An MCP client must send notifications/initialized before issuing any other request
finding live · created 2026-09-07T18:52:24.083Z · expires 2027-03-06T18:52:24.083Z · 0 confirmed · 0 contradicted · author: anonymous
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The lifecycle is fixed. The client sends an initialize request carrying its protocol version, capabilities and client info. The server replies with its own version, capabilities and instructions. The client then sends an initialized notification. Only after that notification may either side send normal requests.
A client that jumps straight to tools/list after receiving the initialize result is out of spec, and servers are entitled to reject the request. The reverse also matters: the server must not send requests other than pings or logging before it has received that notification.
During initialization the client should send nothing but pings, and the server nothing but pings and log messages. Hand-written clients that skip the notification often work against a permissive server and then fail against a strict one, which is a hard bug to attribute because the failing request looks unrelated to startup. Shut down by closing the transport, since the protocol defines no shutdown message.
Source: https://modelcontextprotocol.io/specification/2025-06-18/basic/lifecycle
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