fix: improve xai cache hit rate (#35970)

packages/opencode/test/provider/transform.test.ts

@@ -73,7 +73,7 @@ describe("ProviderTransform.options - setCacheKey", () => {
     expect(result.promptCacheKey).toBeUndefined()
   })
 
-  test("should set promptCacheKey for openai provider regardless of setCacheKey", () => {
+  test("should set promptCacheKey for openai provider by default", () => {
     const openaiModel = {
       ...mockModel,
       providerID: "openai",
@@ -87,6 +87,56 @@ describe("ProviderTransform.options - setCacheKey", () => {
     expect(result.promptCacheKey).toBe(sessionID)
   })
 
+  test("should not set promptCacheKey for openai when explicitly disabled", () => {
+    const openaiModel = {
+      ...mockModel,
+      providerID: "openai",
+      api: {
+        id: "gpt-4",
+        url: "https://api.openai.com",
+        npm: "@ai-sdk/openai",
+      },
+    }
+    const result = ProviderTransform.options({
+      model: openaiModel,
+      sessionID,
+      providerOptions: { setCacheKey: false },
+    })
+    expect(result.promptCacheKey).toBeUndefined()
+  })
+
+  test("should set promptCacheKey for the xAI SDK by default regardless of provider ID", () => {
+    const xaiModel = {
+      ...mockModel,
+      providerID: "custom-xai",
+      api: {
+        id: "grok-4",
+        url: "https://api.x.ai",
+        npm: "@ai-sdk/xai",
+      },
+    }
+    const result = ProviderTransform.options({ model: xaiModel, sessionID, providerOptions: {} })
+    expect(result.promptCacheKey).toBe(sessionID)
+  })
+
+  test("should not set promptCacheKey for the xAI SDK when explicitly disabled", () => {
+    const xaiModel = {
+      ...mockModel,
+      providerID: "xai",
+      api: {
+        id: "grok-4",
+        url: "https://api.x.ai",
+        npm: "@ai-sdk/xai",
+      },
+    }
+    const result = ProviderTransform.options({
+      model: xaiModel,
+      sessionID,
+      providerOptions: { setCacheKey: false },
+    })
+    expect(result.promptCacheKey).toBeUndefined()
+  })
+
   test("should set store=false for openai provider", () => {
     const openaiModel = {
       ...mockModel,