service_tier field is missing from GenerateContentResponseMetadata model

[joneff]

Environment details

• Programming language: Python
• OS: macOS 26.4.1
• Language runtime version: 3.12
• Package version: google-genai>=2.0.0 (actual version 2.2.0)

Issue

GenerateContent endpoint (https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-flash-image:generateContent) has an extra field usageMetadata.serviceTier which is not present in GenerateContentResponseUsageMetadata class.

Since _common.BaseModel has extra='forbid', passing the resulting json to GenerateContentResponse results in error.

Note: the same applies for 3.1 flash image preview. Probably other endpoints as well.