diff --git a/crates/fidc-core/src/stock_pool_execution.rs b/crates/fidc-core/src/stock_pool_execution.rs
index bf5018f..0447d03 100644
--- a/crates/fidc-core/src/stock_pool_execution.rs
+++ b/crates/fidc-core/src/stock_pool_execution.rs
@@ -485,6 +485,7 @@ pub struct StockPoolDecisionConstraints {
     pub frozen_positions: BTreeMap<String, FrozenStockPoolPosition>,
     pub prior_target_weights: BTreeMap<String, i32>,
     /// Sizing ratios from prior plans; integer bps are display/legacy only.
+    #[serde(skip_serializing_if = "BTreeMap::is_empty")]
     pub prior_target_weight_ratios: BTreeMap<String, Decimal>,
     pub pending_entry_symbols: BTreeSet<String>,
     pub next_day_outside_exit_symbols: BTreeSet<String>,
diff --git a/crates/fidc-core/src/stock_pool_execution_tests.rs b/crates/fidc-core/src/stock_pool_execution_tests.rs
index d43694e..35cc534 100644
--- a/crates/fidc-core/src/stock_pool_execution_tests.rs
+++ b/crates/fidc-core/src/stock_pool_execution_tests.rs
@@ -1,5 +1,11 @@
 use super::*;
 
+#[test]
+fn legacy_constraints_do_not_gain_an_empty_precision_field() {
+    let value = serde_json::to_value(StockPoolDecisionConstraints::default()).unwrap();
+    assert!(value.get("prior_target_weight_ratios").is_none());
+}
+
 #[test]
 fn equal_thirty_seats_use_full_precision_at_a_board_lot_boundary() {
     for (equity, price, executable, held) in [("999377.147617", "3.07", "3.070307", 2000), ("995624.8819", "6.42", "6.420642", 900)] {