`to_mcp_server` keeps one ADK session per MCP connection, so successive tool
calls on that connection form a single conversation. It keyed that map on
`ctx.session`.
That key is correct on MCP SDK 1.x, where the server builds one session object
per connection. It is wrong on 2.x: the server builds a fresh `ServerSession`
for every inbound request and holds the connection on the session's private
`_connection`. The key would change on every call, so every tool call would
start a new conversation. Nothing raises. The agent just forgets.
Route the key through `_connection_key`, which reads `_connection` when the SDK
provides it and falls back to the session when it does not. That gives one key
per connection on both versions, and leaves 1.x behaviour unchanged.
The fallback degrades to one session per request on purpose. It must not fall
back to an object shared by all connections, because separate clients would
then share one conversation.
The private attribute is a stopgap. SDK 2.x already defines a public
`mcp.server.context.Context.connection`, but the server does not hand that
class to tool functions yet, so a tool's `Context` has no public route to its
connection.
Co-authored-by: Kathy Wu <wukathy@google.com>
PiperOrigin-RevId: 967462559
Kathy Wu
4e68bad199