[BUG] Headers are not being sent as Authorization or "x-api-key"

[GlauberF]

Bug Description

I am using Next.js version 13. The mcp-server and client are both running within the same application.

I have installed all the mentioned packages to allow sending custom headers (eventsource and extended-eventsource), but even so, the headers are not being sent.

Implementation:

// Create client and connect to server
    const client = new MultiServerMCPClient({
      // Global tool configuration options
      // Whether to throw on errors if a tool fails to load (optional, default: true)
      throwOnLoadError: true,
      // Whether to prefix tool names with the server name (optional, default: true)
      prefixToolNameWithServerName: true,
      // Optional additional prefix for tool names (optional, default: "mcp")
      additionalToolNamePrefix: "mcp",

      // Server configuration
      mcpServers: {
        // adds a STDIO connection to a server named "math"
        // math: {
        //   transport: "stdio",
        //   command: "npx",
        //   args: ["-y", "@modelcontextprotocol/server-math"],
        //   // Restart configuration for stdio transport
        //   restart: {
        //     enabled: true,
        //     maxAttempts: 3,
        //     delayMs: 1000,
        //   },
        // },

        // SSE transport example with reconnection configuration
        saaspilot: {
          transport: "sse",
          url: `${req.headers.origin || "http://localhost:3000"}/api/mcp-server/sse`,
          headers: {
            "x-api-key": req.headers.authorization?.startsWith("Bearer ") ? req.headers.authorization : `Bearer ${req.headers.authorization}`,
          },
          useNodeEventSource: true,
          reconnect: {
            enabled: true,
            maxAttempts: 5,
            delayMs: 2000,
          },
        },
      },
    });

    const tools = await client.getTools();

    // Create an OpenAI model
    const model = new ChatOpenAI({
      modelName: "gpt-4o",
      temperature: 0,
    });

    // Create the React agent
    const agent = createReactAgent({
      llm: model,
      tools,
      prompt: requestData.user_description
    });

Logs

 ▲ Next.js 13.5.11
  - Local:        http://localhost:3000
  - Environments: .env
  - Experiments (use at your own risk):
     · instrumentationHook

 ✓ Ready in 2.2s
 ✓ Compiled /api/agents/v1/answers in 677ms (164 modules)
/api/v1/answers called!
  @langchain/mcp-adapters:client INFO: Initializing connection to server "saaspilot"... +0ms
  @langchain/mcp-adapters:client DEBUG: Creating SSE transport for server "saaspilot" with URL: http://localhost:3000/api/mcp-server/sse +0ms
  @langchain/mcp-adapters:client DEBUG: Using custom headers for SSE transport to server "saaspilot" +0ms
  @langchain/mcp-adapters:client DEBUG: Using Extended EventSource for server "saaspilot" +4ms
  @langchain/mcp-adapters:client DEBUG: Setting headers for Extended EventSource: {"x-api-key":"Bearer saaspk-14dd717c*********************adf32ff"} +1ms

Headers received by my mcp-server:

✓ Compiled /api/mcp-server/sse in 84ms (177 modules)
{
  host: 'localhost:3000',
  connection: 'keep-alive',
  accept: 'text/event-stream',
  'accept-language': '*',
  'sec-fetch-mode': 'cors',
  'user-agent': 'node',
  pragma: 'no-cache',
  'cache-control': 'no-cache',
  'accept-encoding': 'gzip, deflate',
  'x-middleware-invoke': '',
  'x-invoke-path': '/api/mcp-server/sse',
  'x-invoke-query': '%7B%7D',
  'x-forwarded-host': 'localhost:3000',
  'x-forwarded-port': '3000',
  'x-forwarded-proto': 'http',
  'x-forwarded-for': '::ffff:127.0.0.1',
  'x-invoke-output': '/api/mcp-server/sse'
}

Inspector screenshot where the mcp-server works 100%

[botkevin]

+1 Same problem. Looking at the tests in the code, it doesn't seem to be tested. Working through MCP Inspector, but not with langchain mcp-adaptors. Using ngrok to inspect the request, and the headers are being passed when using MCP inspector, but not being passed when called in MultiServerMPCClient. I suspect there's a bug preventing any headers from sending.

For reference, my code is

  // Convert the MCP config to an SSEConnection for the MCP client
  const sseConnections: Record<string, SSEConnection> = {};
  for (const [key, value] of Object.entries(
    state.mcpConnectionConfig.servers
  )) {
    sseConnections[key] = {
      transport: "sse",
      url: value.url,
      headers: value.headers,
      useNodeEventSource: true,
    };
  }

  const clientConfig = {
    mcpServers: sseConnections,
    prefixToolNameWithServerName: false,
    throwOnLoadError: true,
    additionalToolNamePrefix: "",
  };
  console.log("clientConfig", clientConfig.mcpServers);
  console.log("clientConfig headers", clientConfig.mcpServers['mcpServer'].headers);
  // ^ the headers are still here

  const mcpClient = new MultiServerMCPClient(clientConfig);

edit: using latest v0.4.2

[raphaelblum]

Same issue here. This PR from @Qantt fixes it: #58

[benjamincburns]

Fixed as of v0.4.3. v0.4.4 is also out now, which added support for streamable HTTP. Streamable HTTP transports bake in better header support for node, so please prefer this transport in cases when you have control over both the client and server transport.

[torresedgar87]

I am still seeing this issue on the version 0.4.5, added headers do not display in the log

const originalFetch = global.fetch;

global.fetch = async (...args) => {
    console.log("Fetching:", args);
    return originalFetch(args);
}

const token = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWUsImlhdCI6MTUxNjIzOTAyMn0.KMUFsIDTnFmyG3nMiGM6H9FNFUROf3wh7SmqJp-QV30"

const mcpServers = {
    "test-mcp": {
        transport: "sse",
        type: "sse",
        url: "http://localhost:3001/api/mcp/sse",
        headers: {
            Authorization: "Bearer " + token,
        },
        useNodeEventSource: true,
    },
  };

async function main() {
    const client = new MultiServerMCPClient(mcpServers);
    const tools = await client.getTools();
}

main();

output of log

@langchain/mcp-adapters:client INFO: Initializing connection to server "test-mcp"... +0ms
  @langchain/mcp-adapters:client DEBUG: Using custom headers for SSE transport to server "test-mcp" +1ms
(node:32763) [DEP0040] DeprecationWarning: The `punycode` module is deprecated. Please use a userland alternative instead.
(Use `node --trace-deprecation ...` to show where the warning was created)
  @langchain/mcp-adapters:client DEBUG: Using Extended EventSource for server "test-mcp" +3ms
  @langchain/mcp-adapters:client DEBUG: Setting headers for Extended EventSource: {"Authorization":"Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWUsImlhdCI6MTUxNjIzOTAyMn0.KMUFsIDTnFmyG3nMiGM6H9FNFUROf3wh7SmqJp-QV30"}
Fetching: [
  URL {
    href: 'http://localhost:3001/api/mcp/sse',
    origin: 'http://localhost:3001',
    protocol: 'http:',
    username: '',
    password: '',
    host: 'localhost:3001',
    hostname: 'localhost',
    port: '3001',
    pathname: '/api/mcp/sse',
    search: '',
    searchParams: URLSearchParams {},
    hash: ''
  },
  {
    mode: 'cors',
    redirect: 'follow',
    headers: { Accept: 'text/event-stream' },
    cache: 'no-store',
    signal: AbortSignal { aborted: false }
  }
]

[torresedgar87]

Testing directly on the SSEClientTransport, it seems it may be an issue with the client you are using from the @modelcontextprotocol/sdk