diff --git a/packages/gds-framework/gds/helpers.py b/packages/gds-framework/gds/helpers.py
index b51c635..bd47e8c 100644
--- a/packages/gds-framework/gds/helpers.py
+++ b/packages/gds-framework/gds/helpers.py
@@ -7,6 +7,7 @@
 
 from __future__ import annotations
 
+import threading
 from collections.abc import Callable
 from typing import Any
 
@@ -168,6 +169,7 @@ def _to_ports(names: list[str] | None) -> tuple[Port, ...]:
 CheckFn = Callable[[SystemIR], list[Finding]]
 
 _CUSTOM_CHECKS: list[CheckFn] = []
+_CUSTOM_CHECKS_LOCK = threading.Lock()
 
 
 def gds_check(
@@ -177,7 +179,8 @@ def gds_check(
     """Decorator that registers a verification check function.
 
     Attaches ``check_id`` and ``severity`` as function attributes and
-    adds it to the module-level custom check registry.
+    adds it to the module-level custom check registry. Registration is
+    thread-safe.
 
     Usage::
 
@@ -189,19 +192,25 @@ def check_no_orphan_spaces(system: SystemIR) -> list[Finding]:
     def decorator(fn: CheckFn) -> CheckFn:
         fn.check_id = check_id  # type: ignore[attr-defined]
         fn.severity = severity  # type: ignore[attr-defined]
-        _CUSTOM_CHECKS.append(fn)
+        with _CUSTOM_CHECKS_LOCK:
+            _CUSTOM_CHECKS.append(fn)
         return fn
 
     return decorator
 
 
 def get_custom_checks() -> list[CheckFn]:
-    """Return all check functions registered via ``@gds_check``."""
-    return list(_CUSTOM_CHECKS)
+    """Return a snapshot of all check functions registered via ``@gds_check``.
+
+    Returns a copy so callers cannot mutate the internal registry.
+    """
+    with _CUSTOM_CHECKS_LOCK:
+        return list(_CUSTOM_CHECKS)
 
 
 def all_checks() -> list[CheckFn]:
     """Return built-in generic checks + all custom-registered checks."""
     from gds.verification.engine import ALL_CHECKS
 
-    return list(ALL_CHECKS) + list(_CUSTOM_CHECKS)
+    with _CUSTOM_CHECKS_LOCK:
+        return list(ALL_CHECKS) + list(_CUSTOM_CHECKS)
diff --git a/packages/gds-framework/tests/test_helpers.py b/packages/gds-framework/tests/test_helpers.py
index 4d22e29..d244e88 100644
--- a/packages/gds-framework/tests/test_helpers.py
+++ b/packages/gds-framework/tests/test_helpers.py
@@ -20,6 +20,7 @@
 )
 from gds.helpers import (
     _CUSTOM_CHECKS,
+    _CUSTOM_CHECKS_LOCK,
     all_checks,
     entity,
     gds_check,
@@ -245,11 +246,13 @@ def test_empty(self):
 class TestGdsCheck:
     def setup_method(self):
         """Clear custom check registry before each test."""
-        _CUSTOM_CHECKS.clear()
+        with _CUSTOM_CHECKS_LOCK:
+            _CUSTOM_CHECKS.clear()
 
     def teardown_method(self):
         """Clear custom check registry after each test."""
-        _CUSTOM_CHECKS.clear()
+        with _CUSTOM_CHECKS_LOCK:
+            _CUSTOM_CHECKS.clear()
 
     def test_registers_check(self):
         @gds_check("TEST-001", Severity.WARNING)
@@ -309,3 +312,36 @@ def check_b(system: SystemIR) -> list[Finding]:
         assert len(checks) == 2
         assert checks[0] is check_a
         assert checks[1] is check_b
+
+    def test_concurrent_registration(self):
+        """Register checks from multiple threads; all must appear."""
+        import threading
+
+        num_threads = 20
+        barrier = threading.Barrier(num_threads)
+        errors: list[Exception] = []
+
+        def register_check(index: int) -> None:
+            try:
+                barrier.wait(timeout=5)
+
+                @gds_check(f"THREAD-{index:03d}")
+                def _check(system: SystemIR) -> list[Finding]:
+                    return []
+            except Exception as exc:
+                errors.append(exc)
+
+        threads = [
+            threading.Thread(target=register_check, args=(i,))
+            for i in range(num_threads)
+        ]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join(timeout=10)
+
+        assert not errors, f"Thread errors: {errors}"
+        checks = get_custom_checks()
+        assert len(checks) == num_threads
+        check_ids = {c.check_id for c in checks}  # type: ignore[attr-defined]
+        assert check_ids == {f"THREAD-{i:03d}" for i in range(num_threads)}