Add ANTHROPIC_CUSTOM_HEADERS for gateway auth - #6
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Gateways fronted by Azure API Management authenticate on a header meat never sent, so pointing ANTHROPIC_BASE_URL at one returned a 401 with no way to supply the missing credential. Parse $ANTHROPIC_CUSTOM_HEADERS in the same "Name: value" per line format Claude Code uses, and apply the headers after the defaults so a gateway can override one. A malformed line, an invalid field-name, or a control character in a value is a hard error, since a silently dropped auth header resurfaces as an opaque 401. The value is a credential, so it is never echoed in an error. Their presence also counts as configuration: a gateway-authenticated user may have no ANTHROPIC_API_KEY at all, so an empty key no longer falls through to exe.dev discovery or fails the Generate guard.
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Pointing
ANTHROPIC_BASE_URLat a corporate LLM gateway fronted by Azure API Management fails with a 401: the gateway authenticates onOcp-Apim-Subscription-Key, a header meat has no way to send.This adds
$ANTHROPIC_CUSTOM_HEADERS, parsed in the sameName: valueper-line format Claude Code already uses, so the same env var configures both tools.Behavior
anthropic-versionwhen it needs to.ANTHROPIC_API_KEYat all, so an empty key no longer falls through to exe.dev discovery, and no longer trips theGenerateguard. An emptyx-api-keyis not sent.Behavior is unchanged when the variable is unset.
Testing
12 new cases: table-driven parsing and rejection tests, an error-does-not-leak-the-value test, and
httptestassertions that the header reaches the wire, that it overrides a default, that header-only auth works, and that no credential at all still errors.go build ./...,go vet ./..., andgo test -race ./...pass;gofmtclean. Also verified end to end against a real APIM-fronted gateway, where it produced an abridged diff instead of the 401.