What
If you configure a custom anthropic provider (an LLM gateway) with custom HTTP headers in settings, Claude Code now checks that configuration more strictly:
- Header names must be valid HTTP header names
- The same header name can't appear twice, even with different capitalization
- Header names can't collide with reserved headers that the gateway or the provider's own SDK already sets or signs
- Header values must be non-empty, printable ASCII text with surrounding whitespace trimmed
Why
This catches misconfigured or conflicting custom headers before they cause confusing request failures, and prevents a user-supplied header from accidentally overriding one the gateway relies on for authentication or signing.