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Sweep 22 Oct 2026 · 04:32Z Build v2.1.308 527 read Stable v2.1.308 Latest v2.1.308 Next v2.1.308 Feeds RSS JSON llms.txt llms-full.txt Unofficial
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Claude Code v2.1.218 ·

MCP SDK v2 [Feature-Flagged]

Entry
JSON All of v2.1.218
EntryKind: in v2.1.218,
In DevelopmentSection of the release
Status

Server-controlled rollout via tengu_brindle_causeway feature flag (default: disabled). Users can force v2 manually with MCP_SDK_GENERATION=v2.

What

A 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.

Details
  • Both v1 and v2 are fully functional; v2 is currently off by default
  • The generation is selected at arm time and locked for the session
  • A runtime tripwire (MCP_TREE_ID sentinel) confirms the correct module loaded
Evidence

Generation selector and tripwire (search for "MCP runtime accessor tripwire")

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

Read from
Feature flag
tengu_brindle_causeway Off by default, switched on for this account

The shipped code defaults this off, and the flag server returned on for the one account this site reads on this version. That is the reading that makes the entry above worth a second look, and it still says nothing about your account.

This account: on · anonymous baseline: on · compiled default in v2.1.218: 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.

Read once, for one account on one subscription tier, against v2.1.218. It isn't a statement about your account. What a flag value here can and cannot tell you

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.
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 nothing at all, having stopped carrying it 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
One source agreesOne thing we can check says the same as this entry.
The name it cites is new in this buildNew in this build: tengu_brindle_causeway

See this entry in the whole of v2.1.218 →

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