diff --git a/pkg/chat/service.go b/pkg/chat/service.go
index 2aac1ebb..efc194f7 100644
--- a/pkg/chat/service.go
+++ b/pkg/chat/service.go
@@ -461,6 +461,9 @@ func (s *Service) now() time.Time {
 }
 
 func (s *Service) saveSettings(settings ChatSettings) error {
+	if err := s.validateSettings(settings); err != nil {
+		return err
+	}
 	payload := core.JSONMarshalString(settings)
 	return s.store.Set(settingsGroup, settingsKey, payload)
 }
@@ -479,6 +482,9 @@ func (s *Service) loadSettings() ChatSettings {
 }
 
 func (s *Service) selectModel(input selectModelInput) (ChatSettings, error) {
+	if err := s.validateModelName(input.Model); err != nil {
+		return ChatSettings{}, err
+	}
 	settings := s.loadSettings()
 	settings.DefaultModel = input.Model
 	if err := s.saveSettings(settings); err != nil {
@@ -504,6 +510,9 @@ func (s *Service) selectModel(input selectModelInput) (ChatSettings, error) {
 }
 
 func (s *Service) saveConversation(conv Conversation) (Conversation, error) {
+	if err := s.validateConversation(conv); err != nil {
+		return Conversation{}, err
+	}
 	if conv.CreatedAt.IsZero() {
 		conv.CreatedAt = s.now()
 	}
@@ -831,6 +840,9 @@ func (s *Service) send(ctx context.Context, input sendInput) (Conversation, erro
 	for toolRound := 0; toolRound < 3; toolRound++ {
 		effectiveSettings := s.mergedSettings(settings, conv.Settings)
 		conv.Model = s.resolveModel(conv.Model, effectiveSettings.DefaultModel)
+		if err := s.validateAttachmentsForModel(conv.Model, attachmentsForConversationTurn(conv.Messages)); err != nil {
+			return conv, err
+		}
 
 		assistantMessage, err := s.streamAssistant(ctx, conv, effectiveSettings)
 		if err != nil {
@@ -1119,6 +1131,131 @@ func (s *Service) discoverModels() []ModelEntry {
 	return results
 }
 
+func (s *Service) validateSettings(settings ChatSettings) error {
+	if settings.Temperature < 0 || settings.Temperature > 2 {
+		return coreerr.E("chat.settings.save", "temperature must be between 0.0 and 2.0", nil)
+	}
+	if settings.TopP < 0 || settings.TopP > 1 {
+		return coreerr.E("chat.settings.save", "top_p must be between 0.0 and 1.0", nil)
+	}
+	if settings.TopK < 0 || settings.TopK > 200 {
+		return coreerr.E("chat.settings.save", "top_k must be between 0 and 200", nil)
+	}
+	if settings.MaxTokens < 64 || settings.MaxTokens > 32768 {
+		return coreerr.E("chat.settings.save", "max_tokens must be between 64 and 32768", nil)
+	}
+	if !validContextWindow(settings.ContextWindow) {
+		return coreerr.E("chat.settings.save", "context_window must be one of 2048, 4096, 8192, 16384, or 32768", nil)
+	}
+	if err := s.validateOptionalModelName(settings.DefaultModel); err != nil {
+		return err
+	}
+	return nil
+}
+
+func validContextWindow(value int) bool {
+	switch value {
+	case 2048, 4096, 8192, 16384, 32768:
+		return true
+	default:
+		return false
+	}
+}
+
+func (s *Service) validateConversation(conv Conversation) error {
+	if strings.TrimSpace(conv.ID) == "" {
+		return coreerr.E("chat.saveConversation", "conversation id is required", nil)
+	}
+	if err := s.validateOptionalModelName(conv.Model); err != nil {
+		return err
+	}
+	if conv.Settings != nil {
+		if err := s.validateSettings(*conv.Settings); err != nil {
+			return err
+		}
+	}
+	for _, message := range conv.Messages {
+		if err := validateMessageAttachments(message); err != nil {
+			return err
+		}
+	}
+	if err := s.validateAttachmentsForModel(s.resolveModel(conv.Model, s.loadSettings().DefaultModel), attachmentsForConversationTurn(conv.Messages)); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (s *Service) validateModelName(name string) error {
+	if strings.TrimSpace(name) == "" {
+		return coreerr.E("chat.selectModel", "model is required", nil)
+	}
+	if len(s.discoverModels()) == 0 {
+		return nil
+	}
+	if _, ok := s.findModel(name); ok {
+		return nil
+	}
+	return coreerr.E("chat.selectModel", "model is not available: "+name, nil)
+}
+
+func (s *Service) validateOptionalModelName(name string) error {
+	if strings.TrimSpace(name) == "" {
+		return nil
+	}
+	if len(s.discoverModels()) == 0 || strings.EqualFold(strings.TrimSpace(name), "default") {
+		return nil
+	}
+	if _, ok := s.findModel(name); ok {
+		return nil
+	}
+	return coreerr.E("chat.model", "model is not available: "+name, nil)
+}
+
+func (s *Service) findModel(name string) (ModelEntry, bool) {
+	for _, model := range s.discoverModels() {
+		if strings.EqualFold(model.Name, strings.TrimSpace(name)) {
+			return model, true
+		}
+	}
+	return ModelEntry{}, false
+}
+
+func (s *Service) validateAttachmentsForModel(modelName string, attachments []ImageAttachment) error {
+	if len(attachments) == 0 {
+		return nil
+	}
+	model, ok := s.findModel(modelName)
+	if !ok {
+		return coreerr.E("chat.send", "image attachments require a discovered vision-capable model", nil)
+	}
+	if !model.SupportsVision {
+		return coreerr.E("chat.send", "selected model does not support image input: "+model.Name, nil)
+	}
+	return nil
+}
+
+func validateMessageAttachments(message ChatMessage) error {
+	for _, attachment := range message.Attachments {
+		if err := validateImageAttachment(attachment); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func attachmentsForConversationTurn(messages []ChatMessage) []ImageAttachment {
+	if len(messages) == 0 {
+		return nil
+	}
+	for index := len(messages) - 1; index >= 0; index-- {
+		if messages[index].Role != "user" {
+			continue
+		}
+		return messages[index].Attachments
+	}
+	return nil
+}
+
 func discoverModelsOnDisk(root string) []ModelEntry {
 	if strings.TrimSpace(root) == "" {
 		return nil
diff --git a/pkg/chat/service_test.go b/pkg/chat/service_test.go
index 781e3d8f..a20cb951 100644
--- a/pkg/chat/service_test.go
+++ b/pkg/chat/service_test.go
@@ -5,6 +5,7 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -37,25 +38,39 @@ func (m *mockToolExecutor) CallTool(_ context.Context, name string, arguments ma
 	return `{"mode":"left-right"}`, nil
 }
 
-func newChatCore(t *testing.T, handler http.HandlerFunc, toolExecutor ToolExecutor) *core.Core {
+func newChatCore(t *testing.T, handler http.HandlerFunc, toolExecutor ToolExecutor, optionFns ...func(*Options)) *core.Core {
 	t.Helper()
 	server := httptest.NewServer(handler)
 	t.Cleanup(server.Close)
 
+	options := []func(*Options){
+		func(o *Options) { o.APIURL = server.URL },
+		func(o *Options) { o.StorePath = filepath.Join(t.TempDir(), "chat.db") },
+		func(o *Options) { o.ToolExecutor = toolExecutor },
+		func(o *Options) { o.Now = func() time.Time { return time.Unix(1_700_000_000, 0).UTC() } },
+		func(o *Options) { o.ModelRoots = nil },
+	}
+	options = append(options, optionFns...)
+
 	c := core.New(
-		core.WithService(Register(
-			func(o *Options) { o.APIURL = server.URL },
-			func(o *Options) { o.StorePath = filepath.Join(t.TempDir(), "chat.db") },
-			func(o *Options) { o.ToolExecutor = toolExecutor },
-			func(o *Options) { o.Now = func() time.Time { return time.Unix(1_700_000_000, 0).UTC() } },
-			func(o *Options) { o.ModelRoots = nil },
-		)),
+		core.WithService(Register(options...)),
 		core.WithServiceLock(),
 	)
 	require.True(t, c.ServiceStartup(context.Background(), nil).OK)
 	return c
 }
 
+func createDiscoveredModelRoot(t *testing.T, name, architecture string) string {
+	t.Helper()
+	root := t.TempDir()
+	modelDir := filepath.Join(root, name)
+	require.NoError(t, os.MkdirAll(modelDir, 0o755))
+	configJSON := `{"model_type":"` + architecture + `","quantization":{"bits":4,"group_size":32}}`
+	require.NoError(t, os.WriteFile(filepath.Join(modelDir, "config.json"), []byte(configJSON), 0o644))
+	require.NoError(t, os.WriteFile(filepath.Join(modelDir, "weights.safetensors"), []byte("fake"), 0o644))
+	return root
+}
+
 func TestService_Good_SendAndHistory(t *testing.T) {
 	c := newChatCore(t, func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "text/event-stream")
@@ -155,3 +170,61 @@ func TestService_Good_SettingsDefaults(t *testing.T) {
 	require.True(t, actionResult.OK)
 	assert.Equal(t, DefaultSettings(), actionResult.Value.(ChatSettings))
 }
+
+func TestService_Bad_SettingsRejectOutOfRangeValues(t *testing.T) {
+	c := newChatCore(t, func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		_, _ = io.WriteString(w, "data: [DONE]\n\n")
+	}, &mockToolExecutor{})
+
+	result := c.Action("gui.chat.settings.save").Run(context.Background(), core.NewOptions(
+		core.Option{Key: "temperature", Value: float32(2.5)},
+		core.Option{Key: "top_p", Value: float32(0.95)},
+		core.Option{Key: "top_k", Value: 64},
+		core.Option{Key: "max_tokens", Value: 2048},
+		core.Option{Key: "context_window", Value: 8192},
+		core.Option{Key: "system_prompt", Value: "You are a helpful assistant."},
+	))
+	require.False(t, result.OK)
+	require.Error(t, result.Value.(error))
+	assert.Contains(t, result.Value.(error).Error(), "temperature must be between 0.0 and 2.0")
+}
+
+func TestService_Bad_SelectModelRejectsUnknownModel(t *testing.T) {
+	modelRoot := createDiscoveredModelRoot(t, "lemer", "gemma3")
+	c := newChatCore(t, func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		_, _ = io.WriteString(w, "data: [DONE]\n\n")
+	}, &mockToolExecutor{}, func(o *Options) { o.ModelRoots = []string{modelRoot} })
+
+	result := c.Action("gui.chat.selectModel").Run(context.Background(), core.NewOptions(
+		core.Option{Key: "model", Value: "missing-model"},
+	))
+	require.False(t, result.OK)
+	require.Error(t, result.Value.(error))
+	assert.Contains(t, result.Value.(error).Error(), "model is not available")
+}
+
+func TestService_Bad_SendImageRejectsNonVisionModel(t *testing.T) {
+	modelRoot := createDiscoveredModelRoot(t, "lemma", "qwen3")
+	c := newChatCore(t, func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		_, _ = io.WriteString(w, "data: [DONE]\n\n")
+	}, &mockToolExecutor{}, func(o *Options) { o.ModelRoots = []string{modelRoot} })
+
+	attach := c.Action("gui.chat.attachImage").Run(context.Background(), core.NewOptions(
+		core.Option{Key: "filename", Value: "photo.png"},
+		core.Option{Key: "mime_type", Value: "image/png"},
+		core.Option{Key: "data", Value: "ZmFrZQ=="},
+		core.Option{Key: "width", Value: 32},
+		core.Option{Key: "height", Value: 32},
+	))
+	require.True(t, attach.OK)
+
+	send := c.Action("gui.chat.send").Run(context.Background(), core.NewOptions(
+		core.Option{Key: "content", Value: "Describe this image"},
+	))
+	require.False(t, send.OK)
+	require.Error(t, send.Value.(error))
+	assert.Contains(t, send.Value.(error).Error(), "does not support image input")
+}