Live API: is the setup message (system instruction + function declarations) billed in full on every session, and can it be cached?

Gemini API
,
1

Building a realtime voice assistant on the Live API (BidiGenerateContent,
gemini-3.1-flash-live-preview), used by real users in India.

Our BidiGenerateContentSetup carries a large system instruction plus ~120
function declarations. It is byte-identical on every session, and it measures
about 28,000 input tokens. We can see it clearly in our own usage logs: two
sessions that lasted 8 and 11 seconds each still recorded 28,186 input tokens,
so that is the fixed cost of opening a connection.

Four questions:

  1. Does the Live API support context caching (or any equivalent) for the
    system instruction and function declarations in the setup message, so that
    repeated sessions are not billed the full input price each time? The
    context-caching documentation does not say whether it applies to the Live
    API, and the Live API pages do not mention caching at all.

  2. Can function declarations or the system instruction be updated mid-session
    without reconnecting? The docs say all functions must be declared at setup,
    and that configuration can be changed when pausing and resuming via session
    resumption — I would like to confirm that is the only supported route.

  3. When a session is resumed with a resumption token, is the original setup
    (system instruction + function declarations) re-billed as input tokens, or
    is it carried over? This is the one that decides whether a
    small-toolset-then-resume-with-a-larger-one design saves anything.

  4. What is the actual validity window for a session resumption token? One
    page says 2 hours and another says 24 hours.

Happy to share usage numbers if useful. Thanks.

2

Hello @JPSH ,

  1. Caching is not supported on Live API native audio models.
  1. You cannot change system instructions or tool definitions mid-session over an active connection. You must disconnect and reconnect with a new setup configuration using a session resumption handle.
  2. The entire active context (including instructions and active tools) is re-billed on every subsequent turn of the resumed connection. The Live API uses a compounding context window model.
  • For detailed breakdown of how you are billed per turn for the entire active window, see Live API best practices - Pricing and billing.
  1. Resumption tokens are valid for 2 hours after the last session terminates. The references to 24 hours in documentation refer to the maximum duration the data state is kept stored under Zero Data Retention rules.
  • See the token lifetime specifications under Session management with Live API - Session resumption.
  • See state storage duration details under the “Live API” section in Zero data retention.