Minimum Swift version

[benadamdev]

Is there any reason why the minimum support version is Swift 6. I need support for 5.9. Thank you.

[mattt]

Hi @benadamdev. The Swift 6 requirement was chosen primarily out of convenience, because that's what we were using for iMCP. But there's a good chance that we could relax those requirements without significant changes, and we're certainly open to that. I'll look into that this week.

[Lakr233]

I appreciate your work but iOS 17.0 on a fundamental library is not reasonable. It’s not SwiftUI related (I guess) and should work on older devices to have stable behavior. Really hoping to lower the minimum system version.

I am currently providing support to our own iOS ai chat app that works on iOS 16+ (had tool call, planing to switch to MCP) while had an open source edition support 15+ (for geeks to do their own). Lowering to support Swift 5.9 and supporting iOS 16.0+ would be super nice.

I can make a PR if you would like me to do so.

[mattt]

@benadamdev @Lakr233 I just merged #30, which updates platform requirements to support older versions of macOS and iOS, and the other platforms you'd expect. That's now available in 0.6.0

As part of that, I started to backport to Swift 5, but realized that my decision to use Swift Testing instead of XCTest means I can't easily test those changes. As such, the official Swift MCP SDK will continue to require Swift 6+.

If you're unable to upgrade from Swift 5.9, you're welcome to fork and try applying the following diffs (provided without any warranty or guarantees):

Diff

diff --git a/Package@swift-5.swift b/Package@swift-5.swift
new file mode 100644
index 0000000..84c53ec
--- /dev/null
+++ b/Package@swift-5.swift
@@ -0,0 +1,43 @@
+// swift-tools-version:5.9
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "mcp-swift-sdk",
+    platforms: [
+        .macOS("13.0"),
+        .macCatalyst("16.0"),
+        .iOS("16.0"),
+        .watchOS("9.0"),
+        .tvOS("16.0"),
+        .visionOS("1.0"),
+    ],
+    products: [
+        // Products define the executables and libraries a package produces, making them visible to other packages.
+        .library(
+            name: "MCP",
+            targets: ["MCP"])
+    ],
+    dependencies: [
+        .package(url: "https://github.com/apple/swift-system.git", from: "1.0.0"),
+        .package(url: "https://github.com/apple/swift-log.git", from: "1.5.0"),
+    ],
+    targets: [
+        // Targets are the basic building blocks of a package, defining a module or a test suite.
+        // Targets can depend on other targets in this package and products from dependencies.
+        .target(
+            name: "MCP",
+            dependencies: [
+                .product(name: "SystemPackage", package: "swift-system"),
+                .product(name: "Logging", package: "swift-log"),
+            ]),
+        .testTarget(
+            name: "MCPTests",
+            dependencies: [
+                "MCP",
+                .product(name: "SystemPackage", package: "swift-system"),
+                .product(name: "Logging", package: "swift-log"),
+            ]),
+    ]
+)
diff --git a/Sources/MCP/Base/Transports.swift b/Sources/MCP/Base/Transports.swift
index 1cc050c..55df73d 100644
--- a/Sources/MCP/Base/Transports.swift
+++ b/Sources/MCP/Base/Transports.swift
@@ -298,31 +298,20 @@ public actor StdioTransport: Transport {
                 throw MCP.Error.internalError("Failed to encode message")
             }
 
-            // Use a local actor-isolated variable to track continuation state
-            var sendContinuationResumed = false
-
             try await withCheckedThrowingContinuation {
-                [weak self] (continuation: CheckedContinuation<Void, Swift.Error>) in
-                guard let self = self else {
-                    continuation.resume(throwing: MCP.Error.internalError("Transport deallocated"))
-                    return
-                }
-
+                @Sendable (continuation: CheckedContinuation<Void, Swift.Error>) in
                 connection.send(
                     content: data,
-                    completion: .contentProcessed { [weak self] error in
+                    completion: .contentProcessed { @Sendable [weak self] error in
                         guard let self = self else { return }
 
-                        Task { @MainActor in
-                            if !sendContinuationResumed {
-                                sendContinuationResumed = true
-                                if let error = error {
-                                    self.logger.error("Send error: \(error)")
-                                    continuation.resume(
-                                        throwing: MCP.Error.internalError("Send error: \(error)"))
-                                } else {
-                                    continuation.resume()
-                                }
+                        Task {
+                            if let error = error {
+                                self.logger.error("Send error: \(error)")
+                                continuation.resume(
+                                    throwing: MCP.Error.internalError("Send error: \(error)"))
+                            } else {
+                                continuation.resume()
                             }
                         }
                     })
@@ -383,28 +372,19 @@ public actor StdioTransport: Transport {
         }
 
         private func receiveData() async throws -> Data {
-            var receiveContinuationResumed = false
-
-            return try await withCheckedThrowingContinuation {
-                [weak self] (continuation: CheckedContinuation<Data, Swift.Error>) in
-                guard let self = self else {
-                    continuation.resume(throwing: MCP.Error.internalError("Transport deallocated"))
-                    return
-                }
-
+            try await withCheckedThrowingContinuation {
+                @Sendable (continuation: CheckedContinuation<Data, Swift.Error>) in
                 connection.receive(minimumIncompleteLength: 1, maximumLength: 65536) {
+                    @Sendable
                     content, _, _, error in
-                    Task { @MainActor in
-                        if !receiveContinuationResumed {
-                            receiveContinuationResumed = true
-                            if let error = error {
-                                continuation.resume(throwing: MCP.Error.transportError(error))
-                            } else if let content = content {
-                                continuation.resume(returning: content)
-                            } else {
-                                continuation.resume(
-                                    throwing: MCP.Error.internalError("No data received"))
-                            }
+                    Task {
+                        if let error = error {
+                            continuation.resume(throwing: MCP.Error.transportError(error))
+                        } else if let content = content {
+                            continuation.resume(returning: content)
+                        } else {
+                            continuation.resume(
+                                throwing: MCP.Error.internalError("No data received"))
                         }
                     }
                 }