The OpenRouter stamper wrote "unknown" whenever a payload lacked a
top-level provider. That hit every Anthropic /messages event, every
Responses event, and the usage/cost payloads routstr synthesizes at the
end of a stream.
- Read the provider from the Anthropic `message` and Responses
`response` envelopes as well as the top level.
- Carry the provider reported earlier in a stream to later events and
to the synthesized usage/cost payloads.
thefux
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