When an interaction is created with a model alias (e.g. gemini-flash-latest), the response gives no way to know which concrete model served it. The model field echoes the requested alias, and no other field or header carries the version.
generateContent reports it:
POST /v1beta/models/gemini-flash-latest:generateContent
→ "modelVersion": "gemini-3.8-flash"
Interactions doesn’t:
POST /v1beta/interactions {"model": "gemini-flash-latest", "input": "Reply with: ok"}
→ keys: id, status, usage, created, updated, service_tier, steps, object, model
→ "model": "gemini-flash-latest"
usage.model_invocation_token_counts lists token details for each model invocation, but not the model either.
Why it matters: -latest aliases are hot-swapped on new releases, and pricing is per concrete model. Without the served version, requests made through Interactions can’t be attributed to the right model for cost or behavior tracking. Interactions is now the recommended API, so this gap affects all new integrations.
Request: add an output-only field with the concrete model version that served the interaction (e.g. model_version). If one interaction can invoke several models, add it to each entry in model_invocation_token_counts too.
Environment: Gemini API v1beta, API-key auth, @google/genai 2.13.0 (Node), observed 2026-09-27.