{"version":"2.1.218","anchor":"mcp-sdk-version-selection","canonical_anchor":"mcp-sdk-version-selection","heading":"MCP SDK Version Selection","tier":null,"area":null,"scope":null,"heads_up":null,"url":"https:\/\/changelogs.core-directive.com\/v\/2.1.218\/e\/mcp-sdk-version-selection","release_url":"https:\/\/changelogs.core-directive.com\/v\/2.1.218","markdown":"### MCP SDK Version Selection\n\n**What**\n\nThe MCP client SDK can now be switched between v1 and v2 via an environment variable or feature flag.\n\n**Usage**\n\n`MCP_SDK_GENERATION=v2 claude ... # force SDK v2 MCP_SDK_GENERATION=v1 claude ... # force SDK v1 (default)` **Details** - Default is v1 unless the `tengu_brindle_causeway` feature flag is active - Invalid values are ignored with a warning: `\"MCP_SDK_GENERATION=X is invalid; expected 'v1' or 'v2' \u2014 ignoring\"` - A runtime tripwire catches mismatches between the selected generation and the loaded module - The selection source (env\/growthbook\/default) is logged and reported via telemetry **Evidence** New env var and generation selector (search for `\"MCP_SDK_GENERATION\"`)\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."}