diff --git a/RandomizerCore/Logic/LogicManagerBuilder.cs b/RandomizerCore/Logic/LogicManagerBuilder.cs
index dc8b389..923e34e 100644
--- a/RandomizerCore/Logic/LogicManagerBuilder.cs
+++ b/RandomizerCore/Logic/LogicManagerBuilder.cs
@@ -86,6 +86,7 @@ public enum JsonType
             Locations,
             LogicEdit,
             MacroEdit,
+            LogicSubst,
         }
 
         public void DeserializeJson(JsonType type, string s)
@@ -173,6 +174,28 @@ public void DeserializeJson(JsonType type, JsonTextReader jtr)
                         LP.SetMacro(kvp.Key, new LogicClause(lcb));
                     }
                     break;
+
+                case JsonType.LogicSubst:
+                    foreach (RawSubstDef def in JsonUtil.Deserialize<List<RawSubstDef>>(jtr) ?? Enumerable.Empty<RawSubstDef>())
+                    {
+                        TermToken tt = LP.GetTermToken(def.old);
+                        LogicClause lc = LP.ParseInfixToClause(def.replacement);
+                        if (LP.IsMacro(def.name))
+                        {
+                            LogicClauseBuilder lcb = new(LP.GetMacro(def.name));
+                            lcb.Subst(tt, lc);
+                            LP.SetMacro(def.name, new LogicClause(lcb));
+                        }
+                        else if (LogicLookup.TryGetValue(def.name, out LogicClause orig))
+                        {
+                            LogicClauseBuilder lcb = new(orig);
+                            lcb.Subst(tt, lc);
+                            LogicLookup[def.name] = new(lcb);
+                        }
+                        else throw new ArgumentException($"RawSubstDef {def} does not correspond to any known macro or logic.");
+                    }
+                    break;
+
             }
         }
 
@@ -247,6 +270,26 @@ public void DeserializeJson(JsonType type, JToken t)
                         LP.SetMacro(kvp.Key, new LogicClause(lcb));
                     }
                     break;
+                case JsonType.LogicSubst:
+                    foreach (RawSubstDef def in t.ToObject<List<RawSubstDef>>() ?? Enumerable.Empty<RawSubstDef>())
+                    {
+                        TermToken tt = LP.GetTermToken(def.old);
+                        LogicClause lc = LP.ParseInfixToClause(def.replacement);
+                        if (LP.IsMacro(def.name))
+                        {
+                            LogicClauseBuilder lcb = new(LP.GetMacro(def.name));
+                            lcb.Subst(tt, lc);
+                            LP.SetMacro(def.name, new LogicClause(lcb));
+                        }
+                        else if (LogicLookup.TryGetValue(def.name, out LogicClause orig))
+                        {
+                            LogicClauseBuilder lcb = new(orig);
+                            lcb.Subst(tt, lc);
+                            LogicLookup[def.name] = new(lcb);
+                        }
+                        else throw new ArgumentException($"RawSubstDef {def} does not correspond to any known macro or logic.");
+                    }
+                    break;
             }
         }
 
diff --git a/RandomizerCore/Logic/RawSubstDef.cs b/RandomizerCore/Logic/RawSubstDef.cs
new file mode 100644
index 0000000..78ec44a
--- /dev/null
+++ b/RandomizerCore/Logic/RawSubstDef.cs
@@ -0,0 +1,6 @@
+namespace RandomizerCore.Logic
+{
+    public readonly record struct RawSubstDef(string name, string old, string replacement)
+    {
+    }
+}
diff --git a/RandomizerCore/StringLogic/LogicProcessor.cs b/RandomizerCore/StringLogic/LogicProcessor.cs
index 79e12ca..7a1ce46 100644
--- a/RandomizerCore/StringLogic/LogicProcessor.cs
+++ b/RandomizerCore/StringLogic/LogicProcessor.cs
@@ -48,6 +48,8 @@ public void SetMacro(string key, LogicClause c)
             if (lt is null) tokenPool[key] = new MacroToken(key, this);
         }
 
+        public bool IsMacro(string name) => macros.ContainsKey(name);
+
         public LogicClause GetMacro(string name)
         {
             return macros[name];