Index: tests/conftest.py
===================================================================
--- tests/conftest.py	(revision 100c1d25edd93042cee640c536afe12aae1ed4c0)
+++ tests/conftest.py	(revision df459f718c057333d059679915ea06d65dead882)
@@ -3,5 +3,5 @@
 from pathlib import Path
 from datetime import datetime
-from flexoentity import Domain, FlexoSignature
+from flexoentity import Domain, FlexoSignature, DomainManager
 from flexoentity import get_signing_backend, CertificateReference
 
@@ -28,7 +28,12 @@
 
 
+@pytest.fixture
+def sample_domain_manager():
+    return DomainManager()
+
 # ─────────────────────────────────────────────────────────────
 # Basic test data directory + PEM test files
 # ─────────────────────────────────────────────────────────────
+
 
 @pytest.fixture(scope="session")
Index: tests/test_domain.py
===================================================================
--- tests/test_domain.py	(revision df459f718c057333d059679915ea06d65dead882)
+++ tests/test_domain.py	(revision df459f718c057333d059679915ea06d65dead882)
@@ -0,0 +1,56 @@
+import pytest
+from flexoentity import Domain, DomainManager, DuplicateDomainError
+
+
+# ---------------------------------------------------------------
+# Setup/Teardown (clear DomainManager between tests)
+# ---------------------------------------------------------------
+
+
+@pytest.fixture(autouse=True)
+def clear_domain_manager(sample_domain_manager):
+    sample_domain_manager.clear()
+
+
+# ---------------------------------------------------------------
+# Domain creation + registration
+# ---------------------------------------------------------------
+def test_domain_registration(sample_domain_manager, sample_domain):
+
+    sample_domain_manager.add(sample_domain)
+    # Manager must know it now
+    assert sample_domain_manager.get("PY_ARITHM") is sample_domain
+
+
+# ---------------------------------------------------------------
+# Uniqueness: registering the same code twice must fail
+# ---------------------------------------------------------------
+def test_domain_uniqueness_enforced(sample_domain, sample_domain_manager):
+
+    sample_domain_manager.add(sample_domain)
+    with pytest.raises(DuplicateDomainError):
+        sample_domain_manager.add(sample_domain)
+
+
+# ---------------------------------------------------------------
+# Lookup by FlexOID
+# ---------------------------------------------------------------
+def test_lookup_by_oid(sample_domain, sample_domain_manager):
+    sample_domain_manager.add(sample_domain)
+    found = sample_domain_manager.get(sample_domain.domain_id)
+    assert found is sample_domain
+
+
+# ---------------------------------------------------------------
+# JSON roundtrip should preserve identity and not regenerate FlexOID
+# ---------------------------------------------------------------
+def test_domain_json_roundtrip(sample_domain):
+    sample_data = sample_domain.to_dict()
+    loaded = Domain.from_dict(sample_data)
+
+    # Same exact instance
+    assert loaded == sample_domain
+
+    # Same FlexOID
+    assert str(loaded.flexo_id) == str(sample_domain.flexo_id)
+