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Conversation Notes: Fix MCP SSE Server 404 on POST

Date: 2026-02-08 Status: 🗄️ ARCHIVED - Historical reference only

Note: This document describes the SSE server debugging process from February 2026. The implementation details may have evolved since then. For current architecture, see README.md.

Problem

  • Opening the SSE stream at /sse worked and returned a session ID
  • POSTing MCP messages (initialize, tools/list, tools/call) returned 404 page not found

Root Cause

In cmd/mcp-server/main.go, the SSE server was mounted incorrectly:

http.Handle("/sse/", http.StripPrefix("/sse", sseServer))

The mcp-go library's SSEServer.ServeHTTP routes internally using exact path matching:

  • GET /sse -> SSE stream handler
  • POST /message -> message handler

With StripPrefix("/sse", ...), when a client hit GET /sse/sse, the prefix was stripped and the handler saw /sse — so the SSE stream worked. But the message endpoint URL sent back to the client was /message?sessionId=... (no /sse prefix), which didn't match any route on the default mux, resulting in a 404.

Fix Applied

Replaced the manual mux registration with sseServer.Start(), which sets the SSEServer as the root HTTP handler for both endpoints:

// Before (broken)
sseServer := server.NewSSEServer(mcpServer)
http.Handle("/sse/", http.StripPrefix("/sse", sseServer))
http.ListenAndServe(":3333", nil)

// After (fixed)
sseServer := server.NewSSEServer(mcpServer,
    server.WithBaseURL("http://localhost:3333"),
)
sseServer.Start(":3333")

This lets the SSEServer handle routing itself:

  • GET /sse -> SSE stream (returns session ID + message endpoint URL)
  • POST /message?sessionId=... -> MCP message handler

The unused net/http import was also removed.

README Update

Replaced the "What's not working" section with:

  • Endpoint reference table (GET /sse, POST /message)
  • Step-by-step curl testing instructions for the full MCP flow (connect, initialize, tools/list, tools/call)

Configurable Base URL

Added MCP_BASE_URL environment variable support so the same binary works locally and on the VPS:

baseURL := os.Getenv("MCP_BASE_URL")
if baseURL == "" {
    baseURL = "http://localhost:3333"
}

VPS Deployment (vps-01.safecast.jp)

Setup

  • Binary at /root/safecast-mcp-server/safecast-mcp
  • systemd service: /etc/systemd/system/safecast-mcp.service
  • Environment: MCP_BASE_URL=https://vps-01.safecast.jp/mcp
  • Apache reverse proxy in /etc/apache2/sites-available/vps-01.safecast.jp.conf

Apache Proxy (added to existing 443 VirtualHost)

ProxyPass /mcp/ http://localhost:3333/mcp/
ProxyPassReverse /mcp/ http://localhost:3333/mcp/

Key lesson: path consistency

When WithBaseURL includes a path (e.g. /mcp), the Go server expects that path in incoming requests. Apache must forward the full path — use ProxyPass /mcp/ http://localhost:3333/mcp/ (not ProxyPass /mcp/sse http://localhost:3333/sse).

Public endpoints

  • SSE stream: https://vps-01.safecast.jp/mcp/sse
  • POST messages: https://vps-01.safecast.jp/mcp/message?sessionId=...

Files Changed

  • cmd/mcp-server/main.go — fixed server startup, routing, and configurable base URL
  • README.md — updated documentation with working endpoints and test instructions