diff --git a/src/simulator/crates/mc-turn/tests/serde_roundtrip.rs b/src/simulator/crates/mc-turn/tests/serde_roundtrip.rs
index 7b22a4af..1ab46725 100644
--- a/src/simulator/crates/mc-turn/tests/serde_roundtrip.rs
+++ b/src/simulator/crates/mc-turn/tests/serde_roundtrip.rs
@@ -19,6 +19,9 @@ use mc_core::WonderId;
 use mc_economy::Treasury;
 use mc_happiness::pool::{GoldenAgeState, HappinessInput};
 use mc_trade::relation::{Relation, RelationState};
+use mc_trade::{
+    CourierRoute, DiplomaticAgreement, OpenBordersAgreement, SharedMapAgreement, TradeLedger,
+};
 use mc_turn::{GameState, MapUnit, PlayerState, TechState};
 use std::collections::{BTreeMap, BTreeSet};
 
@@ -330,3 +333,75 @@ fn happiness_pool_json_roundtrip_is_stable() {
     assert_eq!(restored_g.golden_age_turns, 7);
     assert_eq!(restored_g.golden_age_count, 2);
 }
+
+/// TradeLedger schema migration (p3-01 c4) — agreements is now
+/// `Vec<DiplomaticAgreement>`. Pin all three variants through JSON round-trip
+/// so a future field addition surfaces immediately.
+#[test]
+fn trade_ledger_diplomatic_agreement_roundtrip() {
+    let mut ledger = TradeLedger {
+        next_agreement_id: 3,
+        agreements: vec![
+            DiplomaticAgreement::LuxurySwap(mc_trade::TradeAgreement {
+                partners: (0, 1),
+                gives_a: "silk".into(),
+                gives_b: "wine".into(),
+                turn_started: 5,
+            }),
+            DiplomaticAgreement::OpenBorders(OpenBordersAgreement {
+                agreement_id: 0,
+                partners: (0, 2),
+                turn_started: 10,
+                turns_remaining: 8,
+                payment_gold: 50,
+                payment_luxury: Some("amber".into()),
+            }),
+            DiplomaticAgreement::SharedMap(SharedMapAgreement {
+                agreement_id: 1,
+                partners: (1, 2),
+                turn_started: 12,
+                duration: 6,
+                share_turns_remaining: 0,
+                payment_gold: 30,
+                payment_luxury: None,
+                courier_route: Some(CourierRoute {
+                    sender: 1,
+                    receiver: 2,
+                    courier_era_tier: 3,
+                    dispatched_turn: 12,
+                    position: (4, 7),
+                    eta_turn: Some(15),
+                    delivered: false,
+                    intercepted: false,
+                }),
+            }),
+        ],
+    };
+    // Allocate one more id to verify counter survives round-trip.
+    let _ = ledger.alloc_agreement_id();
+    assert_eq!(ledger.next_agreement_id, 4);
+
+    let first = serde_json::to_string(&ledger).expect("TradeLedger serializes");
+    let restored: TradeLedger = serde_json::from_str(&first).expect("TradeLedger deserializes");
+    let second = serde_json::to_string(&restored).expect("restored TradeLedger re-serializes");
+    assert_eq!(first, second, "TradeLedger round-trip drifted");
+
+    assert_eq!(restored.agreements.len(), 3);
+    assert_eq!(restored.next_agreement_id, 4);
+
+    // Verify LuxurySwap variant fields survive.
+    assert!(matches!(
+        &restored.agreements[0],
+        DiplomaticAgreement::LuxurySwap(ta) if ta.gives_a == "silk"
+    ));
+
+    // Verify SharedMap courier route fields survive.
+    if let DiplomaticAgreement::SharedMap(sm) = &restored.agreements[2] {
+        let route = sm.courier_route.as_ref().expect("courier_route present");
+        assert_eq!(route.position, (4, 7));
+        assert_eq!(route.courier_era_tier, 3);
+        assert!(!route.delivered);
+    } else {
+        panic!("expected SharedMap variant at index 2");
+    }
+}