UI : fix SSE transport detection and routing through CORS proxy. Assi…

[hrpnr]

DuckDuckGo Search (DDGS) MCP Integration Walkthrough

This document summarizes the changes made to enable connecting the ddgs MCP server to the llama.cpp Web UI using SSE transport over the CORS proxy.

---

1. What was accomplished

We successfully configured the ddgs MCP server to run over SSE transport and integrated it with the llama.cpp Web UI. The Web UI can now connect to the MCP server through llama-server's built-in CORS proxy, discover tools, and execute queries.

---

2. Why changes were required

Standard Stdio Transport vs. Browser Constraints

• ddgs mcp runs on stdio transport by default. However, the browser-based Web UI cannot communicate with local processes via stdio and requires HTTP-based transports (SSE / StreamableHTTP).
• Running ddgs as an SSE server exposed it at http://127.0.0.1:8000/sse.

Transport Selection Bug

• Svelte's createTransport was hardcoded to only check for WebSocket URLs and defaulted all other HTTP schemes to StreamableHTTPClientTransport. The fallback block for SSEClientTransport was never executed since the StreamableHTTPClientTransport constructor does not throw synchronous network errors.

Relative Endpoint Resolution Bug over CORS Proxy

• The browser established the SSE connection correctly through the CORS proxy (http://localhost:8080/cors-proxy?url=...).
• However, when the server responded with a relative message endpoint (e.g., /messages/), the SDK resolved this relative path against the proxy URL, mapping it to the local origin (http://localhost:8080/messages/) instead of wrapping the target through the CORS proxy. This resulted in 404 Not Found errors from llama-server.

---

3. How it was implemented

Step 1: SSE Transport Detection

We modified detectMcpTransportFromUrl to classify URLs ending with /sse, /sse/, or containing /sse? as MCPTransportType.SSE:

if (
	normalized.endsWith('/sse') ||
	normalized.endsWith('/sse/') ||
	normalized.includes('/sse?')
) {
	return MCPTransportType.SSE;
}

Step 2: Explicit SSE Client Transport Instantiation

We updated createTransport to explicitly check for MCPTransportType.SSE and return SSEClientTransport instead of falling through:

if (config.transport === MCPTransportType.SSE) {
	const url = useProxy ? buildProxiedUrl(config.url) : new URL(config.url);
	// ... (diagnostic fetch setup)
	return {
		transport: new SSEClientTransport(url, {
			requestInit,
			fetch: diagnosticFetch,
			eventSourceInit: { fetch: diagnosticFetch }
		}),
		type: MCPTransportType.SSE,
		stopPhaseLogging
	};
}

Step 3: Fetch Interceptor & CORS Proxy URL Rewrite

We added URL rewrite logic at the beginning of the custom fetch handler inside createDiagnosticFetch. If proxy routing is active and an outgoing request erroneously targets the local origin without /cors-proxy, the interceptor reconstructs the destination relative to the original MCP server and routes it through the CORS proxy:

if (useProxy && typeof window !== 'undefined') {
	let requestUrlStr = '';
	if (typeof input === 'string') {
		requestUrlStr = input;
	} else if (input instanceof URL) {
		requestUrlStr = input.href;
	}

	if (requestUrlStr) {
		const parsedRequestUrl = new URL(requestUrlStr, window.location.origin);
		if (
			parsedRequestUrl.origin === window.location.origin &&
			!parsedRequestUrl.pathname.includes('/cors-proxy')
		) {
			const originalConfigUrl = new URL(config.url);
			const realTargetUrl = new URL(
				parsedRequestUrl.pathname + parsedRequestUrl.search,
				originalConfigUrl.origin
			);
			const proxiedUrl = buildProxiedUrl(realTargetUrl.href);

			if (typeof input === 'string') {
				input = proxiedUrl.href;
			} else if (input instanceof URL) {
				input = proxiedUrl;
			}
		}
	}
}

Step 4: UI Packaging

We compiled the changes with:

cd tools/ui
npm run build

This wrote the production static bundle to tools/ui/dist, enabling llama-server to serve it using the --path tools/ui/dist flag.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES, AI built the code with human direction and confirmation

[allozaur]

@hrpnr please fix linting issues

[hrpnr]

@allozaur could you re-review? just fixed linting issues.

[allozaur]

@ServeurpersoCom please check this when u have a moment 😄

[ServeurpersoCom]

LGTM

[engrtipusultan]

Hello @hrpnr I have installed duckduckgo-mcp-server

When I run it with transport sse. It does not work. Can you confirm how does fix apply or I am doing something wrong.

bash  duckduckgo-mcp-server --transport sse
DuckDuckGo MCP Server initialized:
  SafeSearch: MODERATE (kp=-1)
  Default Region: none
  Fetch backend: httpx
Starting DuckDuckGo MCP Server with sse transport
SSE endpoint: http://127.0.0.1:8000/sse
INFO:     Started server process [109640]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     127.0.0.1:34158 - "POST /sse HTTP/1.1" 405 Method Not Allowed

llama.cpp server.

13:20:50
Creating transport for http://127.0.0.1:8000/sse
13:20:50
Transport ready (streamable_http)
13:20:50
Sending initialize request...
13:20:50
HTTP POST http://127.0.0.1:8000/sse
details

13:20:50
HTTP 405 POST http://127.0.0.1:8000/sse (10ms)
details

{
  "response": {
    "url": "http://127.0.0.1:8000/sse",
    "status": 405,
    "statusText": "Method Not Allowed",
    "headers": {
      "content-length": "18",
      "content-type": "text/plain; charset=utf-8"
    },
    "durationMs": 10
  }
}

13:20:50
Protocol error: Streamable HTTP error: Error POSTing to endpoint: Method Not Allowed
details

{
  "error": {
    "name": "Error",
    "message": "Streamable HTTP error: Error POSTing to endpoint: Method Not Allowed",
    "stack": "kv@http://127.0.0.1:8080/_app/immutable/bundle.DEyh8LrV.js:151:1316\nsend@http://127.0.0.1:8080/_app/immutable/bundle.DEyh8LrV.js:151:6899\n"
  }
}

13:20:50
Connection failed during initialize: Streamable HTTP error: Error POSTing to endpoint: Method Not Allowed
details

{
  "error": {
    "name": "Error",
    "message": "Streamable HTTP error: Error POSTing to endpoint: Method Not Allowed",
    "stack": "kv@http://127.0.0.1:8080/_app/immutable/bundle.DEyh8LrV.js:151:1316\nsend@http://127.0.0.1:8080/_app/immutable/bundle.DEyh8LrV.js:151:6899\n"
  },
  "config": {
    "serverName": "57af7ec2-1ec7-4aa5-9690-5d31af43a18a",
    "configuredUrl": "http://127.0.0.1:8000/sse",
    "effectiveUrl": "http://127.0.0.1:8000/sse",
    "transportType": "streamable_http",
    "useProxy": false,
    "headers": {}
  },
  "browser": {
    "location": "http://127.0.0.1:8080/#/mcp-servers",
    "origin": "http://127.0.0.1:8080",
    "protocol": "http:",
    "isSecureContext": true,
    "targetOrigin": "http://127.0.0.1:8000",
    "targetProtocol": "http:",
    "sameOrigin": false,
    "useProxy": false
  },
  "hints": [
    "This is a cross-origin browser request. If the server is reachable from curl or Node but not from the browser, missing CORS headers are the most likely cause."
  ]
}

[hrpnr]

Hi @engrtipusultan , i use DDGS from https://github.com/deedy5/ddgs

1. Install dependencies (one-time, )

pip install -U ddgs[mcp] mcp-proxy

2. Start mcp-proxy wrapping ddgs

& "path\to\mcp-proxy.exe" `
  --port 8000 `
  --allow-origin "http://localhost:8080" `
  -- "path\to\ddgs.exe" mcp

3. Start llama-server (pointing to the same 8080 or other preferred port)

4. In the UI:

http://127.0.0.1:8000/sse

Note: ddgs mcp is stdio-only and will always "Abort!" when run directly in a terminal - that's expected. mcp-proxy handles spawning it correctly as a subprocess.