Add UsageUpdate support to SessionUpdate interface and tests

acp-core/src/main/java/com/agentclientprotocol/sdk/spec/AcpSchema.java

@@ -809,7 +809,8 @@ public record Annotations(@JsonProperty("audience") List<Role> audience, @JsonPr
 			@JsonSubTypes.Type(value = ToolCallUpdateNotification.class, name = "tool_call_update"),
 			@JsonSubTypes.Type(value = Plan.class, name = "plan"),
 			@JsonSubTypes.Type(value = AvailableCommandsUpdate.class, name = "available_commands_update"),
-			@JsonSubTypes.Type(value = CurrentModeUpdate.class, name = "current_mode_update") })
+			@JsonSubTypes.Type(value = CurrentModeUpdate.class, name = "current_mode_update"),
+			@JsonSubTypes.Type(value = UsageUpdate.class, name = "usage_update") })
 	public interface SessionUpdate {
 
 	}
@@ -932,6 +933,20 @@ public CurrentModeUpdate(String sessionUpdate, String currentModeId) {
 		}
 	}
 
+	/**
+	 * Usage update - reports token/resource usage for the session
+	 */
+	@JsonIgnoreProperties(ignoreUnknown = true)
+	@JsonInclude(JsonInclude.Include.NON_NULL)
+	public record UsageUpdate(@JsonProperty("sessionUpdate") String sessionUpdate,
+									@JsonProperty("used") Long used,
+									@JsonProperty("size") Long size,
+									@JsonProperty("_meta") Map<String, Object> meta) implements SessionUpdate {
+		public UsageUpdate(String sessionUpdate, Long used, Long size) {
+			this(sessionUpdate, used, size, null);
+		}
+	}
+
 	// ---------------------------
 	// Tool Call Types
 	// ---------------------------

acp-core/src/test/java/com/agentclientprotocol/sdk/spec/SessionUpdateDeserializationTest.java

@@ -15,7 +15,7 @@
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
- * Tests for deserializing all 8 SessionUpdate types from JSON.
+ * Tests for deserializing all 9 SessionUpdate types from JSON.
  *
  * <p>
  * These tests verify that each SessionUpdate type can be correctly deserialized from JSON,
@@ -258,6 +258,27 @@ void currentModeUpdateDeserialization() throws IOException {
 		assertThat(modeUpdate.currentModeId()).isEqualTo("architect");
 	}
 
+	// ---------------------------
+	// UsageUpdate Tests
+	// ---------------------------
+
+	@Test
+	void usageUpdateDeserialization() throws IOException {
+		String json = loadGolden("session-update-usage-update.json");
+
+		// The golden file is a full JSON-RPC notification; extract the inner update
+		com.fasterxml.jackson.databind.JsonNode root = new com.fasterxml.jackson.databind.ObjectMapper().readTree(json);
+		String updateJson = root.get("params").get("update").toString();
+
+		AcpSchema.SessionUpdate update = deserializeSessionUpdate(updateJson);
+
+		assertThat(update).isInstanceOf(AcpSchema.UsageUpdate.class);
+		AcpSchema.UsageUpdate usageUpdate = (AcpSchema.UsageUpdate) update;
+		assertThat(usageUpdate.sessionUpdate()).isEqualTo("usage_update");
+		assertThat(usageUpdate.used()).isEqualTo(53000L);
+		assertThat(usageUpdate.size()).isEqualTo(200000L);
+	}
+
 	// ---------------------------
 	// All Status Values Tests
 	// ---------------------------

acp-core/src/test/java/com/agentclientprotocol/sdk/test/GoldenFileTest.java

@@ -86,6 +86,20 @@ void parseSessionUpdateNotification() throws IOException {
 		assertThat(node.get("params").get("update").get("sessionUpdate").asText()).isEqualTo("agentMessage");
 	}
 
+	@Test
+	void parseSessionUpdateUsageUpdate() throws IOException {
+		String json = loadGoldenFile("session-update-usage-update.json");
+		JsonNode node = objectMapper.readTree(json);
+
+		assertThat(node.get("method").asText()).isEqualTo("session/update");
+		assertThat(node.has("id")).isFalse();
+		assertThat(node.get("params").get("sessionId").asText()).isEqualTo("sess_abc123");
+		JsonNode update = node.get("params").get("update");
+		assertThat(update.get("sessionUpdate").asText()).isEqualTo("usage_update");
+		assertThat(update.get("used").asLong()).isEqualTo(53000L);
+		assertThat(update.get("size").asLong()).isEqualTo(200000L);
+	}
+
 	@Test
 	void serializeInitializeRequestMatchesExpectedStructure() throws IOException {
 		AcpSchema.InitializeRequest request = new AcpSchema.InitializeRequest(1, new AcpSchema.ClientCapabilities());

acp-core/src/test/resources/golden/session-update-usage-update.json

@@ -0,0 +1,12 @@
+{
+  "jsonrpc": "2.0",
+  "method": "session/update",
+  "params": {
+    "sessionId": "sess_abc123",
+    "update": {
+      "sessionUpdate": "usage_update",
+      "used": 53000,
+      "size": 200000
+    }
+  }
+}