diff --git a/go.mod b/go.mod
index 16afa49..bc6845a 100644
--- a/go.mod
+++ b/go.mod
@@ -47,6 +47,7 @@ require (
 	github.com/rivo/uniseg v0.4.7 // indirect
 	github.com/rogpeppe/go-internal v1.14.1 // indirect
 	github.com/spf13/pflag v1.0.10 // indirect
+	github.com/stretchr/objx v0.5.2 // indirect
 	github.com/tidwall/gjson v1.18.0 // indirect
 	github.com/tidwall/match v1.2.0 // indirect
 	github.com/tidwall/pretty v1.2.1 // indirect
diff --git a/go.sum b/go.sum
index 0afbf4f..0b818dc 100644
--- a/go.sum
+++ b/go.sum
@@ -146,6 +146,8 @@ github.com/spf13/pflag v1.0.10 h1:4EBh2KAYBwaONj6b2Ye1GiHfwjqyROoF4RwYO+vPwFk=
 github.com/spf13/pflag v1.0.10/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/objx v0.4.0/go.mod h1:YvHI0jy2hoMjB+UWwv71VJQ9isScKT/TqJzVSSt89Yw=
+github.com/stretchr/objx v0.5.2 h1:xuMeJ0Sdp5ZMRXx/aWO6RZxdr3beISkG5/G/aIRr3pY=
+github.com/stretchr/objx v0.5.2/go.mod h1:FRsXN1f5AsAjCGJKqEizvkpNtU+EGNCLh3NxZ/8L+MA=
 github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 github.com/stretchr/testify v1.8.0/go.mod h1:yNjHg4UonilssWZ8iaSj1OCr/vHnekPRkoO+kdMU+MU=
 github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
diff --git a/internal/ui/model_test.go b/internal/ui/model_test.go
new file mode 100644
index 0000000..2891853
--- /dev/null
+++ b/internal/ui/model_test.go
@@ -0,0 +1,335 @@
+package ui
+
+import (
+	"context"
+	"testing"
+
+	"github.com/charmbracelet/bubbles/spinner"
+	"github.com/charmbracelet/bubbles/textarea"
+	"github.com/charmbracelet/bubbles/textinput"
+	tea "github.com/charmbracelet/bubbletea"
+	"github.com/cockroachdb/errors"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/mock"
+
+	"github.com/rm-hull/git-commit-summary/internal/interfaces"
+	llmprovider "github.com/rm-hull/git-commit-summary/internal/llm_provider"
+)
+
+// MockLLMProvider is a mock implementation of llmprovider.Provider
+type MockLLMProvider struct {
+	mock.Mock
+}
+
+func (m *MockLLMProvider) Call(ctx context.Context, model string, prompt string) (string, error) {
+	args := m.Called(ctx, model, prompt)
+	return args.String(0), args.Error(1)
+}
+
+func (m *MockLLMProvider) Model() string {
+	args := m.Called()
+	return args.String(0)
+}
+
+// MockGitClient is a mock implementation of interfaces.GitClient
+type MockGitClient struct {
+	mock.Mock
+}
+
+func (m *MockGitClient) IsInWorkTree() error {
+	args := m.Called()
+	return args.Error(0)
+}
+
+func (m *MockGitClient) StagedFiles() ([]string, error) {
+	args := m.Called()
+	return args.Get(0).([]string), args.Error(1)
+}
+
+func (m *MockGitClient) Diff() (string, error) {
+	args := m.Called()
+	return args.String(0), args.Error(1)
+}
+
+func (m *MockGitClient) Commit(message string) error {
+	args := m.Called(message)
+	return args.Error(0)
+}
+
+func TestModel_Update(t *testing.T) {
+	ctx := context.Background()
+	mockLLM := new(MockLLMProvider)
+	mockGit := new(MockGitClient)
+
+	// Common setup for InitialModel
+	initialModel := func() *Model {
+		// Explicitly use the types to avoid "imported and not used" warnings
+		var _ interfaces.GitClient = mockGit
+		var _ llmprovider.Provider = mockLLM
+		return InitialModel(ctx, mockLLM, mockGit, "system prompt", "user message")
+	}
+
+	t.Run("tea.KeyMsg - CtrlC in showSpinner state", func(t *testing.T) {
+		m := initialModel()
+		m.state = showSpinner // Ensure initial state is showSpinner
+
+		updatedModel, cmd := m.Update(tea.KeyMsg{Type: tea.KeyCtrlC})
+
+		assert.Equal(t, Abort, updatedModel.(*Model).action)
+		assert.NotNil(t, cmd)
+		assert.IsType(t, tea.QuitMsg{}, cmd())
+	})
+
+	t.Run("tea.KeyMsg - CtrlC in other states", func(t *testing.T) {
+		m := initialModel()
+		m.state = showCommitView // Set to a state other than showSpinner
+
+		// Mock the sub-model's Update method
+		mockCommitView := new(mockTeaModel)
+		mockCommitView.On("Update", tea.KeyMsg{Type: tea.KeyCtrlC}).Return(mockCommitView, (tea.Cmd)(nil))
+		m.commitView = mockCommitView
+
+		updatedModel, cmd := m.Update(tea.KeyMsg{Type: tea.KeyCtrlC})
+
+		assert.Equal(t, None, updatedModel.(*Model).action) // Action should not be Abort
+		assert.Nil(t, cmd)                                  // No tea.Quit command
+		mockCommitView.AssertCalled(t, "Update", tea.KeyMsg{Type: tea.KeyCtrlC})
+	})
+
+	t.Run("gitCheckMsg - empty (no staged changes)", func(t *testing.T) {
+		m := initialModel()
+		m.state = showSpinner // Ensure initial state is showSpinner
+
+		updatedModel, cmd := m.Update(gitCheckMsg{})
+
+		assert.NotNil(t, updatedModel.(*Model).err)
+		assert.NotNil(t, cmd)
+		assert.IsType(t, tea.QuitMsg{}, cmd())
+	})
+
+	t.Run("gitCheckMsg - non-empty (staged changes)", func(t *testing.T) {
+		m := initialModel()
+		m.state = showSpinner // Ensure initial state is showSpinner
+
+		mockGit.On("Diff").Return("mocked diff content", nil).Once()
+
+		updatedModel, cmd := m.Update(gitCheckMsg{"file1.go", "file2.go"})
+
+		assert.Nil(t, updatedModel.(*Model).err)
+		assert.NotNil(t, cmd)
+		msg := cmd()
+		assert.IsType(t, gitDiffMsg(""), msg)
+		assert.Equal(t, gitDiffMsg("mocked diff content"), msg)
+		mockGit.AssertExpectations(t)
+	})
+
+	t.Run("gitDiffMsg", func(t *testing.T) {
+		m := initialModel()
+		m.state = showSpinner // Ensure initial state is showSpinner
+		mockLLM.On("Model").Return("test-model").Once()
+		// The command returned by Update will execute llmProvider.Call later.
+		// No need to set mockLLM.On("Call") here.
+
+		diffContent := "diff --git a/file.go b/file.go"
+		updatedModel, cmd := m.Update(gitDiffMsg(diffContent))
+
+		assert.Equal(t, diffContent, updatedModel.(*Model).diff)
+		assert.Contains(t, updatedModel.(*Model).spinnerMessage, "Generating commit summary (using: test-model)")
+		assert.IsType(t, tea.Batch(nil), cmd)
+		mockLLM.AssertExpectations(t)
+	})
+
+	t.Run("llmResultMsg - with user message", func(t *testing.T) {
+		m := initialModel()
+		m.state = showSpinner // Ensure initial state is showSpinner
+		llmResult := "This is a summary from LLM."
+		userMsg := "Additional user message."
+
+		m.userMessage = userMsg // Set user message for this test case
+
+		updatedModel, cmd := m.Update(llmResultMsg(llmResult))
+
+		assert.Equal(t, showCommitView, updatedModel.(*Model).state)
+		// Assert that the commitView is set, but not its content directly from Update
+		assert.NotNil(t, updatedModel.(*Model).commitView)
+		assert.NotNil(t, cmd)
+		assert.IsType(t, textarea.Blink(), cmd())
+	})
+
+	t.Run("llmResultMsg - without user message", func(t *testing.T) {
+		m := initialModel()
+		m.state = showSpinner // Ensure initial state is showSpinner
+		llmResult := "This is a summary from LLM."
+		m.userMessage = "" // Ensure no user message
+
+		updatedModel, cmd := m.Update(llmResultMsg(llmResult))
+
+		assert.Equal(t, showCommitView, updatedModel.(*Model).state)
+		// Assert that the commitView is set, but not its content directly from Update
+		assert.NotNil(t, updatedModel.(*Model).commitView)
+		assert.NotNil(t, cmd)
+		assert.IsType(t, textarea.Blink(), cmd())
+	})
+
+	t.Run("commitMsg", func(t *testing.T) {
+		m := initialModel()
+		m.state = showCommitView // Ensure state is showCommitView
+
+		commitContent := "feat: new feature"
+		updatedModel, cmd := m.Update(commitMsg(commitContent))
+
+		assert.Equal(t, Commit, updatedModel.(*Model).action)
+		assert.Equal(t, commitContent, updatedModel.(*Model).commitMessage)
+		assert.NotNil(t, cmd)
+		assert.IsType(t, tea.QuitMsg{}, cmd())
+	})
+
+	t.Run("regenerateMsg", func(t *testing.T) {
+		m := initialModel()
+		m.state = showCommitView // Ensure state is showCommitView
+
+		updatedModel, cmd := m.Update(regenerateMsg{})
+
+		assert.Equal(t, showRegeneratePrompt, updatedModel.(*Model).state)
+		assert.NotNil(t, updatedModel.(*Model).promptView)
+		assert.NotNil(t, cmd)
+		assert.IsType(t, textinput.Blink(), cmd())
+	})
+
+	t.Run("userResponseMsg", func(t *testing.T) {
+		m := initialModel()
+		m.state = showRegeneratePrompt // Ensure state is showRegeneratePrompt
+		mockLLM.On("Model").Return("test-model").Once()
+		// The command returned by Update will execute llmProvider.Call later.
+		// No need to set mockLLM.On("Call") here.
+
+		userResponse := "make it shorter"
+		updatedModel, cmd := m.Update(userResponseMsg(userResponse))
+
+		assert.Equal(t, showSpinner, updatedModel.(*Model).state)
+		assert.Contains(t, updatedModel.(*Model).spinnerMessage, "Re-generating commit summary (using: test-model)")
+		assert.IsType(t, tea.Batch(nil), cmd) // Should return tea.Batch(m.spinner.Tick, m.generateSummary)
+		mockLLM.AssertExpectations(t)
+	})
+
+	t.Run("cancelRegenPromptMsg", func(t *testing.T) {
+		m := initialModel()
+		m.state = showRegeneratePrompt // Ensure state is showRegeneratePrompt
+
+		// Mock the sub-model's Init method
+		mockCommitView := new(mockTeaModel)
+		mockCommitView.On("Init").Return((tea.Cmd)(nil)).Once()
+		m.commitView = mockCommitView
+
+		updatedModel, cmd := m.Update(cancelRegenPromptMsg{})
+
+		assert.Equal(t, showCommitView, updatedModel.(*Model).state)
+		assert.Nil(t, cmd) // Should return m.commitView.Init() which is mocked to return nil
+		mockCommitView.AssertExpectations(t)
+	})
+
+	t.Run("errMsg", func(t *testing.T) {
+		m := initialModel()
+		m.state = showSpinner // Ensure state is showSpinner
+
+		testErr := errors.New("something went wrong")
+		updatedModel, cmd := m.Update(errMsg{err: testErr})
+
+		assert.Equal(t, testErr, updatedModel.(*Model).err)
+		assert.NotNil(t, cmd)
+		assert.IsType(t, tea.QuitMsg{}, cmd())
+	})
+
+	t.Run("abortMsg", func(t *testing.T) {
+		m := initialModel()
+		m.state = showCommitView // Ensure state is showCommitView
+
+		updatedModel, cmd := m.Update(abortMsg{})
+
+		assert.Equal(t, Abort, updatedModel.(*Model).action)
+		assert.NotNil(t, cmd)
+		assert.IsType(t, tea.QuitMsg{}, cmd())
+	})
+
+	t.Run("spinner.Update for showSpinner state", func(t *testing.T) {
+		m := initialModel()
+		m.state = showSpinner
+		// Spinner's Update method is tested by charmbracelet/bubbles,
+		// here we just ensure it's called and returns its cmd.
+		// We can't easily mock spinner.Model directly, so we'll check the cmd.
+		_, cmd := m.Update(spinner.TickMsg{})
+		assert.NotNil(t, cmd)
+		assert.IsType(t, spinner.TickMsg{}, cmd())
+	})
+
+	t.Run("commitView.Update for showCommitView state", func(t *testing.T) {
+		m := initialModel()
+		m.state = showCommitView
+		mockCommitView := new(mockTeaModel)
+		mockCommitView.On("Update", mock.Anything).Return(mockCommitView, (tea.Cmd)(nil)).Once()
+		m.commitView = mockCommitView
+
+		testMsg := tea.KeyMsg{Type: tea.KeyEnter}
+		updatedModel, cmd := m.Update(testMsg)
+
+		assert.NotNil(t, updatedModel)
+		assert.Nil(t, cmd) // Mock returns nil cmd
+		mockCommitView.AssertCalled(t, "Update", testMsg)
+	})
+
+	t.Run("promptView.Update for showRegeneratePrompt state", func(t *testing.T) {
+		m := initialModel()
+		m.state = showRegeneratePrompt
+		mockPromptView := new(mockTeaModel)
+		mockPromptView.On("Update", mock.Anything).Return(mockPromptView, (tea.Cmd)(nil)).Once()
+		m.promptView = mockPromptView
+
+		testMsg := tea.KeyMsg{Type: tea.KeyEnter}
+		updatedModel, cmd := m.Update(testMsg)
+
+		assert.NotNil(t, updatedModel)
+		assert.Nil(t, cmd) // Mock returns nil cmd
+		mockPromptView.AssertCalled(t, "Update", testMsg)
+	})
+}
+
+// mockTeaModel is a generic mock for tea.Model interface
+type mockTeaModel struct {
+	mock.Mock
+}
+
+func (m *mockTeaModel) Init() tea.Cmd {
+	args := m.Called()
+	if len(args) > 0 {
+		if cmd, ok := args.Get(0).(tea.Cmd); ok {
+			return cmd
+		}
+	}
+	return nil
+}
+
+func (m *mockTeaModel) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
+	args := m.Called(msg)
+	var model tea.Model = m
+	var cmd tea.Cmd
+
+	if len(args) > 0 {
+		if m, ok := args.Get(0).(tea.Model); ok {
+			model = m
+		}
+	}
+	if len(args) > 1 {
+		if c, ok := args.Get(1).(tea.Cmd); ok {
+			cmd = c
+		}
+	}
+	return model, cmd
+}
+
+func (m *mockTeaModel) View() string {
+	args := m.Called()
+	if len(args) > 0 {
+		return args.String(0)
+	}
+	return ""
+}