MCP proxy jar fails with "Server does not support tools" when connecting to Burp MCP Server 1.2.0

[jrg09]

Description

The MCP proxy jar fails to connect to the Burp MCP Server extension when used with Claude Desktop.
The SSE server responds correctly to curl requests, but the proxy jar always fails with a 404
error followed by "Server does not support tools".

Environment

• Burp Suite Community Edition: v2026.1.4
• MCP Server Extension: v1.2.0 (updated Feb 16, 2026)
• OS: macOS (Apple Silicon)
• Claude Desktop: latest

Steps to Reproduce

1. Install MCP Server extension v1.2.0 from BApp Store
2. Enable the MCP server in the MCP tab (port 9876)
3. Configure Claude Desktop with the generated claude_desktop_config.json
4. Open Claude Desktop

Expected Behavior

Claude Desktop connects successfully to Burp Suite via the MCP proxy.

Actual Behavior

The proxy jar fails immediately with:

ERROR SseClient - Failed to connect to SSE server at http://localhost:9876/: Expected status code 200 but was 404
ERROR Main - Failed to start proxy: Server does not support tools (required for ToolsList)
java.lang.IllegalStateException: Server does not support tools (required for ToolsList)

Additional Notes

• The SSE server does respond with HTTP 200 to curl:
  curl -i http://localhost:9876/ returns:
  HTTP/1.1 200 OK
  Content-Type: text/event-stream
  event: endpoint
  data: ?sessionId=
• curl works regardless of headers — tested with Accept: text/event-stream and
  User-Agent: Java/21, both return HTTP 200 successfully
• Manual POST to http://localhost:9876/?sessionId=... returns HTTP 202 Accepted,
  but no response is ever received on the SSE stream
• The proxy jar receives HTTP 404 on the same URL that curl receives HTTP 200
• The problem reproduces consistently on every connection attempt
• Tried both http://localhost:9876 and http://127.0.0.1:9876 — same result

[d00xy-hash]

burp": {
"command": "npx",
"args": [
"-y",
"supergateway",
"--sse",
"http://127.0.0.1:9876/"
]

Try to use super gateway it should work

[portswigger-penguin]

This should be fixed again now in the latest version. Using super gateway should work as well.