Bug Report: x-gemini-service-tier missing from CORS Expose-Headers & Streaming Endpoint

Gemini API
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Environment:

  • Model: all supported model that can use flex/priority/standard

  • Endpoint: REST (generateContent) and Streaming (streamGenerateContent)

  • Context: Calling the Gemini API directly from a Client-Side React Application.

Issue 1: Missing from CORS (access-control-expose-headers) When calling the standard generateContent REST endpoint from a browser, the network tab shows that the x-gemini-service-tier header is successfully returned by Google’s server. However, the access-control-expose-headers list does not include x-gemini-service-tier. Because it is not whitelisted, browsers strip the header out, making it impossible to read response.headers.get("x-gemini-service-tier") via client-side JavaScript fetch.

Issue 2: Missing from Streaming (streamGenerateContent) When calling the streamGenerateContent endpoint, the x-gemini-service-tier header is completely missing from the initial HTTP response headers, even in raw curl tests.

Expected Behavior:

  1. x-gemini-service-tier should be added to the access-control-expose-headers CORS whitelist so frontend applications can read their fulfilled billing tier.

  2. The routing tier header should be attached to the initial response headers when establishing an SSE connection via streamGenerateContent.