diff --git a/.changeset/bright-sols-write.md b/.changeset/bright-sols-write.md
new file mode 100644
index 00000000000..b008ad24778
--- /dev/null
+++ b/.changeset/bright-sols-write.md
@@ -0,0 +1,5 @@
+---
+"@opencode-ai/ai": patch
+---
+
+Report OpenAI prompt cache write tokens in normalized usage.
diff --git a/packages/ai/src/protocols/open-responses.ts b/packages/ai/src/protocols/open-responses.ts
index 4cd54573196..249f6a93b43 100644
--- a/packages/ai/src/protocols/open-responses.ts
+++ b/packages/ai/src/protocols/open-responses.ts
@@ -167,7 +167,12 @@ export type OpenResponsesBody = Schema.Schema.Type<typeof OpenResponsesBody>
 
 const OpenResponsesUsage = Schema.Struct({
   input_tokens: Schema.optional(Schema.Number),
-  input_tokens_details: optionalNull(Schema.Struct({ cached_tokens: Schema.optional(Schema.Number) })),
+  input_tokens_details: optionalNull(
+    Schema.Struct({
+      cached_tokens: Schema.optional(Schema.Number),
+      cache_write_tokens: Schema.optional(Schema.Number),
+    }),
+  ),
   output_tokens: Schema.optional(Schema.Number),
   output_tokens_details: optionalNull(Schema.Struct({ reasoning_tokens: Schema.optional(Schema.Number) })),
   total_tokens: Schema.optional(Schema.Number),
@@ -540,19 +545,21 @@ export const fromRequest = Effect.fn("OpenResponses.fromRequest")(function* (
 // Stream Parsing
 // =============================================================================
 // Responses APIs report `input_tokens` (inclusive total) with a
-// `cached_tokens` subset, and `output_tokens` (inclusive total) with a
-// `reasoning_tokens` subset. Pass the totals through and derive the
+// cached-read and cache-write subsets, and `output_tokens` (inclusive total)
+// with a `reasoning_tokens` subset. Pass the totals through and derive the
 // non-cached breakdown.
 const mapUsage = (usage: OpenResponsesUsage | null | undefined, providerMetadataKey: string) => {
   if (!usage) return undefined
   const cached = usage.input_tokens_details?.cached_tokens
+  const cacheWrite = usage.input_tokens_details?.cache_write_tokens
   const reasoning = usage.output_tokens_details?.reasoning_tokens
-  const nonCached = ProviderShared.subtractTokens(usage.input_tokens, cached)
+  const nonCached = ProviderShared.subtractTokens(usage.input_tokens, ProviderShared.sumTokens(cached, cacheWrite))
   return new Usage({
     inputTokens: usage.input_tokens,
     outputTokens: usage.output_tokens,
     nonCachedInputTokens: nonCached,
     cacheReadInputTokens: cached,
+    cacheWriteInputTokens: cacheWrite,
     reasoningTokens: reasoning,
     totalTokens: ProviderShared.totalTokens(usage.input_tokens, usage.output_tokens, usage.total_tokens),
     providerMetadata: { [providerMetadataKey]: usage },
diff --git a/packages/ai/src/protocols/openai-chat.ts b/packages/ai/src/protocols/openai-chat.ts
index 0e327f2abf2..e130f0bdfe4 100644
--- a/packages/ai/src/protocols/openai-chat.ts
+++ b/packages/ai/src/protocols/openai-chat.ts
@@ -131,6 +131,7 @@ const OpenAIChatUsage = Schema.Struct({
   prompt_tokens_details: optionalNull(
     Schema.Struct({
       cached_tokens: Schema.optional(Schema.Number),
+      cache_write_tokens: Schema.optional(Schema.Number),
     }),
   ),
   completion_tokens_details: optionalNull(
@@ -453,20 +454,22 @@ const mapFinishReason = (reason: string | null | undefined): FinishReason => {
 }
 
 // OpenAI Chat reports `prompt_tokens` (inclusive total) with a
-// `cached_tokens` subset, and `completion_tokens` (inclusive total) with
-// a `reasoning_tokens` subset. We pass the inclusive totals through and
-// derive the non-cached breakdown so the `LLM.Usage` contract is
+// cached-read and cache-write subsets, and `completion_tokens` (inclusive
+// total) with a `reasoning_tokens` subset. We pass the inclusive totals
+// through and derive the non-cached breakdown so the `LLM.Usage` contract is
 // satisfied on both sides.
 const mapUsage = (usage: OpenAIChatEvent["usage"]): Usage | undefined => {
   if (!usage) return undefined
   const cached = usage.prompt_tokens_details?.cached_tokens
+  const cacheWrite = usage.prompt_tokens_details?.cache_write_tokens
   const reasoning = usage.completion_tokens_details?.reasoning_tokens
-  const nonCached = ProviderShared.subtractTokens(usage.prompt_tokens, cached)
+  const nonCached = ProviderShared.subtractTokens(usage.prompt_tokens, ProviderShared.sumTokens(cached, cacheWrite))
   return new Usage({
     inputTokens: usage.prompt_tokens,
     outputTokens: usage.completion_tokens,
     nonCachedInputTokens: nonCached,
     cacheReadInputTokens: cached,
+    cacheWriteInputTokens: cacheWrite,
     reasoningTokens: reasoning,
     totalTokens: ProviderShared.totalTokens(usage.prompt_tokens, usage.completion_tokens, usage.total_tokens),
     providerMetadata: { openai: usage },
diff --git a/packages/ai/test/provider/openai-chat.test.ts b/packages/ai/test/provider/openai-chat.test.ts
index 926ea30dba0..57ae8e86530 100644
--- a/packages/ai/test/provider/openai-chat.test.ts
+++ b/packages/ai/test/provider/openai-chat.test.ts
@@ -550,7 +550,7 @@ describe("OpenAI Chat route", () => {
           prompt_tokens: 5,
           completion_tokens: 2,
           total_tokens: 7,
-          prompt_tokens_details: { cached_tokens: 1 },
+          prompt_tokens_details: { cached_tokens: 1, cache_write_tokens: 2 },
           completion_tokens_details: { reasoning_tokens: 0 },
         }),
       )
@@ -558,8 +558,9 @@ describe("OpenAI Chat route", () => {
       const usage = new Usage({
         inputTokens: 5,
         outputTokens: 2,
-        nonCachedInputTokens: 4,
+        nonCachedInputTokens: 2,
         cacheReadInputTokens: 1,
+        cacheWriteInputTokens: 2,
         reasoningTokens: 0,
         totalTokens: 7,
         providerMetadata: {
@@ -567,7 +568,7 @@ describe("OpenAI Chat route", () => {
             prompt_tokens: 5,
             completion_tokens: 2,
             total_tokens: 7,
-            prompt_tokens_details: { cached_tokens: 1 },
+            prompt_tokens_details: { cached_tokens: 1, cache_write_tokens: 2 },
             completion_tokens_details: { reasoning_tokens: 0 },
           },
         },
diff --git a/packages/ai/test/provider/openai-responses.test.ts b/packages/ai/test/provider/openai-responses.test.ts
index cf66e5cbd58..43d19ed42ec 100644
--- a/packages/ai/test/provider/openai-responses.test.ts
+++ b/packages/ai/test/provider/openai-responses.test.ts
@@ -832,7 +832,7 @@ describe("OpenAI Responses route", () => {
               input_tokens: 5,
               output_tokens: 2,
               total_tokens: 7,
-              input_tokens_details: { cached_tokens: 1 },
+              input_tokens_details: { cached_tokens: 1, cache_write_tokens: 2 },
               output_tokens_details: { reasoning_tokens: 0 },
             },
           },
@@ -842,8 +842,9 @@ describe("OpenAI Responses route", () => {
       const usage = new Usage({
         inputTokens: 5,
         outputTokens: 2,
-        nonCachedInputTokens: 4,
+        nonCachedInputTokens: 2,
         cacheReadInputTokens: 1,
+        cacheWriteInputTokens: 2,
         reasoningTokens: 0,
         totalTokens: 7,
         providerMetadata: {
@@ -851,7 +852,7 @@ describe("OpenAI Responses route", () => {
             input_tokens: 5,
             output_tokens: 2,
             total_tokens: 7,
-            input_tokens_details: { cached_tokens: 1 },
+            input_tokens_details: { cached_tokens: 1, cache_write_tokens: 2 },
             output_tokens_details: { reasoning_tokens: 0 },
           },
         },