{"version":"2.1.218","anchor":"mcp-sdk-v2-feature-flagged","canonical_anchor":"mcp-sdk-v2-feature-flagged","heading":"MCP SDK v2 [Feature-Flagged]","tier":null,"area":null,"scope":null,"heads_up":null,"url":"https:\/\/changelogs.core-directive.com\/v\/2.1.218\/e\/mcp-sdk-v2-feature-flagged","release_url":"https:\/\/changelogs.core-directive.com\/v\/2.1.218","markdown":"### MCP SDK v2 [Feature-Flagged]\n\n**Status**\n\nServer-controlled rollout via `tengu_brindle_causeway` feature flag (default: disabled). Users can force v2 manually with `MCP_SDK_GENERATION=v2`.\n\n**What**\n\nA second generation of the MCP client SDK (`v2`) is fully implemented and selectable. V2 brings stateless claude.ai-proxy MCP connections with cached initialization responses to avoid a full round-trip on reconnect (`Yqu`\/`Xqu`), plus module-level separation of transport concerns.\n\n**Details**\n\n- Both v1 and v2 are fully functional; v2 is currently off by default\n- The generation is selected at arm time and locked for the session\n- A runtime tripwire (`MCP_TREE_ID` sentinel) confirms the correct module loaded\n\n**Evidence**\n\nGeneration selector and tripwire (search for `\"MCP runtime accessor tripwire\"`)\n\n- Flag `tengu_brindle_causeway`: Off by default, switched on for this account (read for one account on one subscription tier against v2.1.218; this account: on, anonymous baseline: on, compiled default: off) 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."}