{"version":"2.1.274","anchor":"mcp-client-sdk-runtime-now-defaults-to-v2-not-v1","canonical_anchor":"mcp-sdk-generation-default-flips-to-v2-via-tengu-brindle-cau","heading":"MCP client SDK now defaults to v2","tier":"notice","area":"MCP","scope":null,"heads_up":null,"url":"https:\/\/changelogs.core-directive.com\/v\/2.1.274\/e\/mcp-client-sdk-runtime-now-defaults-to-v2-not-v1","release_url":"https:\/\/changelogs.core-directive.com\/v\/2.1.274","markdown":"### MCP client SDK now defaults to v2\n\nClaude Code's MCP client runtime now defaults to SDK generation v2 instead of v1, controlled by the tengu_brindle_causeway gate.\n\n**Unclear.** No reading of `tengu_brindle_causeway` has been taken under this release yet, so whether it is switched on for any account in this version isn't known; a prior reading (site account and anonymous baseline) showed it on, but that reading predates this release.\n\n**What**\n\n- The function that picks which MCP client SDK generation ('v1' or 'v2') a session uses now defaults to `'v2'` instead of `'v1'` when the `tengu_brindle_causeway` gate is at its default value (the gate's default flipped from off to on this release).\n\n- The `MCP_SDK_GENERATION` environment variable still overrides this in either direction.\n\n**Why** Sessions that don't explicitly set `MCP_SDK_GENERATION` will now use the newer v2 MCP client runtime by default instead of v1.\n\n- Flag `tengu_brindle_causeway`: On for this account, and not off by default (read for one account on one subscription tier against v2.1.274; this account: on, anonymous baseline: on, compiled default: 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.\n- Area: MCP\n- Names: `MCP_SDK_GENERATION`\n- Tier: You'll notice\n- Useful: 3\/5\n- Signal: 2\/5"}