diff --git a/internal/translator/codex/openai/responses/codex_openai-responses_request.go b/internal/translator/codex/openai/responses/codex_openai-responses_request.go
index 828c4d87..f0407149 100644
--- a/internal/translator/codex/openai/responses/codex_openai-responses_request.go
+++ b/internal/translator/codex/openai/responses/codex_openai-responses_request.go
@@ -27,6 +27,9 @@ func ConvertOpenAIResponsesRequestToCodex(modelName string, inputRawJSON []byte,
 	rawJSON, _ = sjson.DeleteBytes(rawJSON, "top_p")
 	rawJSON, _ = sjson.DeleteBytes(rawJSON, "service_tier")
 
+	// Delete the user field as it is not supported by the Codex upstream.
+	rawJSON, _ = sjson.DeleteBytes(rawJSON, "user")
+
 	// Convert role "system" to "developer" in input array to comply with Codex API requirements.
 	rawJSON = convertSystemRoleToDeveloper(rawJSON)
 
diff --git a/internal/translator/codex/openai/responses/codex_openai-responses_request_test.go b/internal/translator/codex/openai/responses/codex_openai-responses_request_test.go
index ea413238..4f562486 100644
--- a/internal/translator/codex/openai/responses/codex_openai-responses_request_test.go
+++ b/internal/translator/codex/openai/responses/codex_openai-responses_request_test.go
@@ -263,3 +263,20 @@ func TestConvertSystemRoleToDeveloper_AssistantRole(t *testing.T) {
 		t.Errorf("Expected third role 'assistant', got '%s'", thirdRole.String())
 	}
 }
+
+func TestUserFieldDeletion(t *testing.T) {  
+	inputJSON := []byte(`{  
+		"model": "gpt-5.2",  
+		"user": "test-user",  
+		"input": [{"role": "user", "content": "Hello"}]  
+	}`)  
+	  
+	output := ConvertOpenAIResponsesRequestToCodex("gpt-5.2", inputJSON, false)  
+	outputStr := string(output)  
+	  
+	// Verify user field is deleted  
+	userField := gjson.Get(outputStr, "user")  
+	if userField.Exists() {
+		t.Errorf("user field should be deleted, but it was found with value: %s", userField.Raw)
+	}
+}