Add streamable HTTP transport for clients (#70)
* Rename file to Transport.swift * Rename file to StdioTransportTests.swift * Add .index-build/ to .gitignore * Implement HTTP client transport * Add test coverage for streamable HTTP client transport * Conditionally import FoundationNetworking * Conditionalize streaming for unsupported platforms * Import FoundationNetworking in tests * Add 5 minute timeout to CI jobs * Remove debug logging * Formatting * Skip SSE tests on platforms that don't support streaming
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20
Sources/MCP/Base/Transport.swift
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20
Sources/MCP/Base/Transport.swift
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import Logging
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import struct Foundation.Data
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/// Protocol defining the transport layer for MCP communication
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public protocol Transport: Actor {
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var logger: Logger { get }
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/// Establishes connection with the transport
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func connect() async throws
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/// Disconnects from the transport
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func disconnect() async
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/// Sends data
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func send(_ data: Data) async throws
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/// Receives data in an async sequence
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func receive() -> AsyncThrowingStream<Data, Swift.Error>
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}
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305
Sources/MCP/Base/Transports/HTTPClientTransport.swift
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305
Sources/MCP/Base/Transports/HTTPClientTransport.swift
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import Foundation
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import Logging
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#if canImport(FoundationNetworking)
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import FoundationNetworking
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#endif
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public actor HTTPClientTransport: Actor, Transport {
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public let endpoint: URL
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private let session: URLSession
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public private(set) var sessionID: String?
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private let streaming: Bool
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private var streamingTask: Task<Void, Never>?
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private var lastEventID: String?
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public nonisolated let logger: Logger
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private var isConnected = false
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private let messageStream: AsyncThrowingStream<Data, Swift.Error>
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private let messageContinuation: AsyncThrowingStream<Data, Swift.Error>.Continuation
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public init(
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endpoint: URL,
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configuration: URLSessionConfiguration = .default,
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streaming: Bool = false,
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logger: Logger? = nil
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) {
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self.init(
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endpoint: endpoint,
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session: URLSession(configuration: configuration),
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streaming: streaming,
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logger: logger
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)
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}
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internal init(
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endpoint: URL,
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session: URLSession,
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streaming: Bool = false,
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logger: Logger? = nil
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) {
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self.endpoint = endpoint
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self.session = session
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self.streaming = streaming
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// Create message stream
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var continuation: AsyncThrowingStream<Data, Swift.Error>.Continuation!
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self.messageStream = AsyncThrowingStream { continuation = $0 }
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self.messageContinuation = continuation
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self.logger =
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logger
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?? Logger(
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label: "mcp.transport.http.client",
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factory: { _ in SwiftLogNoOpLogHandler() }
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)
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}
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/// Establishes connection with the transport
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public func connect() async throws {
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guard !isConnected else { return }
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isConnected = true
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if streaming {
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// Start listening to server events
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streamingTask = Task { await startListeningForServerEvents() }
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}
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logger.info("HTTP transport connected")
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}
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/// Disconnects from the transport
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public func disconnect() async {
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guard isConnected else { return }
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isConnected = false
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// Cancel streaming task if active
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streamingTask?.cancel()
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streamingTask = nil
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// Cancel any in-progress requests
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session.invalidateAndCancel()
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// Clean up message stream
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messageContinuation.finish()
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logger.info("HTTP clienttransport disconnected")
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}
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/// Sends data through an HTTP POST request
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public func send(_ data: Data) async throws {
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guard isConnected else {
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throw MCPError.internalError("Transport not connected")
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}
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var request = URLRequest(url: endpoint)
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request.httpMethod = "POST"
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request.addValue("application/json, text/event-stream", forHTTPHeaderField: "Accept")
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request.addValue("application/json", forHTTPHeaderField: "Content-Type")
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request.httpBody = data
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// Add session ID if available
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if let sessionID = sessionID {
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request.addValue(sessionID, forHTTPHeaderField: "Mcp-Session-Id")
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}
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let (responseData, response) = try await session.data(for: request)
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guard let httpResponse = response as? HTTPURLResponse else {
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throw MCPError.internalError("Invalid HTTP response")
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}
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// Process the response based on content type and status code
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let contentType = httpResponse.value(forHTTPHeaderField: "Content-Type") ?? ""
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// Extract session ID if present
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if let newSessionID = httpResponse.value(forHTTPHeaderField: "Mcp-Session-Id") {
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self.sessionID = newSessionID
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logger.debug("Session ID received", metadata: ["sessionID": "\(newSessionID)"])
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}
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// Handle different response types
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switch httpResponse.statusCode {
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case 200, 201, 202:
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// For SSE, the processing happens in the streaming task
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if contentType.contains("text/event-stream") {
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logger.debug("Received SSE response, processing in streaming task")
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// The streaming is handled by the SSE task if active
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return
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}
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// For JSON responses, deliver the data directly
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if contentType.contains("application/json") && !responseData.isEmpty {
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logger.debug("Received JSON response", metadata: ["size": "\(responseData.count)"])
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messageContinuation.yield(responseData)
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}
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case 404:
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// If we get a 404 with a session ID, it means our session is invalid
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if sessionID != nil {
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logger.warning("Session has expired")
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sessionID = nil
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throw MCPError.internalError("Session expired")
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}
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throw MCPError.internalError("Endpoint not found")
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default:
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throw MCPError.internalError("HTTP error: \(httpResponse.statusCode)")
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}
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}
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/// Receives data in an async sequence
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public func receive() -> AsyncThrowingStream<Data, Swift.Error> {
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return AsyncThrowingStream { continuation in
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Task {
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for try await message in messageStream {
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continuation.yield(message)
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}
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continuation.finish()
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}
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}
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}
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// MARK: - SSE
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/// Starts listening for server events using SSE
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private func startListeningForServerEvents() async {
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guard isConnected else { return }
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// Retry loop for connection drops
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while isConnected && !Task.isCancelled {
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do {
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try await connectToEventStream()
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} catch {
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if !Task.isCancelled {
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logger.error("SSE connection error: \(error)")
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// Wait before retrying
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try? await Task.sleep(nanoseconds: 1_000_000_000) // 1 second
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}
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}
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}
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}
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#if canImport(FoundationNetworking)
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private func connectToEventStream() async throws {
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logger.warning("SSE is not supported on this platform")
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}
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#else
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/// Establishes an SSE connection to the server
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private func connectToEventStream() async throws {
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guard isConnected else { return }
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var request = URLRequest(url: endpoint)
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request.httpMethod = "GET"
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request.addValue("text/event-stream", forHTTPHeaderField: "Accept")
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// Add session ID if available
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if let sessionID = sessionID {
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request.addValue(sessionID, forHTTPHeaderField: "Mcp-Session-Id")
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}
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// Add Last-Event-ID header for resumability if available
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if let lastEventID = lastEventID {
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request.addValue(lastEventID, forHTTPHeaderField: "Last-Event-ID")
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}
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logger.debug("Starting SSE connection")
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// Create URLSession task for SSE
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let (stream, response) = try await session.bytes(for: request)
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guard let httpResponse = response as? HTTPURLResponse else {
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throw MCPError.internalError("Invalid HTTP response")
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}
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// Check response status
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guard httpResponse.statusCode == 200 else {
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throw MCPError.internalError("HTTP error: \(httpResponse.statusCode)")
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}
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// Extract session ID if present
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if let newSessionID = httpResponse.value(forHTTPHeaderField: "Mcp-Session-Id") {
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self.sessionID = newSessionID
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}
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// Process the SSE stream
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var buffer = ""
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var eventType = ""
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var eventID: String?
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var eventData = ""
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for try await byte in stream {
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if Task.isCancelled { break }
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guard let char = String(bytes: [byte], encoding: .utf8) else { continue }
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buffer.append(char)
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// Process complete lines
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while let newlineIndex = buffer.firstIndex(of: "\n") {
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let line = buffer[..<newlineIndex]
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buffer = String(buffer[buffer.index(after: newlineIndex)...])
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// Empty line marks the end of an event
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if line.isEmpty || line == "\r" || line == "\n" || line == "\r\n" {
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if !eventData.isEmpty {
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// Process the event
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if eventType == "id" {
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lastEventID = eventID
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} else {
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// Default event type is "message" if not specified
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if let data = eventData.data(using: .utf8) {
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logger.debug(
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"SSE event received",
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metadata: [
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"type": "\(eventType.isEmpty ? "message" : eventType)",
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"id": "\(eventID ?? "none")",
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])
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messageContinuation.yield(data)
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}
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}
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// Reset for next event
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eventType = ""
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eventData = ""
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}
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continue
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}
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// Lines starting with ":" are comments
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if line.hasPrefix(":") { continue }
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// Parse field: value format
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if let colonIndex = line.firstIndex(of: ":") {
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let field = String(line[..<colonIndex])
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var value = String(line[line.index(after: colonIndex)...])
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// Trim leading space
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if value.hasPrefix(" ") {
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value = String(value.dropFirst())
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}
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// Process based on field
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switch field {
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case "event":
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eventType = value
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case "data":
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if !eventData.isEmpty {
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eventData.append("\n")
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}
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eventData.append(value)
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case "id":
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if !value.contains("\0") { // ID must not contain NULL
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eventID = value
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lastEventID = value
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}
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case "retry":
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// Retry timing not implemented
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break
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default:
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// Unknown fields are ignored per SSE spec
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break
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}
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}
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}
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}
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}
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#endif
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}
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