diff --git a/apps/server/src/vcs/GitVcsDriverCore.test.ts b/apps/server/src/vcs/GitVcsDriverCore.test.ts
index 4a5dff870fd20831f8e82d49d2d69e0f3385a2f9..47ca4e1787001f6be4415eea4bd4525352b803d1 100644
--- a/apps/server/src/vcs/GitVcsDriverCore.test.ts
+++ b/apps/server/src/vcs/GitVcsDriverCore.test.ts
@@ -1449,6 +1449,44 @@ it.layer(TestLayer)("GitVcsDriver core integration", (it) => {
       }),
     );
 
+    for (const splitIndex of [false, true]) {
+      it.effect(`keeps the preceding second cached in review previews (split: ${splitIndex})`, () =>
+        Effect.gen(function* () {
+          const cwd = yield* makeTmpDir();
+          yield* initRepoWithCommit(cwd);
+          const driver = yield* GitVcsDriver.GitVcsDriver;
+          const fs = yield* FileSystem.FileSystem;
+          const path = yield* Path.Path;
+          yield* writeTextFile(cwd, ".gitattributes", "stable.txt filter=probe\n");
+          yield* writeTextFile(cwd, "stable.txt", "unchanged\n");
+          yield* writeTextFile(
+            cwd,
+            ".git/filter.cjs",
+            'require("node:fs").appendFileSync(".git/filter-runs", "read\\n"); process.stdin.pipe(process.stdout);',
+          );
+          yield* git(cwd, ["config", "filter.probe.clean", "node .git/filter.cjs"]);
+          yield* fs.utimes(path.join(cwd, "stable.txt"), 1_699_999_999.5, 1_699_999_999.5);
+          yield* git(cwd, ["add", "."]);
+          yield* git(cwd, ["commit", "-m", "cache stable file"]);
+          if (splitIndex) yield* git(cwd, ["update-index", "--split-index"]);
+          const indexPath = path.join(cwd, ".git", "index");
+          yield* fs.utimes(indexPath, 1_700_000_000, 1_700_000_000);
+          const originalIndex = yield* fs.readFile(indexPath);
+          const originalMtime = (yield* fs.stat(indexPath)).mtime;
+          yield* writeTextFile(cwd, ".git/filter-runs", "");
+          yield* writeTextFile(cwd, "untracked.txt", "new\n");
+          const preview = yield* driver.getReviewDiffPreview({ cwd });
+          assert.deepStrictEqual(
+            preview.sources.find((source) => source.kind === "working-tree")!.files,
+            [{ path: "untracked.txt", previousPath: null, additions: 1, deletions: 0 }],
+          );
+          assert.strictEqual(yield* fs.readFileString(path.join(cwd, ".git/filter-runs")), "");
+          assert.deepStrictEqual(yield* fs.readFile(indexPath), originalIndex);
+          assert.deepStrictEqual((yield* fs.stat(indexPath)).mtime, originalMtime);
+        }),
+      );
+    }
+
     for (const [timestamp, splitIndex] of [
       [1_700_000_000, false],
       [1_700_000_000.9999, false],
diff --git a/apps/server/src/vcs/GitVcsDriverCore.ts b/apps/server/src/vcs/GitVcsDriverCore.ts
index b15cd28b4e927fae2f6eb47825a14775d2c519e6..8ec274a46611ee2b6491aa5785f3578b863338f1 100644
--- a/apps/server/src/vcs/GitVcsDriverCore.ts
+++ b/apps/server/src/vcs/GitVcsDriverCore.ts
@@ -2358,9 +2358,10 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
     if (indexExists) {
       const { mtime } = yield* fileSystem.stat(indexPath);
       yield* fileSystem.copyFile(indexPath, tempIndexPath);
-      // A newer copy timestamp hides racily clean edits. Round down before Git reads or rewrites it.
+      // Node FileSystem.stat truncates bigint timestamps to milliseconds before creating its Date.
+      // Flooring preserves the source second without making preceding-second files racy.
       const indexTime = Option.isSome(mtime)
-        ? Math.max(0, Math.floor((mtime.value.getTime() - 1) / 1000))
+        ? Math.max(0, Math.floor(mtime.value.getTime() / 1000))
         : 0;
       yield* fileSystem.utimes(tempIndexPath, indexTime, indexTime);
     }