diff --git a/backend/tests/test_claude_client.py b/backend/tests/test_claude_client.py
new file mode 100644
index 0000000..29f6669
--- /dev/null
+++ b/backend/tests/test_claude_client.py
@@ -0,0 +1,182 @@
+"""
+Tests for claude_client.py — JSON parsing, payload building, slot parsing,
+UNOWNED marker stripping, and CardEntry construction.
+"""
+import pytest
+from app.services.ai.claude_client import (
+    _parse_json,
+    _parse_slot,
+    _build_payload,
+    CardEntry,
+    CutEntry,
+    DeckPayload,
+    UNOWNED_RE,
+)
+from app.models.deck import CardSlot
+
+
+# ---------------------------------------------------------------------------
+# _parse_json
+# ---------------------------------------------------------------------------
+
+class TestParseJson:
+    def test_valid_json(self):
+        text = '{"deck_name": "Test", "cards": []}'
+        result = _parse_json(text)
+        assert result["deck_name"] == "Test"
+
+    def test_strips_markdown_fences(self):
+        text = '''```json
+{"deck_name": "Test", "cards": []}
+```'''
+        result = _parse_json(text)
+        assert result["deck_name"] == "Test"
+
+    def test_strips_plain_fences(self):
+        text = '''```
+{"deck_name": "Test", "cards": []}
+```'''
+        result = _parse_json(text)
+        assert result["deck_name"] == "Test"
+
+    def test_raises_on_invalid_json(self):
+        with pytest.raises(ValueError, match="not valid JSON"):
+            _parse_json('{"broken: json}')
+
+    def test_raises_on_empty_string(self):
+        with pytest.raises(ValueError):
+            _parse_json("")
+
+
+# ---------------------------------------------------------------------------
+# _parse_slot
+# ---------------------------------------------------------------------------
+
+class TestParseSlot:
+    def test_valid_slots(self):
+        assert _parse_slot("creature") == CardSlot.CREATURE
+        assert _parse_slot("instant") == CardSlot.INSTANT
+        assert _parse_slot("sorcery") == CardSlot.SORCERY
+        assert _parse_slot("enchantment") == CardSlot.ENCHANTMENT
+        assert _parse_slot("artifact") == CardSlot.ARTIFACT
+        assert _parse_slot("planeswalker") == CardSlot.PLANESWALKER
+        assert _parse_slot("land") == CardSlot.LAND
+        assert _parse_slot("battle") == CardSlot.BATTLE
+
+    def test_plural_aliases(self):
+        assert _parse_slot("creatures") == CardSlot.CREATURE
+        assert _parse_slot("instants") == CardSlot.INSTANT
+        assert _parse_slot("sorceries") == CardSlot.SORCERY
+        assert _parse_slot("lands") == CardSlot.LAND
+
+    def test_case_insensitive(self):
+        assert _parse_slot("CREATURE") == CardSlot.CREATURE
+        assert _parse_slot("Land") == CardSlot.LAND
+
+    def test_unknown_defaults_to_creature(self):
+        assert _parse_slot("unknown_type") == CardSlot.CREATURE
+
+
+# ---------------------------------------------------------------------------
+# CardEntry
+# ---------------------------------------------------------------------------
+
+class TestCardEntry:
+    def test_basic_card(self):
+        entry = CardEntry({"name": "Sol Ring", "slot": "artifact", "quantity": 1})
+        assert entry.card_name == "Sol Ring"
+        assert entry.slot == CardSlot.ARTIFACT
+        assert entry.quantity == 1
+        assert entry.is_owned is True
+
+    def test_unowned_marker_stripped(self):
+        entry = CardEntry({"name": "Sol Ring [UNOWNED]", "slot": "artifact", "quantity": 1})
+        assert entry.card_name == "Sol Ring"
+        assert entry.is_owned is False
+
+    def test_unowned_marker_case_insensitive(self):
+        entry = CardEntry({"name": "Sol Ring [unowned]", "slot": "artifact", "quantity": 1})
+        assert entry.card_name == "Sol Ring"
+        assert entry.is_owned is False
+
+    def test_quantity_defaults_to_1(self):
+        entry = CardEntry({"name": "Forest", "slot": "land"})
+        assert entry.quantity == 1
+
+    def test_quantity_minimum_1(self):
+        entry = CardEntry({"name": "Forest", "slot": "land", "quantity": 0})
+        assert entry.quantity == 1
+
+    def test_reasoning_stored(self):
+        entry = CardEntry({"name": "Sol Ring", "slot": "artifact", "reasoning": "Best ramp"})
+        assert entry.reasoning == "Best ramp"
+
+
+# ---------------------------------------------------------------------------
+# _build_payload
+# ---------------------------------------------------------------------------
+
+class TestBuildPayload:
+    def _card(self, name="Sol Ring", slot="artifact"):
+        return {"name": name, "slot": slot, "quantity": 1, "reasoning": "Good card"}
+
+    def test_standard_cards_key(self):
+        data = {
+            "deck_name": "Test Deck",
+            "strategy_summary": "A test deck",
+            "cards": [self._card()]
+        }
+        payload = _build_payload(data)
+        assert payload.deck_name == "Test Deck"
+        assert payload.strategy_summary == "A test deck"
+        assert len(payload.cards) == 1
+        assert payload.cards[0].card_name == "Sol Ring"
+
+    def test_alternative_decklist_key(self):
+        data = {
+            "deck_name": "Test Deck",
+            "decklist": [self._card()]
+        }
+        payload = _build_payload(data)
+        assert len(payload.cards) == 1
+
+    def test_alternative_deck_key(self):
+        data = {
+            "deck_name": "Test Deck",
+            "deck": [self._card()]
+        }
+        payload = _build_payload(data)
+        assert len(payload.cards) == 1
+
+    def test_cards_key_takes_priority_over_deck(self):
+        data = {
+            "cards": [self._card("Sol Ring")],
+            "deck": [self._card("Black Lotus")],
+        }
+        payload = _build_payload(data)
+        assert payload.cards[0].card_name == "Sol Ring"
+
+    def test_default_deck_name(self):
+        payload = _build_payload({})
+        assert payload.deck_name == "Untitled Deck"
+
+    def test_skips_cards_without_name(self):
+        data = {"cards": [{"slot": "artifact"}, self._card()]}
+        payload = _build_payload(data)
+        assert len(payload.cards) == 1
+
+    def test_cuts_parsed(self):
+        data = {
+            "cards": [],
+            "cuts": [{"name": "Swamp", "reasoning": "Too slow"}]
+        }
+        payload = _build_payload(data)
+        assert len(payload.cuts) == 1
+        assert payload.cuts[0].card_name == "Swamp"
+
+    def test_empty_data(self):
+        payload = _build_payload({})
+        assert payload.deck_name == "Untitled Deck"
+        assert payload.strategy_summary == ""
+        assert payload.cards == []
+        assert payload.cuts == []