diff --git a/services/api/tests/test_entity_resolution.py b/services/api/tests/test_entity_resolution.py
new file mode 100644
index 0000000..ecc937b
--- /dev/null
+++ b/services/api/tests/test_entity_resolution.py
@@ -0,0 +1,93 @@
+from services.api.workers.entity_resolution import (
+    EntityCandidate,
+    ResolutionDecision,
+    rank_resolution_candidates,
+    resolve_entity_pair,
+)
+
+
+def organization(entity_id: str, **overrides):
+    values = {
+        "entity_id": entity_id,
+        "entity_type": "organization",
+        "name": "Example Technology AB",
+        "source_ref": f"source:{entity_id}",
+        "registration_country": "SE",
+        "registration_number": None,
+        "website": None,
+        "city": "Stockholm",
+    }
+    values.update(overrides)
+    return EntityCandidate(**values)
+
+
+def person(entity_id: str, **overrides):
+    values = {
+        "entity_id": entity_id,
+        "entity_type": "person",
+        "name": "Anna Andersson",
+        "source_ref": f"source:{entity_id}",
+        "email": None,
+        "organization_id": None,
+    }
+    values.update(overrides)
+    return EntityCandidate(**values)
+
+
+def test_registration_number_is_a_strong_organization_identifier():
+    result = resolve_entity_pair(
+        organization("left", registration_number="556123-4567"),
+        organization("right", registration_number="5561234567"),
+    )
+    assert result.decision is ResolutionDecision.EXACT_LINK
+    assert result.score == 1.0
+    assert result.requires_human_review is False
+
+
+def test_conflicting_registration_numbers_fail_distinct_even_when_names_match():
+    result = resolve_entity_pair(
+        organization("left", registration_number="5561234567"),
+        organization("right", registration_number="5569999999"),
+    )
+    assert result.decision is ResolutionDecision.DISTINCT
+    assert result.score == 0.0
+
+
+def test_same_person_email_is_exact_link():
+    result = resolve_entity_pair(
+        person("left", email="Anna@example.com"),
+        person("right", email=" anna@EXAMPLE.com "),
+    )
+    assert result.decision is ResolutionDecision.EXACT_LINK
+    assert result.requires_human_review is False
+
+
+def test_fuzzy_organization_similarity_never_auto_merges():
+    result = resolve_entity_pair(
+        organization("left", name="Example Technology AB", website="example.se"),
+        organization("right", name="Example Technologies AB", website="https://www.example.se"),
+    )
+    assert result.score > 0.72
+    assert result.decision is ResolutionDecision.REVIEW
+    assert result.requires_human_review is True
+
+
+def test_different_entity_types_are_distinct():
+    result = resolve_entity_pair(
+        organization("org"),
+        person("person"),
+    )
+    assert result.decision is ResolutionDecision.DISTINCT
+
+
+def test_candidate_ranking_prefers_exact_then_review_and_is_stable():
+    query = organization("query", registration_number="5561234567", website="example.se")
+    candidates = [
+        organization("weak", name="Completely Different Company", registration_country="NO", city="Oslo"),
+        organization("review", name="Example Technologies AB", website="example.se"),
+        organization("exact", registration_number="556123-4567"),
+    ]
+    ranked = rank_resolution_candidates(query, candidates)
+    assert [result.right_id for result in ranked] == ["exact", "review", "weak"]
+    assert ranked[0].decision is ResolutionDecision.EXACT_LINK
+    assert ranked[1].decision is ResolutionDecision.REVIEW