CLAUDE_CODE_AGENT_LIST_IN_MESSAGES
A Claude Code environment variable, read out of the shipped bundle. It was in the build from v2.1.138 and was gone by v2.1.178.
Where the Agent tool's list of agent types goes. A true value (1, true, yes, on) moves the list out of the tool description into agent_listing_delta attachments in the conversation, and the description says the types are listed in <system-reminder> messages; a false value (0, false, no, off) keeps the list in the description. Unset or any other value defers to the tengu_agent_list_attach flag, and a truthy CLAUDE_CODE_DISABLE_ATTACHMENTS or CLAUDE_CODE_SIMPLE keeps the list in the description whatever this is set to.
Not Anthropic's. Written by Claude Opus 5.5 from reading the v2.1.177 bundle, and it can be wrong.
In the changelogs
1Releases whose published page names CLAUDE_CODE_AGENT_LIST_IN_MESSAGES.
-
v2.1.84
Agent List in System Messages [Gradual Rollout]
found in this entry's text
First cited
1The earliest release whose published evidence quoted CLAUDE_CODE_AGENT_LIST_IN_MESSAGES was v2.1.84, 25 Mar 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
2Build by build
150One 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.