Fusion MCP Server: Node-based clients fail parsing initialize response (HTTPParserError: invalid chunk size); curl works

[production3NQBF]

Windows 11, Fusion MCP Server enabled (Preferences > General > API), port 27182.
Related thread: "Fusion MCP Server disconnects immediately after initialize - Claude Desktop Windows"

Findings:
- netstat confirms Fusion is LISTENING on 127.0.0.1:27182.
- curl POST initialize to http://127.0.0.1:27182/mcp succeeds for protocolVersion
2025-06-18 and 2025-11-25, with a clean Content-Length response.
- Discovery endpoints (/.well-known/...) return clean 404s; GET /mcp returns a clean 405.
- Claude Desktop's bundled Fusion bridge crashes ~1s after initialize
("Server transport closed unexpectedly").
- Replacing the bridge with mcp-remote (Node/undici) against the same endpoint fails
on the initialize response:
HTTPParserError: Response does not match the HTTP/1.1 protocol
(Invalid character in chunk size)
The error's data field contains the valid initialize result, so the body is correct,
but the HTTP framing sent to this client appears to declare chunked transfer
encoding without a chunked body.
- Not reproducible with curl, even with Node's headers
(Accept-Encoding, User-Agent, Connection: keep-alive).

Conclusion: the server's response framing differs for Node HTTP clients, which breaks
both the official bridge and third-party relays. Happy to provide full logs.

[yonemoto888]

Subject: Fusion MCP Server returns malformed chunked HTTP responses, so Claude Desktop cannot connect

Environment

• Autodesk Fusion: 2703.1.20 x86_64
• OS: Windows 11
• Claude Desktop: 2.1.6120.0
• Node.js v24.19.0, mcp-remote (latest via npx)
• Fusion MCP Server enabled (Preferences → General → API), design open

Summary
The local Fusion MCP Server returns a valid JSON-RPC body in response to initialize, but the HTTP/1.1 chunked transfer framing around it is malformed. Node/undici rejects the response and the client disconnects. This happens with both the Autodesk Fusion extension for Claude Desktop and a manual mcp-remote bridge configured in claude_desktop_config.json.

Error (from Claude Desktop's mcp-server-fusion.log)

Connecting to remote server: http://127.0.0.1:27185/mcp
Connection error: TypeError: terminated
  [cause]: HTTPParserError: Response does not match the HTTP/1.1 protocol (Invalid character in chunk size)

The response body received before the parser error is valid:

{"id":0,"jsonrpc":"2.0","result":{"capabilities":{"resources":{"listChanged":false,"subscribe":false},"tools":{"listChanged":false}},"protocolVersion":"2025-11-25","serverInfo":{"name":"MCP Server Adapter","version":"1.0.0"}}}

Steps to reproduce

1. Start Fusion, open a design, and enable the Fusion MCP Server.
2. Connect Claude Desktop using the Autodesk Fusion extension, or with npx -y mcp-remote http://127.0.0.1:<port>/mcp.
3. Claude sends initialize. Fusion replies, but the response fails HTTP parsing and the server shows as disconnected.

Observations

• Reproduces on the default port 27182 and on a custom port (27185) after restarting Fusion. netstat confirms Fusion is the only listener on that port.
• Fails with both the Autodesk Fusion extension and the mcp-remote bridge.
• Fails on single connection attempts, not only when two connections arrive at once.
• A single manual PowerShell initialize request (Invoke-RestMethod) succeeds, and one standalone manual mcp-remote run from Command Prompt connected successfully. Connections launched by Claude Desktop fail every time.
• On the default port, failed connections were left in CLOSE_WAIT on the server side, which suggests sockets aren't being closed cleanly after the bad response.

Expected: well-formed HTTP/1.1 responses so MCP clients can complete initialize and connect.

[yonemoto888]

Update: the suggested --no-node-snapshot workaround does not resolve the issue

Autodesk Support confirmed this is a known bug in the local Fusion MCP Server's chunked response framing and suggested setting NODE_OPTIONS="--no-node-snapshot" for Claude Desktop as a temporary workaround.

I tested it, and it does not help. With the option added to the mcp-remote entry in claude_desktop_config.json and Claude Desktop fully restarted, every connection still fails on the initialize response with the same error:

HTTPParserError: Response does not match the HTTP/1.1 protocol (Invalid character in chunk size)
data: '{"id":0,"jsonrpc":"2.0","result":{...,"protocolVersion":"2025-11-25","serverInfo":{"name":"MCP Server Adapter","version":"1.0.0"}}}'

--no-node-snapshot only affects Node's startup snapshot, not HTTP parsing, and Node 24 has no legacy HTTP parser to fall back to. So the fix needs to come from Fusion's side.

Environment: Fusion 2703.1.20 x86_64, Windows 11, Claude Desktop 2.1.6120.0, Node.js v24.19.0, mcp-remote (latest), MCP server on port 27185 (also reproduced on 27182).

If anyone has found a working client-side workaround in the meantime, I'd be glad to hear it. Otherwise, I'm subscribed here and will test as soon as a fixed build is available.