The x-anthropic-billing-header sent with every API request now includes cc_is_subagent=true; when the request originates from a subagent session rather than the top-level interactive session. The main CLI session is tagged isMainSession: true internally; any agent that lacks that flag gets the subagent marker appended.
This is an internal attribution change with no user-visible behavior. It helps Anthropic distinguish API usage generated directly by the user from usage generated by orchestrated subagents, which improves billing analytics and usage reporting.
Attribution header builder now appends cc_is_subagent=true; for non-main-session callers (search for "cc_is_subagent=true;" in the source); main session agent config now sets isMainSession: !0.
Strings lifted out of the shipped bundle, so the claim above can be checked against them.