diff --git a/src/simulator/crates/mc-trade/src/lib.rs b/src/simulator/crates/mc-trade/src/lib.rs
index 0681bfe4..41722944 100644
--- a/src/simulator/crates/mc-trade/src/lib.rs
+++ b/src/simulator/crates/mc-trade/src/lib.rs
@@ -194,6 +194,102 @@ pub fn break_trades_on_war(ledger: &mut TradeLedger, at_war_a: u8, at_war_b: u8)
     broken
 }
 
+// ── Diplomacy actions ──────────────────────────────────────────────────────
+
+/// Outcome of a diplomacy action, returned to the turn processor for logging.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub enum DiplomacyEvent {
+    WarDeclared { by: u8, against: u8 },
+    PeaceOffered { by: u8, to: u8 },
+    PeaceAccepted { by: u8, with: u8 },
+    /// EA: AI always rejects player-initiated peace offers.
+    PeaceRejected { by: u8, with: u8 },
+    TradeOfferAccepted { by: u8, with: u8, gold: u32, luxury_id: String },
+    /// EA: AI always rejects gold-for-luxury offers.
+    TradeOfferRejected { by: u8, with: u8 },
+}
+
+/// A player-initiated offer of gold in exchange for a luxury resource.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct TradeOffer {
+    pub from: u8,
+    pub to: u8,
+    /// Gold the offering player pays.
+    pub gold: u32,
+    /// Luxury ID the offering player wants in return.
+    pub luxury_id: String,
+}
+
+/// Declare war between two players. Sets relation to War immediately,
+/// breaks all active trades between them, resets counters.
+/// Returns the event plus any broken agreements.
+pub fn declare_war(
+    relations: &mut BTreeMap<(u8, u8), RelationState>,
+    ledger: &mut TradeLedger,
+    by: u8,
+    against: u8,
+) -> (DiplomacyEvent, Vec<TradeAgreement>) {
+    let key = pair_key(by, against);
+    let rs = relations.entry(key).or_default();
+    rs.relation = Relation::War;
+    rs.peaceful_turns = 0;
+    rs.trade_turns = 0;
+    rs.war_idle_turns = 0;
+    let broken = break_trades_on_war(ledger, by, against);
+    (DiplomacyEvent::WarDeclared { by, against }, broken)
+}
+
+/// Offer peace to an opponent. In Early Access the AI always rejects, so
+/// this returns `PeaceRejected`. The caller may choose to accept on behalf
+/// of a human opponent by calling `accept_peace` directly.
+pub fn offer_peace(by: u8, to: u8) -> DiplomacyEvent {
+    DiplomacyEvent::PeaceRejected { by, with: to }
+}
+
+/// Force-accept a peace offer (used when the receiving side is human or when
+/// the armistice auto-fires). Sets relation to Neutral so the peace counter
+/// can accumulate toward Peace.
+pub fn accept_peace(
+    relations: &mut BTreeMap<(u8, u8), RelationState>,
+    by: u8,
+    with: u8,
+) -> DiplomacyEvent {
+    let key = pair_key(by, with);
+    let rs = relations.entry(key).or_default();
+    rs.relation = Relation::Neutral;
+    rs.war_idle_turns = 0;
+    DiplomacyEvent::PeaceAccepted { by, with }
+}
+
+/// Evaluate a `TradeOffer` (gold ↔ luxury). In EA the AI always rejects
+/// player-initiated offers — automated luxury swaps go through `evaluate_trades`.
+/// Returns `TradeOfferRejected`. Deduct gold only on acceptance; callers
+/// handle the ledger update if they override this for tests.
+pub fn evaluate_trade_offer(offer: &TradeOffer) -> DiplomacyEvent {
+    DiplomacyEvent::TradeOfferRejected { by: offer.to, with: offer.from }
+}
+
+/// Apply an accepted trade offer: deduct gold from `from`, credit `luxury_id`
+/// into `to`'s `traded_luxuries`. Called only when the offer is accepted
+/// (human vs human, or test override).
+pub fn apply_trade_offer(
+    from_gold: &mut i32,
+    to_traded_luxuries: &mut BTreeSet<String>,
+    offer: &TradeOffer,
+) -> DiplomacyEvent {
+    *from_gold -= offer.gold as i32;
+    if *from_gold < 0 {
+        *from_gold = 0;
+    }
+    to_traded_luxuries.insert(offer.luxury_id.clone());
+    DiplomacyEvent::TradeOfferAccepted {
+        by: offer.to,
+        with: offer.from,
+        gold: offer.gold,
+        luxury_id: offer.luxury_id.clone(),
+    }
+}
+
 /// Advance all relation states for one turn.
 ///
 /// `combat_pairs`: set of `pair_key(a, b)` where combat occurred this turn.