diff --git a/backend/internal/handler/openai_gateway_handler.go b/backend/internal/handler/openai_gateway_handler.go
index 70131417..5400da3f 100644
--- a/backend/internal/handler/openai_gateway_handler.go
+++ b/backend/internal/handler/openai_gateway_handler.go
@@ -11,7 +11,6 @@ import (
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
-	"github.com/Wei-Shaw/sub2api/internal/pkg/openai"
 	middleware2 "github.com/Wei-Shaw/sub2api/internal/server/middleware"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 
@@ -92,17 +91,7 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 		return
 	}
 
-	// For non-Codex CLI requests, set default instructions
 	userAgent := c.GetHeader("User-Agent")
-	if !openai.IsCodexCLIRequest(userAgent) {
-		reqBody["instructions"] = openai.DefaultInstructions
-		// Re-serialize body
-		body, err = json.Marshal(reqBody)
-		if err != nil {
-			h.errorResponse(c, http.StatusInternalServerError, "api_error", "Failed to process request")
-			return
-		}
-	}
 
 	// Track if we've started streaming (for error handling)
 	streamStarted := false
diff --git a/backend/internal/service/openai_codex_transform.go b/backend/internal/service/openai_codex_transform.go
index a52c88b5..e6c71775 100644
--- a/backend/internal/service/openai_codex_transform.go
+++ b/backend/internal/service/openai_codex_transform.go
@@ -135,7 +135,7 @@ func codexModeEnabled() bool {
 	}
 }
 
-func applyCodexOAuthTransform(reqBody map[string]any, codexMode bool) codexTransformResult {
+func applyCodexOAuthTransform(reqBody map[string]any) codexTransformResult {
 	result := codexTransformResult{}
 
 	model := ""
@@ -151,16 +151,13 @@ func applyCodexOAuthTransform(reqBody map[string]any, codexMode bool) codexTrans
 		result.NormalizedModel = normalizedModel
 	}
 
-	reqBody["store"] = false
-	reqBody["stream"] = true
-	result.Modified = true
-
-	instructions := getCodexInstructions(normalizedModel)
-	if instructions != "" {
-		if existing, ok := reqBody["instructions"].(string); !ok || existing != instructions {
-			reqBody["instructions"] = instructions
-			result.Modified = true
-		}
+	if v, ok := reqBody["store"].(bool); !ok || v {
+		reqBody["store"] = false
+		result.Modified = true
+	}
+	if v, ok := reqBody["stream"].(bool); !ok || !v {
+		reqBody["stream"] = true
+		result.Modified = true
 	}
 
 	if _, ok := reqBody["max_output_tokens"]; ok {
@@ -180,49 +177,30 @@ func applyCodexOAuthTransform(reqBody map[string]any, codexMode bool) codexTrans
 		result.PromptCacheKey = strings.TrimSpace(v)
 	}
 
+	instructions := strings.TrimSpace(getCodexInstructions(normalizedModel))
+	existingInstructions, _ := reqBody["instructions"].(string)
+	existingInstructions = strings.TrimSpace(existingInstructions)
+
+	if instructions != "" {
+		if existingInstructions != "" && existingInstructions != instructions {
+			if input, ok := reqBody["input"].([]any); ok {
+				reqBody["input"] = prependSystemInstruction(input, existingInstructions)
+				result.Modified = true
+			}
+		}
+		if existingInstructions != instructions {
+			reqBody["instructions"] = instructions
+			result.Modified = true
+		}
+	}
+
 	if input, ok := reqBody["input"].([]any); ok {
 		input = filterCodexInput(input)
-		if codexMode {
-			cachedPrompt := getOpenCodeCodexPrompt()
-			input = filterOpenCodeSystemPromptsWithCachedPrompt(input, cachedPrompt)
-			if hasTools(reqBody) {
-				input = addCodexBridgeMessage(input)
-			}
-		} else if hasTools(reqBody) {
-			input = addToolRemapMessage(input)
-		}
 		input = normalizeOrphanedToolOutputs(input)
 		reqBody["input"] = input
 		result.Modified = true
 	}
 
-	effort, summary := resolveCodexReasoning(reqBody, normalizedModel)
-	if effort != "" || summary != "" {
-		reasoning := ensureMap(reqBody["reasoning"])
-		if effort != "" {
-			reasoning["effort"] = effort
-		}
-		if summary != "" {
-			reasoning["summary"] = summary
-		}
-		reqBody["reasoning"] = reasoning
-		result.Modified = true
-	}
-
-	textVerbosity := resolveTextVerbosity(reqBody)
-	if textVerbosity != "" {
-		text := ensureMap(reqBody["text"])
-		text["verbosity"] = textVerbosity
-		reqBody["text"] = text
-		result.Modified = true
-	}
-
-	include := resolveInclude(reqBody)
-	if include != nil {
-		reqBody["include"] = include
-		result.Modified = true
-	}
-
 	return result
 }
 
@@ -487,6 +465,19 @@ func filterCodexInput(input []any) []any {
 	return filtered
 }
 
+func prependSystemInstruction(input []any, instructions string) []any {
+	message := map[string]any{
+		"role": "system",
+		"content": []any{
+			map[string]any{
+				"type": "input_text",
+				"text": instructions,
+			},
+		},
+	}
+	return append([]any{message}, input...)
+}
+
 func filterOpenCodeSystemPromptsWithCachedPrompt(input []any, cachedPrompt string) []any {
 	if len(input) == 0 {
 		return input
diff --git a/backend/internal/service/openai_gateway_service.go b/backend/internal/service/openai_gateway_service.go
index 8f59110d..33244330 100644
--- a/backend/internal/service/openai_gateway_service.go
+++ b/backend/internal/service/openai_gateway_service.go
@@ -21,6 +21,7 @@ import (
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/openai"
 	"github.com/Wei-Shaw/sub2api/internal/util/responseheaders"
 	"github.com/Wei-Shaw/sub2api/internal/util/urlvalidator"
 	"github.com/gin-gonic/gin"
@@ -530,20 +531,28 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 	reqModel, _ := reqBody["model"].(string)
 	reqStream, _ := reqBody["stream"].(bool)
 	promptCacheKey := ""
+	if v, ok := reqBody["prompt_cache_key"].(string); ok {
+		promptCacheKey = strings.TrimSpace(v)
+	}
 
 	// Track if body needs re-serialization
 	bodyModified := false
 	originalModel := reqModel
 
-	// Apply model mapping
-	mappedModel := account.GetMappedModel(reqModel)
-	if mappedModel != reqModel {
-		reqBody["model"] = mappedModel
-		bodyModified = true
+	isCodexCLI := openai.IsCodexCLIRequest(c.GetHeader("User-Agent"))
+
+	// Apply model mapping (skip for Codex CLI for transparent forwarding)
+	mappedModel := reqModel
+	if !isCodexCLI {
+		mappedModel = account.GetMappedModel(reqModel)
+		if mappedModel != reqModel {
+			reqBody["model"] = mappedModel
+			bodyModified = true
+		}
 	}
 
-	if account.Type == AccountTypeOAuth {
-		codexResult := applyCodexOAuthTransform(reqBody, codexModeEnabled())
+	if account.Type == AccountTypeOAuth && !isCodexCLI {
+		codexResult := applyCodexOAuthTransform(reqBody)
 		if codexResult.Modified {
 			bodyModified = true
 		}