Attribution header can override the environment opt-out

One first-party request path sends the attribution header even if you opted out via env var.

Under the hood Notable Internal Changes

JSON All of v2.1.229

Under the hoodTier: how much it should matter to you

3Useful: my rating, 1 to 5

4Signal: worth watching, 1 to 5

TelemetryArea: what it touches

Internal ChangesKind: in v2.1.229, 12 Aug 26

What probably matters to youSection of the release

What

The builder for the attribution header now takes an options argument, and one specific first-party, non-unix-socket path passes ignoreEnvOptOut, which makes the header be sent even when CLAUDE_CODE_ATTRIBUTION_HEADER is set to opt out.

Evidence

ignoreEnvOptOut

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

Read from

Names in the bundleCLAUDE_CODE_ATTRIBUTION_HEADER

How sure we are

One source agreesOne thing we can check says the same as this entry.

Anthropic's release notes agreeFixed auto mode failing on every tool call for users who disable the attribution header via CLAUDE_CODE_ATTRIBUTION_HEADER (direct Anthropic…

See this entry in the whole of v2.1.229 →

The build it came inv2.1.229

12 Aug 2026 · 205 entries, this one is #14.

Use it now23
You'll notice87
Nothing to try yet28
Under the hood67

Either side of it

v2.1.228Older
v2.1.231Newer

Open the release →

Names in this entry

envCLAUDE_CODE_ATTRIBUTION_HEADER
Set to 0 to omit the attribution block, which carries the client version and a prompt fingerprint, from the start of the system prompt. Caching on a direct connection to the Anthropic API is unaffected either way. In some direct-connection setups, Claude Code keeps the block on auto mode classifier requests even when you set 0. In System prompt attribution block, check which connections and credentials this covers. Before v2.1.181 the block included a per-request token on custom base URLs and Microsoft Foundry connections, so on those versions set it to 0 when your LLM gateway caches on the request body or forwards requests to a third-party provider, or when you connect to Microsoft Foundry directly

Anthropic's note

Upstream's own release notes for v2.1.229 name something this entry names, so the two are probably about the same change. Probably: the pairing is made on a shared name rather than on the words around it.

Fixed auto mode failing on every tool call for users who disable the attribution header via CLAUDE_CODE_ATTRIBUTION_HEADER (direct Anthropic API connections)

Is this right?

Other releases about the same thing. Found by shared names or similar wording; neither means one caused the other.

v2.1.227 Billing attribution header carries a prompt id and validates its values
Both mention attribution header
v2.1.299 Managed settings can hold their CLAUDE_CODE_ATTRIBUTION_HEADER value against a host program
Both mention attribution header
v2.1.302 Attribution block can name a project, and one fork subagent check no longer reads its variable
Both mention attribution header