Stdio MCP servers (tool servers that run as a local program on your machine) now use automatic protocol negotiation, where Claude Code probes the server to agree on a protocol version. They fall back to the older handshake only when the server-side setting comes back explicitly off. Before, with no value from the server, they used the older handshake. The setting has read on for this site's account and for the anonymous baseline since 2026-09-29.
Other details:
- In automatic mode, stdio servers can skip an earlier check that probed the older listing method
- HTTP servers already used negotiation by default
- claude.ai proxy and remote proxy connections still fall back to the older handshake
- The log line for the older HTTP and server-sent events fallback now says "this server" instead of "this connector"
Local MCP servers may now connect with the newer protocol directly, without going through the older probe first.
tengu_mcp_protocol_negotiation_stdio On for this account, and not off by defaultThe 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 v2.1.292: 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.
tengu_mcp_legacy_sse_fallback Not enough to sayNothing here resolved what this flag was doing on this version, so nothing here should be read as on or off.
This account: no value returned · anonymous baseline: no value returned · compiled default in v2.1.292: 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 v2.1.292. It isn't a statement about your account. What a flag value here can and cannot tell you
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.
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.