tengu_mcp_legacy_sse_fallback
A Claude Code feature flag, read out of the shipped bundle. It has been in the build since v2.1.265, including the newest one read.
MCP servers using newer HTTP now automatically retry with an older HTTP+SSE connection method if the newer one fails
Not Anthropic's. This is the line from the earliest changelog entry here that named it, v2.1.265.
In the changelogs
11Releases whose published page names tengu_mcp_legacy_sse_fallback.
-
v2.1.317
Some web-based MCP servers no longer get disconnected after three fatal errors
Behind a switch that defaults to off, some HTTP MCP servers no longer count fatal connection errors toward the three-error disconnect
gate cited by this entry
-
v2.1.307
MCP connection timeouts now measure time with a clock that tracks awake time
Timeouts when connecting to MCP servers now use a clock that keeps track of time the computer spends awake, not plain wall-clock time
gate cited by this entry
-
v2.1.292
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
-
v2.1.290
Stricter failure handling when MCP servers reconnect
MCP reconnects now restore earlier state if the older HTTP+SSE fallback fails, and a refused sign-in step-up fails with
adopt_refusedgate cited by this entry
-
v2.1.288
MCP connections survive oversized responses and report errors after sign-in prompts
An MCP server sending a too-large response no longer drops the connection, and malformed replies after a sign-in prompt get their own reason
gate cited by this entry
-
v2.1.288
An oversized reply from an MCP server no longer drops the connection
For MCP servers reached over HTTP, an oversized reply now fails only that request and the connection to the server stays open
gate cited by this entry
-
v2.1.286
Extra reconnect attempt for older Java MCP servers that reject elicitation
If an older Java MCP server rejects elicitation again after reconnecting, Claude Code now tries once more in legacy mode
gate cited by this entry
-
v2.1.285
MCP tool calls made from task code can now connect to a server
The link that lets task code call MCP tools can now also connect to a named MCP server, and one callback's errors are logged and ignored
gate cited by this entry
-
v2.1.274
MCP connect-timeout retry and legacy SSE fallback gates referenced
MCP connection code references two settings controlling connect-timeout retries and legacy SSE fallback
named in this entry, gate cited by this entry, found in this entry's text
-
v2.1.265
MCP connections gain legacy HTTP+SSE fallback on initialize failure
MCP servers using newer HTTP now automatically retry with an older HTTP+SSE connection method if the newer one fails
gate cited by this entry
-
v2.1.265
Legacy HTTP+SSE fallback for MCP servers rewritten with a dedicated gate
MCP servers can fall back to a legacy HTTP+SSE connection method if the normal connection fails
gate cited by this entry, found in this entry's text
Gate readings
9| Release | Resolved | This account | Anonymous | Compiled default |
|---|---|---|---|---|
| v2.1.317 read on another version | Not enough to say | no value returned | no value returned | on |
| v2.1.307 read on another version | Not enough to say | no value returned | no value returned | on |
| v2.1.292 read on another version | Not enough to say | no value returned | no value returned | on |
| v2.1.290 read on another version | Not enough to say | no value returned | no value returned | on |
| v2.1.288 read on another version | Not enough to say | no value returned | no value returned | on |
| v2.1.286 read on another version | Not enough to say | no value returned | no value returned | on |
| v2.1.285 read on another version | Not enough to say | no value returned | no value returned | on |
| v2.1.274 read on another version | Not enough to say | no value returned | no value returned | on |
| v2.1.265 | Not enough to say | no value returned | no value returned | 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.
First cited
2The earliest release whose published evidence quoted tengu_mcp_legacy_sse_fallback was v2.1.265, 8 Sep 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.
Presence across releases
1Build by build
53One 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.