StreamableHTTP transport timeout issue.

[piyushgithub15]

Describe the bug
The Go MCP SDK client experiences a 60-second timeout (504 Gateway Timeout) when calling tools on the remote MCP server at mcp.on-demand.io, while the Node.js MCP SDK client successfully completes the same request in ~1 second. The issue appears to be a server-side compatibility problem with how the Go SDK's requests are processed.

To Reproduce

1. Install the official Go MCP SDK:

go get github.com/modelcontextprotocol/go-sdk/mcp

2. Create a Go client connecting to the remote MCP server:

   client := mcp.NewClient(&mcp.Implementation{
       Name:    "mcp-mermaid-client",
       Version: "1.0.0",
   }, nil)
   transport := &mcp.StreamableClientTransport{
       Endpoint:   "https://mcp.on-demand.io/instances/presight-mermaid-mcp/mcp?apiKey=<REDACTED>",
       HTTPClient: &http.Client{Timeout: 0},
       MaxRetries: 3,
   }
   session, err := client.Connect(ctx, transport, nil)

3. Call the generate_mermaid_diagram tool:

   params := &mcp.CallToolParams{
       Name: "generate_mermaid_diagram",
       Arguments: map[string]any{
           "mermaid": "graph TD\n    A-->B",
           "outputType": "mermaid",
       },
   }
   
   result, err := session.CallTool(ctx, params)

Expected behavior
The Go client should successfully call the generate_mermaid_diagram tool and receive the mermaid diagram response within a reasonable time (~1-2 seconds), similar to the Node.js client behavior.

Logs

2026/01/28 12:50:42 Calling generate_mermaid_diagram tool...
2026/01/28 12:50:42 Starting tool call...
2026/01/28 12:50:42 === HTTP REQUEST ===
POST /instances/presight-mermaid-mcp/mcp?apiKey=<REDACTED> HTTP/1.1
Host: mcp.on-demand.io
User-Agent: Go-http-client/1.1
Content-Length: 165
Accept: application/json, text/event-stream
Content-Type: application/json
Mcp-Protocol-Version: 2025-06-18
Accept-Encoding: gzip

{"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"generate_mermaid_diagram","arguments":{"mermaid":"graph TD\n    A--\u003eB","outputType":"mermaid"}}}

2026/01/28 12:51:42 === HTTP RESPONSE (after 1m0.316470083s) ===
HTTP/2.0 504 Gateway Timeout
Content-Length: 102
Access-Control-Allow-Origin: *
Content-Security-Policy: default-src 'none'; frame-ancestors 'none'
Content-Type: application/json; charset=utf-8
Date: Wed, 28 Jan 2026 07:21:42 GMT

2026/01/28 12:51:42 CallTool failed after 1m0.316810209s: calling "tools/call": rejected by transport: sending "tools/call": Gateway Timeout

[maciej-kisiel]

Hi @piyushgithub15, thanks for reporting the issue. In the future, I would advise to be careful with sharing API keys, as they could be misused. I redacted them from your initial issue content, but they were actually useful in reproducing the issue.

Could you also share your Node.js client code and the logged HTTP requests and responses so we can compare them?

[piyushgithub15]

import { Client } from '@modelcontextprotocol/sdk/client/index.js';
import { StreamableHTTPClientTransport } from '@modelcontextprotocol/sdk/client/streamableHttp.js';

async function main() {
  console.log('Creating MCP client...');
  
  // Create a new MCP client
  const client = new Client(
    {
      name: 'mcp-mermaid-node-client',
      version: '1.0.0',
    },
    {
      capabilities: {},
    }
  );

  // Create the transport for the remote server
  const serverUrl = new URL('https://mcp.on-demand.io/instances/presight-mermaid-mcp/mcp?apiKey=<apiKey>');
  const transport = new StreamableHTTPClientTransport(serverUrl);

  try {
    console.log('Connecting to MCP server...');
    await client.connect(transport);
    console.log('Connected successfully!');

    // Get server info
    const serverInfo = client.getServerVersion();
    console.log(`Server info: ${serverInfo?.name} v${serverInfo?.version}`);

    // List available tools
    console.log('\nListing tools...');
    const toolsResult = await client.listTools();
    console.log(`Available tools: ${toolsResult.tools.length}`);
    toolsResult.tools.forEach(tool => {
      console.log(`  - ${tool.name}: ${tool.description}`);
    });

    // Call the generate_mermaid_diagram tool
    console.log('\nCalling generate_mermaid_diagram tool...');
    console.log('With flowchart:');
    console.log(`flowchart TD
    A[User] --> B[Login Request]
    B --> C{Credentials Valid?}
    C -->|Yes| D[Generate Token]
    C -->|No| E[Return Error]`);

    const startTime = Date.now();
    const result = await client.callTool({
      name: 'generate_mermaid_diagram',
      arguments: {
        mermaid: `flowchart TD
    A[User] --> B[Login Request]
    B --> C{Credentials Valid?}
    C -->|Yes| D[Generate Token]
    C -->|No| E[Return Error]`,
        outputType: 'svg',
        theme: 'default',
        backgroundColor: 'white',
      },
    });
    const endTime = Date.now();

    console.log(`\nTool call completed in ${endTime - startTime}ms`);
    console.log(`IsError: ${result.isError || false}`);
    console.log(`Number of content items: ${result.content.length}`);

    // Display the result
    result.content.forEach((content, i) => {
      if (content.type === 'text') {
        console.log(`\nContent[${i}] (text):`);
        console.log(content.text);
      } else if (content.type === 'image') {
        console.log(`\nContent[${i}] (image):`);
        console.log(`  MimeType: ${content.mimeType}`);
        console.log(`  Data length: ${content.data.length} characters`);
        // Show first 100 characters of the data
        console.log(`  Data preview: ${content.data.substring(0, 100)}...`);
      } else if (content.type === 'resource') {
        console.log(`\nContent[${i}] (resource):`);
        console.log(`  URI: ${content.resource.uri}`);
        console.log(`  MimeType: ${content.resource.mimeType}`);
      } else {
        console.log(`\nContent[${i}] (${content.type}):`, JSON.stringify(content, null, 2));
      }
    });

    console.log('\n✅ Test completed successfully!');
  } catch (error) {
    console.error('❌ Error:', error.message);
    console.error('Stack:', error.stack);
  } finally {
    await client.close();
    console.log('\nClient closed.');
  }
}

main().catch(console.error);

HTTP logs for node client

Request 1: Initialize

Request:

POST https://mcp.on-demand.io/instances/presight-mermaid-mcp/mcp?apiKey=<REDACTED>
Content-Type: application/json
Accept: application/json, text/event-stream

{
  "method": "initialize",
  "params": {
    "protocolVersion": "2025-11-25",
    "capabilities": {},
    "clientInfo": {
      "name": "mcp-mermaid-node-client",
      "version": "1.0.0"
    }
  },
  "jsonrpc": "2.0",
  "id": 0
}

Response:

HTTP/1.1 200 OK
Content-Type: text/event-stream
Cache-Control: no-cache

event: message
data: {
  "jsonrpc": "2.0",
  "id": 0,
  "result": {
    "protocolVersion": "2025-06-18",
    "capabilities": {
      "tools": {}
    },
    "serverInfo": {
      "name": "mcp-mermaid",
      "version": "0.1.3"
    }
  }
}

Duration: 1256ms

---

Request 2: Initialized Notification

Request:

POST https://mcp.on-demand.io/instances/presight-mermaid-mcp/mcp?apiKey=<REDACTED>
Content-Type: application/json
Accept: application/json, text/event-stream
MCP-Protocol-Version: 2025-06-18

{
  "method": "notifications/initialized",
  "jsonrpc": "2.0"
}

Response:

HTTP/1.1 202 Accepted
Content-Length: 0

Duration: 1075ms

---

Request 3: SSE Connection (Long-polling)

Request:

GET https://mcp.on-demand.io/instances/presight-mermaid-mcp/mcp?apiKey=<REDACTED>
Accept: text/event-stream
MCP-Protocol-Version: 2025-06-18

Response:

HTTP/1.1 404 Not Found
Content-Length: 9

Not Found

Duration: 367ms
Note: This request appears to fail, but the client continues to work.

---

Request 4: List Tools

Request:

POST https://mcp.on-demand.io/instances/presight-mermaid-mcp/mcp?apiKey=<REDACTED>
Content-Type: application/json
Accept: application/json, text/event-stream
MCP-Protocol-Version: 2025-06-18

{
  "method": "tools/list",
  "jsonrpc": "2.0",
  "id": 1
}

Response:

HTTP/1.1 200 OK
Content-Type: text/event-stream

event: message
data: {
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "tools": [
      {
        "name": "generate_mermaid_diagram",
        "description": "Generate mermaid diagram and chart with mermaid syntax dynamically...",
        "inputSchema": {
          "type": "object",
          "properties": {
            "mermaid": { "type": "string", "minLength": 1, "description": "..." },
            "theme": { "type": "string", "enum": ["default", "base", "forest", "dark", "neutral"], "default": "default" },
            "backgroundColor": { "type": "string", "default": "white" },
            "outputType": { "type": "string", "enum": ["png", "svg", "mermaid"], "default": "png" }
          },
          "required": ["mermaid"],
          "$schema": "http://json-schema.org/draft-07/schema#"
        }
      }
    ]
  }
}

Duration: 1189ms

---

Request 5: Call Tool

Request:

POST https://mcp.on-demand.io/instances/presight-mermaid-mcp/mcp?apiKey=<REDACTED>
Content-Type: application/json
Accept: application/json, text/event-stream
MCP-Protocol-Version: 2025-06-18

{
  "method": "tools/call",
  "params": {
    "name": "generate_mermaid_diagram",
    "arguments": {
      "mermaid": "flowchart TD\n    A[User] --> B[Login Request]\n    B --> C{Credentials Valid?}\n    C -->|Yes| D[Generate Token]\n    C -->|No| E[Return Error]",
      "outputType": "svg",
      "theme": "default",
      "backgroundColor": "white"
    }
  },
  "jsonrpc": "2.0",
  "id": 2
}

Response:

HTTP/1.1 200 OK
Content-Type: text/event-stream

event: message
data: {
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "<svg aria-roledescription=\"flowchart-v2\" ...>...</svg>"
      }
    ]
  }
}

This is my node client and logs

[maciej-kisiel]

It looks like this is cause by automatic HTML-escaping that is done by json.Marshal. It was affecting the ">" character and the server was not handling it well. I think we shouldn't do it, it doesn't seem to be required by any relevant specifications and I checked that Typescript SDK doesn't do it. The fix that made it work can be found in #769. However, it's a behavior change, @findleyr WDYT? Is it warranted?

[findleyr]

@maciej-kisiel behavior changes are OK if they're fixing a bug, which this arguably is. I don't think the reasons for escaping apply here, and this is clearly affecting at least one user.