error starting sse type mcp server

[abhi-parate-st3]

Version: 1.100.0-insider (Universal)
Commit: c73f020
Date: 2025-04-15T05:04:09.439Z
Electron: 34.4.1
ElectronBuildId: 11317338
Chromium: 132.0.6834.210
Node.js: 20.18.3
V8: 13.2.152.41-electron.0
OS: Darwin arm64 24.0.0

Steps to Reproduce:

1. add any sse type mcp server in the mcp server config and then try to start that server
2. getting error in output channel Error: MPC -32601: Not found

I can confirm that the same server was working on two of my machines before, but after upgrading to the this version it stoped working on both the machines
i can also confirm that other types of server are working file like docker and npx.

Also i think there is a typo in the error message as it shows MPC instead of MCP, well if it is intentional it certainly causes confusion.

vscode/src/vs/workbench/contrib/mcp/common/mcpTypes.ts

Lines 408 to 412 in a0ce446

	export class MpcResponseError extends Error {
	constructor(message: string, public readonly code: number, public readonly data: unknown) {
	super(`MPC ${code}: ${message}`);
	}
	}

[abhi-parate-st3]

@connor4312

[connor4312]

Please:

1. Run "Developer: Set Log Level" -> "Trace"
2. Start your MCP server and reproduce the error
3. Show the MCP server's output ("MCP: List Servers" > your server > "Show Output")
4. Share that

[abhi-parate-st3]

here is the server's output :

2025-04-16 08:10:40.118 [info] Starting server mcp-intro
2025-04-16 08:10:40.118 [info] Connection state: Starting
2025-04-16 08:10:40.125 [info] Starting server from LocalProcess extension host
2025-04-16 08:10:40.125 [info] Connection state: Running
2025-04-16 08:10:40.125 [debug] [editor -> server] {"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-03-26","capabilities":{"roots":{"listChanged":true}},"clientInfo":{"name":"Visual Studio Code - Insiders","version":"1.100.0-insider"}}}
2025-04-16 08:10:40.475 [debug] [server <- editor] {"jsonrpc":"2.0","id":1,"error":{"code":-32601,"message":"Not found"}}
2025-04-16 08:10:40.475 [error] Error: MPC -32601: Not found
    at BDi.F (vscode-file://vscode-app/Users/staytuned/Documents/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:1997:30982)
    at BDi.C (vscode-file://vscode-app/Users/staytuned/Documents/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:1997:30777)
    at ane.value (vscode-file://vscode-app/Users/staytuned/Documents/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:1997:29798)
    at E.B (vscode-file://vscode-app/Users/staytuned/Documents/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:29:2392)
    at E.fire (vscode-file://vscode-app/Users/staytuned/Documents/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:29:2611)
    at N0s.pushMessage (vscode-file://vscode-app/Users/staytuned/Documents/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:1296:112366)
    at VYt.$onDidReceiveMessage (vscode-file://vscode-app/Users/staytuned/Documents/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:1296:111954)
    at xyt.S (vscode-file://vscode-app/Users/staytuned/Documents/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:3233:40354)
    at xyt.Q (vscode-file://vscode-app/Users/staytuned/Documents/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:3233:40132)
    at xyt.M (vscode-file://vscode-app/Users/staytuned/Documents/Visual%20Studio%20Code%20-%20Insiders.app/Contents/Resources/app/out/vs/workbench/workbench.desktop.main.js:3233:39220)

working in :

Version: 1.100.0-insider
Commit: 1799c2beaa0669db243c05b6cfc2656162086eba
Date: 2025-04-14T07:40:24.894Z
Electron: 34.4.1
ElectronBuildId: 11317338
Chromium: 132.0.6834.210
Node.js: 20.18.3
V8: 13.2.152.41-electron.0
OS: Linux arm64 6.11.0-1010-raspi

not working in:

Version: 1.100.0-insider (Universal)
Commit: c73f02083edc88e5c72a8c6cc301fb8a6430493f
Date: 2025-04-15T05:04:09.439Z
Electron: 34.4.1
ElectronBuildId: 11317338
Chromium: 132.0.6834.210
Node.js: 20.18.3
V8: 13.2.152.41-electron.0
OS: Darwin arm64 24.0.0

[connor4312]

What SSE server are you using specifically where you see this bug?

[abhi-parate-st3]

Hi @connor4312,

The SSE server in question is one that we have developed internally.

I performed some additional investigation comparing the two Insider versions mentioned previously:

• Working Version: 1.100.0-insider (Commit: 1799c2b)
  • Sends a GET request to the configured SSE URL (http://mcp.localhost:5500/sse based on my config below).
• Non-Working Version: 1.100.0-insider (Commit: c73f020)
  • Sends a POST request to the same SSE URL.

Here is the relevant part of my configuration:

"local-intro": {
    "type": "sse",
    "url": "http://mcp.localhost:5500/sse",
    "headers": {
        "token": "${input:intro_token}"
    }
}

Our internal server is currently implemented to handle GET requests for the SSE connection, but not POST requests. This difference in the HTTP method used by VS Code appears to be the cause of the failure in the newer version.

Could you please clarify if this change from GET to POST for SSE type MCP servers is an intentional and permanent change? If it is, we will proceed with updating our server implementation to accommodate POST requests.

Thank you for your assistance.

[abhi-parate-st3]

Just wanted to point out that MCP specification generally relies on the standard W3C/WHATWG Server-Sent Events (SSE) mechanism when using HTTP for transport. Looking at the WHATWG HTML Standard for SSE, the process defined for initiating an EventSource connection effectively requires using an HTTP GET request (via the underlying Fetch standard default).

Using POST might deviate from the standard SSE connection process. Perhaps it's worth double-checking if GET would be more appropriate here to align with the underlying SSE specification that MCP utilizes for this transport?

Just a thought based on the specs!

[connor4312]

Could you please clarify if this change from GET to POST for SSE type MCP servers is an intentional and permanent change?

Yes, because SSE was removed from the spec and replaced with streamable HTTP in the latest version. If POSTing to the endpoint results in a 4xx error code (most appropriately, 405 Method Not Allowed) then VS Code will automatically fall back to legacy SSE mode and retry with a GET. This works with the out-of-the-box behaviors that the official MCP SDK's use.

It seems like in your case, the server accepts the POST but then responds with a an MCP error?

[abhi-parate-st3]

Thank you @connor4312 for clarifying the situation regarding the SSE to streamable HTTP change.
I appreciate the detailed explanation about the protocol updates and fallback mechanism.

I will update my MCP server to properly handle the new POST requests for streamable HTTP as per the specification.
This change makes sense from a protocol standardization perspective.

Thanks for the quick support! I'll close this issue once I've implemented the changes.