column: "done",
+        dependencies: [],
+        steps: [],
+        currentStep: 0,
+        log: [],
+        createdAt: "2026-05-14T00:00:00.000Z",
+        updatedAt: "2026-05-14T00:00:00.000Z",
+        columnMovedAt: "2026-05-14T00:00:00.000Z",
+        baseBranch: "main",
+        mergeDetails: {
+          commitSha: squashCommit,
+          filesChanged: 108,
+          insertions: 999,
+          deletions: 999,
+        },
+      } as Task);
+
+      const response = await getDoneDiff(store);
+      expect(response.status).toBe(200);
+      expect(response.body.stats).toEqual(expected);
+      expect(response.body.stats).not.toEqual({ filesChanged: 108, additions: 999, deletions: 999 });
+    } finally {
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  });
+
+  it("returns zero stats when merge SHA is unresolvable even if mergeDetails has stored counts", async () => {
+    const rootDir = mkdtempSync(join(tmpdir(), "fn-4527-done-no-merge-sha-"));
+
+    try {
+      git(rootDir, "init", "-b", "main");
+      git(rootDir, "config", "user.email", "fusion@example.com");
+      git(rootDir, "config", "user.name", "Fusion");
+      commitFile(rootDir, "base.ts", "export const base = 1;\n", "base");
+
+      const store = new RealGitStore(rootDir);
+      store.addTask({
+        id: "FN-4524",
+        title: "missing sha test",
+        description: "missing sha test",
+        column: "done",
+        dependencies: [],
+        steps: [],
+        currentStep: 0,
+        log: [],
+        createdAt: "2026-05-14T00:00:00.000Z",
+        updatedAt: "2026-05-14T00:00:00.000Z",
+        columnMovedAt: "2026-05-14T00:00:00.000Z",
+        baseBranch: "main",
+        mergeDetails: {
+          filesChanged: 108,
+          insertions: 999,
+          deletions: 999,
+        },
+      } as Task);
+
+      const response = await getDoneDiff(store);
+      expect(response.status).toBe(200);
+      expect(response.body.stats).toEqual({ filesChanged: 0, additions: 0, deletions: 0 });
+    } finally {
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  });
+
   it("matches shortstat when hunks include ++/-- content lines", async () => {
     const rootDir = mkdtempSync(join(tmpdir(), "fn-4524-done-plusminus-"));
 
diff --git a/packages/dashboard/src/routes/register-session-diff-routes.ts b/packages/dashboard/src/routes/register-session-diff-routes.ts
index 2086f8b90..9b15d708f 100644
--- a/packages/dashboard/src/routes/register-session-diff-routes.ts
+++ b/packages/dashboard/src/routes/register-session-diff-routes.ts
@@ -197,6 +197,17 @@ type AggregatedDoneTaskFile = {
   patch: string;
 };
 
+function parseGitShortstat(output: string): { filesChanged: number; additions: number; deletions: number } {
+  const filesMatch = output.match(/(\d+) files? changed/);
+  const additionsMatch = output.match(/(\d+) insertions?\(\+\)/);
+  const deletionsMatch = output.match(/(\d+) deletions?\(-\)/);
+  return {
+    filesChanged: filesMatch ? Number(filesMatch[1]) : 0,
+    additions: additionsMatch ? Number(additionsMatch[1]) : 0,
+    deletions: deletionsMatch ? Number(deletionsMatch[1]) : 0,
+  };
+}
+
 function statusPriority(status: DoneTaskFileStatus): number {
   switch (status) {
     case "added":
@@ -641,13 +652,14 @@ export function registerSessionDiffRoutes(router: Router, deps: SessionDiffRoute
         }
 
         if (!resolvedMergeSha) {
-          const md = task.mergeDetails;
+          // FN-4527: mergeDetails summary stats can be stale after post-merge
+          // rebase-and-push (FN-4526). Never echo stored values from /diff.
           res.json({
             files: [],
             stats: {
-              filesChanged: md?.filesChanged ?? 0,
-              additions: md?.insertions ?? 0,
-              deletions: md?.deletions ?? 0,
+              filesChanged: 0,
+              additions: 0,
+              deletions: 0,
             },
           });
           return;
@@ -697,17 +709,13 @@ export function registerSessionDiffRoutes(router: Router, deps: SessionDiffRoute
           return;
         }
 
-        const patch = await runGitCommand(["diff", diffSpec.range], rootDir, 10000).catch(() => "");
-        const filesChanged = (await runGitCommand(["diff", "--name-only", diffSpec.range], rootDir, 10000)
-          .then((output) => output.split("\n").filter(Boolean).length)
-          .catch(() => 0));
+        const shortstat = await runGitCommand(["show", "--shortstat", "--format=", resolvedMergeSha], rootDir, 10000)
+          .then((output) => parseGitShortstat(output))
+          .catch(() => ({ filesChanged: 0, additions: 0, deletions: 0 }));
 
         res.json({
           files: [],
-          stats: {
-            filesChanged,
-            ...countPatchLines(patch),
-          },
+          stats: shortstat,
         });
         return;
       }