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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.

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Claude Code v2.1.221

MCP clients record a negotiated protocol era

Under the hood
Useful2 Signal0
MCP

Claude now negotiates a protocol era with MCP servers and only uses the experimental permission channel for older ones.

What

Connected MCP client records now carry a protocol era and the negotiated protocol version, derived from a server/discover-based negotiation, and that era decides which permission channel is used.

Details
  • New fields protocolEra and negotiatedProtocolVersion on connected client records.
  • Connection telemetry emits both alongside negotiationMode.
  • The experimental claude/channel/permission path is now used only for servers whose era is not modern (r.protocolEra !== "modern").
  • Tool-schema compatibility checks take the era as an argument.
Evidence

protocolEra

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

See this entry in the whole of v2.1.221 →