Streamable HTTP transport hangs during initialization

[andrey-star]

The client hangs during the first initialize message to a (public) Shopify MCP https://my-best-shoes.myshopify.com/api/mcp

I can successfully connect https://my-best-shoes.myshopify.com/api/mcp from npx @modelcontextprotocol/inspector.

Note that #396 describes that the client transport hangs during sendNotification for notifications/initialized. In this case, the client hangs even earlier - during initialize.

Steps to reproduce:

String url = "https://my-best-shoes.myshopify.com/api";
McpClientTransport transport = HttpClientStreamableHttpTransport.builder(url).endpoint("/mcp").build();

McpSyncClient client = McpClient.sync(transport)
    .requestTimeout(Duration.ofSeconds(20))
    .initializationTimeout(Duration.ofSeconds(20))
    .build();

try {
    client.initialize();

    McpSchema.ListToolsResult tools = client.listTools();
    logger.log(Level.INFO, "Available tools: {0}", tools.tools());

} catch (Exception e) {
    logger.log(Level.SEVERE, "Failed to initialize MCP client from DebugServlet", e);
}

[tzolov]

What is https://my-best-shoes.myshopify.com/api ? how to create the shopify server to reproduce the issue?

[andrey-star]

This is a publicly available MCP server provided for Shopify shops through a setting https://shopify.dev/docs/apps/build/storefront-mcp/servers/storefront that opt-in. They're publicly available, so you can connect directly to https://my-best-shoes.myshopify.com/api/mcp from npx @modelcontextprotocol/inspector

[tzolov]

Thank you @andrey-star

Apparently this MCP server has couple MCP & HTTP compliance issues (see the description in the PR).
Perhaps you can report those to the server maintainer.

The #413 improves the robustness of the java clients to tolerate and recover in case non-compliant messages.

[andrey-star]

Awesome, thanks @tzolov!