diff --git a/packages/ai/src/providers/openai-completions.ts b/packages/ai/src/providers/openai-completions.ts
index 01bc6e78..de4c5239 100644
--- a/packages/ai/src/providers/openai-completions.ts
+++ b/packages/ai/src/providers/openai-completions.ts
@@ -997,17 +997,17 @@ function parseChunkUsage(
 	model: Model<"openai-completions">,
 ): AssistantMessage["usage"] {
 	const promptTokens = rawUsage.prompt_tokens || 0;
-	const reportedCachedTokens = rawUsage.prompt_tokens_details?.cached_tokens ?? rawUsage.prompt_cache_hit_tokens ?? 0;
+	const cacheReadTokens = rawUsage.prompt_tokens_details?.cached_tokens ?? rawUsage.prompt_cache_hit_tokens ?? 0;
 	const cacheWriteTokens = rawUsage.prompt_tokens_details?.cache_write_tokens || 0;
 
-	// Normalize to pi-ai semantics:
-	// - cacheRead: hits from cache created by previous requests only
-	// - cacheWrite: tokens written to cache in this request
-	// Some OpenAI-compatible providers (observed on OpenRouter) report cached_tokens
-	// as (previous hits + current writes). In that case, remove cacheWrite from cacheRead.
-	const cacheReadTokens =
-		cacheWriteTokens > 0 ? Math.max(0, reportedCachedTokens - cacheWriteTokens) : reportedCachedTokens;
-
+	// Follow documented OpenAI/OpenRouter semantics: cached_tokens is cache-read
+	// tokens (hits). OpenAI does not document or emit cache_write_tokens, but
+	// OpenRouter-compatible providers can include it as a separate write count.
+	// OpenRouter's own provider/tests affirm the separate mapping:
+	// https://github.com/OpenRouterTeam/ai-sdk-provider/pull/409
+	// Do not subtract writes from cached_tokens, otherwise spec-compliant
+	// providers are under-reported. DS4 mirrors this contract too:
+	// https://github.com/antirez/ds4/pull/29
 	const input = Math.max(0, promptTokens - cacheReadTokens - cacheWriteTokens);
 	// OpenAI completion_tokens already includes reasoning_tokens.
 	const outputTokens = rawUsage.completion_tokens || 0;
diff --git a/packages/ai/test/openai-completions-tool-choice.test.ts b/packages/ai/test/openai-completions-tool-choice.test.ts
index c9c8ec9f..7b79b2da 100644
--- a/packages/ai/test/openai-completions-tool-choice.test.ts
+++ b/packages/ai/test/openai-completions-tool-choice.test.ts
@@ -819,7 +819,7 @@ describe("openai-completions tool_choice", () => {
 		expect(response.usage.totalTokens).toBe(43);
 	});
 
-	it("preserves prompt_tokens_details.cache_write_tokens from chunk usage", async () => {
+	it("preserves prompt_tokens_details cache read/write fields from chunk usage", async () => {
 		mockState.chunks = [
 			{
 				id: "chatcmpl-cache-write",
@@ -853,13 +853,14 @@ describe("openai-completions tool_choice", () => {
 			{ apiKey: "test" },
 		).result();
 
-		expect(response.usage.input).toBe(50);
-		expect(response.usage.cacheRead).toBe(20);
+		// cached_tokens is documented as cache reads; cache_write_tokens is separate.
+		expect(response.usage.input).toBe(20);
+		expect(response.usage.cacheRead).toBe(50);
 		expect(response.usage.cacheWrite).toBe(30);
 		expect(response.usage.totalTokens).toBe(105);
 	});
 
-	it("preserves prompt_tokens_details.cache_write_tokens from choice usage fallback", async () => {
+	it("preserves prompt_tokens_details cache read/write fields from choice usage fallback", async () => {
 		mockState.chunks = [
 			{
 				id: "chatcmpl-cache-write-choice",
@@ -898,8 +899,9 @@ describe("openai-completions tool_choice", () => {
 			{ apiKey: "test" },
 		).result();
 
-		expect(response.usage.input).toBe(50);
-		expect(response.usage.cacheRead).toBe(20);
+		// cached_tokens is documented as cache reads; cache_write_tokens is separate.
+		expect(response.usage.input).toBe(20);
+		expect(response.usage.cacheRead).toBe(50);
 		expect(response.usage.cacheWrite).toBe(30);
 		expect(response.usage.totalTokens).toBe(105);
 	});