diff --git a/server/local_git.py b/server/local_git.py
index 3a34a1b8..d72310de 100644
--- a/server/local_git.py
+++ b/server/local_git.py
@@ -9,9 +9,16 @@
 CREATE_NO_WINDOW = 0x08000000
 
 # Patches are parsed and displayed by Amadeus, so their format must not follow
-# the user's diff configuration: an external diff driver, forced color, or
-# diff.noprefix/diff.mnemonicPrefix would change what the parser receives.
-PATCH_FORMAT_ARGS = ("--no-ext-diff", "--no-color", "--src-prefix=a/", "--dst-prefix=b/")
+# the user's diff configuration: an external diff driver, forced color,
+# diff.noprefix/diff.mnemonicPrefix, or diff.submodule=log/diff would change
+# what the parser receives.
+PATCH_FORMAT_ARGS = (
+    "--no-ext-diff",
+    "--no-color",
+    "--src-prefix=a/",
+    "--dst-prefix=b/",
+    "--submodule=short",
+)
 
 
 async def run_git(cwd: str, args: list[str]) -> dict[str, Any]:
diff --git a/tests/test_work_artifact_registry.py b/tests/test_work_artifact_registry.py
index 8ef84c58..0359e026 100644
--- a/tests/test_work_artifact_registry.py
+++ b/tests/test_work_artifact_registry.py
@@ -308,6 +308,78 @@ async def run() -> None:
     asyncio.run(run())
 
 
+@pytest.mark.parametrize("submodule_format", ["log", "diff"])
+def test_attempt_patch_ignores_user_submodule_diff_format(submodule_format: str) -> None:
+    async def run() -> None:
+        with tempfile.TemporaryDirectory(prefix="work_artifact_submodule_") as temp:
+            library = Path(temp) / "library"
+            library.mkdir()
+            _git(library, "init")
+            _git(library, "config", "user.email", "amadeus-test@example.invalid")
+            _git(library, "config", "user.name", "Amadeus Test")
+            (library / "lib.txt").write_text("base\n", encoding="utf-8")
+            _git(library, "add", ".")
+            _git(library, "commit", "-m", "library baseline")
+            root = Path(temp) / "project"
+            root.mkdir()
+            _git(root, "init")
+            _git(root, "config", "user.email", "amadeus-test@example.invalid")
+            _git(root, "config", "user.name", "Amadeus Test")
+            (root / "app.txt").write_text("base\n", encoding="utf-8")
+            _git(
+                root,
+                "-c",
+                "protocol.file.allow=always",
+                "submodule",
+                "add",
+                str(library),
+                "vendor",
+            )
+            _git(root, "add", ".")
+            _git(root, "commit", "-m", "baseline")
+            _git(root, "config", "diff.submodule", submodule_format)
+
+            store = WorkLedgerStore(Path(temp) / "ledger.sqlite3")
+            project = store.create_or_get_project(root)
+            item = store.create_work_item(project.project_id, title="Update vendor")
+            attempt = store.create_attempt(
+                item.work_item_id,
+                provider="codex",
+                task="Update vendor",
+            )
+            registry = WorkArtifactRegistry(store)
+            await registry.capture_baseline(attempt, item)
+            vendor = root / "vendor"
+            _git(vendor, "config", "user.email", "amadeus-test@example.invalid")
+            _git(vendor, "config", "user.name", "Amadeus Test")
+            (vendor / "lib.txt").write_text("base\nchanged\n", encoding="utf-8")
+            _git(vendor, "commit", "-am", "library change")
+            (root / "app.txt").write_text("base\nchanged\n", encoding="utf-8")
+
+            current_attempt = store.get_attempt(attempt.attempt_id)
+            assert current_attempt is not None
+            delta = await registry.finalize_attempt(current_attempt, item)
+            store.close()
+
+        assert "Submodule vendor" not in delta["patch"]
+        assert "lib.txt" not in delta["patch"]
+        parsed = parse_unified_diff(
+            delta["patch"],
+            changed_files=delta["changed_files"],
+            untracked=delta["untracked"],
+        )
+        assert [(entry["path"], entry["additions"], entry["deletions"]) for entry in parsed["files"]] == [
+            ("app.txt", 1, 0),
+            ("vendor", 1, 1),
+        ]
+        assert parsed["files"][0]["hunks"][0]["lines"] == [
+            {"kind": "context", "oldLine": 1, "newLine": 1, "text": "base"},
+            {"kind": "add", "newLine": 2, "text": "changed"},
+        ]
+
+    asyncio.run(run())
+
+
 def test_non_git_workspace_registers_new_files_without_claiming_preexisting_changes() -> None:
     async def run() -> None:
         with tempfile.TemporaryDirectory(prefix="work_artifact_filesystem_") as temp: