fix(gemini): honor pro-model thinking constraints

pkg/providers/gemini_provider.go

@@ -347,12 +347,21 @@ func buildGeminiThinkingConfig(model string, options map[string]any) map[string]
 	config["includeThoughts"] = includeThoughts
 
 	if isGemini25Model(model) {
+		if isGemini25ProModel(model) && (rawLevel == "off" || rawLevel == "minimal") {
+			// Gemini 2.5 Pro cannot disable thinking; keep model-default thinking.
+			return config
+		}
 		if budget, ok := mapGeminiThinkingBudget(rawLevel); ok {
 			config["thinkingBudget"] = budget
 		}
 		return config
 	}
 
+	if isGemini3ProModel(model) && (rawLevel == "off" || rawLevel == "minimal") {
+		// Gemini 3.x Pro does not support minimal thinking level.
+		return config
+	}
+
 	if thinkingLevel := mapGeminiThinkingLevel(rawLevel); thinkingLevel != "" {
 		config["thinkingLevel"] = thinkingLevel
 	}
@@ -369,6 +378,16 @@ func isGemini25Model(model string) bool {
 	return strings.Contains(lowerModel, "gemini-2.5") || strings.Contains(lowerModel, "gemini-25")
 }
 
+func isGemini25ProModel(model string) bool {
+	lowerModel := strings.ToLower(strings.TrimSpace(model))
+	return isGemini25Model(lowerModel) && strings.Contains(lowerModel, "pro")
+}
+
+func isGemini3ProModel(model string) bool {
+	lowerModel := strings.ToLower(strings.TrimSpace(model))
+	return strings.Contains(lowerModel, "gemini-3") && strings.Contains(lowerModel, "pro")
+}
+
 func mapGeminiThinkingBudget(level string) (int, bool) {
 	level = strings.ToLower(strings.TrimSpace(level))
 	if level == "" {

pkg/providers/gemini_provider_test.go

@@ -362,6 +362,56 @@ func TestGeminiProvider_BuildRequestBody_DefaultsThinkingOffForGemini3(t *testin
 	}
 }
 
+func TestGeminiProvider_BuildRequestBody_DefaultsThinkingOffForGemini25Pro(t *testing.T) {
+	provider := NewGeminiProvider("test-key", "https://example.com/v1beta", "", "", 0, nil, nil)
+	body := provider.buildRequestBody(
+		[]Message{{Role: "user", Content: "hello"}},
+		nil,
+		"gemini-2.5-pro",
+		nil,
+	)
+
+	generationConfig, ok := body["generationConfig"].(map[string]any)
+	if !ok {
+		t.Fatalf("generationConfig = %#v, want map", body["generationConfig"])
+	}
+	thinkingConfig, ok := generationConfig["thinkingConfig"].(map[string]any)
+	if !ok {
+		t.Fatalf("thinkingConfig = %#v, want map", generationConfig["thinkingConfig"])
+	}
+	if includeThoughts, ok := thinkingConfig["includeThoughts"].(bool); !ok || includeThoughts {
+		t.Fatalf("includeThoughts = %#v, want false for default/off", thinkingConfig["includeThoughts"])
+	}
+	if _, hasBudget := thinkingConfig["thinkingBudget"]; hasBudget {
+		t.Fatalf("thinkingBudget should be omitted for Gemini 2.5 Pro default/off: %#v", thinkingConfig)
+	}
+}
+
+func TestGeminiProvider_BuildRequestBody_DefaultsThinkingOffForGemini31Pro(t *testing.T) {
+	provider := NewGeminiProvider("test-key", "https://example.com/v1beta", "", "", 0, nil, nil)
+	body := provider.buildRequestBody(
+		[]Message{{Role: "user", Content: "hello"}},
+		nil,
+		"gemini-3.1-pro",
+		nil,
+	)
+
+	generationConfig, ok := body["generationConfig"].(map[string]any)
+	if !ok {
+		t.Fatalf("generationConfig = %#v, want map", body["generationConfig"])
+	}
+	thinkingConfig, ok := generationConfig["thinkingConfig"].(map[string]any)
+	if !ok {
+		t.Fatalf("thinkingConfig = %#v, want map", generationConfig["thinkingConfig"])
+	}
+	if includeThoughts, ok := thinkingConfig["includeThoughts"].(bool); !ok || includeThoughts {
+		t.Fatalf("includeThoughts = %#v, want false for default/off", thinkingConfig["includeThoughts"])
+	}
+	if _, hasLevel := thinkingConfig["thinkingLevel"]; hasLevel {
+		t.Fatalf("thinkingLevel should be omitted for Gemini 3.1 Pro default/off: %#v", thinkingConfig)
+	}
+}
+
 func TestGeminiProvider_BuildRequestBody_PreservesMultipleSystemMessages(t *testing.T) {
 	provider := NewGeminiProvider("test-key", "https://example.com/v1beta", "", "", 0, nil, nil)
 	body := provider.buildRequestBody(