Add UsageUpdate support to SessionUpdate types

Add UsageUpdate (unstable spec feature) for reporting context window and
cost usage during sessions. Includes Cost record with amount/currency
fields per the ACP unstable schema.

Fixes PR #2: aligns indentation, adds Cost type, uses bare golden file
format, and adds UNSTABLE annotations.

Co-authored-by: Huang YunKun <htynkn@gmail.com>

Author: Mark Pollack

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

@@ -934,19 +934,28 @@ public CurrentModeUpdate(String sessionUpdate, String currentModeId) {
 	}
 
 	/**
-	 * Usage update - reports token/resource usage for the session
+	 * Usage update - context window and cost update for the session (UNSTABLE)
 	 */
 	@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 {
+			@JsonProperty("used") Long used, @JsonProperty("size") Long size,
+			@JsonProperty("cost") Cost cost,
+			@JsonProperty("_meta") Map<String, Object> meta) implements SessionUpdate {
 		public UsageUpdate(String sessionUpdate, Long used, Long size) {
-			this(sessionUpdate, used, size, null);
+			this(sessionUpdate, used, size, null, null);
 		}
 	}
 
+	/**
+	 * Cost information for a session (UNSTABLE)
+	 */
+	@JsonIgnoreProperties(ignoreUnknown = true)
+	@JsonInclude(JsonInclude.Include.NON_NULL)
+	public record Cost(@JsonProperty("amount") Double amount,
+			@JsonProperty("currency") String currency) {
+	}
+
 	// ---------------------------
 	// 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 9 SessionUpdate types from JSON.
+ * Tests for deserializing all SessionUpdate types from JSON.
  *
  * <p>
  * These tests verify that each SessionUpdate type can be correctly deserialized from JSON,
@@ -266,17 +266,37 @@ void currentModeUpdateDeserialization() throws IOException {
 	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);
+		AcpSchema.SessionUpdate update = deserializeSessionUpdate(json);
 
 		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);
+		assertThat(usageUpdate.cost()).isNull();
+	}
+
+	@Test
+	void usageUpdateWithCostDeserialization() throws IOException {
+		String json = """
+				{
+				  "sessionUpdate": "usage_update",
+				  "used": 53000,
+				  "size": 200000,
+				  "cost": {
+				    "amount": 0.42,
+				    "currency": "USD"
+				  }
+				}
+				""";
+
+		AcpSchema.SessionUpdate update = deserializeSessionUpdate(json);
+
+		assertThat(update).isInstanceOf(AcpSchema.UsageUpdate.class);
+		AcpSchema.UsageUpdate usageUpdate = (AcpSchema.UsageUpdate) update;
+		assertThat(usageUpdate.cost()).isNotNull();
+		assertThat(usageUpdate.cost().amount()).isEqualTo(0.42);
+		assertThat(usageUpdate.cost().currency()).isEqualTo("USD");
 	}
 
 	// ---------------------------

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

@@ -86,20 +86,6 @@ 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

@@ -1,12 +1,5 @@
 {
-  "jsonrpc": "2.0",
-  "method": "session/update",
-  "params": {
-    "sessionId": "sess_abc123",
-    "update": {
-      "sessionUpdate": "usage_update",
-      "used": 53000,
-      "size": 200000
-    }
-  }
-}
+  "sessionUpdate": "usage_update",
+  "used": 53000,
+  "size": 200000
+}