tengu_mcp_protocol_negotiation_stdio

A Claude Code feature flag, read out of the shipped bundle. It has been in the build since v2.1.221, including the newest one read.

Releases whose published page names tengu_mcp_protocol_negotiation_stdio.

1. v2.1.292 6 Oct 2026
   Local MCP servers get one retry with the newer handshake when the old one fails

   If a stdio MCP server remembered as old-style fails to start, Claude Code forgets that and restarts it once with the newer handshake

   gate cited by this entry

2. v2.1.292 6 Oct 2026
   Local MCP servers now use protocol negotiation by default

   Stdio MCP servers now use automatic protocol negotiation unless a server-side setting explicitly turns it off

   gate cited by this entry

3. v2.1.292 6 Oct 2026
   Slow-starting local MCP servers are no longer remembered as old-protocol

   A stdio MCP server that answers the protocol check late is no longer remembered as speaking the older protocol

   gate cited by this entry

4. v2.1.292 6 Oct 2026
   Slow-starting local MCP servers are no longer wrongly remembered as older servers

   A local MCP server that answered the protocol check late is no longer remembered as an older server after a connection timeout

   gate cited by this entry

5. v2.1.292 6 Oct 2026
   Slow resource listings no longer hold up MCP tools on HTTP and SSE servers

   After connecting to an HTTP or SSE MCP server, Claude Code can now register its tools without waiting for its resource listing to finish

   gate cited by this entry

6. v2.1.292 6 Oct 2026
   Protocol negotiation for local MCP servers now defaults on, with a safer memory file

   Negotiating the protocol version with local stdio MCP servers now falls back to on, and a broken record of past probes is ignored rather than trusted

   gate cited by this entry

7. v2.1.292 6 Oct 2026
   Stdio MCP servers get a stable identity, and protocol negotiation defaults to on

   Local stdio MCP servers now get an identity built from their command, and the stdio protocol-negotiation switch now defaults to on

   gate cited by this entry

8. v2.1.274 16 Sep 2026
   New stdio MCP protocol-negotiation gate added

   A new gate lets stdio MCP connections also try the newer protocol-negotiation handshake

   named in this entry, gate cited by this entry

9. v2.1.221 3 Aug 2026
   MCP 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, found in this entry's text

10. v2.1.221 3 Aug 2026
    New 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, found in this entry's text

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.

One row per unbroken stretch of one reading, newest first, from snapshots taken every twenty-five minutes or so. No longer carried is not off: it means the flag server stopped returning the gate at all, which is what a retired experiment looks like. The version column is what was installed when the reading was taken, not what caused it.

The earliest release whose published evidence quoted tengu_mcp_protocol_negotiation_stdio was v2.1.221, 3 Aug 2026. That is the oldest release this project wrote about it, so it is a floor on the name's age and not the release that introduced it. It is not a presence reading: which builds carried the name is the table below.

One row per release since the first build this name was read out of. Absent means the build was read and the name was not in it, never mined means no bundle for that release was ever archived, and a declared type or a default is only ever what that release's own bundle stated.

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.