diff --git a/Sources/SafeDICore/Generators/ScopeGenerator.swift b/Sources/SafeDICore/Generators/ScopeGenerator.swift
index e7aa1c4b..a081aaa4 100644
--- a/Sources/SafeDICore/Generators/ScopeGenerator.swift
+++ b/Sources/SafeDICore/Generators/ScopeGenerator.swift
@@ -865,8 +865,10 @@ actor ScopeGenerator: CustomStringConvertible, Sendable {
 		static let configurationStructName = "SafeDIMockConfiguration"
 
 		/// The qualified configuration type name for references (e.g., `ChildA.SafeDIMockConfiguration`).
+		/// Uses the concrete fulfilling type so the struct can be nested in a concrete type
+		/// extension. Protocol extensions cannot contain nested type declarations.
 		var configurationTypeName: String {
-			"\(instantiatedTypeDescription.asSource).\(Self.configurationStructName)"
+			"\(concreteType.asSource).\(Self.configurationStructName)"
 		}
 
 		/// The builder closure type as a Swift source string (unlabeled parameters).
@@ -918,7 +920,7 @@ actor ScopeGenerator: CustomStringConvertible, Sendable {
 		let indent = Self.standardIndent
 		return uniqueTypes.map { node in
 			(
-				typeName: node.instantiatedTypeDescription.asSource,
+				typeName: node.concreteType.asSource,
 				structCode: Self.generateConfigurationStruct(for: node, indent: indent),
 			)
 		}
@@ -1090,8 +1092,11 @@ actor ScopeGenerator: CustomStringConvertible, Sendable {
 	}
 
 	/// Collects all unique types from the `MockParameterNode` tree, deduplicated
-	/// by `instantiatedTypeDescription`. Returns nodes in depth-first order
-	/// (children before parents) so that referenced types appear before their referrers.
+	/// by `concreteType`. Returns nodes in depth-first order (children before
+	/// parents) so that referenced types appear before their referrers.
+	/// Uses `concreteType` (not `instantiatedTypeDescription`) because config
+	/// structs are nested in concrete type extensions — protocol extensions
+	/// cannot contain nested type declarations.
 	/// When the same type appears in both sendable and non-sendable contexts,
 	/// the sendable version is preferred (`@Sendable` closures work in both contexts).
 	private static func collectUniqueConfigurationTypes(
@@ -1101,7 +1106,7 @@ actor ScopeGenerator: CustomStringConvertible, Sendable {
 		var result = [MockParameterNode]()
 
 		func walk(_ node: MockParameterNode, ancestorTypes: Set<String> = []) {
-			let key = node.instantiatedTypeDescription.asSource
+			let key = node.concreteType.asSource
 			// Skip nodes whose type matches an ancestor — self-referencing cycle.
 			guard !ancestorTypes.contains(key) else { return }
 			var childAncestors = ancestorTypes
@@ -1117,7 +1122,7 @@ actor ScopeGenerator: CustomStringConvertible, Sendable {
 				// replace it — @Sendable closures are compatible in both contexts.
 				if node.requiresSendable,
 				   let existingIndex = result.firstIndex(where: {
-				   	$0.instantiatedTypeDescription.asSource == key && !$0.requiresSendable
+				   	$0.concreteType.asSource == key && !$0.requiresSendable
 				   })
 				{
 					result[existingIndex] = node
@@ -1207,7 +1212,7 @@ actor ScopeGenerator: CustomStringConvertible, Sendable {
 		// Exclude children whose type matches this node — they'd create a recursive
 		// value type. These are self-referencing Instantiators (lazy cycles).
 		let nonCycleChildren = node.children.filter {
-			$0.instantiatedTypeDescription != node.instantiatedTypeDescription
+			$0.concreteType != node.concreteType
 		}
 		let childLabelMap = disambiguatePropertyLabels(for: nonCycleChildren)
 		for child in nonCycleChildren {
diff --git a/Tests/SafeDIToolTests/SafeDIToolMockGenerationTests.swift b/Tests/SafeDIToolTests/SafeDIToolMockGenerationTests.swift
index ef8cf2cc..bcf6f364 100644
--- a/Tests/SafeDIToolTests/SafeDIToolMockGenerationTests.swift
+++ b/Tests/SafeDIToolTests/SafeDIToolMockGenerationTests.swift
@@ -4106,6 +4106,23 @@ struct SafeDIToolMockGenerationTests: ~Copyable {
 		    }
 		}
 		#endif
+
+		#if DEBUG
+		extension DefaultAuthService {
+		    struct SafeDIMockConfiguration {
+		        init(
+		            networkService: (() -> DefaultNetworkService)? = nil,
+		            _ safeDIBuilder: ((NetworkService, NetworkService) -> DefaultAuthService)? = nil
+		        ) {
+		            self.networkService = networkService
+		            self.safeDIBuilder = safeDIBuilder
+		        }
+
+		        let networkService: (() -> DefaultNetworkService)?
+		        let safeDIBuilder: ((NetworkService, NetworkService) -> DefaultAuthService)?
+		    }
+		}
+		#endif
 		""", "Unexpected output \(output.mockFiles["DefaultAuthService+SafeDIMock.swift"] ?? "")")
 
 		#expect(output.mockFiles["DefaultNetworkService+SafeDIMock.swift"] == """
@@ -4132,14 +4149,14 @@ struct SafeDIToolMockGenerationTests: ~Copyable {
 		    struct SafeDIParameters {
 		        init(
 		            networkService: (() -> DefaultNetworkService)? = nil,
-		            authService: AuthService.SafeDIMockConfiguration = .init()
+		            authService: DefaultAuthService.SafeDIMockConfiguration = .init()
 		        ) {
 		            self.networkService = networkService
 		            self.authService = authService
 		        }
 
 		        let networkService: (() -> DefaultNetworkService)?
-		        let authService: AuthService.SafeDIMockConfiguration
+		        let authService: DefaultAuthService.SafeDIMockConfiguration
 		    }
 
 		    static func mock(
@@ -4162,22 +4179,6 @@ struct SafeDIToolMockGenerationTests: ~Copyable {
 		// Any modifications made to this file will be overwritten on subsequent builds.
 		// Please refrain from editing this file directly.
 
-		#if DEBUG
-		extension AuthService {
-		    struct SafeDIMockConfiguration {
-		        init(
-		            networkService: (() -> DefaultNetworkService)? = nil,
-		            _ safeDIBuilder: ((NetworkService, NetworkService) -> DefaultAuthService)? = nil
-		        ) {
-		            self.networkService = networkService
-		            self.safeDIBuilder = safeDIBuilder
-		        }
-
-		        let networkService: (() -> DefaultNetworkService)?
-		        let safeDIBuilder: ((NetworkService, NetworkService) -> DefaultAuthService)?
-		    }
-		}
-		#endif
 
 		""", "Unexpected output \(output.mockConfigurationFile ?? "")")
 	}
@@ -6221,12 +6222,12 @@ struct SafeDIToolMockGenerationTests: ~Copyable {
 		extension Root {
 		    struct SafeDIParameters {
 		        init(
-		            service: AnyService.SafeDIMockConfiguration = .init()
+		            service: ConcreteService.SafeDIMockConfiguration = .init()
 		        ) {
 		            self.service = service
 		        }
 
-		        let service: AnyService.SafeDIMockConfiguration
+		        let service: ConcreteService.SafeDIMockConfiguration
 		    }
 
 		    static func mock(
@@ -6248,22 +6249,6 @@ struct SafeDIToolMockGenerationTests: ~Copyable {
 		// Any modifications made to this file will be overwritten on subsequent builds.
 		// Please refrain from editing this file directly.
 
-		#if DEBUG
-		extension AnyService {
-		    struct SafeDIMockConfiguration {
-		        init(
-		            helper: (() -> Helper)? = nil,
-		            _ safeDIBuilder: ((Helper) -> ConcreteService)? = nil
-		        ) {
-		            self.helper = helper
-		            self.safeDIBuilder = safeDIBuilder
-		        }
-
-		        let helper: (() -> Helper)?
-		        let safeDIBuilder: ((Helper) -> ConcreteService)?
-		    }
-		}
-		#endif
 
 		""", "Unexpected output \(output.mockConfigurationFile ?? "")")
 	}