diff --git a/src/shared/providers/definitions/models/openai-responses.test.ts b/src/shared/providers/definitions/models/openai-responses.test.ts
index 3a68d66019..276539f641 100644
--- a/src/shared/providers/definitions/models/openai-responses.test.ts
+++ b/src/shared/providers/definitions/models/openai-responses.test.ts
@@ -55,10 +55,11 @@ function createModel(overrides: Partial<ConstructorParameters<typeof OpenAIRespo
 }
 
 describe('OpenAIResponses call settings', () => {
-  it('forces store=false for stateless responses while preserving user OpenAI provider options', () => {
+  it('forces store=false and uses the stable session ID as the prompt cache key for stateless responses', () => {
     const openaiResponses = createModel({ forceStatelessResponses: true })
 
     const settings = openaiResponses.exposeCallSettings({
+      sessionId: 'session-123',
       providerOptions: {
         openai: {
           reasoningEffort: 'high',
@@ -69,6 +70,19 @@ describe('OpenAIResponses call settings', () => {
     expect(settings.providerOptions).toEqual({
       openai: {
         reasoningEffort: 'high',
+        promptCacheKey: 'session-123',
+        store: false,
+      },
+    })
+  })
+
+  it('does not inject an empty prompt cache key when a stateless request has no session ID', () => {
+    const openaiResponses = createModel({ forceStatelessResponses: true })
+
+    const settings = openaiResponses.exposeCallSettings()
+
+    expect(settings.providerOptions).toEqual({
+      openai: {
         store: false,
       },
     })
diff --git a/src/shared/providers/definitions/models/openai-responses.ts b/src/shared/providers/definitions/models/openai-responses.ts
index 41cbf5c2a1..8c6a8d84c9 100644
--- a/src/shared/providers/definitions/models/openai-responses.ts
+++ b/src/shared/providers/definitions/models/openai-responses.ts
@@ -42,6 +42,7 @@ export default class OpenAIResponses extends AbstractAISDKModel {
 
   protected getCallSettings(options: CallChatCompletionOptions) {
     const openaiProviderOptions = options.providerOptions?.openai
+    const promptCacheKey = this.options.forceStatelessResponses ? options.sessionId : undefined
 
     return {
       temperature: this.options.temperature,
@@ -54,6 +55,7 @@ export default class OpenAIResponses extends AbstractAISDKModel {
               openai: {
                 ...openaiProviderOptions,
                 ...(this.options.forceStatelessResponses ? { store: false } : {}),
+                ...(promptCacheKey ? { promptCacheKey } : {}),
               },
             }
           : undefined,