fix(mcp): the serve path and bind failures were both invisible

[vietanhdev]

Two defects that each make the MCP feature look like it works when it doesn't.

1. Every config the app hands out points at a dead endpoint

The UI's client-config snippets — and all eight examples in docs/guide/mcp.md — pointed at /sse with "type": "sse". That's rmcp 0.1.5's transport. The rmcp 2 migration (#29) mounts Streamable HTTP at /mcp and serves nothing at /sse.

So a user starts the server, copies the Claude Desktop config displayed in the app, and every connection attempt 404s. The feature is unusable via its own documented setup path. The docs were additionally self-contradictory — claude mcp add --transport http ... /sse pairs a Streamable HTTP transport with an SSE path.

The path is now a constant the router is built from and McpStatus reports, so the UI renders whatever is actually being served rather than a second copy free to drift. "type" is corrected to "http", which is what selects Streamable HTTP.

2. A failed bind reports success

tokio::spawn(async move {
    let listener = match TcpListener::bind(addr).await {
        Err(e) => { eprintln!("[MCP] Failed to bind {}: {}", addr, e); return; }
        ...
});
s.cancel_token = Some(ct);          // unconditional
Ok(ApiResponse { success: true, ... })

The bind happened inside the spawned task, so its error had nowhere to go but stderr while the command had already returned success and stored a cancel token.

The port field is user-editable. Enter 80 (EACCES) or a port something else holds: the task exits immediately, nothing is listening, but the UI shows "Running on 127.0.0.1:80" — and because the cancel token was stored, every retry is refused with "MCP server is already running" until the user presses Stop.

Binding before the spawn puts the error back in the caller's hands.

Verification

99 tests pass, clippy clean, frontend lint + format clean.

The guard against re-hardcoding the path is mutation-verified — restoring the nest_service("/mcp", ...) literal fails it:

router_path_is_not_hardcoded_alongside_the_constant ... FAILED
  nest_service should be given MCP_PATH, not a literal

(It's scoped to code above the test module specifically so it can't match its own assertion string.)

Found while auditing the backend for runtime defects.

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Status	Name	Latest Commit	Updated (UTC)
❌ Deployment failed View logs	thinkutils	c18f577	Jul 20 2026, 03:05 AM