Fixing Fatal Deadlock Error With Mcp.Client.Connect() And Non-MCP Endpoints

Jul 17, 2025 by gitftunila 76 views

Calling mcp.Client.Connect() Causes a Fatal Error: All Goroutines Are Asleep - Deadlock

This article addresses a critical bug encountered when using the mcp.Client.Connect() function from the modelcontextprotocol/go-sdk. Specifically, calling this function with a StreamableClientTransport that points to a non-MCP HTTP endpoint leads to a fatal error: "all goroutines are asleep - deadlock!". This issue can halt the execution of your Go applications and requires careful attention. This comprehensive guide will explore the root cause of this deadlock, provide a step-by-step breakdown of how to reproduce the bug, analyze the error logs, and ultimately, offer insights into how to prevent this issue in your Go-based Model Context Protocol (MCP) applications.

Understanding the Bug: Deadlock in Goroutines

Before diving into the specifics of the MCP client, it's crucial to grasp the fundamental concept of a deadlock in concurrent programming. In Go, which heavily relies on goroutines for concurrency, a deadlock occurs when two or more goroutines are blocked indefinitely, waiting for each other to release resources. This creates a standstill where no progress can be made, ultimately leading to the "all goroutines are asleep - deadlock!" error. The error message itself is a strong indicator of a severe concurrency issue that needs immediate attention. Recognizing this error as a deadlock situation is the first step towards resolving it. Identifying the root cause within the intricate interactions of goroutines requires careful examination of the code and the execution flow. In the context of the MCP client, the deadlock arises from a specific interaction between the StreamableClientTransport and the underlying JSON-RPC communication mechanism when connected to an incompatible endpoint.

Bug Description

The core issue lies in the incompatibility between the StreamableClientTransport and non-MCP HTTP endpoints. The StreamableClientTransport is designed to facilitate communication with MCP-compliant servers, which adhere to a specific protocol for exchanging messages. When this transport is used to connect to a standard HTTP endpoint (like https://www.baidu.com in the provided example), the communication patterns deviate from the expected MCP protocol. This deviation leads to a situation where the goroutines involved in the connection and message handling get stuck waiting for responses that will never arrive in the expected format. The mcp.Client.Connect() function, when used with an inappropriate transport, inadvertently triggers this deadlock scenario. The StreamableClientTransport's internal mechanisms for reading and writing messages are predicated on the assumption of an MCP-compliant server. When this assumption is violated, the goroutines responsible for these operations become blocked, resulting in the fatal error. This bug highlights the importance of ensuring that the chosen transport mechanism aligns with the type of endpoint being targeted.

Steps to Reproduce: Triggering the Deadlock

To effectively address a bug, it's essential to be able to reliably reproduce it. The following Go code snippet demonstrates how to trigger the "all goroutines are asleep - deadlock!" error when calling mcp.Client.Connect() with a non-MCP HTTP endpoint:

package main

import (
	"context"
	"time"

	"github.com/modelcontextprotocol/go-sdk/mcp"
)

func main() {
	client := mcp.NewClient(&mcp.Implementation{}, nil)
	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
	defer cancel()
	session, err := client.Connect(ctx, mcp.NewStreamableClientTransport("https://www.baidu.com", nil))
	if err != nil {
		panic(err)
	}
	session.Close()
}

This code performs the following actions:

It imports the necessary packages, including the mcp package from the modelcontextprotocol/go-sdk. The mcp package is the core component for interacting with MCP services.
It creates a new MCP client using mcp.NewClient(). The mcp.Implementation{} serves as a placeholder for a concrete implementation of the MCP interface, and nil indicates that no additional client options are being provided.
It creates a context.Context with a timeout of 2 seconds. This context is used to manage the lifecycle of the connection attempt and prevent indefinite blocking. The defer cancel() ensures that the context is canceled when the main function exits, releasing any associated resources.
The crucial step is the call to client.Connect(). This is where the bug is triggered. It attempts to establish a connection using mcp.NewStreamableClientTransport("https://www.baidu.com", nil). This creates a transport that is configured to communicate over a stream-based connection, but it's pointed at https://www.baidu.com, a standard HTTP endpoint that doesn't adhere to the MCP protocol. The mismatch between the transport and the endpoint is the root cause of the deadlock.
If the connection attempt fails (which it will in this case due to the deadlock), the code panics with the error. This behavior is intentional to clearly signal the occurrence of the bug.
The session.Close() call is intended to close the connection if it were successfully established. However, due to the deadlock, this line is typically not reached.

By running this code, you will consistently encounter the "all goroutines are asleep - deadlock!" error. This reproducible scenario allows for a deeper understanding of the issue and facilitates the development of effective solutions.

Analyzing the Error Logs: Tracing the Deadlock

The error logs provide valuable clues for understanding the deadlock. Let's examine the provided log output:

go
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [chan receive]:
github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).Wait(0xc000160680)
	C:/Users/rkonfj/go/pkg/mod/github.com/modelcontextprotocol/[email protected]/internal/jsonrpc2/conn.go:472 +0x27
github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).Close(0xc000160680)
	C:/Users/rkonfj/go/pkg/mod/github.com/modelcontextprotocol/[email protected]/internal/jsonrpc2/conn.go:491 +0x29
github.com/modelcontextprotocol/go-sdk/mcp.(*ClientSession).Close(0x5ae120?)
	C:/Users/rkonfj/go/pkg/mod/github.com/modelcontextprotocol/[email protected]/mcp/client.go:188 +0x2f
github.com/modelcontextprotocol/go-sdk/mcp.(*Client).Connect(0xc000128100, {0x5afec0, 0xc00012a230}, {0x5abdc0?, 0xc000008738?})
	C:/Users/rkonfj/go/pkg/mod/github.com/modelcontextprotocol/[email protected]/mcp/client.go:129 +0x18e
main.main()
	C:/Users/rkonfj/Documents/mcpurl/cmd/minreproduce/main.go:14 +0x145

goroutine 7 [select]:
github.com/modelcontextprotocol/go-sdk/mcp.(*streamableClientConn).Read(0xc000128180, {0x5b02d0, 0xc00002ad50})
	C:/Users/rkonfj/go/pkg/mod/github.com/modelcontextprotocol/[email protected]/mcp/streamable.go:666 +0xa5
github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).readIncoming(0xc000160680, {0x5b02d0, 0xc00002ad50}, {0x15d5c0b1178, 0xc000128180}, {0x5abfc0, 0xc00005c1b8})
	C:/Users/rkonfj/go/pkg/mod/github.com/modelcontextprotocol/[email protected]/internal/jsonrpc2/conn.go:500 +0x72
created by github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.NewConnection.(*Connection).start.func1 in goroutine 1
	C:/Users/rkonfj/go/pkg/mod/github.com/modelcontextprotocol/[email protected]/internal/jsonrpc2/conn.go:282 +0x106
exit status 2

The logs reveal two key goroutines involved in the deadlock:

Goroutine 1: This goroutine is the main goroutine that calls client.Connect() and subsequently session.Close(). The stack trace shows that it's blocked in github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).Wait(), which suggests it's waiting for a response on the JSON-RPC connection. The subsequent calls to (*Connection).Close() and (*ClientSession).Close() indicate that the deadlock occurs during the connection closing process, likely because the connection is in an inconsistent state.
Goroutine 7: This goroutine is responsible for reading incoming messages from the stream. The stack trace shows that it's blocked in github.com/modelcontextprotocol/go-sdk/mcp.(*streamableClientConn).Read(), which is the read operation for the StreamableClientTransport. It's waiting for data to arrive on the connection. This goroutine is spawned by the JSON-RPC connection's readIncoming function, further highlighting the interplay between the transport and the JSON-RPC layer.

The deadlock occurs because Goroutine 1 is waiting for Goroutine 7 to process a response, but Goroutine 7 is blocked waiting for data that will never arrive in the expected MCP format from the non-MCP HTTP endpoint. This creates a circular dependency, resulting in the deadlock.

Root Cause Analysis: Mismatched Protocols

The fundamental cause of this deadlock is the attempt to use a StreamableClientTransport, designed for MCP-compliant servers, with a standard HTTP endpoint. The StreamableClientTransport relies on a specific communication pattern and message format defined by the MCP protocol. When it connects to a non-MCP endpoint, the endpoint doesn't respond in the expected way, leading to a protocol mismatch. This mismatch manifests as blocked goroutines waiting for data or responses that never conform to the MCP specifications.

The StreamableClientTransport internally uses JSON-RPC 2.0 for communication. JSON-RPC 2.0 is a lightweight remote procedure call protocol that uses JSON as a data format. When the transport connects to a non-JSON-RPC endpoint, the responses are not valid JSON-RPC messages. This causes the JSON-RPC client to get stuck waiting for valid messages, leading to the deadlock. The protocol mismatch disrupts the expected flow of communication, preventing the goroutines from making progress. The connection remains in a state of limbo, unable to send or receive data in a way that satisfies the MCP protocol requirements.

Preventing the Deadlock: Choosing the Right Transport

The key to preventing this deadlock is to ensure that the correct transport is used for the target endpoint. The StreamableClientTransport should only be used when connecting to MCP-compliant servers. For standard HTTP endpoints, alternative transport mechanisms should be employed. Selecting the appropriate transport is crucial for the stability and reliability of your MCP client applications. Using the wrong transport can lead to unexpected behavior, including deadlocks and other communication errors. In cases where you're interacting with a non-MCP HTTP endpoint, you'll need to use a different approach, such as a standard HTTP client, to communicate with the server.

If you need to interact with a non-MCP HTTP endpoint, you should use Go's standard net/http package or a similar HTTP client library. These libraries are designed to handle standard HTTP communication and will not attempt to interpret the responses as MCP messages. Using the standard HTTP client ensures that you're communicating with the endpoint in a way that it understands, preventing the protocol mismatch that leads to the deadlock. It's important to choose the right tool for the job, and in this case, a standard HTTP client is the appropriate choice for interacting with a standard HTTP endpoint.

Version Information

This bug was observed with the following versions:

Go: go1.24.5
github.com/modelcontextprotocol/go-sdk: v0.2.0

This information is crucial for developers who may be encountering this issue in their projects. Knowing the specific versions involved helps in identifying whether the bug is present in their environment. If you are using these versions or a similar configuration, you are likely to encounter this deadlock if you attempt to connect a StreamableClientTransport to a non-MCP HTTP endpoint. Being aware of the affected versions allows developers to take appropriate preventative measures or consider upgrading to a version where the bug may be resolved.

Conclusion: Avoiding Deadlocks in MCP Applications

The "all goroutines are asleep - deadlock!" error encountered when calling mcp.Client.Connect() with an incompatible transport highlights the importance of understanding the underlying communication protocols and choosing the correct transport mechanism. By ensuring that the StreamableClientTransport is only used with MCP-compliant servers and employing standard HTTP clients for other endpoints, you can effectively prevent this deadlock and maintain the stability of your Go applications. A deep understanding of the MCP protocol and its interaction with the transport layer is essential for building robust and reliable applications. This article has provided a comprehensive guide to understanding, reproducing, and preventing this specific deadlock, empowering you to build more resilient MCP-based systems.

By carefully considering the endpoint type and selecting the appropriate transport, you can avoid this common pitfall and ensure the smooth operation of your MCP client applications. Remember that the StreamableClientTransport is specifically designed for MCP communication, and using it with other types of endpoints will likely lead to unexpected issues. Always verify the endpoint type and choose the transport accordingly to prevent deadlocks and other communication errors. This proactive approach will contribute to the overall stability and reliability of your Go applications that utilize the Model Context Protocol.