diff --git a/internal/runtime/executor/aistudio_executor.go b/internal/runtime/executor/aistudio_executor.go
index 042704ffb..0cefe601d 100644
--- a/internal/runtime/executor/aistudio_executor.go
+++ b/internal/runtime/executor/aistudio_executor.go
@@ -488,9 +488,37 @@ func (e *AIStudioExecutor) translateRequest(ctx context.Context, req cliproxyexe
 	}
 	payload, _ = sjson.DeleteBytes(payload, "session_id")
 	payload = helps.EnsureGeminiLeadingUserContent(payload, "contents")
+	payload = normalizeAIStudioThinkingLevel(payload)
 	return payload, translatedPayload{payload: payload, action: action, toFormat: to}, nil
 }
 
+// normalizeAIStudioThinkingLevel normalizes thinking levels to Google's canonical uppercase enum values.
+// The AI Studio upstream validates generationConfig.thinkingConfig.thinkingLevel case-sensitively
+// and rejects lowercase values with a 400 invalid argument error.
+func normalizeAIStudioThinkingLevel(payload []byte) []byte {
+	const path = "generationConfig.thinkingConfig.thinkingLevel"
+	level := gjson.GetBytes(payload, path)
+	if level.Type != gjson.String {
+		return payload
+	}
+
+	normalized := strings.ToUpper(level.String())
+	switch normalized {
+	case "MINIMAL", "LOW", "MEDIUM", "HIGH":
+	default:
+		return payload
+	}
+	if normalized == level.String() {
+		return payload
+	}
+
+	result, err := sjson.SetBytes(payload, path, normalized)
+	if err != nil {
+		return payload
+	}
+	return result
+}
+
 func (e *AIStudioExecutor) buildEndpoint(model, action, alt string) string {
 	base := fmt.Sprintf("%s/%s/models/%s:%s", glEndpoint, glAPIVersion, model, action)
 	if action == "streamGenerateContent" {
diff --git a/internal/runtime/executor/aistudio_executor_test.go b/internal/runtime/executor/aistudio_executor_test.go
index e14aff80e..27e6b36b2 100644
--- a/internal/runtime/executor/aistudio_executor_test.go
+++ b/internal/runtime/executor/aistudio_executor_test.go
@@ -39,6 +39,84 @@ func TestAIStudioTranslateRequestPreservesSummaryFromOriginalRequest(t *testing.
 	}
 }
 
+func TestAIStudioTranslateRequestNormalizesThinkingLevel(t *testing.T) {
+	executor := NewAIStudioExecutor(&config.Config{}, "aistudio", nil)
+	req := cliproxyexecutor.Request{
+		Model: "gemini-3.7-flash",
+		Payload: []byte(`{
+			"contents":[{"role":"user","parts":[{"text":"Human: Hi"}]}],
+			"model":"gemini-3.7-flash",
+			"generationConfig":{"thinkingConfig":{"thinkingLevel":"high","includeThoughts":true}}
+		}`),
+	}
+	opts := cliproxyexecutor.Options{SourceFormat: sdktranslator.FormatGemini}
+
+	payload, _, err := executor.translateRequest(context.Background(), req, opts, false)
+	if err != nil {
+		t.Fatalf("translateRequest() error = %v", err)
+	}
+	if got := gjson.GetBytes(payload, "generationConfig.thinkingConfig.thinkingLevel").String(); got != "HIGH" {
+		t.Fatalf("thinkingLevel = %q, want HIGH; payload=%s", got, payload)
+	}
+	if !gjson.GetBytes(payload, "generationConfig.thinkingConfig.includeThoughts").Bool() {
+		t.Fatalf("includeThoughts was lost: %s", payload)
+	}
+}
+
+func TestAIStudioTranslateRequestNormalizesThinkingLevelAfterPayloadOverride(t *testing.T) {
+	executor := NewAIStudioExecutor(&config.Config{Payload: config.PayloadConfig{Override: []config.PayloadRule{{
+		Models: []config.PayloadModelRule{{Name: "gemini-3.7-flash", Protocol: "gemini"}},
+		Params: map[string]any{"generationConfig.thinkingConfig.thinkingLevel": "medium"},
+	}}}}, "aistudio", nil)
+	req := cliproxyexecutor.Request{
+		Model:   "gemini-3.7-flash",
+		Payload: []byte(`{"contents":[{"role":"user","parts":[{"text":"hi"}]}],"model":"gemini-3.7-flash"}`),
+	}
+	opts := cliproxyexecutor.Options{SourceFormat: sdktranslator.FormatGemini}
+
+	payload, _, err := executor.translateRequest(context.Background(), req, opts, false)
+	if err != nil {
+		t.Fatalf("translateRequest() error = %v", err)
+	}
+	if got := gjson.GetBytes(payload, "generationConfig.thinkingConfig.thinkingLevel").String(); got != "MEDIUM" {
+		t.Fatalf("thinkingLevel = %q, want MEDIUM; payload=%s", got, payload)
+	}
+}
+
+func TestNormalizeAIStudioThinkingLevel(t *testing.T) {
+	tests := []struct {
+		name      string
+		levelJSON string
+		want      string
+		unchanged bool
+	}{
+		{name: "minimal", levelJSON: `"minimal"`, want: "MINIMAL"},
+		{name: "low", levelJSON: `"low"`, want: "LOW"},
+		{name: "medium", levelJSON: `"medium"`, want: "MEDIUM"},
+		{name: "mixed case high", levelJSON: `"hIgH"`, want: "HIGH"},
+		{name: "already uppercase", levelJSON: `"HIGH"`, want: "HIGH", unchanged: true},
+		{name: "none", levelJSON: `"none"`, want: "none", unchanged: true},
+		{name: "xhigh", levelJSON: `"xhigh"`, want: "xhigh", unchanged: true},
+		{name: "max", levelJSON: `"max"`, want: "max", unchanged: true},
+		{name: "custom", levelJSON: `"custom"`, want: "custom", unchanged: true},
+		{name: "non-string", levelJSON: `1`, want: "1", unchanged: true},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			input := []byte(fmt.Sprintf(`{"generationConfig":{"thinkingConfig":{"thinkingLevel":%s}},"marker":true}`, test.levelJSON))
+			output := normalizeAIStudioThinkingLevel(input)
+			level := gjson.GetBytes(output, "generationConfig.thinkingConfig.thinkingLevel")
+			if got := level.String(); got != test.want {
+				t.Fatalf("thinkingLevel = %q, want %q; payload=%s", got, test.want, output)
+			}
+			if test.unchanged && string(output) != string(input) {
+				t.Fatalf("payload changed from %s to %s", input, output)
+			}
+		})
+	}
+}
+
 func TestAIStudioTranslateRequestPrependsLeadingUserForIssue4959ResponsesHistory(t *testing.T) {
 	executor := NewAIStudioExecutor(&config.Config{}, "aistudio", nil)
 	_, body, err := executor.translateRequest(context.Background(), cliproxyexecutor.Request{