Hey @Andrew_Pashynnyk, thanks for the report and the reproduction.
We were able to confirm what you described: when Grok Bot reuses an existing MCP session, it doesn’t restore the negotiated protocol version, so those requests go out without the MCP-Protocol-Version header. I’ve flagged this to the team along with your suggested fix.
In the meantime, updating T3 Code to the build that includes the Effect 4.0.2 compatibility fix should get tool calls working again, since that server then accepts requests without the header. Reconnecting or re-authorizing in Grok Bot won’t help on its own, because a fresh connection gets reused the same way.