From f6e401aa971aa9e43900d3299b160d20934879c3 Mon Sep 17 00:00:00 2001
From: Ayal Spitz <github@blackgecko.com>
Date: Sun, 27 Apr 2025 09:50:08 -0400
Subject: [PATCH] Fix event stream line parsing to handle CRLF and empty lines
 correctly (#89)

* Optimized code in `connectToEventStream` to use `.lines` computed value of `AsyncSequence` to produce a `AsyncLineSequence` instead of building a lines manually.

* - Reverter use of `lines` (it swallows newlines)
- Fiexed original issue I experienced with line 229 `while let newlineIndex = buffer.firstIndex(of: "\n") {` not handling "\r\n" properly
- Added unit test to test for "\r\n" scenario
- Simplified `end of event` test - `line` creation trims newline, added code trims carriage return. Only thing that should be left is an empty line
---
 .../Base/Transports/HTTPClientTransport.swift | 14 ++++++--
 Tests/MCPTests/HTTPClientTransportTests.swift | 34 +++++++++++++++++++
 2 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/Sources/MCP/Base/Transports/HTTPClientTransport.swift b/Sources/MCP/Base/Transports/HTTPClientTransport.swift
index 5a68b61..33d033a 100644
--- a/Sources/MCP/Base/Transports/HTTPClientTransport.swift
+++ b/Sources/MCP/Base/Transports/HTTPClientTransport.swift
@@ -226,12 +226,16 @@ public actor HTTPClientTransport: Actor, Transport {
                 buffer.append(char)
 
                 // Process complete lines
-                while let newlineIndex = buffer.firstIndex(of: "\n") {
-                    let line = buffer[..<newlineIndex]
+                while let newlineIndex = buffer.utf8.firstIndex(where: { $0 == 10 }) {
+                    var line = buffer[..<newlineIndex]
+                    if line.hasSuffix("\r") {
+                        line = line.dropLast()
+                    }
+
                     buffer = String(buffer[buffer.index(after: newlineIndex)...])
 
                     // Empty line marks the end of an event
-                    if line.isEmpty || line == "\r" || line == "\n" || line == "\r\n" {
+                    if line.isEmpty {
                         if !eventData.isEmpty {
                             // Process the event
                             if eventType == "id" {
@@ -256,6 +260,10 @@ public actor HTTPClientTransport: Actor, Transport {
                         continue
                     }
 
+                    if line.hasSuffix("\r") {
+                        line = line.dropLast()
+                    }
+                    
                     // Lines starting with ":" are comments
                     if line.hasPrefix(":") { continue }
 
diff --git a/Tests/MCPTests/HTTPClientTransportTests.swift b/Tests/MCPTests/HTTPClientTransportTests.swift
index 9656c28..fdbff85 100644
--- a/Tests/MCPTests/HTTPClientTransportTests.swift
+++ b/Tests/MCPTests/HTTPClientTransportTests.swift
@@ -411,6 +411,40 @@ import Testing
                 let expectedData = #"{"key":"value"}"#.data(using: .utf8)!
                 let receivedData = try await iterator.next()
 
+                #expect(receivedData == expectedData)
+            }
+            @Test("Receive Server-Sent Event (SSE) (CR-NL)", .httpClientTransportSetup)
+            func testReceiveSSE_CRNL() async throws {
+                let configuration = URLSessionConfiguration.ephemeral
+                configuration.protocolClasses = [MockURLProtocol.self]
+
+                let transport = HTTPClientTransport(
+                    endpoint: testEndpoint, configuration: configuration, streaming: true,
+                    logger: nil)
+
+                let eventString = "id: event1\r\ndata: {\"key\":\"value\"}\r\n\n"
+                let sseEventData = eventString.data(using: .utf8)!
+
+                await MockURLProtocol.requestHandlerStorage.setHandler {
+                    [testEndpoint] (request: URLRequest) in
+                    #expect(request.url == testEndpoint)
+                    #expect(request.httpMethod == "GET")
+                    #expect(request.value(forHTTPHeaderField: "Accept") == "text/event-stream")
+                    let response = HTTPURLResponse(
+                        url: testEndpoint, statusCode: 200, httpVersion: "HTTP/1.1",
+                        headerFields: ["Content-Type": "text/event-stream"])!
+                    return (response, sseEventData)
+                }
+
+                try await transport.connect()
+                try await Task.sleep(for: .milliseconds(100))
+
+                let stream = await transport.receive()
+                var iterator = stream.makeAsyncIterator()
+
+                let expectedData = #"{"key":"value"}"#.data(using: .utf8)!
+                let receivedData = try await iterator.next()
+
                 #expect(receivedData == expectedData)
             }
         #endif  // !canImport(FoundationNetworking)