Claude Code's MCP client runtime now defaults to SDK generation v2 instead of v1, controlled by the tengu_brindle_causeway gate.
What
- The function that picks which MCP client SDK generation ('v1' or 'v2') a session uses now defaults to
'v2'instead of'v1'when thetengu_brindle_causewaygate is at its default value (the gate's default flipped from off to on this release). - The
MCP_SDK_GENERATIONenvironment variable still overrides this in either direction.
Why
Sessions that don't explicitly set MCP_SDK_GENERATION will now use the newer v2 MCP client runtime by default instead of v1.
tengu_brindle_causeway On for this account, and not off by defaultThe flag server returned on for the one account this site reads, and nothing in this release compiles it off by default. The compiled default is shown below, and says which it is when we cannot read one: a fifth of gates compile in a string or a number rather than on or off, and most published releases have no gate table behind them at all. No client can see what the server returns for your account.
This account: on · anonymous baseline: on · compiled default in v2.1.274: on
These values were read against a different version of Claude Code, so treat them as the nearest reading available instead of one taken on this release.
Read once, for one account on one subscription tier, against v2.1.274. It isn't a statement about your account. What a flag value here can and cannot tell you
Changed Bedrock, Vertex, Foundry and telemetry-disabled installs to use the v2 MCP client and MCP 2026-07-28 negotiation with direct HTTP…