diff --git a/packages/ai/src/providers/faux.ts b/packages/ai/src/providers/faux.ts
index c3771cc2c..0f4188e60 100644
--- a/packages/ai/src/providers/faux.ts
+++ b/packages/ai/src/providers/faux.ts
@@ -263,11 +263,12 @@ function withUsageEstimate(
 		if (previousPrompt) {
 			const cachedChars = commonPromptPrefixLength(previousPrompt, prompt);
 			cacheRead = Math.ceil(cachedChars / 4);
-			cacheWrite = Math.ceil((promptLength - cachedChars) / 4);
-			input = Math.max(0, promptTokens - cacheRead);
+			cacheWrite = Math.max(0, promptTokens - cacheRead);
 		} else {
 			cacheWrite = promptTokens;
 		}
+		// Every prompt token is either read from or written to the simulated cache.
+		input = 0;
 		promptCache.set(sessionId, prompt);
 	}
 
diff --git a/packages/ai/test/faux-provider.test.ts b/packages/ai/test/faux-provider.test.ts
index 6f4178111..ad23ed938 100644
--- a/packages/ai/test/faux-provider.test.ts
+++ b/packages/ai/test/faux-provider.test.ts
@@ -332,17 +332,18 @@ describe("faux provider", () => {
 		const user = (content: string) => ({ role: "user" as const, content, timestamp: 1 });
 
 		// Prompt texts: "user:hello world" (16 characters), then 16 + 2 + "user:next" (9) = 27.
-		await complete(registration.getModel(), { messages: [user("hello world")] }, options);
+		const first = await complete(registration.getModel(), { messages: [user("hello world")] }, options);
+		expect(first.usage).toMatchObject({ input: 0, cacheRead: 0, cacheWrite: 4 });
 		const extended = await complete(
 			registration.getModel(),
 			{ messages: [user("hello world"), user("next")] },
 			options,
 		);
-		expect(extended.usage).toMatchObject({ input: 3, cacheRead: 4, cacheWrite: 3 });
+		expect(extended.usage).toMatchObject({ input: 0, cacheRead: 4, cacheWrite: 3 });
 
 		// The first message now differs after "user:hello w" (12 characters): "user:hello wide" + 2 + 9 = 26.
 		const edited = await complete(registration.getModel(), { messages: [user("hello wide"), user("next")] }, options);
-		expect(edited.usage).toMatchObject({ input: 4, cacheRead: 3, cacheWrite: 4 });
+		expect(edited.usage).toMatchObject({ input: 0, cacheRead: 3, cacheWrite: 4 });
 	});
 
 	it("does not simulate caching when cacheRetention is none", async () => {