MCP server could not be started: MPC -32602: Unsupported protocol version

[MorningStar-825]

Hello!Recently, I've encountered a protocol version mismatch issue when running the server.Is there any plan to update to version 2025.3.26 in the near future?

[hkr04]

Yes. And for temporary solution, it might be useful to remove the version negotiation:

cpp-mcp/src/mcp_server.cpp

Lines 719 to 730 in 0fa4096

	if (requested_version != MCP_VERSION) {
	LOG_ERROR("Unsupported protocol version: ", requested_version, ", server supports: ", MCP_VERSION);
	return response::create_error(
	req.id,
	error_code::invalid_params,
	"Unsupported protocol version",
	{
	{"supported", {MCP_VERSION}},
	{"requested", params["protocolVersion"]}
	}
	).to_json();
	}

[MorningStar-825]

It works!!! thank you!

[AstroTeng]

Same,and my solution is to modify constexpr const char* MCP_VERSION = "2024-11-05"; in cpp-mcp\include\mcp_message.h, to constexpr const char* MCP_VERSION = "2025-03-26";

[hkr04]

@AstroTeng Yes both are OK as long as it can pass the version negotiation. But actually I'm not sure if 2024-11-05 version is well compatible with 2025-03-26 version.

For me, the most significant difference is about stateless HTTP transportation. It seems to be compatible with current SSE implementation (could downgrade to it). I haven't dived into 2025-03-26 version so far since I'm a little bit busy recent days. But if you find some incompatible behaviors, please write a issue to let me know, then I'll raise it's priority in developing :)

[Artem535]

Hi! I'm encountering the same error, but with version 2025-06-18. The suggested solution doesn't work properly for me.
I'm trying to connect @modelcontextprotocol/inspector to my MCP server, but I get the following error:

Connection Error - Check if your MCP server is running and proxy token is correct

However, the server logs show no issues — everything appears to be running normally. Is there any plan to update to version 2025-06-18 in the near future?

[gladish]

Same or similar error when trying to use the openai agents SDK

Name: openai-agents
Version: 0.3.1
Summary: OpenAI Agents SDK
Home-page: https://openai.github.io/openai-agents-python/

2025-10-10 12:39:24 [ERROR] Unsupported protocol version: 2025-06-18, server supports: 2024-11-05

[hkr04]

@gladish Unfortunately, I've been quite busy with my studies recently, so I haven’t upgraded to the 2025-06-18 version yet. However, due to backward compatibility, it should still work if you manually set MCP_VERSION to 2025-06-18 to pass the version negotiation.
See: hkr04/cpp-mcp#23 (comment)