Update transports to send and receive data instead of strings (modelcontextprotocol#42)

* Update transports to send and receive data instead of strings

* Rename MockTransport helper methods

* Adopt Task.sleep(for:...) instead of variant taking nanoseconds

* Update comment

* Revert inadvertant change to visibility of send method

Author: mattt

Sources/MCP/Base/Transports.swift

@@ -19,11 +19,11 @@ public protocol Transport: Actor {
     /// Disconnects from the transport
     func disconnect() async
 
-    /// Sends a message string
-    func send(_ message: String) async throws
+    /// Sends data
+    func send(_ data: Data) async throws
 
-    /// Receives message strings as an async sequence
-    func receive() -> AsyncThrowingStream<String, Swift.Error>
+    /// Receives data in an async sequence
+    func receive() -> AsyncThrowingStream<Data, Swift.Error>
 }
 
 /// Standard input/output transport implementation
@@ -33,8 +33,8 @@ public actor StdioTransport: Transport {
     public nonisolated let logger: Logger
 
     private var isConnected = false
-    private let messageStream: AsyncStream<String>
-    private let messageContinuation: AsyncStream<String>.Continuation
+    private let messageStream: AsyncStream<Data>
+    private let messageContinuation: AsyncStream<Data>.Continuation
 
     public init(
         input: FileDescriptor = FileDescriptor.standardInput,
@@ -50,7 +50,7 @@ public actor StdioTransport: Transport {
                 factory: { _ in SwiftLogNoOpLogHandler() })
 
         // Create message stream
-        var continuation: AsyncStream<String>.Continuation!
+        var continuation: AsyncStream<Data>.Continuation!
         self.messageStream = AsyncStream { continuation = $0 }
         self.messageContinuation = continuation
     }
@@ -105,15 +105,13 @@ public actor StdioTransport: Transport {
                     let messageData = pendingData[..<newlineIndex]
                     pendingData = pendingData[(newlineIndex + 1)...]
 
-                    if let message = String(data: messageData, encoding: .utf8),
-                        !message.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty
-                    {
-                        logger.debug("Message received", metadata: ["message": "\(message)"])
-                        messageContinuation.yield(message)
+                    if !messageData.isEmpty {
+                        logger.debug("Message received", metadata: ["size": "\(messageData.count)"])
+                        messageContinuation.yield(Data(messageData))
                     }
                 }
             } catch let error where Error.isResourceTemporarilyUnavailable(error) {
-                try? await Task.sleep(nanoseconds: 10_000_000)  // 10ms backoff
+                try? await Task.sleep(for: .milliseconds(10))
                 continue
             } catch {
                 if !Task.isCancelled {
@@ -133,17 +131,16 @@ public actor StdioTransport: Transport {
         logger.info("Transport disconnected")
     }
 
-    public func send(_ message: String) async throws {
+    public func send(_ message: Data) async throws {
         guard isConnected else {
             throw Error.transportError(Errno.socketNotConnected)
         }
 
-        let message = message + "\n"
-        guard let data = message.data(using: .utf8) else {
-            throw Error.transportError(Errno.invalidArgument)
-        }
+        // Add newline as delimiter
+        var messageWithNewline = message
+        messageWithNewline.append(UInt8(ascii: "\n"))
 
-        var remaining = data
+        var remaining = messageWithNewline
         while !remaining.isEmpty {
             do {
                 let written = try remaining.withUnsafeBytes { buffer in
@@ -153,15 +150,15 @@ public actor StdioTransport: Transport {
                     remaining = remaining.dropFirst(written)
                 }
             } catch let error where Error.isResourceTemporarilyUnavailable(error) {
-                try await Task.sleep(nanoseconds: 10_000_000)  // 10ms backoff
+                try await Task.sleep(for: .milliseconds(10))
                 continue
             } catch {
                 throw Error.transportError(error)
             }
         }
     }
 
-    public func receive() -> AsyncThrowingStream<String, Swift.Error> {
+    public func receive() -> AsyncThrowingStream<Data, Swift.Error> {
         return AsyncThrowingStream { continuation in
             Task {
                 for await message in messageStream {
@@ -182,8 +179,8 @@ public actor StdioTransport: Transport {
         public nonisolated let logger: Logger
 
         private var isConnected = false
-        private let messageStream: AsyncThrowingStream<String, Swift.Error>
-        private let messageContinuation: AsyncThrowingStream<String, Swift.Error>.Continuation
+        private let messageStream: AsyncThrowingStream<Data, Swift.Error>
+        private let messageContinuation: AsyncThrowingStream<Data, Swift.Error>.Continuation
 
         // Track connection state for continuations
         private var connectionContinuationResumed = false
@@ -198,7 +195,7 @@ public actor StdioTransport: Transport {
                 )
 
             // Create message stream
-            var continuation: AsyncThrowingStream<String, Swift.Error>.Continuation!
+            var continuation: AsyncThrowingStream<Data, Swift.Error>.Continuation!
             self.messageStream = AsyncThrowingStream { continuation = $0 }
             self.messageContinuation = continuation
         }
@@ -289,14 +286,14 @@ public actor StdioTransport: Transport {
             logger.info("Network transport disconnected")
         }
 
-        public func send(_ message: String) async throws {
+        public func send(_ message: Data) async throws {
             guard isConnected else {
                 throw MCP.Error.internalError("Transport not connected")
             }
 
-            guard let data = (message + "\n").data(using: .utf8) else {
-                throw MCP.Error.internalError("Failed to encode message")
-            }
+            // Add newline as delimiter
+            var messageWithNewline = message
+            messageWithNewline.append(UInt8(ascii: "\n"))
 
             // Use a local actor-isolated variable to track continuation state
             var sendContinuationResumed = false
@@ -309,7 +306,7 @@ public actor StdioTransport: Transport {
                 }
 
                 connection.send(
-                    content: data,
+                    content: messageWithNewline,
                     completion: .contentProcessed { [weak self] error in
                         guard let self = self else { return }
 
@@ -329,7 +326,7 @@ public actor StdioTransport: Transport {
             }
         }
 
-        public func receive() -> AsyncThrowingStream<String, Swift.Error> {
+        public func receive() -> AsyncThrowingStream<Data, Swift.Error> {
             return AsyncThrowingStream { continuation in
                 Task {
                     do {
@@ -357,11 +354,10 @@ public actor StdioTransport: Transport {
                         let messageData = buffer[..<newlineIndex]
                         buffer = buffer[(newlineIndex + 1)...]
 
-                        if let message = String(data: messageData, encoding: .utf8),
-                            !message.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty
-                        {
-                            logger.debug("Message received", metadata: ["message": "\(message)"])
-                            messageContinuation.yield(message)
+                        if !messageData.isEmpty {
+                            logger.debug(
+                                "Message received", metadata: ["size": "\(messageData.count)"])
+                            messageContinuation.yield(Data(messageData))
                         }
                     }
                 } catch let error as NWError {

Sources/MCP/Client/Client.swift

@@ -181,15 +181,10 @@ public actor Client {
 
                 do {
                     let stream = await connection.receive()
-                    for try await string in stream {
+                    for try await data in stream {
                         if Task.isCancelled { break }  // Check inside loop too
 
-                        // Decode and handle incoming message
-                        guard let data = string.data(using: .utf8) else {
-                            throw Error.parseError("Invalid UTF-8 data")
-                        }
-
-                        // Attempt to decode string data as AnyResponse or AnyMessage
+                        // Attempt to decode data as AnyResponse or AnyMessage
                         let decoder = JSONDecoder()
                         if let response = try? decoder.decode(AnyResponse.self, from: data),
                             let request = pendingRequests[response.id]
@@ -207,7 +202,7 @@ public actor Client {
                         }
                     }
                 } catch let error where Error.isResourceTemporarilyUnavailable(error) {
-                    try? await Task.sleep(nanoseconds: 10_000_000)  // 10ms
+                    try? await Task.sleep(for: .milliseconds(10))
                     continue
                 } catch {
                     await logger?.error(
@@ -256,22 +251,19 @@ public actor Client {
         }
 
         let requestData = try JSONEncoder().encode(request)
-        guard let requestString = String(data: requestData, encoding: .utf8) else {
-            throw Error.internalError("Failed to encode request")
-        }
 
+        // Store the pending request first
         return try await withCheckedThrowingContinuation { continuation in
-            // Store the pending request first
             Task {
                 self.addPendingRequest(
                     id: request.id,
                     continuation: continuation,
                     type: M.Result.self
                 )
 
-                // Send the request
+                // Send the request data
                 do {
-                    try await connection.send(requestString)
+                    try await connection.send(requestData)
                 } catch {
                     continuation.resume(throwing: error)
                     self.removePendingRequest(id: request.id)

Sources/MCP/Server/Server.swift

@@ -172,15 +172,11 @@ public actor Server {
         task = Task {
             do {
                 let stream = await transport.receive()
-                for try await string in stream {
+                for try await data in stream {
                     if Task.isCancelled { break }  // Check cancellation inside loop
 
                     var requestID: ID?
                     do {
-                        guard let data = string.data(using: .utf8) else {
-                            throw Error.parseError("Invalid UTF-8 data")
-                        }
-
                         // Attempt to decode string data as AnyRequest or AnyMessage
                         let decoder = JSONDecoder()
                         if let request = try? decoder.decode(AnyRequest.self, from: data) {
@@ -203,7 +199,7 @@ public actor Server {
                         }
                     } catch let error where Error.isResourceTemporarilyUnavailable(error) {
                         // Resource temporarily unavailable, retry after a short delay
-                        try? await Task.sleep(nanoseconds: 10_000_000)  // 10ms
+                        try? await Task.sleep(for: .milliseconds(10))
                         continue
                     } catch {
                         await logger?.error(
@@ -266,19 +262,17 @@ public actor Server {
 
     // MARK: - Sending
 
-    /// Send a response to a client
+    /// Send a response to a request
     public func send<M: Method>(_ response: Response<M>) async throws {
         guard let connection = connection else {
             throw Error.internalError("Server connection not initialized")
         }
+
         let encoder = JSONEncoder()
         encoder.outputFormatting = [.sortedKeys, .withoutEscapingSlashes]
 
         let responseData = try encoder.encode(response)
-
-        if let responseStr = String(data: responseData, encoding: .utf8) {
-            try await connection.send(responseStr)
-        }
+        try await connection.send(responseData)
     }
 
     /// Send a notification to connected clients
@@ -291,10 +285,7 @@ public actor Server {
         encoder.outputFormatting = [.sortedKeys, .withoutEscapingSlashes]
 
         let notificationData = try encoder.encode(notification)
-
-        if let notificationStr = String(data: notificationData, encoding: .utf8) {
-            try await connection.send(notificationStr)
-        }
+        try await connection.send(notificationData)
     }
 
     // MARK: -
@@ -407,7 +398,7 @@ public actor Server {
 
             // Send initialized notification after a short delay
             Task {
-                try? await Task.sleep(nanoseconds: 100_000_000)  // 100ms
+                try? await Task.sleep(for: .milliseconds(10))
                 try? await self.notify(InitializedNotification.message())
             }
 

Tests/MCPTests/ClientTests.swift

@@ -27,27 +27,27 @@ struct ClientTests {
 
         try await client.connect(transport: transport)
         // Small delay to ensure message loop is started
-        try await Task.sleep(nanoseconds: 10_000_000)  // 10ms
+        try await Task.sleep(for: .milliseconds(10))
 
         // Create a task for initialize that we'll cancel
         let initTask = Task {
             try await client.initialize()
         }
 
         // Give it a moment to send the request
-        try await Task.sleep(nanoseconds: 10_000_000)  // 10ms
+        try await Task.sleep(for: .milliseconds(10))
 
         #expect(await transport.sentMessages.count == 1)
-        #expect(await transport.sentMessages[0].contains(Initialize.name))
-        #expect(await transport.sentMessages[0].contains(client.name))
-        #expect(await transport.sentMessages[0].contains(client.version))
+        #expect(await transport.sentMessages.first?.contains(Initialize.name) == true)
+        #expect(await transport.sentMessages.first?.contains(client.name) == true)
+        #expect(await transport.sentMessages.first?.contains(client.version) == true)
 
         // Cancel the initialize task
         initTask.cancel()
 
         // Disconnect client to clean up message loop and give time for continuation cleanup
         await client.disconnect()
-        try await Task.sleep(nanoseconds: 50_000_000)  // 50ms
+        try await Task.sleep(for: .milliseconds(50))
     }
 
     @Test(
@@ -60,25 +60,25 @@ struct ClientTests {
 
         try await client.connect(transport: transport)
         // Small delay to ensure message loop is started
-        try await Task.sleep(nanoseconds: 10_000_000)  // 10ms
+        try await Task.sleep(for: .milliseconds(10))
 
         // Create a task for the ping that we'll cancel
         let pingTask = Task {
             try await client.ping()
         }
 
         // Give it a moment to send the request
-        try await Task.sleep(nanoseconds: 10_000_000)  // 10ms
+        try await Task.sleep(for: .milliseconds(10))
 
         #expect(await transport.sentMessages.count == 1)
-        #expect(await transport.sentMessages[0].contains(Ping.name))
+        #expect(await transport.sentMessages.first?.contains(Ping.name) == true)
 
         // Cancel the ping task
         pingTask.cancel()
 
         // Disconnect client to clean up message loop and give time for continuation cleanup
         await client.disconnect()
-        try await Task.sleep(nanoseconds: 50_000_000)  // 50ms
+        try await Task.sleep(for: .milliseconds(50))
     }
 
     @Test("Connection failure handling")
@@ -168,7 +168,7 @@ struct ClientTests {
 
         // Wait a bit for any setup to complete
         try await Task.sleep(for: .milliseconds(10))
-        
+
         // Send the listPrompts request and immediately provide an error response
         let promptsTask = Task {
             do {
@@ -187,7 +187,7 @@ struct ClientTests {
                         id: decodedRequest.id,
                         error: Error.methodNotFound("Test: Prompts capability not available")
                     )
-                    try await transport.queueResponse(errorResponse)
+                    try await transport.queue(response: errorResponse)
 
                     // Try the request now that we have a response queued
                     do {