diff --git a/Sources/Firestore/FirestoreClient.swift b/Sources/Firestore/FirestoreClient.swift
index 991718b..e8832d8 100644
--- a/Sources/Firestore/FirestoreClient.swift
+++ b/Sources/Firestore/FirestoreClient.swift
@@ -930,3 +930,6 @@ extension FirestoreClient {
         }
     }
 }
+
+// MARK: - FirestoreClientType Conformance
+extension FirestoreClient: FirestoreClientType {}
diff --git a/Sources/Firestore/FirestoreClientType.swift b/Sources/Firestore/FirestoreClientType.swift
new file mode 100644
index 0000000..2dca368
--- /dev/null
+++ b/Sources/Firestore/FirestoreClientType.swift
@@ -0,0 +1,392 @@
+//
+//  FirestoreClientType.swift
+//  EasyFirebaseFirestore
+//
+//  Created by Fumiya Tanaka on 2025/02/11.
+//
+
+import FirebaseFirestore
+
+/// A protocol abstracting FirestoreClient for dependency injection and testing.
+///
+/// All methods are marked `async` so that both actor-isolated implementations
+/// (where actor isolation satisfies the async requirement per SE-0306)
+/// and regular class-based mocks can conform.
+///
+/// `writeTransaction` is intentionally excluded due to its complex signature
+/// (`WritableKeyPath` + tuple closure). It can be added later if needed.
+///
+/// @mockable
+public protocol FirestoreClientType {
+
+    // MARK: - FirestoreModel - Write
+
+    @discardableResult
+    func write<Model: FirestoreModel>(
+        _ model: Model,
+        newDocumentIdIfNotExists: String?
+    ) async throws -> DocumentReference
+
+    // MARK: - FirestoreModel - Get
+
+    func get<Model: FirestoreModel>(
+        documentId: String,
+        includeCache: Bool
+    ) async throws -> Model
+
+    func get<Model: FirestoreModel>(
+        filter: [FirestoreQueryFilter],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder],
+        limit: Int?
+    ) async throws -> [Model]
+
+    // MARK: - FirestoreModel - Listen
+
+    func listen<Model: FirestoreModel>(
+        documentId: String,
+        includeCache: Bool
+    ) async -> AsyncThrowingStream<Model, Error>
+
+    func listen<Model: FirestoreModel>(
+        filter: [FirestoreQueryFilter],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder],
+        limit: Int?
+    ) async -> AsyncThrowingStream<[Model], Error>
+
+    func listenChanges<Model: FirestoreModel>(
+        filter: [FirestoreQueryFilter],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder],
+        limit: Int?
+    ) async -> AsyncThrowingStream<[FirestoreDocumentChange<Model>], Error>
+
+    // MARK: - FirestoreModel - Delete
+
+    func delete<Model: FirestoreModel>(_ model: Model) async throws
+
+    // MARK: - SubCollectionModel - Create/Update
+
+    func create<Model: SubCollectionModel>(_ model: Model) async throws
+
+    func update<Model: FirestoreModel & SubCollectionModel>(
+        _ model: Model
+    ) async throws
+
+    // MARK: - SubCollectionModel - Get
+
+    func get<Model: SubCollectionModel>(
+        parent parentDocumentId: String,
+        filter: [FirestoreQueryFilter],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder],
+        limit: Int?
+    ) async throws -> [Model]
+
+    func get<Model: SubCollectionModel>(
+        documentId: String,
+        parent parentUid: String,
+        includeCache: Bool
+    ) async throws -> Model
+
+    // MARK: - SubCollectionModel - Listen
+
+    func listen<Model: FirestoreModel & SubCollectionModel>(
+        parentDocumentId: String,
+        documentId: String,
+        filter: [FirestoreQueryFilter],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder],
+        limit: Int?
+    ) async -> AsyncThrowingStream<Model, Error>
+
+    func listen<Model: FirestoreModel & SubCollectionModel>(
+        parent: String,
+        superParent: String?,
+        filter: [FirestoreQueryFilter],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder],
+        limit: Int?
+    ) async -> AsyncThrowingStream<[Model], Error>
+
+    func listenChanges<Model: FirestoreModel & SubCollectionModel>(
+        parent: String,
+        superParent: String?,
+        filter: [FirestoreQueryFilter],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder],
+        limit: Int?
+    ) async -> AsyncThrowingStream<[FirestoreDocumentChange<Model>], Error>
+
+    // MARK: - CollectionGroup
+
+    func getCollectionGroup<Model: FirestoreModel>(
+        filter: [FirestoreQueryFilter],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder],
+        limit: Int?
+    ) async throws -> [Model]
+
+    func listenCollectionGroup<Model: FirestoreModel>(
+        collectionName: String,
+        filter: [FirestoreQueryFilter],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder],
+        limit: Int?
+    ) async -> AsyncThrowingStream<[Model], Error>
+
+    func listenCollectionGroupChanges<Model: FirestoreModel>(
+        collectionName: String,
+        filter: [FirestoreQueryFilter],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder],
+        limit: Int?
+    ) async -> AsyncThrowingStream<[FirestoreDocumentChange<Model>], Error>
+
+    // MARK: - Listener Management
+
+    func stopListening<Model: FirestoreModel>(type: Model.Type) async
+    func stopListening<Model: FirestoreModel>(
+        type: Model.Type,
+        documentID: String
+    ) async
+    func stopListening(ref: DocumentReference) async
+    func stopListeningAll() async
+}
+
+// MARK: - Convenience Methods (Default Parameters)
+
+extension FirestoreClientType {
+
+    // MARK: FirestoreModel - Write
+
+    @discardableResult
+    public func write<Model: FirestoreModel>(
+        _ model: Model
+    ) async throws -> DocumentReference {
+        try await write(model, newDocumentIdIfNotExists: nil)
+    }
+
+    // MARK: FirestoreModel - Get
+
+    public func get<Model: FirestoreModel>(
+        documentId: String
+    ) async throws -> Model {
+        try await get(documentId: documentId, includeCache: true)
+    }
+
+    public func get<Model: FirestoreModel>(
+        filter: [FirestoreQueryFilter] = [],
+        order: [FirestoreQueryOrder] = [],
+        limit: Int? = nil
+    ) async throws -> [Model] {
+        try await get(
+            filter: filter,
+            includeCache: true,
+            order: order,
+            limit: limit
+        )
+    }
+
+    public func get<Model: FirestoreModel>(
+        filter: [FirestoreQueryFilter] = [],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder] = [],
+        limit: Int? = nil
+    ) async throws -> [Model] {
+        try await get(
+            filter: filter,
+            includeCache: includeCache,
+            order: order,
+            limit: limit
+        )
+    }
+
+    // MARK: FirestoreModel - Listen
+
+    public func listen<Model: FirestoreModel>(
+        documentId: String
+    ) async -> AsyncThrowingStream<Model, Error> {
+        await listen(documentId: documentId, includeCache: true)
+    }
+
+    public func listen<Model: FirestoreModel>(
+        filter: [FirestoreQueryFilter] = [],
+        order: [FirestoreQueryOrder] = [],
+        limit: Int? = nil
+    ) async -> AsyncThrowingStream<[Model], Error> {
+        await listen(
+            filter: filter,
+            includeCache: true,
+            order: order,
+            limit: limit
+        )
+    }
+
+    public func listen<Model: FirestoreModel>(
+        filter: [FirestoreQueryFilter] = [],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder] = [],
+        limit: Int? = nil
+    ) async -> AsyncThrowingStream<[Model], Error> {
+        await listen(
+            filter: filter,
+            includeCache: includeCache,
+            order: order,
+            limit: limit
+        )
+    }
+
+    public func listenChanges<Model: FirestoreModel>(
+        filter: [FirestoreQueryFilter] = [],
+        order: [FirestoreQueryOrder] = [],
+        limit: Int? = nil
+    ) async -> AsyncThrowingStream<[FirestoreDocumentChange<Model>], Error> {
+        await listenChanges(
+            filter: filter,
+            includeCache: true,
+            order: order,
+            limit: limit
+        )
+    }
+
+    public func listenChanges<Model: FirestoreModel>(
+        filter: [FirestoreQueryFilter] = [],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder] = [],
+        limit: Int? = nil
+    ) async -> AsyncThrowingStream<[FirestoreDocumentChange<Model>], Error> {
+        await listenChanges(
+            filter: filter,
+            includeCache: includeCache,
+            order: order,
+            limit: limit
+        )
+    }
+
+    // MARK: SubCollectionModel - Get
+
+    public func get<Model: SubCollectionModel>(
+        parent parentDocumentId: String,
+        filter: [FirestoreQueryFilter] = [],
+        order: [FirestoreQueryOrder] = [],
+        limit: Int? = nil
+    ) async throws -> [Model] {
+        try await get(
+            parent: parentDocumentId,
+            filter: filter,
+            includeCache: true,
+            order: order,
+            limit: limit
+        )
+    }
+
+    public func get<Model: SubCollectionModel>(
+        parent parentDocumentId: String,
+        filter: [FirestoreQueryFilter] = [],
+        includeCache: Bool,
+        order: [FirestoreQueryOrder] = [],
+        limit: Int? = nil
+    ) async throws -> [Model] {
+        try await get(
+            parent: parentDocumentId,
+            filter: filter,
+            includeCache: includeCache,
+            order: order,
+            limit: limit
+        )
+    }
+
+    public func get<Model: SubCollectionModel>(
+        documentId: String,
+        parent parentUid: String
+    ) async throws -> Model {
+        try await get(
+            documentId: documentId,
+            parent: parentUid,
+            includeCache: true
+        )
+    }
+
+    // MARK: SubCollectionModel - Listen
+
+    public func listen<Model: FirestoreModel & SubCollectionModel>(
+        parent: String,
+        superParent: String? = nil,
+        filter: [FirestoreQueryFilter],
+        order: [FirestoreQueryOrder],
+        limit: Int?
+    ) async -> AsyncThrowingStream<[Model], Error> {
+        await listen(
+            parent: parent,
+            superParent: superParent,
+            filter: filter,
+            includeCache: true,
+            order: order,
+            limit: limit
+        )
+    }
+
+    public func listenChanges<Model: FirestoreModel & SubCollectionModel>(
+        parent: String,
+        superParent: String? = nil,
+        filter: [FirestoreQueryFilter],
+        order: [FirestoreQueryOrder],
+        limit: Int?
+    ) async -> AsyncThrowingStream<[FirestoreDocumentChange<Model>], Error> {
+        await listenChanges(
+            parent: parent,
+            superParent: superParent,
+            filter: filter,
+            includeCache: true,
+            order: order,
+            limit: limit
+        )
+    }
+
+    // MARK: CollectionGroup
+
+    public func getCollectionGroup<Model: FirestoreModel>(
+        filter: [FirestoreQueryFilter] = [],
+        order: [FirestoreQueryOrder] = [],
+        limit: Int? = nil
+    ) async throws -> [Model] {
+        try await getCollectionGroup(
+            filter: filter,
+            includeCache: true,
+            order: order,
+            limit: limit
+        )
+    }
+
+    public func listenCollectionGroup<Model: FirestoreModel>(
+        collectionName: String,
+        filter: [FirestoreQueryFilter] = [],
+        order: [FirestoreQueryOrder] = [],
+        limit: Int? = nil
+    ) async -> AsyncThrowingStream<[Model], Error> {
+        await listenCollectionGroup(
+            collectionName: collectionName,
+            filter: filter,
+            includeCache: true,
+            order: order,
+            limit: limit
+        )
+    }
+
+    public func listenCollectionGroupChanges<Model: FirestoreModel>(
+        collectionName: String,
+        filter: [FirestoreQueryFilter] = [],
+        order: [FirestoreQueryOrder] = [],
+        limit: Int? = nil
+    ) async -> AsyncThrowingStream<[FirestoreDocumentChange<Model>], Error> {
+        await listenCollectionGroupChanges(
+            collectionName: collectionName,
+            filter: filter,
+            includeCache: true,
+            order: order,
+            limit: limit
+        )
+    }
+}