diff --git a/Cargo.toml b/Cargo.toml
index 592903db..0bc08b32 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -22,6 +22,7 @@ unstable = [
     "unstable_session_list",
     "unstable_session_model",
     "unstable_session_resume",
+    "unstable_session_usage",
 ]
 unstable_cancel_request = []
 unstable_session_config_options = []
@@ -30,6 +31,7 @@ unstable_session_info_update = []
 unstable_session_list = []
 unstable_session_model = []
 unstable_session_resume = []
+unstable_session_usage = []
 
 [[bin]]
 name = "generate"
diff --git a/src/agent.rs b/src/agent.rs
index 6f5e5627..0db402a0 100644
--- a/src/agent.rs
+++ b/src/agent.rs
@@ -2118,6 +2118,10 @@ impl PromptRequest {
 pub struct PromptResponse {
     /// Indicates why the agent stopped processing the turn.
     pub stop_reason: StopReason,
+    /// Token usage for this turn (optional).
+    #[cfg(feature = "unstable_session_usage")]
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub usage: Option<Usage>,
     /// The _meta property is reserved by ACP to allow clients and agents to attach additional
     /// metadata to their interactions. Implementations MUST NOT make assumptions about values at
     /// these keys.
@@ -2132,10 +2136,20 @@ impl PromptResponse {
     pub fn new(stop_reason: StopReason) -> Self {
         Self {
             stop_reason,
+            #[cfg(feature = "unstable_session_usage")]
+            usage: None,
             meta: None,
         }
     }
 
+    /// Token usage for this turn.
+    #[cfg(feature = "unstable_session_usage")]
+    #[must_use]
+    pub fn usage(mut self, usage: impl IntoOption<Usage>) -> Self {
+        self.usage = usage.into_option();
+        self
+    }
+
     /// The _meta property is reserved by ACP to allow clients and agents to attach additional
     /// metadata to their interactions. Implementations MUST NOT make assumptions about values at
     /// these keys.
@@ -2175,6 +2189,65 @@ pub enum StopReason {
     Cancelled,
 }
 
+/// Token usage information for a prompt turn.
+#[cfg(feature = "unstable_session_usage")]
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
+#[serde(rename_all = "camelCase")]
+#[non_exhaustive]
+pub struct Usage {
+    /// Sum of all token types across session.
+    pub total_tokens: u64,
+    /// Total input tokens across all turns.
+    pub input_tokens: u64,
+    /// Total output tokens across all turns.
+    pub output_tokens: u64,
+    /// Total thought/reasoning tokens (for o1/o3 models).
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub thought_tokens: Option<u64>,
+    /// Total cache read tokens.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub cached_read_tokens: Option<u64>,
+    /// Total cache write tokens.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub cached_write_tokens: Option<u64>,
+}
+
+#[cfg(feature = "unstable_session_usage")]
+impl Usage {
+    #[must_use]
+    pub fn new(total_tokens: u64, input_tokens: u64, output_tokens: u64) -> Self {
+        Self {
+            total_tokens,
+            input_tokens,
+            output_tokens,
+            thought_tokens: None,
+            cached_read_tokens: None,
+            cached_write_tokens: None,
+        }
+    }
+
+    /// Total thought/reasoning tokens (for o1/o3 models).
+    #[must_use]
+    pub fn thought_tokens(mut self, thought_tokens: impl IntoOption<u64>) -> Self {
+        self.thought_tokens = thought_tokens.into_option();
+        self
+    }
+
+    /// Total cache read tokens.
+    #[must_use]
+    pub fn cached_read_tokens(mut self, cached_read_tokens: impl IntoOption<u64>) -> Self {
+        self.cached_read_tokens = cached_read_tokens.into_option();
+        self
+    }
+
+    /// Total cache write tokens.
+    #[must_use]
+    pub fn cached_write_tokens(mut self, cached_write_tokens: impl IntoOption<u64>) -> Self {
+        self.cached_write_tokens = cached_write_tokens.into_option();
+        self
+    }
+}
+
 // Model
 
 /// **UNSTABLE**
diff --git a/src/client.rs b/src/client.rs
index 94cf5644..74369049 100644
--- a/src/client.rs
+++ b/src/client.rs
@@ -104,6 +104,9 @@ pub enum SessionUpdate {
     #[cfg(feature = "unstable_session_info_update")]
     /// Session metadata has been updated (title, timestamps, custom metadata)
     SessionInfoUpdate(SessionInfoUpdate),
+    /// Context window and cost update for the session.
+    #[cfg(feature = "unstable_session_usage")]
+    UsageUpdate(UsageUpdate),
 }
 
 /// The current mode of the session has changed
@@ -245,6 +248,82 @@ impl SessionInfoUpdate {
     }
 }
 
+/// Context window and cost update for a session.
+#[cfg(feature = "unstable_session_usage")]
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema, PartialEq)]
+#[serde(rename_all = "camelCase")]
+#[non_exhaustive]
+pub struct UsageUpdate {
+    /// Tokens currently in context.
+    pub used: u64,
+    /// Total context window size in tokens.
+    pub size: u64,
+    /// Cumulative session cost (optional).
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub cost: Option<Cost>,
+    /// The _meta property is reserved by ACP to allow clients and agents to attach additional
+    /// metadata to their interactions. Implementations MUST NOT make assumptions about values at
+    /// these keys.
+    ///
+    /// See protocol docs: [Extensibility](https://agentclientprotocol.com/protocol/extensibility)
+    #[serde(skip_serializing_if = "Option::is_none", rename = "_meta")]
+    pub meta: Option<Meta>,
+}
+
+#[cfg(feature = "unstable_session_usage")]
+impl UsageUpdate {
+    #[must_use]
+    pub fn new(used: u64, size: u64) -> Self {
+        Self {
+            used,
+            size,
+            cost: None,
+            meta: None,
+        }
+    }
+
+    /// Cumulative session cost (optional).
+    #[must_use]
+    pub fn cost(mut self, cost: impl IntoOption<Cost>) -> Self {
+        self.cost = cost.into_option();
+        self
+    }
+
+    /// The _meta property is reserved by ACP to allow clients and agents to attach additional
+    /// metadata to their interactions. Implementations MUST NOT make assumptions about values at
+    /// these keys.
+    ///
+    /// See protocol docs: [Extensibility](https://agentclientprotocol.com/protocol/extensibility)
+    #[must_use]
+    pub fn meta(mut self, meta: impl IntoOption<Meta>) -> Self {
+        self.meta = meta.into_option();
+        self
+    }
+}
+
+/// Cost information for a session.
+#[cfg(feature = "unstable_session_usage")]
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema, PartialEq)]
+#[serde(rename_all = "camelCase")]
+#[non_exhaustive]
+pub struct Cost {
+    /// Total cumulative cost for session.
+    pub amount: f64,
+    /// ISO 4217 currency code (e.g., "USD", "EUR").
+    pub currency: String,
+}
+
+#[cfg(feature = "unstable_session_usage")]
+impl Cost {
+    #[must_use]
+    pub fn new(amount: f64, currency: impl Into<String>) -> Self {
+        Self {
+            amount,
+            currency: currency.into(),
+        }
+    }
+}
+
 /// A streamed item of content
 #[derive(Debug, Clone, Serialize, Deserialize, JsonSchema, PartialEq)]
 #[serde(rename_all = "camelCase")]