tengu_bg_attach_outcome
A Claude Code feature flag, read out of the shipped bundle. No mined inventory carries it: everything below comes from what this site has written about it.
Background attach events now record cold starts, surface and first frame details.
Not Anthropic's. This is the line from the earliest changelog entry here that named it, v2.1.221.
In the changelogs
5Releases whose published page names tengu_bg_attach_outcome.
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v2.1.227
Recovered background attaches report the same way on both code paths
Recovered background attaches report identically whether found by the old or new sweep.
gate cited by this entry, found in this entry's text
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v2.1.221
Background attach telemetry gets cold-start and surface dimensions
Background attach events now record cold starts, surface and first frame details.
gate cited by this entry, found in this entry's text
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v2.1.221
Background-attach outcomes are journaled to disk and recovered later
Background session attaches now write a small journal file so crashed attaches still report what happened later.
gate cited by this entry, found in this entry's text
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v2.1.221
Much richer background-attach timing telemetry
Attaching to a background session now records detailed timing, including a 30-second timeout flag.
gate cited by this entry, found in this entry's text
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v2.1.221
claude attachnow measures time-to-interactiveAttaching to a background session now times how long it takes to paint and reach an idle prompt.
gate cited by this entry, found in this entry's text
Gate readings
0No release here has a gate table behind it for this flag. Most published releases have none at all: the readings exist only for versions the pipeline captured flag state on.
First cited
1The earliest release whose published evidence quoted tengu_bg_attach_outcome was v2.1.227, 10 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
0The miner has never read a build containing this name. It is here because a changelog entry of ours names it, which is evidence that it existed and not evidence of which releases carried it. The inventory starts at v2.1.138 and everything older than that was never mined.
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.