Remote (SSE/OAuth) source type POSTs directly to configured URL instead of implementing two-step SSE handshake

[penradioGables]

Description

The Remote (SSE/OAuth) source type does not implement the legacy two-step SSE handshake used by older MCP servers. Instead it POSTs directly to the configured URL, which produces a 405 Method Not Allowed response from any server that follows the original SSE transport spec.

Expected behavior

For servers using the old-spec SSE transport, the client should:

1. GET /sse — server returns a session-scoped messages endpoint URL in the stream
2. POST to that session endpoint for subsequent messages

Actual behavior

Arc Relay POSTs directly to the configured root URL. Against an old-spec SSE server this yields 405, and following the redirect/error chain then yields 404.

Steps to reproduce

1. Add a Remote (SSE/OAuth) server pointed at an MCP server that uses the legacy two-step SSE handshake (e.g. seynurmammad0v/obsidian-mcp)
2. Attempt any tool call
3. Observe 405 on the configured URL, then 404 chasing the session endpoint

Notes

This is a real gap in remote SSE support — the source type name implies SSE compatibility, but the implementation does not match the handshake. Streamable HTTP (the newer transport) does not have this issue.

Arc Relay version: v0.1.1