You'll noticeTier: how much it should matter to you
1Useful: my rating, 1 to 5
1Signal: worth watching, 1 to 5
MCPArea: what it touches
ImprovementsKind: in v2.1.307,
ImprovementsSection of the release
What
MCP servers are add-on programs that provide tools. When the connection to one closes or its session expires, the error used to always read: 'MCP session expired for ... send mcp_reconnect and retry'. Error texts are now built by a shared helper and depend on what happened:
Connection closed during the call: 'connection closed before the tool call was answered. The tool may have run: check whether it did before running it again.'
Connection closed before the call ran, or the session expired: Claude is told to send mcp_reconnect and retry.
The McpSessionExpiredError now records what kind of expiry it was, with new kinds connection_closed_call_refused and connection_closed_call_not_run alongside the older session_expired_404 and connection_closed_http. Whether each call was sent or refused is tracked per call, and mcp_call errors use the same helper.
Why
Claude and you can tell whether retrying a tool is safe, which helps avoid running something with side effects twice.
