fix: prompt_cache_key openai chat -> openai responses (#6861)

relaykit/relayconvert/internal/oai_chat/to_oai_responses_req.go

@@ -380,6 +380,14 @@ func ChatCompletionsRequestToResponsesRequest(req *dto.GeneralOpenAIRequest) (*d
 		presencePenaltyRaw, _ = kitutil.Marshal(req.PresencePenalty)
 	}
 
+	var promptCacheKeyRaw json.RawMessage
+	if req.PromptCacheKey != "" {
+		promptCacheKeyRaw, err = kitutil.Marshal(req.PromptCacheKey)
+		if err != nil {
+			return nil, fmt.Errorf("marshal prompt_cache_key: %w", err)
+		}
+	}
+
 	out := &dto.OpenAIResponsesRequest{
 		Model:             req.Model,
 		Input:             inputRaw,
@@ -396,6 +404,7 @@ func ChatCompletionsRequestToResponsesRequest(req *dto.GeneralOpenAIRequest) (*d
 		ParallelToolCalls: parallelToolCallsRaw,
 		Store:             req.Store,
 		Metadata:          req.Metadata,
+		PromptCacheKey:    promptCacheKeyRaw,
 		EnableThinking:    req.EnableThinking,
 		ThinkingBudget:    req.ThinkingBudget,
 	}

relaykit/relayconvert/internal/oai_chat/to_oai_responses_req_test.go

@@ -39,6 +39,38 @@ func TestChatCompletionsRequestToResponsesRequestInstructionsAndTools(t *testing
 	assert.Equal(t, "function_call_output", gjson.GetBytes(got.Input, "3.type").String())
 }
 
+func TestChatCompletionsRequestToResponsesRequestPreservesPromptCacheKey(t *testing.T) {
+	t.Run("present", func(t *testing.T) {
+		key := "session-\"quoted\"\\path\n世界"
+		got, err := ChatCompletionsRequestToResponsesRequest(&dto.GeneralOpenAIRequest{
+			Model:          "gpt-test",
+			Messages:       []dto.Message{{Role: "user", Content: "hello"}},
+			PromptCacheKey: key,
+		})
+		require.NoError(t, err)
+
+		keyRaw, err := kitutil.Marshal(key)
+		require.NoError(t, err)
+		assert.Equal(t, keyRaw, []byte(got.PromptCacheKey))
+
+		encoded, err := kitutil.Marshal(got)
+		require.NoError(t, err)
+		assert.Equal(t, key, gjson.GetBytes(encoded, "prompt_cache_key").String())
+	})
+
+	t.Run("absent", func(t *testing.T) {
+		got, err := ChatCompletionsRequestToResponsesRequest(&dto.GeneralOpenAIRequest{
+			Model:    "gpt-test",
+			Messages: []dto.Message{{Role: "user", Content: "hello"}},
+		})
+		require.NoError(t, err)
+
+		encoded, err := kitutil.Marshal(got)
+		require.NoError(t, err)
+		assert.False(t, gjson.GetBytes(encoded, "prompt_cache_key").Exists())
+	})
+}
+
 func TestChatCompletionsRequestToResponsesRequestPreservesQwenThinkingBudget(t *testing.T) {
 	tests := []struct {
 		name   string