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Sweep 28 Aug 2026 · 00:00Z Build v2.1.250 478 read Stable v2.1.236 Latest v2.1.250 Next v2.1.250 Feeds RSS JSON llms.txt

DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

Reference / Feature flags

tengu_mcp_protocol_negotiation_stdio

A Claude Code feature flag, read out of the shipped bundle. It was in the build from v2.1.221 and was gone by v2.1.238.

What it is

A newer MCP protocol handshake exists per transport but every gate falls back to legacy here.

Not Anthropic's. This is the line from the earliest changelog entry here that named it, v2.1.221.

Presence across releases

ReleasesReadingDeclared typeDefault
2.1.245 – 2.1.250 5 releases never mined — —
2.1.238 – 2.1.243 6 releases not in the build — —
2.1.221 – 2.1.237 16 releases in the build — bool:false

A row reading never mined is a release with no archived bundle behind it. Nothing was read out of it, so nothing here says whether the name was in it. A declared type or a default is only ever what that release's own bundle stated.

Gate readings

ReleaseResolvedThis accountAnonymousCompiled default
v2.1.221 read on another version On for this account, and not off by default on on on

Two arms, because they disagree: this account is the one account this site reads and anonymous is a baseline with no identity attached. A reading says nothing about your account, and off cannot rule out a rollout both readings sit outside of. What these readings can and cannot show.

Changelog entries naming it

  1. v2.1.221MCP protocol-era negotiation (2026-07-28 "modern" revision) added behind per-transport gates

    A newer MCP protocol handshake exists per transport but every gate falls back to legacy here.

    gate cited by this entry

  2. v2.1.221New MCP protocol-era negotiation with probe and legacy fallback

    In auto mode MCP connections would probe for a modern protocol and fall back to legacy, but nothing enables it.

    gate cited by this entry

Documentation pages

None of Anthropic's documentation pages this site has a copy of carries this name.

Names beside it

Every feature flag in the inventory starting tengu_mcp_protocol_negotiation, up to twelve of them.

tengu_mcp_protocol_negotiation_MCP servers using a newer protocol era no longer get custom notification handlers, and you see a warning. never mined tengu_mcp_protocol_negotiation_ccrThe ccr-proxy MCP transport can negotiate protocol versions, but stays on legacy unless a flag is turned on. 2.1.236 tengu_mcp_protocol_negotiation_claudeaiA newer MCP protocol handshake exists per transport but every gate falls back to legacy here. 2.1.221 tengu_mcp_protocol_negotiation_httpA newer MCP protocol handshake exists per transport but every gate falls back to legacy here. 2.1.221

Read out of the published npm bundle release by release, and out of Anthropic's own documentation as this site captured it. Nothing on this page is a description anybody here wrote about what the feature flag does. All feature flags.