Crash in NetworkTransport on reconnect

Describe the bug
Came across a runtime crash in NetworkTransport when a MCP client reconnects (mcp-swift-sdk version 0.9.0)

To Reproduce
I haven't yet been able to reliably reproduce it. In my case the host macOS app was running, when I launched Zed where I had the MCP server configured. Zed seems to reconnect, but that caused a runtime crash in the host macOS app.

Logs

macOS crash log snippet:

0x1a76fa1f8 _assertionFailure(_:_:file:line:flags:) + 176
1   libswift_Concurrency.dylib    	       0x27ecd64e4 CheckedContinuation.resume(throwing:) + 348
2   MyHostApp                      	       0x1003beb94 closure #1 in NetworkTransport.handleReconnection(error:continuation:context:) + 180

Relevant code snippet:

        private func handleReconnection(
            error: Swift.Error,
            continuation: CheckedContinuation<Void, Swift.Error>,
            context: String
        ) async {
            if !isStopping,
                reconnectionConfig.enabled,
                reconnectAttempt < reconnectionConfig.maxAttempts
            {
                // Try to reconnect with exponential backoff
                reconnectAttempt += 1
                logger.info(
                    "Attempting reconnection after \(context) (\(reconnectAttempt)/\(reconnectionConfig.maxAttempts))..."
                )

                // Calculate backoff delay
                let delay = reconnectionConfig.backoffDelay(for: reconnectAttempt)

                // Schedule reconnection attempt after delay
                Task {
                    try? await Task.sleep(for: .seconds(delay))
                    if !isStopping {
                        // Cancel the current connection before attempting to reconnect.
                        self.connection.cancel()
                        // Resume original continuation with error; outer logic or a new call to connect() will handle retry.
                        continuation.resume(throwing: error)
                    } else {
                        continuation.resume(throwing: error)  // Stopping, so fail.
                    }
                }
            } else {
                // Not configured to reconnect, exceeded max attempts, or stopping
                self.connection.cancel()  // Ensure connection is cancelled
                continuation.resume(throwing: error)
            }
        }

Additional context

The crash is triggered when continuation.resume(throwing: ...) gets called multiple times for the same continuation. I haven't yet wrapped my head around the entire flow here and how this is possible, and it's hard to debug as it seems intermittent.

Suggestions appreciated for how to address this.

Ran into the same issue

Re-opening as I only merged that in my fork. It isn't yet merged in this repo. I've opened #147 for that.

In my PR #175, I replaced the continuation-passing pattern with AsyncStream:

private func waitForConnectionReady() async throws {
    let stateStream = AsyncStream<NWConnection.State> { continuation in
        connection.stateUpdateHandler = { state in
            continuation.yield(state)
            switch state {
            case .ready, .failed, .cancelled:
                continuation.finish()
            default:
                break
            }
        }

        continuation.onTermination = { [weak self] _ in
            self?.connection.stateUpdateHandler = nil
        }
    }

    connection.start(queue: .main)

    for await state in stateStream {
        switch state {
        case .ready:
            return
        case .failed(let error):
            throw error
        case .cancelled:
            throw MCPError.internalError("Connection cancelled")
        case .waiting(let error):
            logger.debug("Connection waiting: \(error)")
        case .preparing:
            logger.debug("Connection preparing...")
        case .setup:
            logger.debug("Connection setup...")
        @unknown default:
            logger.warning("Unknown connection state")
        }
    }

    throw MCPError.internalError("Connection stream ended unexpectedly")
}

This is safe because:

AsyncStream.Continuation is thread-safe
finish() is idempotent
yield() after finish() is ignored
The stream finishes atomically on terminal states

Changes

Removed:

connectionContinuationResumed property
handleConnectionReady(continuation:)
handleConnectionFailed(error:continuation:)
handleConnectionCancelled(continuation:)
handleReconnection(error:continuation:context:)

Added:

waitForConnectionReady() using AsyncStream

Simplified:

connect() now calls waitForConnectionReady() directly

fix: Update MCP Swift SDK to 0.11.0 to fix NetworkTransport crash

This seems to be fixed in 0.11 and up, see #199. Closing.

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Crash in `NetworkTransport` on reconnect #137

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Crash in NetworkTransport on reconnect #137

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provencher commented on Jun 19, 2025

nighthawk commented on Jul 14, 2025

DePasqualeOrg commented on Jan 4, 2026

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nighthawk commented on Mar 27, 2026

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Crash in `NetworkTransport` on reconnect #137