Cline MCP: "Failed to open SSE stream: Not Found" after VS Code update

Question

After updating VS Code, Cline 3.38.1 on Windows 11 fails to connect to a local MCP server with "Streamable HTTP error: Failed to open SSE stream: Not Found". The server is configured as type "streamableHttp" at http://localhost:3000/mcp. What causes this error and how do I fix it?

Originally asked on GitHub: https://github.com/cline/cline/issues/7577

AgentArmorHQ team · Accepted Answer

This is a known issue caused by the MCP SDK upgrade from 1.22.0 → 1.25.1 in Cline v3.46.0, which made SSE stream initialization stricter. The SDK now sends a GET request to your MCP endpoint and expects either an SSE stream or HTTP 405 Method Not Allowed; many servers respond with 404 Not Found, which the SDK treats as a hard failure. Cline opened PR #8321 to normalize 404 → 405 client-side, and the issue is closed as completed.

If you control the MCP server, the spec-compliant fix confirmed in the thread is to return HTTP 405 for GET requests on the `/mcp` endpoint when SSE streaming isn't supported — one user verified this resolves the error.

If you don't control the server, switching Cline's config to the legacy SSE transport worked for another user (if your server supports it):

```json
{
  "cnc": {
    "type": "sse",
    "url": "http://localhost:3000/mcp",
    "disabled": false,
    "timeout": 60
  }
}
```

Note that errors appearing only after ~an hour of use (e.g. `SSE stream disconnected: terminated` or `Body Timeout Error`) may be a separate session/reconnection timeout issue, not the 404 problem; the maintainer suggested filing a new issue with those details.

AgentArmorHQ team · Answer

**What @bmingles tried on GitHub** (Worked)

Modified the MCP server to return HTTP 405 for GET requests on the /mcp endpoint (instead of 404), matching what the MCP SDK expects when SSE streaming isn't offered. User confirmed this fixed the issue.

Source: https://github.com/cline/cline/issues/7577#issuecomment-3683137680

AgentArmorHQ team · Answer

**What @bmingles tried on GitHub** (Worked)

Changed the MCP server to support the legacy SSE protocol and used it with Cline. User reported getting things working this way, though they noted it shouldn't be required.

Source: https://github.com/cline/cline/issues/7577#issuecomment-3676543371

AgentArmorHQ team · Answer

**What a GitHub user tried on GitHub** (Didn't work)

After updating VS Code, ran Cline 3.38.1 on Windows 11 with a streamableHttp MCP config pointing at http://localhost:3000/mcp; the 'Failed to open SSE stream: Not Found' error reproduced.

Source: https://github.com/cline/cline/issues/7577

AgentArmorHQ team · Answer

**What @schaveyt tried on GitHub** (Outcome unclear)

With a Microsoft C# MCP SDK v0.4.0-preview server, tried both 'streamableHttp' and 'sse' transport types in Cline. Initial connection succeeded, but after a while both failed with reconnect errors ('SSE stream disconnected: terminated', 'Bad Request', 'Body Timeout').

Source: https://github.com/cline/cline/issues/7577#issuecomment-3688182547

Cline MCP: "Failed to open SSE stream: Not Found" after VS Code update

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