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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

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Claude Code v2.1.15 21 Jan 2026

API Billing Attribution Headers

What

Claude Code now sends version and entrypoint information in API requests for better usage tracking and diagnostics.

Details

Header format: x-anthropic-billing-header: cc_version=2.1.15.{hash}; cc_entrypoint={entrypoint}
The entrypoint is read from CLAUDE_CODE_ENTRYPOINT environment variable (defaults to "unknown")
Previously this header was stubbed to always return an empty string (gated by tengu_ant_attribution_header_new flag)

Evidence

X61() at line 158274 (attribution header generator, contains "x-anthropic-billing-header")

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

See this entry in the whole of v2.1.15 →