Struct ContextUpdate 

#[non_exhaustive]
pub struct ContextUpdate {
    pub system_instruction: Option<Content>,
    pub tools: Option<Vec<Tool>>,
}

A mid-session replacement of the model preamble: the system instruction, the declared tools, or both. Sent as a contextUpdate message.

Only the fields that are set change. tools: None keeps the current tools; Some(vec![]) clears them. A new system instruction replaces the one given at setup. Updates are processed in order with the rest of the client’s input, so an update sent before a tool response is in effect when the model reads that response.

Both fields are part of the model preamble, so every update invalidates the server’s prefix cache. Send one only when something changed.

Gemini 3.8 Live accepts it on Google AI and Vertex AI; Gemini 2.5 Live does not (see SessionConfig::supports_context_update).

let update = ContextUpdate::new()
    .system_instruction("The caller is verified. Never read balances aloud.")
    .tools(vec![Tool::functions(vec![FunctionDeclaration {
        name: "get_balance".into(),
        description: "Fetch the verified caller's balance.".into(),
        parameters: None,
        behavior: None,
    }])]);
assert!(!update.is_empty());
assert!(ContextUpdate::new().is_empty());

Fields (Non-exhaustive)

Non-exhaustive structs could have additional fields added in future. Therefore, non-exhaustive structs cannot be constructed in external crates using the traditional Struct { .. } syntax; cannot be matched against without a wildcard ..; and struct update syntax will not work.

system_instruction: Option<Content>

Replacement system instruction. None keeps the current one.

tools: Option<Vec<Tool>>

Replacement tool list. None keeps the current tools; an empty list clears them.

Implementations

impl ContextUpdate

pub fn new() -> Self

An update that changes nothing until a field is set.

pub fn system_instruction(self, instruction: impl Into<String>) -> Self

Replace the system instruction with instruction.

pub fn tools(self, tools: Vec<Tool>) -> Self

Replace the whole tool list with tools.

pub fn clear_tools(self) -> Self

Remove every declared tool.

pub fn is_empty(&self) -> bool

true when the update would change nothing.

Trait Implementations

impl Clone for ContextUpdate

fn clone(&self) -> ContextUpdate

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for ContextUpdate

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for ContextUpdate

fn default() -> ContextUpdate

Returns the “default value” for a type.

impl PartialEq for ContextUpdate

fn eq(&self, other: &ContextUpdate) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for ContextUpdate