diff --git a/fixtures/decision-reader/legacy.json b/fixtures/decision-reader/legacy.json
index d6dc46c..a376e32 100644
--- a/fixtures/decision-reader/legacy.json
+++ b/fixtures/decision-reader/legacy.json
@@ -25,6 +25,38 @@
     "accepts": true,
     "kind": "legacy"
   },
+  {
+    "name": "legacy-choice-total-lower-edge",
+    "answer": {
+      "type": "choice",
+      "choice": "b",
+      "probabilities": {
+        "a": 0.060817,
+        "b": 0.532085,
+        "c": 0.407097
+      },
+      "confidence": 0.125,
+      "calibration_version": "unpublished"
+    },
+    "accepts": true,
+    "kind": "legacy"
+  },
+  {
+    "name": "legacy-choice-total-lower-edge-unordered-keys",
+    "answer": {
+      "type": "choice",
+      "choice": "b",
+      "probabilities": {
+        "b": 0.611317,
+        "1": 0.032434,
+        "a": 0.356248
+      },
+      "confidence": 0.255,
+      "calibration_version": "unpublished"
+    },
+    "accepts": true,
+    "kind": "legacy"
+  },
   {
     "name": "legacy-score",
     "answer": {
diff --git a/python/src/system_zero/decisions.py b/python/src/system_zero/decisions.py
index 93f536e..cdb1184 100644
--- a/python/src/system_zero/decisions.py
+++ b/python/src/system_zero/decisions.py
@@ -86,7 +86,10 @@ def _distribution(answer: Mapping[str, Any], canonical: bool) -> Dict[str, Union
         if not isinstance(key, str) or not _probability(value):
             _invalid("probabilities must be finite numbers in [0, 1]")
         values[key] = _units(value) if canonical else value
-    total = sum(values.values())
+    # Explicit sequential addition in UTF-8 key order matches TypeScript; sum() is compensated on Python 3.12+.
+    total = 0
+    for key in sorted(values, key=lambda key: key.encode("utf-8")):
+        total += values[key]
     
     if (canonical and (abs(total - _MILLION) > 1 or (len(values) == 1 and total != _MILLION))) or (not canonical and abs(total - 1) > 0.000001):
         _invalid("probabilities must sum to one")
diff --git a/typescript/src/decisions.ts b/typescript/src/decisions.ts
index 122f38f..2c99688 100644
--- a/typescript/src/decisions.ts
+++ b/typescript/src/decisions.ts
@@ -61,13 +61,13 @@ function units(value: unknown): number {
 function distribution(answer: Record<string, unknown>, canonical: boolean): Map<string, number> {
   if (!record(answer.probabilities) || Object.keys(answer.probabilities).length === 0) invalid('missing probabilities');
   const values = new Map<string, number>();
-  let total = 0;
   for (const [key, value] of Object.entries(answer.probabilities)) {
     if (!probability(value)) invalid('probabilities must be finite numbers in [0, 1]');
-    const amount = canonical ? units(value) : value;
-    values.set(key, amount);
-    total += amount;
+    values.set(key, canonical ? units(value) : value);
   }
+  // Add in UTF-8 key order, the reader's tie order, so a legacy float total never depends on property enumeration order.
+  let total = 0;
+  for (const key of [...values.keys()].sort((a, b) => Buffer.compare(Buffer.from(a), Buffer.from(b)))) total += values.get(key)!;
    
   if (canonical ? Math.abs(total - million) > 1 || (values.size === 1 && total !== million) : Math.abs(total - 1) > 0.000001) invalid('probabilities must sum to one');
   return values;