feat(ai): add totalFromProvider flag to usage cost (#11178)

Add Usage.cost.totalFromProvider to record whether cost.total is the
billed cost reported by the provider or an estimate from catalog rates.
Catalog-derived totals set it to false; a missing value also means
false. combineUsage marks a sum as provider-reported only when both
parts are.

packages/coding-agent/src/core/usage-totals.ts

@@ -47,6 +47,8 @@ export function combineUsage(first: Usage, second: Usage): Usage {
 			cacheRead: first.cost.cacheRead + second.cost.cacheRead,
 			cacheWrite: first.cost.cacheWrite + second.cost.cacheWrite,
 			total: first.cost.total + second.cost.total,
+			// A sum is provider-reported only when every part is.
+			totalFromProvider: first.cost.totalFromProvider === true && second.cost.totalFromProvider === true,
 		},
 	};
 }

packages/coding-agent/test/compaction-summary-reasoning.test.ts

@@ -314,7 +314,7 @@ describe("generateSummary reasoning options", () => {
 			input: 20,
 			output: 20,
 			totalTokens: 40,
-			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0, totalFromProvider: false },
 		});
 	});
 });

packages/coding-agent/test/usage-totals.test.ts

@@ -0,0 +1,30 @@
+import type { Usage } from "@earendil-works/pi-ai/compat";
+import { describe, expect, it } from "vitest";
+import { combineUsage } from "../src/core/usage-totals.ts";
+
+function usage(total: number, totalFromProvider?: boolean): Usage {
+	return {
+		input: 1,
+		output: 1,
+		cacheRead: 0,
+		cacheWrite: 0,
+		totalTokens: 2,
+		cost: {
+			input: 0,
+			output: 0,
+			cacheRead: 0,
+			cacheWrite: 0,
+			total,
+			...(totalFromProvider === undefined ? {} : { totalFromProvider }),
+		},
+	};
+}
+
+describe("combineUsage", () => {
+	it("marks the combined total as provider-reported only when both totals are", () => {
+		expect(combineUsage(usage(1, true), usage(2, true)).cost).toMatchObject({ total: 3, totalFromProvider: true });
+		expect(combineUsage(usage(1, true), usage(2)).cost.totalFromProvider).toBe(false);
+		expect(combineUsage(usage(1), usage(2, true)).cost.totalFromProvider).toBe(false);
+		expect(combineUsage(usage(1), usage(2)).cost.totalFromProvider).toBe(false);
+	});
+});