What
MCP (Model Context Protocol) servers connect Claude Code to outside tools and data. This change affects MCP servers reached over the http and claudeai-proxy transports. A transport is the way messages travel between Claude Code and the server.
- An oversized response now raises a new kind of error and logs a message saying the server sent a response larger than a set number of MB. Claude Code stops reading it and fails the request it answered. The connection stays open.
- If a malformed message arrives while an authentication challenge is open, the transport closes with the reason 'malformed message after an auth challenge'. An authentication challenge is a request from the server to log in. Before, every malformed response closed with 'malformed JSON-RPC message (response truncated)'.
- The OAuth login provider now tracks whether an authentication challenge is open (
authChallengeOpen). - The probe timeout is now passed through when connecting.
Why
One very large reply from an MCP server no longer cuts off the whole server, so its other tools keep working. When a connection breaks during a login step, the close reason now says so, which makes the problem easier to track down.
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 v2.1.288: 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 v2.1.288. It isn't a statement about your account. What a flag value here can and cannot tell you
It is not clear whether the legacy SSE fallback switch plays any part in these changes.