CORS Issue

[jnovak-wq]

Hi everyone,
For reference I am very new to this and am working on a project for work (not a cs student or anything so super new to me) and I've been trying to run this server for our mediaWiki and then connect to a local LLM (model: Llama bartowski 3.2 8B Q4_K_M downloaded from hf) I have been using WSL2 and running through Zed. I keep getting a failed to fetch error which I believe is CORS related. I'm not entirely sure how to fix it. I checked with the MCP inspector tool to make sure the literal server is running and I believe it is. I've been researching through forums and other sources and haven't found an applicable fix so I was hoping anyone can help/point in the right direction.
Thanks!!

[alistair3149]

Do you have more info such as logs?

[jnovak-wq]

http://127.0.0.1:8081/mcp

Failed to fetch

Connection Log (7)
10:44:54
Creating transport for http://127.0.0.1:8081/mcp
10:44:54
Transport ready (streamable_http)
10:44:54
Sending initialize request...
10:44:54
HTTP POST http://127.0.0.1:8081/mcp
details

{
  "serverName": "455881af-007f-4fcf-a7b7-b2f6434cbfff",
  "request": {
    "url": "http://127.0.0.1:8081/mcp",
    "method": "POST",
    "headers": {
      "accept": "application/json, text/event-stream",
      "content-type": "application/json"
    },
    "body": {
      "kind": "string",
      "size": 189
    },
    "jsonRpcMethods": [
      "initialize"
    ]
  }
}

10:44:56
HTTP POST http://127.0.0.1:8081/mcp failed: Failed to fetch (check CORS?)
details

{
  "serverName": "455881af-007f-4fcf-a7b7-b2f6434cbfff",
  "request": {
    "url": "http://127.0.0.1:8081/mcp",
    "method": "POST",
    "headers": {
      "accept": "application/json, text/event-stream",
      "content-type": "application/json"
    },
    "body": {
      "kind": "string",
      "size": 189
    },
    "jsonRpcMethods": [
      "initialize"
    ]
  },
  "error": {
    "name": "TypeError",
    "message": "Failed to fetch",
    "stack": "TypeError: Failed to fetch\n    at window.fetch (http://127.0.0.1:8080/bundle.js:3:12909)\n    at fetch (http://127.0.0.1:8080/bundle.js:113:2110)\n    at r8.send (http://127.0.0.1:8080/bundle.js:111:4901)"
  },
  "browser": {
    "location": "http://127.0.0.1:8080/#/mcp-servers",
    "origin": "http://127.0.0.1:8080",
    "protocol": "http:",
    "isSecureContext": true,
    "targetOrigin": "http://127.0.0.1:8081",
    "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.",
    "\"Failed to fetch\" is a browser-level network failure. Common causes are CORS rejection, mixed-content blocking, certificate/TLS errors, DNS failures, or nothing listening on the target port."
  ],
  "durationMs": 2038
}

10:44:56
Protocol error: Failed to fetch
details

{
  "error": {
    "name": "TypeError",
    "message": "Failed to fetch",
    "stack": "TypeError: Failed to fetch\n    at window.fetch (http://127.0.0.1:8080/bundle.js:3:12909)\n    at fetch (http://127.0.0.1:8080/bundle.js:113:2110)\n    at r8.send (http://127.0.0.1:8080/bundle.js:111:4901)"
  }
}

10:44:56
Connection failed during initialize: Failed to fetch
details

{
  "error": {
    "name": "TypeError",
    "message": "Failed to fetch",
    "stack": "TypeError: Failed to fetch\n    at window.fetch (http://127.0.0.1:8080/bundle.js:3:12909)\n    at fetch (http://127.0.0.1:8080/bundle.js:113:2110)\n    at r8.send (http://127.0.0.1:8080/bundle.js:111:4901)"
  },
  "config": {
    "serverName": "455881af-007f-4fcf-a7b7-b2f6434cbfff",
    "configuredUrl": "http://127.0.0.1:8081/mcp",
    "effectiveUrl": "http://127.0.0.1:8081/mcp",
    "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://127.0.0.1:8081",
    "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.",
    "\"Failed to fetch\" is a browser-level network failure. Common causes are CORS rejection, mixed-content blocking, certificate/TLS errors, DNS failures, or nothing listening on the target port."
  ]
}

These are the error logs I get. I've tried different ports like 3000 8081 etc

[alistair3149]

Thanks for the detailed logs — they actually point straight at the cause, and the good news is this isn't a problem with the server for your setup.

Your log is coming from the MCP Inspector's web UI (http://127.0.0.1:8080/#/mcp-servers) making a browser fetch() to the server on http://127.0.0.1:8081/mcp. Those are two different origins (different ports), so the browser applies CORS, and since this server doesn't send CORS response headers, the browser blocks the response and reports Failed to fetch. CORS is a browser-only mechanism — it's deliberate here, because opening a local server to arbitrary web origins would let any website you visit talk to it.

The key thing: Zed doesn't use a browser, so CORS never applies to it. The error you're seeing is the Inspector's browser UI, not Zed. For your actual goal (Zed + a local model), point Zed straight at the server over stdio and you skip all of this.

Configure Zed with a stdio command server. In Zed's settings.json:

{
  "context_servers": {
    "mediawiki-mcp-server": {
      "source": "custom",
      "command": "npx",
      "args": ["@professional-wiki/mediawiki-mcp-server@latest"],
      "env": {
        "CONFIG": "/absolute/path/to/config.json"
      }
    }
  }
}

Create config.json from config.example.json to point at your wiki. You don't need MCP_TRANSPORT=http at all for this — stdio is the default and the simplest path. (Zed's exact config keys have shifted between versions; if the above doesn't match your build, check Zed's context-server docs.)

If you still want to verify with the Inspector, use its default proxy connection (browser → local Node proxy → server) rather than a browser-direct connection — the proxy isn't a browser, so no CORS.

One WSL2 note: run the server on the same side as whatever launches it. If Zed runs on Windows, it needs to be able to run the npx command (Node installed where Zed launches it); if you run Zed inside WSL2, the config above works as-is.

Let me know how you get on — happy to help get the Zed config working.