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Claude Code v2.1.274 ·

MCP client SDK now defaults to v2

Claude Code's MCP client runtime now defaults to SDK generation v2 instead of v1, controlled by the tengu_brindle_causeway gate.

Group of 2 You'll notice Notable Improvements
JSON All of v2.1.274
You'll noticeTier: how much it should matter to you
3Useful: my rating, 1 to 5
2Signal: worth watching, 1 to 5
MCPArea: what it touches
ImprovementsKind: in v2.1.274,
What probably matters to youSection of the release

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.

What

  • 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).
  • The MCP_SDK_GENERATION environment 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.

Read from
Names in the bundleMCP_SDK_GENERATION
Feature flag
tengu_brindle_causeway On for this account, and not off by default

The 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

Since it was published

The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.

Confirmed since Anthropic's documentation has since written up MCP_SDK_GENERATION, on Error reference. * To connect without changing the server, start Claude Code with [`MCP_SDK_GENERATION=v1`](/docs/en/env-vars), whose [runtime](/docs/en/mcp#mcp-client-runtimes) doesn't apply this rule and sends the token request over plain HTTP. That choi… errors see the edit
What has happened since
Flag reading moved The flag server now returns on for tengu_brindle_causeway, read as this account. A reading is one sample. Claude Code evaluates its flags remotely, so no client sees the targeting rule behind a value and this says nothing about your account.
Flag reading moved The flag server now returns off for tengu_brindle_causeway, read as this account. A reading is one sample. Claude Code evaluates its flags remotely, so no client sees the targeting rule behind a value and this says nothing about your account.

See this across every release →

How sure we are
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubtNo reading of `tengu_brindle_causeway` has been taken under this release yet, so whether it is switched on for any account in this version…
Anthropic's documentation agreesAnthropic's documentation has since written up MCP_SDK_GENERATION, on Error reference.
Anthropic's release notes agreeChanged Bedrock, Vertex, Foundry and telemetry-disabled installs to use the v2 MCP client and MCP 2026-07-28 negotiation with direct HTTP…

See this entry in the whole of v2.1.274 →

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