diff --git a/Documentation/Manual.md b/Documentation/Manual.md
index 51af55bc..c33dd269 100644
--- a/Documentation/Manual.md
+++ b/Documentation/Manual.md
@@ -508,7 +508,7 @@ Set `mockConditionalCompilation` to `nil` to generate mocks without conditional
 
 Each `@Instantiable` type gets a `mock()` static method that builds its full dependency subtree. Types with `generateMock: true` must not also contain a hand-written `mock()` method — the macro will emit an error with a fix-it to remove `generateMock: true`. If you provide your own `mock()` method (without `generateMock: true`), parent types that instantiate the child will call `ChildType.mock(...)` instead of `ChildType(...)` when constructing it, threading mock parameters through your custom method. Note that mocks defined in separate extensions are not detected; the method must be in the `@Instantiable`-decorated declaration body.
 
-Your user-defined `mock()` method must be `public` (or `open`) and must accept parameters for each of the type's `@Instantiated`, `@Received`, and `@Forwarded` dependencies. It may also accept additional parameters with default values. On concrete type declarations the return type must be `Self` or the type name; on extension-based `@Instantiable` types the return type must match the extended type (e.g. `-> Container<Bool>`), mirroring the corresponding `instantiate()` method. The `@Instantiable` macro validates these requirements and provides fix-its for any issues.
+Your user-defined `mock()` method must be `public` (or `open`) and must accept parameters for each of the type's `@Instantiated`, `@Received`, and `@Forwarded` dependencies. It may also accept additional parameters with default values. On concrete type declarations the return type must be `Self`, the type name, or a type listed in `fulfillingAdditionalTypes`; on extension-based `@Instantiable` types the return type must match the extended type (e.g. `-> Container<Bool>`) or a `fulfillingAdditionalTypes` entry, mirroring the corresponding `instantiate()` method. The `@Instantiable` macro validates these requirements and provides fix-its for any issues.
 
 ```swift
 #if DEBUG
diff --git a/Sources/SafeDIMacros/Macros/InstantiableMacro.swift b/Sources/SafeDIMacros/Macros/InstantiableMacro.swift
index 2490640e..c71a9402 100644
--- a/Sources/SafeDIMacros/Macros/InstantiableMacro.swift
+++ b/Sources/SafeDIMacros/Macros/InstantiableMacro.swift
@@ -432,9 +432,11 @@ public struct InstantiableMacro: MemberMacro {
 				let typeName = concreteDeclaration.name.text
 				let instantiableTypeStrippingGenerics = visitor.instantiableType?.strippingGenerics
 				let mockReturnType = mockSyntax.signature.returnClause?.type.typeDescription
+				let additionalTypesStrippingGenerics = (visitor.additionalInstantiables ?? []).map(\.strippingGenerics)
 				let isSelfReturnType = mockReturnType == .simple(name: "Self", generics: [])
 				let returnTypeMatchesTypeName = isSelfReturnType
 					|| mockReturnType?.strippingGenerics == instantiableTypeStrippingGenerics
+					|| additionalTypesStrippingGenerics.contains(where: { $0 == mockReturnType?.strippingGenerics })
 				if !returnTypeMatchesTypeName {
 					var fixedMockSyntax = mockSyntax
 					if let existingReturnClause = mockSyntax.signature.returnClause {
@@ -620,7 +622,9 @@ public struct InstantiableMacro: MemberMacro {
 			var seenMockReturnTypes = [TypeDescription: FunctionDeclSyntax]()
 			for mockFunction in allMockFunctions {
 				let mockReturnType = mockFunction.signature.returnClause?.type.typeDescription
+				let additionalTypesStrippingGenerics = (visitor.additionalInstantiables ?? []).map(\.strippingGenerics)
 				let returnTypeMatchesExtendedType = mockReturnType?.strippingGenerics == extendedTypeStrippingGenerics
+					|| additionalTypesStrippingGenerics.contains(where: { $0 == mockReturnType?.strippingGenerics })
 				if !returnTypeMatchesExtendedType {
 					var fixedMockFunction = mockFunction
 					if let existingReturnClause = mockFunction.signature.returnClause {
diff --git a/Tests/SafeDIMacrosTests/InstantiableMacroTests.swift b/Tests/SafeDIMacrosTests/InstantiableMacroTests.swift
index 19347fe8..2e8f19cc 100644
--- a/Tests/SafeDIMacrosTests/InstantiableMacroTests.swift
+++ b/Tests/SafeDIMacrosTests/InstantiableMacroTests.swift
@@ -4933,6 +4933,58 @@ import Testing
 			)
 		}
 
+		@Test
+		func mockMethodReturningFulfillingAdditionalTypeProducesNoDiagnostic() {
+			assertMacroExpansion(
+				"""
+				@Instantiable(fulfillingAdditionalTypes: [MyServiceProtocol.self])
+				public struct MyService: MyServiceProtocol, Instantiable {
+				    public init() {}
+
+				    public static func mock() -> MyServiceProtocol {
+				        MyService()
+				    }
+				}
+				""",
+				expandedSource: """
+				public struct MyService: MyServiceProtocol, Instantiable {
+				    public init() {}
+
+				    public static func mock() -> MyServiceProtocol {
+				        MyService()
+				    }
+				}
+				""",
+				macros: instantiableTestMacros,
+			)
+		}
+
+		@Test
+		func extension_mockMethodReturningFulfillingAdditionalTypeProducesNoDiagnostic() {
+			assertMacroExpansion(
+				"""
+				@Instantiable(fulfillingAdditionalTypes: [MyServiceProtocol.self])
+				extension MyService: Instantiable {
+				    public static func instantiate() -> MyService { MyService() }
+
+				    public static func mock() -> MyServiceProtocol {
+				        MyService()
+				    }
+				}
+				""",
+				expandedSource: """
+				extension MyService: Instantiable {
+				    public static func instantiate() -> MyService { MyService() }
+
+				    public static func mock() -> MyServiceProtocol {
+				        MyService()
+				    }
+				}
+				""",
+				macros: instantiableTestMacros,
+			)
+		}
+
 		@Test
 		func extension_mockMethodNotPublicProducesDiagnostic() {
 			assertMacroExpansion(