MCP Server not starting

[matteohoeren]

Describe the Bug

Tried everything to get this running, however i always fail with this error message:

Error POSTing to endpoint (HTTP 406): {"jsonrpc":"2.0","error":{"code":-32000,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"},"id":null}',

Link to the code that reproduces this issue

https://github.com/matteohoeren/payload-test-mcp

Reproduction Steps

Clone, pnpm install, pnpm dev, create an MCP API key, connect with the following configuration:

pnpm install throws:
pnpm add @payloadcms/plugin-mcp Packages: +70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Progress: resolved 932, reused 795, downloaded 0, added 0, done  WARN  Failed to create bin at /Users/matteo/dev/test-mcp/node_modules/.pnpm/@payloadcms+plugin-mcp@3.68.5_hono@4.11.1_next@15.4.10_@babel+core@7.28.5_@playwright+t_ebad8d8d647a11389cdf94811bde71eb/node_modules/@payloadcms/plugin-mcp/node_modules/.bin/mcp-adapter. ENOENT: no such file or directory, open '/Users/matteo/dev/test-mcp/node_modules/.pnpm/@vercel+mcp-adapter@1.0.0_@modelcontextprotocol+sdk@1.25.1_hono@4.11.1_zod@3.25.76__nex_f4ab38b306e71f0c415e2d1a19b60b51/node_modules/@vercel/mcp-adapter/dist/cli/index.js'  WARN  Failed to create bin at /Users/matteo/dev/test-mcp/node_modules/.pnpm/@payloadcms+plugin-mcp@3.68.5_hono@4.11.1_next@15.4.10_@babel+core@7.28.5_@playwright+t_ebad8d8d647a11389cdf94811bde71eb/node_modules/@payloadcms/plugin-mcp/node_modules/.bin/create-mcp-route. ENOENT: no such file or directory, open '/Users/matteo/dev/test-mcp/node_modules/.pnpm/@vercel+mcp-adapter@1.0.0_@modelcontextprotocol+sdk@1.25.1_hono@4.11.1_zod@3.25.76__nex_f4ab38b306e71f0c415e2d1a19b60b51/node_modules/@vercel/mcp-adapter/dist/cli/index.js'

    "payload": {
      "command": "npx",
      "args": [
        "-y",
        "mcp-remote@latest",
        "http://127.0.0.1:3000/api/mcp",
        "--debug",
        "--header",
        "Authorization: Bearer TOKEN"
      ]
    }

Result:

2025-12-17 09:45:27.963 [info] Starting server payload 2025-12-17 09:45:27.964 [info] Connection state: Starting 2025-12-17 09:45:27.973 [info] Starting server from LocalProcess extension host 2025-12-17 09:45:27.978 [info] Connection state: Starting 2025-12-17 09:45:27.978 [info] Connection state: Running 2025-12-17 09:45:29.502 [warning] [server stderr] [78365] Debug mode enabled - detailed logs will be written to ~/.mcp-auth/ 2025-12-17 09:45:29.502 [warning] [server stderr] [DEBUG LOG ERROR] global.currentServerUrlHash is not set. Cannot write debug log. 2025-12-17 09:45:29.503 [warning] [server stderr] [2025-12-17T08:45:29.502Z][78365] Starting mcp-remote with server URL: http://127.0.0.1:3000/api/mcp 2025-12-17 09:45:29.506 [warning] [server stderr] [78365] Using automatically selected callback port: 37765 2025-12-17 09:45:29.506 [warning] [server stderr] [2025-12-17T08:45:29.505Z][78365] Using automatically selected callback port: 37765 2025-12-17 09:45:29.506 [warning] [server stderr] [78365] Using custom headers: {"Authorization":"Bearer b4eeb455-56f1-442e-8c00-96b6fada7b8e"} 2025-12-17 09:45:29.506 [warning] [server stderr] [2025-12-17T08:45:29.506Z][78365] Using custom headers: {"Authorization":"Bearer b4eeb455-56f1-442e-8c00-96b6fada7b8e"} 2025-12-17 09:45:29.506 [warning] [server stderr] [78365] Discovering OAuth server configuration... 2025-12-17 09:45:29.506 [warning] [server stderr] [2025-12-17T08:45:29.506Z][78365] Discovering OAuth server configuration... 2025-12-17 09:45:29.507 [warning] [server stderr] [2025-12-17T08:45:29.506Z][78365] Starting OAuth server discovery { serverUrl: 'http://127.0.0.1:3000/api/mcp' } 2025-12-17 09:45:29.508 [warning] [server stderr] [2025-12-17T08:45:29.507Z][78365] Probing MCP server for WWW-Authenticate header 2025-12-17 09:45:29.555 [warning] [server stderr] [2025-12-17T08:45:29.554Z][78365] Starting Protected Resource Metadata discovery { 2025-12-17 09:45:29.555 [warning] [server stderr] resourceUrl: 'http://127.0.0.1:3000/api/mcp', 2025-12-17 09:45:29.555 [warning] [server stderr] hasWWWAuthenticateHeader: false 2025-12-17 09:45:29.555 [warning] [server stderr] } 2025-12-17 09:45:29.555 [warning] [server stderr] [2025-12-17T08:45:29.554Z][78365] Built Protected Resource Metadata URLs { 2025-12-17 09:45:29.555 [warning] [server stderr] resourceUrl: 'http://127.0.0.1:3000/api/mcp', 2025-12-17 09:45:29.555 [warning] [server stderr] urls: [ 2025-12-17 09:45:29.555 [warning] [server stderr] 'http://127.0.0.1:3000/.well-known/oauth-protected-resource/api/mcp', 2025-12-17 09:45:29.555 [warning] [server stderr] 'http://127.0.0.1:3000/.well-known/oauth-protected-resource' 2025-12-17 09:45:29.555 [warning] [server stderr] ] 2025-12-17 09:45:29.555 [warning] [server stderr] } 2025-12-17 09:45:29.555 [warning] [server stderr] [2025-12-17T08:45:29.555Z][78365] Fetching Protected Resource Metadata { 2025-12-17 09:45:29.555 [warning] [server stderr] metadataUrl: 'http://127.0.0.1:3000/.well-known/oauth-protected-resource/api/mcp' 2025-12-17 09:45:29.555 [warning] [server stderr] } 2025-12-17 09:45:30.709 [warning] [server stderr] [2025-12-17T08:45:30.708Z][78365] Protected Resource Metadata not found (404) { 2025-12-17 09:45:30.709 [warning] [server stderr] metadataUrl: 'http://127.0.0.1:3000/.well-known/oauth-protected-resource/api/mcp' 2025-12-17 09:45:30.709 [warning] [server stderr] } 2025-12-17 09:45:30.709 [warning] [server stderr] [2025-12-17T08:45:30.708Z][78365] Fetching Protected Resource Metadata { 2025-12-17 09:45:30.709 [warning] [server stderr] metadataUrl: 'http://127.0.0.1:3000/.well-known/oauth-protected-resource' 2025-12-17 09:45:30.709 [warning] [server stderr] } 2025-12-17 09:45:30.727 [warning] [server stderr] [2025-12-17T08:45:30.726Z][78365] Protected Resource Metadata not found (404) { 2025-12-17 09:45:30.727 [warning] [server stderr] metadataUrl: 'http://127.0.0.1:3000/.well-known/oauth-protected-resource' 2025-12-17 09:45:30.727 [warning] [server stderr] } 2025-12-17 09:45:30.727 [warning] [server stderr] [2025-12-17T08:45:30.726Z][78365] Protected Resource Metadata discovery failed - no metadata found 2025-12-17 09:45:30.727 [warning] [server stderr] [2025-12-17T08:45:30.727Z][78365] No Protected Resource Metadata found, falling back to server URL as authorization server 2025-12-17 09:45:30.727 [warning] [server stderr] [2025-12-17T08:45:30.727Z][78365] Fetching authorization server metadata { 2025-12-17 09:45:30.727 [warning] [server stderr] serverUrl: 'http://127.0.0.1:3000/api/mcp', 2025-12-17 09:45:30.727 [warning] [server stderr] metadataUrl: 'http://127.0.0.1:3000/.well-known/oauth-authorization-server' 2025-12-17 09:45:30.727 [warning] [server stderr] } 2025-12-17 09:45:30.746 [warning] [server stderr] [2025-12-17T08:45:30.744Z][78365] Authorization server metadata endpoint not found (404) { 2025-12-17 09:45:30.746 [warning] [server stderr] metadataUrl: 'http://127.0.0.1:3000/.well-known/oauth-authorization-server' 2025-12-17 09:45:30.746 [warning] [server stderr] } 2025-12-17 09:45:30.746 [warning] [server stderr] [2025-12-17T08:45:30.744Z][78365] No Protected Resource Metadata found, using server URL as authorization server 2025-12-17 09:45:30.746 [warning] [server stderr] [78365] [78365] Connecting to remote server: http://127.0.0.1:3000/api/mcp 2025-12-17 09:45:30.746 [warning] [server stderr] [2025-12-17T08:45:30.745Z][78365] [78365] Connecting to remote server: http://127.0.0.1:3000/api/mcp 2025-12-17 09:45:30.746 [warning] [server stderr] [78365] Using transport strategy: http-first 2025-12-17 09:45:30.746 [warning] [server stderr] [2025-12-17T08:45:30.745Z][78365] Using transport strategy: http-first 2025-12-17 09:45:30.747 [warning] [server stderr] [2025-12-17T08:45:30.745Z][78365] Attempting to connect to remote server { sseTransport: false } 2025-12-17 09:45:30.747 [warning] [server stderr] [2025-12-17T08:45:30.745Z][78365] Starting transport directly 2025-12-17 09:45:30.747 [warning] [server stderr] [2025-12-17T08:45:30.746Z][78365] Creating test transport for HTTP-only connection test 2025-12-17 09:45:30.753 [warning] [server stderr] [2025-12-17T08:45:30.752Z][78365] Reading OAuth tokens 2025-12-17 09:45:30.754 [warning] [server stderr] [2025-12-17T08:45:30.754Z][78365] Token request stack trace: Error 2025-12-17 09:45:30.754 [warning] [server stderr] at NodeOAuthClientProvider.tokens (file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/chunk-F76MHFRJ.js:19688:44) 2025-12-17 09:45:30.754 [warning] [server stderr] at StreamableHTTPClientTransport._commonHeaders (file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/chunk-F76MHFRJ.js:18268:47) 2025-12-17 09:45:30.754 [warning] [server stderr] at StreamableHTTPClientTransport.send (file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/chunk-F76MHFRJ.js:18454:34) 2025-12-17 09:45:30.754 [warning] [server stderr] at file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/chunk-F76MHFRJ.js:16250:23 2025-12-17 09:45:30.754 [warning] [server stderr] at new Promise (<anonymous>) 2025-12-17 09:45:30.754 [warning] [server stderr] at Client.request (file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/chunk-F76MHFRJ.js:16183:12) 2025-12-17 09:45:30.754 [warning] [server stderr] at Client.connect (file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/chunk-F76MHFRJ.js:16563:33) 2025-12-17 09:45:30.755 [warning] [server stderr] at process.processTicksAndRejections (node:internal/process/task_queues:103:5) 2025-12-17 09:45:30.755 [warning] [server stderr] at async connectToRemoteServer (file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/chunk-F76MHFRJ.js:19139:9) 2025-12-17 09:45:30.755 [warning] [server stderr] at async runProxy (file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/proxy.js:157:29) 2025-12-17 09:45:30.755 [warning] [server stderr] [2025-12-17T08:45:30.754Z][78365] Token result: Not found 2025-12-17 09:45:31.028 [warning] [server stderr] [78365] Connection error: Error: Error POSTing to endpoint (HTTP 406): {"jsonrpc":"2.0","error":{"code":-32000,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"},"id":null} 2025-12-17 09:45:31.029 [warning] [server stderr] at StreamableHTTPClientTransport.send (file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/chunk-F76MHFRJ.js:18516:15) 2025-12-17 09:45:31.029 [warning] [server stderr] at process.processTicksAndRejections (node:internal/process/task_queues:103:5) 2025-12-17 09:45:31.029 [warning] [server stderr] [2025-12-17T08:45:31.028Z][78365] Connection error: Error: Error POSTing to endpoint (HTTP 406): {"jsonrpc":"2.0","error":{"code":-32000,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"},"id":null} 2025-12-17 09:45:31.029 [warning] [server stderr] at StreamableHTTPClientTransport.send (file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/chunk-F76MHFRJ.js:18516:15) 2025-12-17 09:45:31.029 [warning] [server stderr] at process.processTicksAndRejections (node:internal/process/task_queues:103:5) 2025-12-17 09:45:31.029 [warning] [server stderr] [2025-12-17T08:45:31.028Z][78365] Connection error { 2025-12-17 09:45:31.029 [warning] [server stderr] errorMessage: 'Error POSTing to endpoint (HTTP 406): {"jsonrpc":"2.0","error":{"code":-32000,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"},"id":null}', 2025-12-17 09:45:31.029 [warning] [server stderr] stack: 'Error: Error POSTing to endpoint (HTTP 406): {"jsonrpc":"2.0","error":{"code":-32000,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"},"id":null}\n' + 2025-12-17 09:45:31.029 [warning] [server stderr] ' at StreamableHTTPClientTransport.send (file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/chunk-F76MHFRJ.js:18516:15)\n' + 2025-12-17 09:45:31.029 [warning] [server stderr] ' at process.processTicksAndRejections (node:internal/process/task_queues:103:5)', 2025-12-17 09:45:31.029 [warning] [server stderr] transportType: 'StreamableHTTPClientTransport' 2025-12-17 09:45:31.029 [warning] [server stderr] } 2025-12-17 09:45:31.029 [warning] [server stderr] [78365] Fatal error: Error: Error POSTing to endpoint (HTTP 406): {"jsonrpc":"2.0","error":{"code":-32000,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"},"id":null} 2025-12-17 09:45:31.029 [warning] [server stderr] at StreamableHTTPClientTransport.send (file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/chunk-F76MHFRJ.js:18516:15) 2025-12-17 09:45:31.030 [warning] [server stderr] at process.processTicksAndRejections (node:internal/process/task_queues:103:5) 2025-12-17 09:45:31.030 [warning] [server stderr] [2025-12-17T08:45:31.029Z][78365] Fatal error: Error: Error POSTing to endpoint (HTTP 406): {"jsonrpc":"2.0","error":{"code":-32000,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"},"id":null} 2025-12-17 09:45:31.030 [warning] [server stderr] at StreamableHTTPClientTransport.send (file:///Users/matteo/.npm/_npx/1a3c4333f3a90708/node_modules/mcp-remote/dist/chunk-F76MHFRJ.js:18516:15) 2025-12-17 09:45:31.030 [warning] [server stderr] at process.processTicksAndRejections (node:internal/process/task_queues:103:5) 2025-12-17 09:45:31.048 [info] Connection state: Error Process exited with code 1

Which area(s) are affected?

plugin: mcp

Environment Info

Binaries:
  Node: 25.2.1
  npm: 11.6.2
  Yarn: 1.22.22
  pnpm: 10.24.0
Relevant Packages:
  payload: 3.68.5
  next: 15.4.10
  @payloadcms/db-mongodb: 3.68.5
  @payloadcms/graphql: 3.68.5
  @payloadcms/next/utilities: 3.68.5
  @payloadcms/plugin-mcp: 3.68.5
  @payloadcms/richtext-lexical: 3.68.5
  @payloadcms/translations: 3.68.5
  @payloadcms/ui/shared: 3.68.5
  react: 19.2.1
  react-dom: 19.2.1
Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.1.0: Thu Oct 10 21:03:15 PDT 2024; root:xnu-11215.41.3~2/RELEASE_ARM64_T6000
  Available memory (MB): 32768
  Available CPU cores: 10

[kendelljoseph]

What MCP client are you using to test this?
If you are testing with a curl are you including the propper Accept header?

Take a look at this example here in the tests.

payload/test/plugin-mcp/int.spec.ts

Line 169 in ef710e3

const response = await restClient.POST('/mcp', {

You must include text/event-stream in your Accept param in your headers, as MCP servers are required to stream content to clients in accordance with the protocol -- (see 'sending messages to the server').

2. The client MUST include an Accept header, listing both application/json and text/event-stream as supported content types.

Your headers should look something like this:

headers: {
  Authorization: `Bearer ${apiKey}`,
  Accept: 'application/json, text/event-stream', // Include BOTH
  'Content-Type': 'application/json',
},

[matteohoeren]

@kendelljoseph Tried both CC and Copilot.

  curl -v -X POST \
    -H "Authorization: Bearer b4eeb455-56f1-442e-8c00-96b6fada7b8e" \
    -H "Accept: application/json, text/event-stream" \
    -H "Content-Type: application/json" \
    -d '{
      "jsonrpc": "2.0",
      "id": 1,
      "method": "initialize",
      "params": {
        "protocolVersion": "2025-06-18",
        "capabilities": {},
        "clientInfo": {"name": "test", "version": "1.0.0"}
      }
    }' \
    http://127.0.0.1:3000/api/mcp

This fails with the same error, i think this request is correct though. I tried really hard for 2-3 hours and couldn't get it working.
Do you need further information to investigate? Does my Repo run on your machine correctly with MCP Server?

[robert-sarosi]

same here with 3.67.0.

npx -y mcp-remote http://localhost:3000/api/mcp --header "Authorization: Bearer xxxxxx" --header "Accept: 'application/json, text/event-stream'" --header "Content-Type: 'application/json'"

[78922] Using automatically selected callback port: 30177
[78922] Using custom headers: {"Authorization":"Bearer xxxxxx","Accept":"'application/json, text/event-stream'","Content-Type":"'application/json'"}
[78922] Discovering OAuth server configuration...
[78922] [78922] Connecting to remote server: http://localhost:3000/api/mcp
[78922] Using transport strategy: http-first
[78922] Connection error: Error: Error POSTing to endpoint (HTTP 406): {"jsonrpc":"2.0","error":{"code":-32000,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"},"id":null}

[Rot4tion]

same here with 3.67.0.

npx -y mcp-remote http://localhost:3000/api/mcp --header "Authorization: Bearer xxxxxx" --header "Accept: 'application/json, text/event-stream'" --header "Content-Type: 'application/json'"

[78922] Using automatically selected callback port: 30177
[78922] Using custom headers: {"Authorization":"Bearer xxxxxx","Accept":"'application/json, text/event-stream'","Content-Type":"'application/json'"}
[78922] Discovering OAuth server configuration...
[78922] [78922] Connecting to remote server: http://localhost:3000/api/mcp
[78922] Using transport strategy: http-first
[78922] Connection error: Error: Error POSTing to endpoint (HTTP 406): {"jsonrpc":"2.0","error":{"code":-32000,"message":"Not Acceptable: Client must accept both application/json and text/event-stream"},"id":null}

same issue with 3.68.5