diff --git a/apps/server/src/vcs/GitVcsDriverCore.test.ts b/apps/server/src/vcs/GitVcsDriverCore.test.ts
index 76b1fb4f898b5..8f9fd37f110eb 100644
--- a/apps/server/src/vcs/GitVcsDriverCore.test.ts
+++ b/apps/server/src/vcs/GitVcsDriverCore.test.ts
@@ -7396,30 +7396,49 @@ it.layer(layerTest)("GitVcsDriver core integration", (it) => {
     );
 
     it.effect.each([
-      { location: "root", conflict: false },
-      { location: "nested", conflict: false },
-      { location: "worktree", conflict: false },
-      { location: "root", conflict: true },
-      { location: "nested", conflict: true },
-      { location: "worktree", conflict: true },
+      { location: "root", conflict: false, encoding: "utf8" },
+      { location: "nested", conflict: false, encoding: "utf8" },
+      { location: "worktree", conflict: false, encoding: "utf8" },
+      { location: "root", conflict: true, encoding: "utf8" },
+      { location: "nested", conflict: true, encoding: "utf8" },
+      { location: "worktree", conflict: true, encoding: "utf8" },
+      { location: "root", conflict: true, encoding: "utf16le" },
+      { location: "root", conflict: true, encoding: "utf16le", large: true },
     ])(
-      "rejects selected-file merge commits without mutation ($location, conflict: $conflict)",
-      ({ location, conflict }) =>
+      "rejects selected-file merge commits without mutation ($location, conflict: $conflict, $encoding)",
+      ({ location, conflict, encoding, large }) =>
         Effect.gen(function* () {
           let cwd = yield* makeTmpDir();
           const { initialBranch } = yield* initRepoWithCommit(cwd);
+          const writeReadme = (text: string) =>
+            Effect.sync(() =>
+              NodeFS.writeFileSync(
+                NodePath.join(cwd, "README.md"),
+                encoding === "utf16le" ? Buffer.from(`\ufeff${text}`, "utf16le") : text,
+              ),
+            );
+          if (encoding === "utf16le") {
+            yield* writeTextFile(
+              cwd,
+              ".gitattributes",
+              "README.md text working-tree-encoding=UTF-16\n",
+            );
+            yield* writeReadme("base\n");
+            yield* git(cwd, ["add", "."]);
+            yield* git(cwd, ["commit", "-m", "Declare UTF-16 encoding"]);
+          }
           const driver = yield* GitVcsDriver.GitVcsDriver;
           const fs = yield* FileSystem.FileSystem;
           const path = yield* Path.Path;
           yield* git(cwd, ["checkout", "-b", "feature"]);
           yield* writeTextFile(cwd, "feature.txt", "feature\n");
-          if (conflict) yield* writeTextFile(cwd, "README.md", "feature version\n");
+          if (conflict) yield* writeReadme("feature version\n");
           yield* git(cwd, ["add", "."]);
           yield* git(cwd, ["commit", "-m", "Feature"]);
           const featureHead = yield* git(cwd, ["rev-parse", "HEAD"]);
           yield* git(cwd, ["checkout", initialBranch]);
           yield* writeTextFile(cwd, "main.txt", "main\n");
-          if (conflict) yield* writeTextFile(cwd, "README.md", "main version\n");
+          if (conflict) yield* writeReadme("main version\n");
           yield* git(cwd, ["add", "."]);
           yield* git(cwd, ["commit", "-m", "Main"]);
           const mainHead = yield* git(cwd, ["rev-parse", "HEAD"]);
@@ -7468,6 +7487,7 @@ it.layer(layerTest)("GitVcsDriver core integration", (it) => {
           assert.equal(yield* git(cwd, ["rev-parse", "HEAD"]), mainHead);
           assert.equal(yield* git(cwd, ["ls-files", "--unmerged"]), unmerged);
 
+          const resolvedText = `Heading\n=======\n\nresolved${large ? "x".repeat(17 * 1024 * 1024) : ""}\n`;
           if (conflict) {
             const unresolved = yield* driver
               .commit(commitCwd, "Unresolved merge", "", { stage: {} })
@@ -7476,8 +7496,10 @@ it.layer(layerTest)("GitVcsDriver core integration", (it) => {
             assert.equal(yield* git(cwd, ["rev-parse", "HEAD"]), mainHead);
             assert.equal(yield* git(cwd, ["ls-files", "--unmerged"]), unmerged);
             // A lone "=======" is a heading underline, not a conflict boundary.
-            yield* writeTextFile(cwd, "README.md", "Heading\n=======\n\nresolved\n");
-            assert.isNotNull(yield* driver.prepareCommitContext(commitCwd));
+            yield* writeReadme(resolvedText);
+            assert.isNotNull(
+              yield* driver.prepareCommitContext(commitCwd, undefined, { includePatch: false }),
+            );
           }
           yield* driver.commit(commitCwd, "Finish merge", "", { stage: {} });
           assert.equal(
@@ -7487,7 +7509,12 @@ it.layer(layerTest)("GitVcsDriver core integration", (it) => {
           assert.isFalse(NodeFS.existsSync(path.join(gitDir, "MERGE_HEAD")));
           assert.equal(yield* git(cwd, ["show", "HEAD:feature.txt"]), "feature");
           assert.equal(yield* git(cwd, ["show", "HEAD:unrelated.txt"]), "unrelated");
-          if (conflict)
+          if (large) {
+            assert.equal(
+              yield* git(cwd, ["cat-file", "-s", "HEAD:README.md"]),
+              String(Buffer.byteLength(resolvedText)),
+            );
+          } else if (conflict)
             assert.equal(
               yield* git(cwd, ["show", "HEAD:README.md"]),
               "Heading\n=======\n\nresolved",
diff --git a/apps/server/src/vcs/GitVcsDriverCore.ts b/apps/server/src/vcs/GitVcsDriverCore.ts
index 280664cf8a136..bf58b74b6f067 100644
--- a/apps/server/src/vcs/GitVcsDriverCore.ts
+++ b/apps/server/src/vcs/GitVcsDriverCore.ts
@@ -2696,7 +2696,7 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
         );
         const reports = check.stdout.split("\n");
         const conflicted: string[] = [];
-        let topLevel: string | undefined;
+        let emptyTree: string | undefined;
         for (const file of unmergedPaths) {
           const markerLines = new Set(
             reports.flatMap((line) => {
@@ -2707,11 +2707,26 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
             }),
           );
           if (markerLines.size === 0) continue;
-          topLevel ??= (yield* runGitStdout("GitVcsDriver.commit.topLevel", cwd, [
-            "rev-parse",
-            "--show-toplevel",
-          ])).trim();
-          const lines = (yield* fileSystem.readFileString(path.join(topLevel, file)).pipe(
+          emptyTree ??= yield* readEmptyTreeHash(cwd);
+          // --check numbers lines after working-tree-encoding and clean filters; a diff from the
+          // empty tree prints that view. Read it in full so large resolved files can still commit.
+          const fileDiff = yield* runGitStdoutWithOptions(
+            "GitVcsDriver.commit.conflictMarkers",
+            cwd,
+            [
+              "-c",
+              "diff.relative=false",
+              "diff",
+              "--no-color",
+              "--no-ext-diff",
+              "--no-textconv",
+              "--text",
+              emptyTree,
+              "--",
+              `:(top,literal)${file}`,
+            ],
+            { ...(env ? { env } : {}), maxOutputBytes: Number.POSITIVE_INFINITY },
+          ).pipe(
             Effect.mapError(
               (cause) =>
                 new GitCommandError({
@@ -2722,7 +2737,12 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
                   cause,
                 }),
             ),
-          )).split("\n");
+          );
+          const diffLines = fileDiff.split("\n");
+          const lines = diffLines
+            .slice(diffLines.findIndex((line) => line.startsWith("@@")) + 1)
+            .filter((line) => line.startsWith("+"))
+            .map((line) => line.slice(1));
           // Git also reports a lone "=======", which is valid content such as a heading underline.
           const reported = (marker: string) =>
             lines.some((line, index) => markerLines.has(index) && line.startsWith(marker));
diff --git a/scripts/scenarios/utf16-merge-conflict-blocks-commit.e2e.ts b/scripts/scenarios/utf16-merge-conflict-blocks-commit.e2e.ts
new file mode 100644
index 0000000000000..047d0f9a9e695
--- /dev/null
+++ b/scripts/scenarios/utf16-merge-conflict-blocks-commit.e2e.ts
@@ -0,0 +1,52 @@
+import { expect } from "e2e";
+import * as Effect from "effect/Effect";
+import * as FileSystem from "effect/FileSystem";
+
+import { run } from "./shell.ts";
+import { runPromise, test, WEB } from "./t3.ts";
+
+const utf16 = (text: string) => Buffer.from(`\ufeff${text}`, "utf16le");
+
+test(
+  "The commit dialog refuses a merge whose UTF-16 file still has conflict markers",
+  WEB,
+  async ({ t3, app, screen }) => {
+    const project = await t3.project("demo", {
+      ".gitattributes": "README.md text working-tree-encoding=UTF-16\n",
+      "README.md": utf16("base\n"),
+    });
+    const git = (args: ReadonlyArray<string>, allowExit: ReadonlyArray<number> = [0]) =>
+      runPromise(
+        run(
+          "git",
+          ["-c", "user.name=Scenario", "-c", "user.email=scenario@example.invalid", ...args],
+          { cwd: project, allowExit },
+        ),
+      );
+    const commitReadme = async (text: string) => {
+      await runPromise(
+        Effect.flatMap(FileSystem.FileSystem, (fs) =>
+          fs.writeFile(`${project}/README.md`, utf16(text)),
+        ),
+      );
+      await git(["commit", "--quiet", "--all", "--message", text.trim()]);
+    };
+    await git(["checkout", "--quiet", "-b", "feature"]);
+    await commitReadme("feature version\n");
+    await git(["checkout", "--quiet", "main"]);
+    await commitReadme("main version\n");
+    await git(["merge", "--quiet", "feature"], [1]);
+    const thread = await t3.thread({ project, title: "Merge", messages: ["merge"] });
+
+    await t3.open(thread);
+    await screen.getByRole("button", "Git action options").tap();
+    await screen.getByRole("menuitem", "Commit").tap();
+    await screen.getByPlaceholder("Leave empty to auto-generate").fill("Finish merge");
+    await screen.getByRole("button", "Commit", { exact: true }).tap();
+    // On a failed assertion e2e captures the committed merge itself.
+    await expect(
+      screen.getByText("unresolved merge conflicts in README.md", { exact: false }),
+    ).toBeVisible();
+    await app.screenshot("refused");
+  },
+);