diff --git a/tests/test_long_schema_version.py b/tests/test_long_schema_version.py
new file mode 100644
index 0000000..dcbbfd1
--- /dev/null
+++ b/tests/test_long_schema_version.py
@@ -0,0 +1,80 @@
+"""Regression: arbitrarily long ASCII schema majors must not leak Python int limits.
+
+Issue #15: the reader permits a decimal spelling of major 1 (including leading
+zeroes), but every unsupported major must raise ContractError rather than an
+interpreter-dependent ValueError. No external dependencies or schema changes.
+"""
+from __future__ import annotations
+
+import json
+import unittest
+from pathlib import Path
+
+from tools.contract_validation import (
+    ContractError,
+    parse_major,
+    validate_repository_role,
+    validate_source_manifest,
+    validate_source_ref,
+    validate_workspace_snapshot,
+)
+
+EXAMPLES = Path(__file__).resolve().parents[1] / "examples"
+VALIDATORS = (
+    ("source_ref.git.json", validate_source_ref),
+    ("source_ref.drive.json", validate_source_ref),
+    ("repository_role.json", validate_repository_role),
+    ("source_manifest.json", validate_source_manifest),
+    ("workspace_snapshot.json", validate_workspace_snapshot),
+)
+# Above CPython 3.11+'s default 4,300-digit integer-conversion limit.
+UNSUPPORTED_MAJORS = ("9" * 5000, "0" * 5000 + "2")
+SUPPORTED_PADDED_ONE = "0" * 5000 + "1"
+
+
+class LongSchemaMajorTests(unittest.TestCase):
+    def test_oversized_unsupported_major_is_contract_error(self):
+        for major in UNSUPPORTED_MAJORS:
+            with self.subTest(major_length=len(major), suffix=major[-3:]):
+                with self.assertRaisesRegex(ContractError, "^unsupported schema major$"):
+                    parse_major(major + ".0")
+
+    def test_oversized_leading_zero_major_one_is_still_supported(self):
+        for minor in ("0", "15"):
+            with self.subTest(minor=minor):
+                self.assertEqual(parse_major(SUPPORTED_PADDED_ONE + "." + minor), 1)
+
+    def test_all_reader_shapes_reject_oversized_unsupported_major(self):
+        for name, validate in VALIDATORS:
+            for major in UNSUPPORTED_MAJORS:
+                with self.subTest(example=name, suffix=major[-3:]):
+                    source = json.loads((EXAMPLES / name).read_text(encoding="utf-8"))
+                    source["schema_version"] = major + ".7"
+                    with self.assertRaisesRegex(ContractError, "^unsupported schema major$"):
+                        validate(source)
+
+    def test_all_reader_shapes_accept_oversized_leading_zero_one(self):
+        for name, validate in VALIDATORS:
+            with self.subTest(example=name):
+                source = json.loads((EXAMPLES / name).read_text(encoding="utf-8"))
+                source["schema_version"] = SUPPORTED_PADDED_ONE + ".7"
+                output = validate(source)
+                self.assertEqual(output["schema_version"], source["schema_version"])
+                self.assertIsNot(output, source)
+                self.assertEqual(source["schema_version"], SUPPORTED_PADDED_ONE + ".7")
+
+    def test_short_version_contract_unchanged(self):
+        for version in ("1.0", "01.0", "0001.123"):
+            self.assertEqual(parse_major(version), 1)
+        for version in ("0.0", "2.0", "02.1"):
+            with self.subTest(version=version):
+                with self.assertRaisesRegex(ContractError, "^unsupported schema major$"):
+                    parse_major(version)
+        for version in ("1", "+1.0", "1.0.0", "1.0\n"):
+            with self.subTest(version=version):
+                with self.assertRaisesRegex(ContractError, "^invalid schema_version$"):
+                    parse_major(version)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/tools/contract_validation.py b/tools/contract_validation.py
index 3b25c68..a6226db 100644
--- a/tools/contract_validation.py
+++ b/tools/contract_validation.py
@@ -22,9 +22,9 @@ def parse_major(version: str) -> int:
     if not isinstance(version,str): raise ContractError('schema_version must be a string')
     m=VER.fullmatch(version)
     if not m: raise ContractError('invalid schema_version')
-    major=int(m.group(1))
-    if major!=1: raise ContractError('unsupported schema major')
-    return major
+    # Decimal normalization avoids CPython's configurable int digit limit.
+    if m.group(1).lstrip('0')!='1': raise ContractError('unsupported schema major')
+    return 1
 
 def _copy(data: Any) -> dict:
     if not isinstance(data,dict): raise ContractError('contract object required')