diff --git a/extensions/catalog.json b/extensions/catalog.json
index 1c423d4707..7c6c542b50 100644
--- a/extensions/catalog.json
+++ b/extensions/catalog.json
@@ -51,7 +51,7 @@
     "git": {
       "name": "Git Branching Workflow",
       "id": "git",
-      "version": "1.0.2",
+      "version": "1.0.3",
       "description": "Feature branch creation, numbering (sequential/timestamp), validation, and Git remote detection",
       "author": "spec-kit-core",
       "repository": "https://github.com/github/spec-kit",
diff --git a/extensions/git/extension.yml b/extensions/git/extension.yml
index 2d762a126a..cf3f728b77 100644
--- a/extensions/git/extension.yml
+++ b/extensions/git/extension.yml
@@ -3,7 +3,7 @@ schema_version: "1.0"
 extension:
   id: git
   name: "Git Branching Workflow"
-  version: "1.0.2"
+  version: "1.0.3"
   description: "Feature branch creation, numbering (sequential/timestamp), templating, validation, and Git remote detection"
   author: spec-kit-core
   repository: https://github.com/github/spec-kit
diff --git a/extensions/git/scripts/python/create_new_feature_branch.py b/extensions/git/scripts/python/create_new_feature_branch.py
index 0cfb6a5c09..7073f3ef7d 100644
--- a/extensions/git/scripts/python/create_new_feature_branch.py
+++ b/extensions/git/scripts/python/create_new_feature_branch.py
@@ -24,6 +24,7 @@
 
 SCRIPT_DIR = Path(__file__).resolve().parent
 MAX_BRANCH_LENGTH = 244  # GitHub enforces a 244-byte limit on branch names
+MAX_FEATURE_NUMBER = 2**63 - 1
 
 USAGE = (
     "Usage: create_new_feature_branch.py [--json] [--dry-run] "
@@ -76,6 +77,16 @@ def _persist_hint(var_name: str, value: str) -> str:
     return f"export {var_name}={escaped_value}"
 
 
+def _int64_from_digits(value: str) -> int | None:
+    normalized = value.lstrip("0") or "0"
+    maximum = str(MAX_FEATURE_NUMBER)
+    if len(normalized) > len(maximum) or (
+        len(normalized) == len(maximum) and normalized > maximum
+    ):
+        return None
+    return int(normalized, 10)
+
+
 @dataclass
 class Args:
     json_mode: bool = False
@@ -113,6 +124,14 @@ def parse_args(argv: list[str]) -> Args:
             if not re.fullmatch(r"[0-9]+", args.branch_number):
                 _err("Error: --number must be a non-negative integer")
                 raise SystemExit(1)
+            normalized_number = _int64_from_digits(args.branch_number)
+            if normalized_number is None:
+                _err(
+                    "Error: --number must be between 0 and "
+                    f"{MAX_FEATURE_NUMBER}, got '{args.branch_number}'"
+                )
+                raise SystemExit(1)
+            args.branch_number = str(normalized_number)
         elif arg == "--timestamp":
             args.use_timestamp = True
         elif arg in ("--help", "-h"):
@@ -530,7 +549,21 @@ def build_branch_name(feature_num: str, branch_suffix: str) -> str:
                 else:
                     branch_number = get_highest_from_specs(specs_dir) + 1
 
-            feature_num = f"{int(branch_number):03d}"
+            try:
+                feature_number = int(branch_number)
+            except ValueError:
+                _err(
+                    "Error: feature number must be between 0 and "
+                    f"{MAX_FEATURE_NUMBER}, got '{branch_number}'"
+                )
+                return 1
+            if not 0 <= feature_number <= MAX_FEATURE_NUMBER:
+                _err(
+                    "Error: feature number must be between 0 and "
+                    f"{MAX_FEATURE_NUMBER}, got '{branch_number}'"
+                )
+                return 1
+            feature_num = f"{feature_number:03d}"
             branch_name = build_branch_name(feature_num, branch_suffix)
 
     branch_byte_len = _byte_length(branch_name)
diff --git a/tests/extensions/git/test_git_extension.py b/tests/extensions/git/test_git_extension.py
index 538925a8c7..8e1c2652a4 100644
--- a/tests/extensions/git/test_git_extension.py
+++ b/tests/extensions/git/test_git_extension.py
@@ -179,7 +179,7 @@ def test_manifest_validates(self):
 
         m = ExtensionManifest(EXT_DIR / "extension.yml")
         assert m.id == "git"
-        assert m.version == "1.0.2"
+        assert m.version == "1.0.3"
 
     def test_manifest_commands(self):
         """Manifest declares expected commands."""
diff --git a/tests/extensions/git/test_git_extension_python_parity.py b/tests/extensions/git/test_git_extension_python_parity.py
index 64d9bdbe19..b8cfd57b23 100644
--- a/tests/extensions/git/test_git_extension_python_parity.py
+++ b/tests/extensions/git/test_git_extension_python_parity.py
@@ -322,6 +322,92 @@ def test_explicit_number(self, tmp_path: Path):
         _assert_parity(b, p)
         assert json.loads(p.stdout)["FEATURE_NUM"] == "042"
 
+    def test_explicit_number_max_int64_accepted(self, tmp_path: Path):
+        _, py_proj = _twin_projects(tmp_path)
+        p = _run_py(
+            "create-new-feature-branch",
+            py_proj,
+            "--json",
+            "--dry-run",
+            "--number",
+            str(2**63 - 1),
+            "some feature",
+        )
+        assert p.returncode == 0, p.stderr
+        assert json.loads(p.stdout)["FEATURE_NUM"] == str(2**63 - 1)
+
+    def test_explicit_number_zero_padded_input_accepted(self, tmp_path: Path):
+        _, py_proj = _twin_projects(tmp_path)
+        p = _run_py(
+            "create-new-feature-branch",
+            py_proj,
+            "--json",
+            "--dry-run",
+            "--number",
+            "0" * 5000 + "1",
+            "some feature",
+        )
+        assert p.returncode == 0, p.stderr
+        assert json.loads(p.stdout)["FEATURE_NUM"] == "001"
+
+    def test_explicit_number_above_max_int64_rejected(self, tmp_path: Path):
+        """Pin Python-side bound checking; bash currently has no equivalent guard."""
+        _, py_proj = _twin_projects(tmp_path)
+        too_large = str(2**63)
+        p = _run_py(
+            "create-new-feature-branch",
+            py_proj,
+            "--json",
+            "--dry-run",
+            "--number",
+            too_large,
+            "some feature",
+        )
+        assert p.returncode == 1
+        assert (
+            "Error: --number must be between 0 and 9223372036854775807, "
+            f"got '{too_large}'"
+        ) in p.stderr
+        assert "Traceback" not in p.stderr
+
+    def test_explicit_number_oversized_digits_rejected_without_traceback(
+        self, tmp_path: Path
+    ):
+        _, py_proj = _twin_projects(tmp_path)
+        oversized = "9" * 5000
+        p = _run_py(
+            "create-new-feature-branch",
+            py_proj,
+            "--json",
+            "--dry-run",
+            "--number",
+            oversized,
+            "some feature",
+        )
+        assert p.returncode == 1
+        assert (
+            "Error: --number must be between 0 and 9223372036854775807, "
+            f"got '{oversized}'"
+        ) in p.stderr
+        assert "Traceback" not in p.stderr
+
+    def test_auto_detected_number_above_max_int64_rejected(self, tmp_path: Path):
+        _, py_proj = _twin_projects(tmp_path)
+        (py_proj / "specs" / f"{2**63 - 1}-existing").mkdir(parents=True)
+        p = _run_py(
+            "create-new-feature-branch",
+            py_proj,
+            "--json",
+            "--dry-run",
+            "some feature",
+        )
+        assert p.returncode == 1
+        assert (
+            "Error: feature number must be between 0 and 9223372036854775807, "
+            "got '9223372036854775808'"
+        ) in p.stderr
+        assert "Traceback" not in p.stderr
+
     def test_timestamp_mode_format(self, tmp_path: Path):
         _, py_proj = _twin_projects(tmp_path)
         p = _run_py(