🐛 Bug: MCP server rejects `ping` message before `initialize`

[antoniordz96]

Summary

When running the AWS Agent Core MCP server backed by quarkus-mcp-server, the MCP client receives an error when it sends a ping message before completing the initialize handshake.

---

💥 Observed Behavior

Server logs show:

{
  "jsonrpc" : "2.0",
  "method" : "ping",
  "params" : null,
  "id" : 4
}

2025-10-25 02:23:21,753 INFO  [io.qua.mcp.ser.traffic] (vert.x-eventloop-thread-0) MCP message sent [...]:
{
  "jsonrpc" : "2.0",
  "id" : 4,
  "error" : {
    "code" : -32601,
    "message" : "The first message from the client must be \"initialize\": ping"
  }
}

As a result, the server rejects legitimate ping requests that occur before initialize.

---

📜 Expected Behavior (per MCP spec)

According to the [Model Context Protocol Specification (2025-06-18)](https://modelcontextprotocol.io/specification/2025-06-18/basic/lifecycle):

The client SHOULD NOT send requests other than pings before the server has responded to the initialize request.
The server SHOULD NOT send requests other than pings and logging before receiving the initialized notification.

Therefore, ping should be accepted at any point in the lifecycle, including before initialization completes.

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⚙️ Environment

• MCP server: Quarkus MCP Server (io.quarkiverse.mcp.server)
• Deployment: AWS Agent Core
• Protocol: Streamable HTTP v1.7.0

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🧩 Likely Root Cause

McpMessageHandler.initializeNew(...) currently enforces:

if (!INITIALIZE.equals(method)) {
    String msg = "The first message from the client must be \"initialize\": " + method;
    ...
}

This check blocks legitimate pre-initialize ping requests, contrary to the latest spec.

---

✅ Suggested Fix

Permit ping in the NEW connection state (before initialize) — for example:

if (PING.equals(method)) {
    return ping(message, mcpRequest);
}

before the if (!INITIALIZE.equals(method)) check in initializeNew(...).

This would make the behavior compliant with the 2025-06-18 MCP lifecycle specification.

---

[mkouba]

@antoniordz96 I don't think that your analysis is correct. If you read the error message it states: "The first message from the client must be "initialize": ping". This indicates that the client did not send the initialize message at all. In other words, it did not start the initialization phase yet. And the spec is clear that "The initialization phase MUST be the first interaction between client and server.".

Now if you look at the spec wording: "The client SHOULD NOT send requests other than pings before the server has responded to the initialize request.". Here the spec is talking about the phase where the the initialize message was sent from the client but the server did not receive the initialized notification from the client yet. In other words, clients should not send pings before the initialize message.

If you're using the HTTP server you can use the quarkus-smallrye-health extension to run the health checks periodically instead of ping messages and check the health status of the server.