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gRPC-Web through Envoy: CORS filters and preflight for protobuf browsers
Envoy routes gRPC-Web to backends; enable CORS on the HTTP connection manager before gRPC transcoding.
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grpc-webenvoycors
Filter order
Place cors filter before router so preflight short-circuits with correct headers.
Keep max-age aligned with CDN caches to reduce redundant OPTIONS on repeat visitors.
Debugging
Log :method and :authority for failing preflights; grpc-web clients often send custom metadata headers.
Verify grpc-timeout and grpc-message are not blocked by overly strict exposed header lists.