diff --git a/backend/tests/test_constraints.py b/backend/tests/test_constraints.py
new file mode 100644
index 0000000..c50f3b1
--- /dev/null
+++ b/backend/tests/test_constraints.py
@@ -0,0 +1,71 @@
+"""
+Tests for constraints.py — all four owned/budget combinations.
+"""
+import pytest
+from unittest.mock import MagicMock
+from app.services.ai.constraints import build_constraint_context, build_owned_card_list
+from app.schemas.deck import DeckConstraints
+
+
+def make_constraints(prefer_owned=False, budget_enabled=False, budget_amount=None, budget_scope="purchase"):
+    c = MagicMock(spec=DeckConstraints)
+    c.prefer_owned = prefer_owned
+    c.budget_enabled = budget_enabled
+    c.budget_amount = budget_amount
+    c.budget_scope = budget_scope
+    return c
+
+
+class TestBuildConstraintContext:
+    def test_no_constraints(self):
+        c = make_constraints()
+        result = build_constraint_context(c, None)
+        assert "No special constraints" in result
+
+    def test_prefer_owned_with_cards(self):
+        c = make_constraints(prefer_owned=True)
+        result = build_constraint_context(c, ["Sol Ring", "Command Tower"])
+        assert "Prefer cards the user already owns" in result
+        assert "[UNOWNED]" in result
+
+    def test_prefer_owned_without_cards(self):
+        c = make_constraints(prefer_owned=True)
+        result = build_constraint_context(c, [])
+        assert "No special constraints" in result
+
+    def test_budget_only(self):
+        c = make_constraints(budget_enabled=True, budget_amount=100.0, budget_scope="purchase")
+        result = build_constraint_context(c, None)
+        assert "$100.00" in result
+        assert "cards to purchase" in result
+
+    def test_budget_total_scope(self):
+        c = make_constraints(budget_enabled=True, budget_amount=200.0, budget_scope="total")
+        result = build_constraint_context(c, None)
+        assert "total deck" in result
+
+    def test_owned_and_budget(self):
+        c = make_constraints(prefer_owned=True, budget_enabled=True, budget_amount=50.0)
+        result = build_constraint_context(c, ["Sol Ring"])
+        assert "Prefer cards" in result
+        assert "$50.00" in result
+
+    def test_budget_disabled_no_amount(self):
+        c = make_constraints(budget_enabled=False, budget_amount=100.0)
+        result = build_constraint_context(c, None)
+        assert "$100.00" not in result
+
+
+class TestBuildOwnedCardList:
+    def test_empty_list(self):
+        result = build_owned_card_list([])
+        assert result == ""
+
+    def test_cards_sorted(self):
+        result = build_owned_card_list(["Sol Ring", "Command Tower", "Atraxa"])
+        assert result.index("Atraxa") < result.index("Command Tower") < result.index("Sol Ring")
+
+    def test_format(self):
+        result = build_owned_card_list(["Sol Ring"])
+        assert "OWNED CARDS:" in result
+        assert "- Sol Ring" in result