ANTHROPIC_CUSTOM_HEADERS is read by a shared helper

Claude Code now reads ANTHROPIC_CUSTOM_HEADERS through a single shared function instead of separate inline code

Under the hood Internal Changes

JSON All of v2.1.287

Under the hoodTier: how much it should matter to you

1Useful: my rating, 1 to 5

0Signal: worth watching, 1 to 5

APIArea: what it touches

Internal ChangesKind: in v2.1.287, 01 Oct 26

Internal ChangesSection of the release

What

ANTHROPIC_CUSTOM_HEADERS is an environment variable for adding your own headers to Claude Code's requests. Claude Code now reads it with one shared function. Before, each place split the value into lines and each line at its first colon.

Why

Most values should read the same as before. Unusual values, near the edges of the Name: Value format, could now be read differently.

Read from

Names in the bundleANTHROPIC_CUSTOM_HEADERS

How sure we are

Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.

The writer flagged doubtIt is not clear whether the shared function reads any value differently from the old line-and-colon split.

Anthropic's release notes agreeBedrock, Vertex, Mantle: Fixed model availability checks under CLAUDE_CODE_SKIP_*_AUTH sending a different Authorization header than real…

See this entry in the whole of v2.1.287 →

The build it came inv2.1.287

1 Oct 2026 · 448 entries, this one is #391.

Use it now30
You'll notice260
Nothing to try yet9
Under the hood149

Either side of it

v2.1.286Older
v2.1.288Newer

Open the release →

Names in this entry

envANTHROPIC_CUSTOM_HEADERS
Custom headers to add to requests (Name: Value format, newline-separated for multiple headers). If a name or value contains a character an HTTP header can't carry, such as a curly quote or a zero-width space, the request fails with an error that identifies the pair by position. Requires Claude Code v2.1.227 or later. Invalid request header value lists the exact character set and where the check runs. A value that sets a credential, org or tenant, routing, or API-behavior header, such as Authorization or Host, counts as a setting that needs approval when server-managed settings deliver it. From project or local settings, such a value follows the rules for when env values apply

Anthropic's note

Upstream's own release notes for v2.1.287 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.

Bedrock, Vertex, Mantle: Fixed model availability checks under CLAUDE_CODE_SKIP_*_AUTH sending a different Authorization header than real requests when ANTHROPIC_CUSTOM_HEADERS repeats it

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 Auth failures now name the credential source
Both mention custom header
v2.1.227 Auth token and custom header failures now say which value is wrong
Both mention custom header
v2.1.251 Custom headers from settings are now validated before reaching the environment
Both mention custom header