MCP: "Failed to parse message: ''" warning from empty SSE priming frames

[lqdflying]

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Does this issue occur when all extensions are disabled?: Yes

• VS Code Version: 1.99.0 (or check Help → About)
• OS Version: Windows / macOS / Linux

Steps to Reproduce:

1. Set up a FastMCP server with streamable-http transport in stateful mode (e.g. mcp.http_app(event_store=..., stateless_http=False))
2. Add the server to VS Code as an MCP server in settings.json
3. Open VS Code Output panel → MCP logs

Expected: No warnings during SSE stream initialization

Actual: Repeated warnings logged:
Failed to parse message: ""

Root Cause:
The MCP Python SDK sends an empty SSE priming frame (data: \n\n) when a client opens a GET /mcp SSE stream. VS Code's SSE message parser attempts to JSON-parse every frame including empty ones, producing this warning.

Suggested Fix:
In the SSE message handler, skip empty frames before attempting JSON parsing:
if (!data || data.trim() === '') return;

Additional Context:

• MCP server: FastMCP 3.1.x, streamable-http transport, stateful mode
• MCP Python SDK: 1.x
• Despite the warnings, the server functions correctly — 14 tools discovered, OAuth works normally
• The warning does not appear in stateless mode (stateless_http=True) since GET /mcp returns 405 and no SSE stream is opened

[vs-code-engineering]

Thanks for creating this issue! It looks like you may be using an old version of VS Code; the latest stable release is 1.112.0. Please try upgrading to the latest version and checking whether this issue remains.

Happy Coding!