diff --git a/src/commander_ai_lab/deck_builder/core/rules_engine.py b/src/commander_ai_lab/deck_builder/core/rules_engine.py
index a3aaa8d..3350418 100644
--- a/src/commander_ai_lab/deck_builder/core/rules_engine.py
+++ b/src/commander_ai_lab/deck_builder/core/rules_engine.py
@@ -1 +1,202 @@
-# Rules engine (color identity, singleton, 99-card) — Phase 5
+"""
+Rules engine for the Commander AI Deck Builder.
+
+Validates decks against Commander format rules:
+  - Color identity compliance
+  - Singleton rule (basic lands exempt)
+  - Exactly 99 cards + 1 commander
+  - Minimum card type ratios
+  - Ban list checking
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import Dict, List, Set, Tuple
+
+from .models import CardEntry, CommanderDeck, DeckRatios
+
+logger = logging.getLogger(__name__)
+
+# Commander ban list (partial — keep updated)
+BANNED_CARDS: Set[str] = {
+    "Ancestral Recall", "Balance", "Biorhythm", "Black Lotus",
+    "Braids, Cabal Minion", "Channel", "Chaos Orb", "Coalition Victory",
+    "Emrakul, the Aeons Torn", "Erayo, Soratami Ascendant",
+    "Falling Star", "Fastbond", "Flash", "Gifts Ungiven",
+    "Golos, Tireless Pilgrim", "Griselbrand", "Hullbreacher",
+    "Iona, Shield of Emeria", "Karakas", "Leovold, Emissary of Trest",
+    "Library of Alexandria", "Limited Resources", "Lutri, the Spellchaser",
+    "Mox Emerald", "Mox Jet", "Mox Pearl", "Mox Ruby", "Mox Sapphire",
+    "Panoptic Mirror", "Paradox Engine", "Primeval Titan",
+    "Prophet of Kruphix", "Recurring Nightmare", "Rofellos, Llanowar Emissary",
+    "Shahrazad", "Sundering Titan", "Sway of the Stars",
+    "Sylvan Primordial", "Time Vault", "Time Walk", "Tinker",
+    "Tolarian Academy", "Trade Secrets", "Upheaval", "Worldfire",
+}
+
+BASIC_LANDS: Set[str] = {
+    "Plains", "Island", "Swamp", "Mountain", "Forest",
+    "Wastes", "Snow-Covered Plains", "Snow-Covered Island",
+    "Snow-Covered Swamp", "Snow-Covered Mountain", "Snow-Covered Forest",
+}
+
+
+# ── color identity validation ────────────────────────────────────
+def validate_color_identity(
+    card: CardEntry,
+    commander_identity: Set[str],
+) -> bool:
+    """Check if a card's color identity is within the commander's."""
+    if not card.color_identity:
+        return True  # colorless cards are always legal
+    return card.color_identity.issubset(commander_identity)
+
+
+def filter_by_color_identity(
+    cards: List[CardEntry],
+    commander_identity: Set[str],
+) -> Tuple[List[CardEntry], List[CardEntry]]:
+    """
+    Split cards into valid and invalid based on color identity.
+
+    Returns (valid_cards, rejected_cards).
+    """
+    valid = []
+    rejected = []
+    for card in cards:
+        if validate_color_identity(card, commander_identity):
+            valid.append(card)
+        else:
+            rejected.append(card)
+    return valid, rejected
+
+
+# ── singleton validation ─────────────────────────────────────────
+def validate_singleton(cards: List[CardEntry]) -> List[str]:
+    """
+    Check for singleton violations.
+
+    Returns a list of card names that appear more than once
+    (excluding basic lands).
+    """
+    seen: Dict[str, int] = {}
+    violations: List[str] = []
+
+    for card in cards:
+        if card.name in BASIC_LANDS:
+            continue
+        seen[card.name] = seen.get(card.name, 0) + card.quantity
+        if seen[card.name] > 1 and card.name not in violations:
+            violations.append(card.name)
+
+    return violations
+
+
+# ── ban list check ───────────────────────────────────────────────
+def check_ban_list(cards: List[CardEntry]) -> List[str]:
+    """Return names of any banned cards found in the list."""
+    return [c.name for c in cards if c.name in BANNED_CARDS]
+
+
+# ── card count validation ────────────────────────────────────────
+def validate_card_count(cards: List[CardEntry], expected: int = 99) -> int:
+    """Return the difference from expected count (0 = valid)."""
+    total = sum(c.quantity for c in cards)
+    return total - expected
+
+
+# ── ratio validation ─────────────────────────────────────────────
+def validate_ratios(
+    cards: List[CardEntry],
+    ratios: DeckRatios,
+) -> Dict[str, Dict[str, int]]:
+    """
+    Check if card categories meet minimum targets.
+
+    Returns a dict of category -> {"target": int, "actual": int, "diff": int}.
+    """
+    counts: Dict[str, int] = {}
+    for card in cards:
+        cat = card.category
+        counts[cat] = counts.get(cat, 0) + card.quantity
+
+    result: Dict[str, Dict[str, int]] = {}
+    targets = {
+        "lands": ratios.lands,
+        "ramp": ratios.ramp,
+        "card_draw": ratios.card_draw,
+        "removal": ratios.removal,
+        "protection": ratios.protection,
+        "synergy": ratios.synergy,
+        "wincon": ratios.wincon,
+        "uncategorized": ratios.uncategorized,
+    }
+
+    for cat, target in targets.items():
+        actual = counts.get(cat, 0)
+        result[cat] = {
+            "target": target,
+            "actual": actual,
+            "diff": actual - target,
+        }
+
+    return result
+
+
+# ── full deck validation ─────────────────────────────────────────
+def validate_deck(
+    commander: CardEntry,
+    cards: List[CardEntry],
+    ratios: DeckRatios,
+) -> Dict[str, any]:
+    """
+    Run all validation checks on a deck.
+
+    Returns a dict with:
+      - "valid": bool
+      - "errors": list of fatal errors
+      - "warnings": list of non-fatal warnings
+      - "ratio_report": category ratio details
+    """
+    errors: List[str] = []
+    warnings: List[str] = []
+
+    # Color identity
+    commander_ci = commander.color_identity
+    _, rejected = filter_by_color_identity(cards, commander_ci)
+    if rejected:
+        names = [c.name for c in rejected]
+        errors.append(f"Color identity violations: {', '.join(names)}")
+
+    # Singleton
+    dupes = validate_singleton(cards)
+    if dupes:
+        errors.append(f"Singleton violations: {', '.join(dupes)}")
+
+    # Ban list
+    banned = check_ban_list(cards)
+    if banned:
+        errors.append(f"Banned cards: {', '.join(banned)}")
+
+    # Card count
+    diff = validate_card_count(cards)
+    if diff != 0:
+        direction = "too many" if diff > 0 else "too few"
+        errors.append(f"Card count: {abs(diff)} {direction} (need exactly 99)")
+
+    # Ratios
+    ratio_report = validate_ratios(cards, ratios)
+    for cat, info in ratio_report.items():
+        if info["diff"] < 0:
+            warnings.append(
+                f"{cat}: {info['actual']}/{info['target']} "
+                f"({abs(info['diff'])} short)"
+            )
+
+    return {
+        "valid": len(errors) == 0,
+        "errors": errors,
+        "warnings": warnings,
+        "ratio_report": ratio_report,
+    }