fix: complete extension usage accounting

closes #6509

packages/coding-agent/test/agent-session-stats.test.ts

@@ -5,6 +5,7 @@ import { AgentSession } from "../src/core/agent-session.ts";
 import { AuthStorage } from "../src/core/auth-storage.ts";
 import { SessionManager } from "../src/core/session-manager.ts";
 import { SettingsManager } from "../src/core/settings-manager.ts";
+import { getUsageCostBreakdown } from "../src/core/usage-totals.ts";
 import { createInMemoryModelRegistry, getModelRuntime } from "./model-runtime-test-utils.ts";
 import { createTestResourceLoader } from "./utilities.ts";
 
@@ -224,6 +225,31 @@ describe("AgentSession.getSessionStats", () => {
 		}
 	});
 
+	it("groups tool and summary usage separately from model-attributed usage", () => {
+		const sessionManager = SessionManager.inMemory();
+		const rootId = sessionManager.appendMessage(createUserMessage("hello", 1));
+		sessionManager.appendMessage({
+			...createAssistantMessage("response", 100, 2),
+			usage: { ...createUsage(100), cost: { ...createUsage(100).cost, total: 0.5 } },
+		});
+		sessionManager.appendMessage(
+			createToolResultMessage({ ...createUsage(100), cost: { ...createUsage(100).cost, total: 1 } }),
+		);
+		sessionManager.appendCompaction("summary", rootId, 100, undefined, false, {
+			...createUsage(100),
+			cost: { ...createUsage(100).cost, total: 2 },
+		});
+		sessionManager.branchWithSummary(null, "branch summary", undefined, false, {
+			...createUsage(100),
+			cost: { ...createUsage(100).cost, total: 3 },
+		});
+
+		expect(getUsageCostBreakdown(sessionManager.getEntries())).toEqual([
+			{ key: "Tools/summaries", cost: 6, tokens: 300 },
+			{ key: `${model.provider}/${model.id}`, cost: 0.5, tokens: 100 },
+		]);
+	});
+
 	it("ignores zero-usage messages when checking for post-compaction context usage", async () => {
 		const { session, sessionManager } = await createSession();