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
3Releases whose published page names tengu_mcp_legacy_sse_fallback.
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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
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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
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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
2| Release | Resolved | This account | Anonymous | Compiled default |
|---|---|---|---|---|
| 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
15One 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.