diff --git a/build-system/SwiftTL/Sources/SwiftTL/Resolution.swift b/build-system/SwiftTL/Sources/SwiftTL/Resolution.swift
index 5fbd3a90d9..431300a431 100644
--- a/build-system/SwiftTL/Sources/SwiftTL/Resolution.swift
+++ b/build-system/SwiftTL/Sources/SwiftTL/Resolution.swift
@@ -199,7 +199,114 @@ enum Resolver {
         
         return types.values.sorted(by: { $0.name < $1.name })
     }
-    
+
+    static func resolveLayeredTypes(
+        layers: [(layerNumber: Int, constructors: [DescriptionParser.ConstructorDescription])]
+    ) throws -> [(layerNumber: Int, types: [SumType])] {
+        // Running state: for each constructor name, the target type name and the raw description.
+        // We keep raw descriptions (not resolved forms) because a later-layer constructor may
+        // introduce new target-type names, and resolveTypeReference needs the final target-type set.
+        var liveConstructors: [QualifiedName: (typeName: QualifiedName, description: DescriptionParser.ConstructorDescription)] = [:]
+        var result: [(layerNumber: Int, types: [SumType])] = []
+
+        for (layerNumber, layerConstructors) in layers {
+            // Apply this layer's constructors to the running map with last-wins semantics.
+            for constructorDescription in layerConstructors {
+                switch constructorDescription.type {
+                case let .type(name):
+                    if !name.value[name.value.startIndex].isUppercase {
+                        throw ResolutionError(text: "Type constructor \(constructorDescription.name) -> \(name): the resulting type name should begin with a capital letter")
+                    }
+                    liveConstructors[constructorDescription.name] = (name, constructorDescription)
+                case let .generic(name, argumentType):
+                    throw ResolutionError(text: "Type constructor \(constructorDescription.name) can not be used to construct a generic type \(name)<\(argumentType)>")
+                }
+            }
+
+            // Note: a constructor reassigned to a different target type in a later layer is
+            // removed from its old type's constructor set. If that drops the old type to zero
+            // constructors, it vanishes from this snapshot, and any unrelated argument that
+            // still references the old type via boxedType(...) will fail to resolve here.
+            // secret_scheme.tl does not exercise this case (same-name constructors always
+            // target the same type), but the rule applies if a future layered schema does.
+            // Snapshot: group by target type, resolve.
+            var constructedTypes: [QualifiedName: [DescriptionParser.ConstructorDescription]] = [:]
+            var constructorNameToType: [QualifiedName: QualifiedName] = [:]
+            for (ctorName, entry) in liveConstructors {
+                constructedTypes[entry.typeName, default: []].append(entry.description)
+                constructorNameToType[ctorName] = entry.typeName
+            }
+
+            func resolveTypeReference(description: DescriptionParser.TypeReferenceDescription) throws -> TypeReference {
+                switch description {
+                case let .type(name):
+                    if let resolvedBuiltinType = resolveBuiltinType(name: name) {
+                        return resolvedBuiltinType
+                    }
+                    if name.value[name.value.startIndex].isUppercase {
+                        if let _ = constructedTypes[name] {
+                            return .boxedType(name)
+                        } else {
+                            throw ResolutionError(text: "Unresolved type \(name) in layer \(layerNumber)")
+                        }
+                    } else {
+                        if let typeName = constructorNameToType[name] {
+                            return .bareConstructor(typeName: typeName, name: name)
+                        } else {
+                            throw ResolutionError(text: "Unresolved type constructor \(name) in layer \(layerNumber)")
+                        }
+                    }
+                case let .generic(name, argumentType):
+                    if name == "vector" {
+                        return .bareVector(try resolveTypeReference(description: .type(name: argumentType)))
+                    } else if name == "Vector" {
+                        return .boxedVector(try resolveTypeReference(description: .type(name: argumentType)))
+                    } else {
+                        throw ResolutionError(text: "Unresolved generic type \(name) in layer \(layerNumber)")
+                    }
+                }
+            }
+
+            func resolveArgument(existingArguments: [Argument], description: DescriptionParser.ArgumentDescription) throws -> Argument {
+                return Argument(
+                    name: description.name,
+                    type: try resolveTypeReference(description: description.type),
+                    condition: try description.condition.flatMap { condition -> Argument.Condition in
+                        if !existingArguments.contains(where: { $0.name == condition.fieldName }) {
+                            throw ResolutionError(text: "Unresolved conditional field reference to \(condition.fieldName) in layer \(layerNumber)")
+                        }
+                        return Argument.Condition(fieldName: condition.fieldName, bitIndex: condition.bitIndex)
+                    }
+                )
+            }
+
+            var types: [QualifiedName: SumType] = [:]
+            for (typeName, constructorDescriptions) in constructedTypes {
+                let type = SumType(name: typeName)
+                for constructorDescription in constructorDescriptions {
+                    var arguments: [Argument] = []
+                    for argumentDescription in constructorDescription.arguments {
+                        arguments.append(try resolveArgument(existingArguments: arguments, description: argumentDescription))
+                    }
+                    guard let id = constructorDescription.explicitId else {
+                        throw ResolutionError(text: "Constructor \(constructorDescription.name) does not have an id")
+                    }
+                    type.constructors[constructorDescription.name] = SumType.Constructor(
+                        name: constructorDescription.name,
+                        id: id,
+                        arguments: arguments
+                    )
+                }
+                types[type.name] = type
+            }
+
+            let sortedTypes = types.values.sorted(by: { $0.name < $1.name })
+            result.append((layerNumber, sortedTypes))
+        }
+
+        return result
+    }
+
     static func resolveFunctions(types: [SumType], functionDescriptions: [DescriptionParser.ConstructorDescription]) throws -> [Function] {
         var functions: [QualifiedName: Function] = [:]