diff --git a/internal/translator/gemini/openai/responses/gemini_openai-responses_request.go b/internal/translator/gemini/openai/responses/gemini_openai-responses_request.go
index 29d66df5..f7d0f18a 100644
--- a/internal/translator/gemini/openai/responses/gemini_openai-responses_request.go
+++ b/internal/translator/gemini/openai/responses/gemini_openai-responses_request.go
@@ -119,7 +119,7 @@ func ConvertOpenAIResponsesRequestToGemini(modelName string, inputRawJSON []byte
 
 			switch itemType {
 			case "message":
-				if strings.EqualFold(itemRole, "system") {
+				if strings.EqualFold(itemRole, "system") || strings.EqualFold(itemRole, "developer") {
 					if contentArray := item.Get("content"); contentArray.Exists() {
 						systemInstr := []byte(`{"parts":[]}`)
 						if systemInstructionResult := gjson.GetBytes(out, "systemInstruction"); systemInstructionResult.Exists() {
diff --git a/internal/translator/gemini/openai/responses/gemini_openai-responses_request_test.go b/internal/translator/gemini/openai/responses/gemini_openai-responses_request_test.go
index 35418689..0693b63d 100644
--- a/internal/translator/gemini/openai/responses/gemini_openai-responses_request_test.go
+++ b/internal/translator/gemini/openai/responses/gemini_openai-responses_request_test.go
@@ -55,6 +55,80 @@ func TestConvertOpenAIResponsesRequestToGemini_ReasoningSignatureCompatibility(t
 	}
 }
 
+func TestConvertOpenAIResponsesRequestToGemini_SystemAndDeveloperRoles(t *testing.T) {
+	tests := []struct {
+		name     string
+		role     string
+		wantText string
+	}{
+		{
+			name:     "system role",
+			role:     "system",
+			wantText: "System message text",
+		},
+		{
+			name:     "developer role",
+			role:     "developer",
+			wantText: "Developer message text",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			input := []byte(`{
+				"instructions": "Be a helpful assistant",
+				"input": [
+					{
+						"type": "message",
+						"role": "` + tt.role + `",
+						"content": [
+							{
+								"type": "input_text",
+								"text": "` + tt.wantText + `"
+							}
+						]
+					},
+					{
+						"type": "message",
+						"role": "user",
+						"content": [
+							{
+								"type": "input_text",
+								"text": "Hello"
+							}
+						]
+					}
+				]
+			}`)
+
+			output := ConvertOpenAIResponsesRequestToGemini("gemini-3.5-flash", input, false)
+			result := gjson.ParseBytes(output)
+
+			systemInstruction := result.Get("systemInstruction")
+			if !systemInstruction.Exists() {
+				t.Fatalf("systemInstruction missing. Output: %s", output)
+			}
+			parts := systemInstruction.Get("parts")
+			if got := parts.Get("#").Int(); got != 2 {
+				t.Fatalf("systemInstruction parts = %d, want 2. Output: %s", got, output)
+			}
+			if got := parts.Get("0.text").String(); got != "Be a helpful assistant" {
+				t.Fatalf("first systemInstruction part = %q, want %q. Output: %s", got, "Be a helpful assistant", output)
+			}
+			if got := parts.Get("1.text").String(); got != tt.wantText {
+				t.Fatalf("second systemInstruction part = %q, want %q. Output: %s", got, tt.wantText, output)
+			}
+
+			result.Get("contents").ForEach(func(_, value gjson.Result) bool {
+				if role := value.Get("role").String(); role == tt.role {
+					t.Fatalf("role %q leaked into contents array. Output: %s", tt.role, output)
+				}
+				return true
+			})
+		})
+	}
+}
+
 func validResponsesGPTReasoningSignature() string {
 	raw := make([]byte, 1+8+16+16+32)
 	raw[0] = 0x80