The previous _current_session_id() implementation tried to import a
non-existent get_context() helper, so the production code path always
fell through to the empty _TEST_SESSION_ID ContextVar — meaning every
MCP client shared the empty-string identity and there was no per-session
isolation in production.
Use Context.session (the long-lived ServerSession object) as the source
of identity. id(ctx.session) is stable across every tool call the same
client makes within a Streamable HTTP session, which is exactly what the
busy tracker needs to renew leases.
Wire FastMCP to inject the Context into the wrapper by setting
tool.context_kwarg = "ctx" after swapping tool.fn; wrap the swap in a
defensive try/except that surfaces a FastMcpSdkIncompatibilityError on
future SDK layout drift.
Add 4 tests covering the production path: stability across calls in the
same session, isolation between sessions, fallback to _TEST_SESSION_ID
when no Context is supplied, and verification that the registered tool
declares context_kwarg="ctx".
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
q792602257andClaude Opus 4.6
98089b6748