CLAUDE_CODE_EMIT_STARTUP_TIMING
A Claude Code environment variable, read out of the shipped bundle. It has been in the build since v2.1.274, including the newest one read.
A new CLAUDE_CODE_EMIT_STARTUP_TIMING environment variable turns on startup-timing telemetry outside remote sessions
Not Anthropic's. This is the line from the earliest changelog entry here that named it, v2.1.274.
In the changelogs
7Releases whose published page names CLAUDE_CODE_EMIT_STARTUP_TIMING.
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v2.1.319
Startup timing now records how long hook setup takes for each origin
With CLAUDE_CODE_REMOTE or CLAUDE_CODE_EMIT_STARTUP_TIMING set, startup timing now notes the longest hook setup time for each origin
found in this entry's text
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v2.1.315
Startup timing breakdown now names four more phases
Claude Code's breakdown of time spent before a turn's first request now names four more phases that used to be counted as other
found in this entry's text
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v2.1.295
Startup timing data can leave out the process start time
Startup timing data now omits the process's absolute start time under one condition
found in this entry's text, named in this entry
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v2.1.288
Turn timing data adds three more phases
Timing data for the start of a turn now includes message_prep, model_config_wait and auto_mode_context_wait
found in this entry's text, named in this entry
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v2.1.287
Timing reports now split the resume step at the start of a turn into smaller parts
Turn timing data now breaks the resume step into several parts and records what kind of resume it was
found in this entry's text
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v2.1.285
SDK result messages can now report a breakdown of time before the first model request
Successful SDK results gain timing fields that break down the delay before the first model request, under the same conditions as startup timing
found in this entry's text
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v2.1.274
New environment variable CLAUDE_CODE_EMIT_STARTUP_TIMING
A new CLAUDE_CODE_EMIT_STARTUP_TIMING environment variable turns on startup-timing telemetry outside remote sessions
named in this entry, found in this entry's text
First cited
1The earliest release whose published evidence quoted CLAUDE_CODE_EMIT_STARTUP_TIMING was v2.1.274, 16 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
42One 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 environment variable does. All environment variables.