Streamable-HTTP MCP connector: 405 on SSE GET surfaces fatal "Client server capabilities not available" toast

What happened

Claude Desktop repeatedly shows a toast "Client server capabilities not available" when a remote Streamable-HTTP MCP connector is enabled (in my case an Atlassian remote connector, but the failure is in the client transport layer, not the server).

Enabling DevTools (developer_settings.json → {"allowDevTools": true}) and watching the renderer console shows the toast corresponds 1:1 with this repeating error:

GET https://claude.ai/v1/toolbox/shttp/mcp/<connector-id>  405 (Method Not Allowed)
    at s
    at _startOrAuthSse
    at send
    at notification
    at connect
    at refreshClient
    at useClient
    at listTools

So the MCP client, while (re)connecting the connector, calls _startOrAuthSse to open the optional GET / SSE notification stream against the streamable-HTTP endpoint. The claude.ai proxy route for that connector answers 405 Method Not Allowed (it accepts POST only). The client treats that 405 as a failed connect → capabilities never load → the toast fires.

Per the MCP spec, a 405 on the standalone GET (SSE) stream is explicitly allowed and the client is supposed to simply not use that stream and continue with POST. Here it is being treated as a fatal connection failure instead.

Triggered from AutoSyncConnectorsToSession / refreshClient (the app re-syncing remote connectors to a session), and it recurs continuously.

Impact

Mostly cosmetic but persistent and noisy. The actual tool calls (POSTs) do succeed — e.g. the same connector's tools work when invoked, and a parallel probe of the same connector from the Claude Code CLI completed normally in ≤17s. Only the capability-refresh path fails, so the user gets a stream of scary "capabilities not available" toasts even though tools are working. It also appears to correlate with occasional multi-minute stalls on the first calls after a (re)connect.

Notable

• No 401 / auth / token error anywhere in the console — it is not OAuth/credential related.
• Only affects the remote Streamable-HTTP connector; local mcp-remote stdio bridges on the same machine are unaffected.

Steps to reproduce

1. Enable a remote Streamable-HTTP MCP connector routed through claude.ai/v1/toolbox/shttp/mcp/<id> whose endpoint returns 405 on a GET (SSE stream open).
2. Use it in a session so the app runs AutoSyncConnectorsToSession / refreshClient.
3. Observe the recurring "Client server capabilities not available" toast; DevTools console shows the 405 (Method Not Allowed) on _startOrAuthSse → connect.

Tried / did not help

• Disconnect + reconnect the connector.
• Full machine reboot.

Both leave the toast recurring.

Expected

Client should tolerate a 405 on the optional SSE GET stream (per MCP spec), fall back to POST-only operation silently, and not surface a fatal "capabilities not available" toast when tool calls are actually succeeding.

Environment

• Claude Desktop 1.21459.3.0 (x64, Windows)
• Embedded Claude Code 2.1.209
• Windows 11 Pro (26200)

(Connector ID, cloud ID, and internal item keys redacted — the repro is transport-layer and connector-agnostic.)