tengu_foamy_spring
A Claude Code feature flag, read out of the shipped bundle. It was in the build from v2.1.251 and was gone by v2.1.309.
A new gate is on by default and stays on even if its reader fails.
Not Anthropic's. This is the line from the earliest changelog entry here that named it, v2.1.248.
In the changelogs
7Releases whose published page names tengu_foamy_spring.
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v2.1.309
A widely used server switch now counts as on even when it cannot be read
A server switch checked in many places now counts as on when no value can be read at all, not only when the server sends nothing
gate cited by this entry, found in this entry's text
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v2.1.281
On resume, split assistant responses are regrouped, behind tengu_foamy_spring (defaults on)
When a conversation is resumed, messages that were split away from the response they belong to are moved back next to it
found in this entry's text
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v2.1.281
Reminder-fold decision (tengu_chair_sermon) is now fixed per session and passed to subagents
Whether system reminders are folded is now fixed once per session and handed down to subagents
found in this entry's text
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v2.1.281
Reminder-fold, tool-input-copy and context-announcement telemetry
New usage reporting covers reminder folding, copied tool inputs and the estimated size of each attachment
found in this entry's text
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v2.1.259
Declared-tools filtering for always-loaded claude.ai MCP tools
Always-loaded claude.ai MCP tools are held out of the declared tool set and loaded lazily until the tool-use decision settles.
gate cited by this entry
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v2.1.251
First-party behavior gated by CLAUDE_CODE_DAPPER_LAGOON
An environment variable overrides whether something counts as first-party.
gate cited by this entry, found in this entry's text
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v2.1.248
New gate
tengu_foamy_spring, on by default and fail-openA new gate is on by default and stays on even if its reader fails.
gate cited by this entry, found in this entry's text
Gate readings
1| Release | Resolved | This account | Anonymous | Compiled default |
|---|---|---|---|---|
| v2.1.309 read on another version | Not enough to say | no value returned | no value returned | not a boolean we can read |
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_foamy_spring was v2.1.248, 27 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.
Presence across releases
4Build by build
50One 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.