Reported by mac0923: Agent Space rejects Responses probes with anthropic-version as 400 invalid_headers. Add the default version only for Anthropic Messages while preserving custom headers through Sign.
TestProviderProbesUseProtocolHeaders fails before the fix across endpoint tests, model tests and both detection paths, then passes with the fix, including 20 race-detector runs. go vet and macOS, Linux and Windows builds pass.