[Bug] Auto-discovery fails on Windows - discover_servers returns 406 due to missing Accept header

[zoranbebic-ct]

Environment

Operating System:

• Windows 11 (MINGW64_NT-10.0-26200)

Versions:

• next-devtools-mcp: 0.3.3 (latest)
• Next.js: 16.0.3
• Node.js: v24.9.0
• Bun: 1.3.0 (used as package manager)
• React: 19.2.0

MCP Configuration (.mcp.json):
{
"servers": {
"next-devtools": {
"type": "stdio",
"command": "bunx",
"args": ["next-devtools-mcp@latest"],
"env": {}
}
}
}

Next.js Configuration:
const nextConfig: NextConfig = {
reactCompiler: true,
cacheComponents: true,
};

---

Issue Description

The discover_servers action consistently fails to detect running Next.js dev servers on Windows, despite the server running correctly on port 3000 and the MCP
endpoint being fully functional.

---

Steps to Reproduce

1. Start Next.js 16.0.3 dev server on Windows 11:
   bun run dev
2. Verify server is running:
   netstat -ano | findstr :3000

Output shows: LISTENING on port 3000 (PID ****)

3. Attempt MCP server discovery:
   nextjs_runtime({ action: "discover_servers" })
4. Result: No servers found

---

Expected Behavior

Auto-discovery should detect the Next.js dev server running on port 3000 and return available MCP tools.

---

Actual Behavior

Discovery fails with zero servers detected. Manual testing reveals the root cause:

Test 1: Standard HTTP Request (Fails)

curl -v http://localhost:3000/_next/mcp
Result: HTTP/1.1 406 Not Acceptable

Test 2: JSON-RPC without proper headers (Fails)

curl -X POST http://localhost:3000/_next/mcp
-H "Content-Type: application/json"
-d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{}}'
Response:
{
"jsonrpc": "2.0",
"error": {
"code": -32000,
"message": "Not Acceptable: Client must accept both application/json and text/event-stream"
},
"id": null
}

Test 3: With correct Accept header (Success)

curl -X POST http://localhost:3000/_next/mcp
-H "Content-Type: application/json"
-H "Accept: application/json, text/event-stream"
-d '{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}'
Result: ✅ Success - Returns 6 tools with full metadata

---

Root Cause Analysis

The auto-discovery mechanism fails because it does not send the required Accept: application/json, text/event-stream header when probing ports. The Next.js MCP
endpoint strictly requires both content types in the Accept header, even for standard JSON-RPC requests (not SSE streams).

Windows-Specific Factors:

• Windows resolves localhost to IPv6 (::1) before IPv4 (127.0.0.1)
• Potential stricter header validation in Windows networking stack
• This issue was partially addressed in v0.2.4 ("Fix Next.js runtime discovery on Windows"), but the header issue persists

---

Workaround

Manually specify the port instead of relying on auto-discovery:

nextjs_runtime({ action: "list_tools", port: "3000" })

This successfully connects and returns all 6 MCP tools:

• get_project_metadata
• get_errors
• get_page_metadata
• get_logs
• get_server_action_by_id
• get_routes

---

Proposed Solutions

Option 1: Fix Discovery Probe (Recommended)

Update the port scanning mechanism to include the required Accept header:
headers: {
'Content-Type': 'application/json',
'Accept': 'application/json, text/event-stream'
}

Option 2: Add Configuration Support

Allow users to specify custom port in .mcp.json (assume default 3000 in absence of env variable):
{
"servers": {
"next-devtools": {
"command": "bunx",
"args": ["next-devtools-mcp@latest"],
"env": {
"NEXTJS_DEFAULT_PORT": "3000"
}
}
}
}

Option 3: Relax Header Requirements

Consider whether the text/event-stream requirement is necessary for standard JSON-RPC requests (vs SSE streams).

---

Additional Context

This appears to be related to a broader MCP protocol implementation pattern where HTTP endpoints require text/event-stream even for non-streaming requests.
Similar issues exist in other MCP implementations:

• Client must accept both application/json and text/event-stream in streamable-http transport mode junjiem/dify-plugin-tools-mcp_sse#70
• [BUG] HTTP endpoint incorrectly requires text/event-stream Accept header tadata-org/fastapi_mcp#233

---

Impact

• Users on Windows cannot use auto-discovery functionality
• Requires manual port specification in every MCP tool call
• Reduces developer experience and adds friction to Windows workflows

[gaojude]

Can you confirm whether the issue still persists in the latest version (0.3.5 as of now)?

[ZoranBebic]

Updated next.js package to 16.0.4
Updated devDependencies for next-devtools-mcp@0.3.5 (not sure if this dev dependency is really required)

prompt: next-devtools-mcp init #command executes OK

prompt: use next-devtool-mcp and check for errors #fails

nextjs_call to check for errors.
⎿  {
"success": false,
"error": "No running Next.js dev servers with MCP enabled found",
"hint": "Make sure you're running Next.js 16+ (MCP is enabled by default). Start the dev server with 'npm run dev'. If on Next.js 15 or earlier, upgrade using the
'upgrade-nextjs-16' prompt.",
"servers": []
}

however when we explicitly tell it to use port 3000 it generates toolcall with additional params that seem to help

next-dev-tools - nextjs_call (MCP)(port: "3000", toolName: "get_errors")

[AlexHoff97]

I am encountering this same issue on next.js 16.0.6 with next-devtools-mcp 0.3.5 installed.

prompt: next-devtools-mcp init - succeeds

prompt: Next Devtools, show me the structure of my routes - fails to find mcp server

nextjs_call with parameters port=3000, toolName=get_routes - succeeds

With the same project on my mac auto discovery works.