Problem downcasting request in Client.handleResponse

[zats]

I will prephrase my question by saying I am not sure at all that any of my findings are accurate.
I think I am seeing consistent failure of the client to handle responses due to 2 issues:

First of all it appears that we are passing response twice under

https://github.com/loopwork-ai/mcp-swift-sdk/blob/main/Sources/MCP/Client/Client.swift#L132-L133

Where instead we should be getting appropriate request from pandingRequests like so

if let response = try? decoder.decode(AnyResponse.self, from: data), let request = pendingRequests[response.id] {
  await handleResponse(response, for: request)
} ... 

Even fixing that locally downcast here https://github.com/loopwork-ai/mcp-swift-sdk/blob/main/Sources/MCP/Client/Client.swift#L329 fails

guard let typedRequest = request as? PendingRequest<Any> else { return }

Not sure if there is a way to tell Swift "don't worry about generic the original value was created with", it seem to consistently fail. Specifically I am debugging initialize sequence and so when I fix passing pending request, $ po request prints MCP.Client.PendingRequest<MCP.Initialize.Result>) , tho guard let typedRequest = request as? PendingRequest<Any> else { return } fails

Any ideas if I am holding it wrong? or we might need to store type-erased pending requests instead of Any?

[zats]

Hmm I think I have a hacky fix locally (that's if I understand the problem 🙂)
There is probably a more elegant way to achieve this.
Happy to open pull request if this makes sense

diff --git a/Sources/MCP/Base/Value.swift b/Sources/MCP/Base/Value.swift
--- a/Sources/MCP/Base/Value.swift
+++ b/Sources/MCP/Base/Value.swift
@@ -400,3 +400,16 @@
         }
     }
 }
+
+
+
+// MARK: - Decode specific type from Value
+
+public extension JSONDecoder {
+  func decode<T: Decodable>(_ type: T.Type, from value: Value) throws -> T {
+    guard let data = try? JSONEncoder().encode(value) else {
+      throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: [], debugDescription: "Failed to encode value"))
+    }    
+    return try self.decode(T.self, from: data)
+  }
+}
diff --git a/Sources/MCP/Client/Client.swift b/Sources/MCP/Client/Client.swift
--- a/Sources/MCP/Client/Client.swift
+++ b/Sources/MCP/Client/Client.swift
@@ -5,6 +5,7 @@
 import struct Foundation.Date
 import class Foundation.JSONDecoder
 import class Foundation.JSONEncoder
+import Foundation
 
 /// Model Context Protocol client
 public actor Client {
@@ -87,12 +88,43 @@
     /// The task for the message handling loop
     private var task: Task<Void, Never>?
 
+  private struct AnyPendingRequest {
+      private let _resume: (Result<Any, Swift.Error>) -> Void
+      
+    init<T: Sendable & Decodable>(_ request: PendingRequest<T>) {
+          _resume = { result in
+              switch result {
+              case .success(let value):
+                if let typedValue = value as? T {
+                  request.continuation.resume(returning: typedValue)
+                } else if let value = value as? Value,
+                          let decoded = try? JSONDecoder().decode(T.self, from: value) {
+                  request.continuation.resume(returning: decoded)
+                } else {
+                  request.continuation.resume(throwing: TypeMismatchError())
+                }
+              case .failure(let error):
+                request.continuation.resume(throwing: error)
+              }
+          }
+      }
+    func resume(returning value: Any) {
+      _resume(.success(value))
+    }
+    
+    func resume(throwing error: Swift.Error) {
+      _resume(.failure(error))
+    }
+  }
+
+  private struct TypeMismatchError: Swift.Error {}
+
     /// A pending request with a continuation for the result
     private struct PendingRequest<T> {
         let continuation: CheckedContinuation<T, Swift.Error>
     }
     /// A dictionary of type-erased pending requests, keyed by request ID
-    private var pendingRequests: [ID: Any] = [:]
+    private var pendingRequests: [ID: AnyPendingRequest] = [:]
 
     public init(
         name: String,
@@ -129,8 +161,8 @@
 
                         // Attempt to decode string data as AnyResponse or AnyMessage
                         let decoder = JSONDecoder()
-                        if let response = try? decoder.decode(AnyResponse.self, from: data) {
-                            await handleResponse(response, for: response)
+                      if let response = try? decoder.decode(AnyResponse.self, from: data), let request = pendingRequests[response.id] {
+                            await handleResponse(response, for: request)
                         } else if let message = try? decoder.decode(AnyMessage.self, from: data) {
                             await handleMessage(message)
                         } else {
@@ -220,12 +252,12 @@
         }
     }
 
-    private func addPendingRequest<T>(
+  private func addPendingRequest<T: Sendable & Decodable>(
         id: ID,
         continuation: CheckedContinuation<T, Swift.Error>,
         type: T.Type
     ) {
-        pendingRequests[id] = PendingRequest(continuation: continuation)
+        pendingRequests[id] = AnyPendingRequest(PendingRequest(continuation: continuation))
     }
 
     private func removePendingRequest(id: ID) {
@@ -320,21 +352,19 @@
 
     // MARK: -
 
-    private func handleResponse(_ response: Response<AnyMethod>, for request: Any) async {
+    private func handleResponse(_ response: Response<AnyMethod>, for request: AnyPendingRequest) async {
         await logger?.debug(
             "Processing response",
             metadata: ["id": "\(response.id)"])
 
         // We know this cast is safe because we only store PendingRequest values
-        guard let typedRequest = request as? PendingRequest<Any> else { return }
-
-        switch response.result {
-        case .success(let value):
-            typedRequest.continuation.resume(returning: value)
-        case .failure(let error):
-            typedRequest.continuation.resume(throwing: error)
-        }
-
+      
+      switch response.result {
+      case .success(let value):
+        request.resume(returning: value)
+      case .failure(let error):
+        request.resume(throwing: error)
+      }
         removePendingRequest(id: response.id)
     }

[zats]

Addressed in #8

[mattt]

Hey @zats! Thanks for digging into this. I'm taking a look at your PR now.

[stallent]

@mattt @zats i am now hitting a new issue that seems to be right in this same place. Actually attempting to call a tool is failing in Client.AnyPendingRequest._resume which comes from the exact line where you fix fixed something else.

[stallent]

hit send too soon. Its failing on line 100 attempting to cast the successful tool response as T

[mattt]

Fixed by #8 🎉