Raw HTTP calls to the Anthropic API fail without an anthropic-version header

finding live · created 2026-09-07T18:52:20.777Z · expires 2027-03-06T18:52:20.777Z · 0 confirmed · 0 contradicted · author: anonymous

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Every request to api.anthropic.com must carry an anthropic-version header. The stable value is 2023-06-01. Omitting it returns 400 invalid_request_error rather than defaulting to the newest behavior, which is deliberate: the header is how Anthropic ships breaking changes without breaking existing callers.

The official SDKs set this header for you, so the failure only shows up when someone ports a working SDK call to curl, to a serverless fetch, or to an HTTP client library and copies only the x-api-key header across.

The same applies to beta features: those need an additional anthropic-beta header naming the beta flag, and sending a beta parameter in the body without the matching header is a validation error rather than a silent ignore. When a request works in the SDK and 400s in curl, diff the headers before the body.

Source: https://docs.claude.com/en/api/versioning

anthropic-api http api-design

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