diff --git a/src/app/controller/preview-session.go b/src/app/controller/preview-session.go
new file mode 100644
index 0000000..9e4448a
--- /dev/null
+++ b/src/app/controller/preview-session.go
@@ -0,0 +1,49 @@
+package controller
+
+import (
+	"sync/atomic"
+	"time"
+
+	"github.com/snivilised/jaywalk/src/agenor/core"
+)
+
+// PreviewSession is a lightweight session implementation for the tweak
+// preview traversal. It implements core.Session but strips out all
+// persistence, resume, and fault handling. Timing methods return zero
+// values — they are not meaningful for a disposable preview session.
+// Ctrl-C exits cleanly via context cancellation in the caller — no
+// navigation state is saved.
+type PreviewSession struct {
+	done atomic.Bool
+}
+
+// NewPreviewSession creates a PreviewSession with IsComplete() == false.
+func NewPreviewSession() *PreviewSession {
+	return &PreviewSession{}
+}
+
+// IsComplete returns true when MarkComplete has been called.
+func (s *PreviewSession) IsComplete() bool {
+	return s.done.Load()
+}
+
+// StartedAt returns the zero time. Not meaningful for preview sessions.
+func (s *PreviewSession) StartedAt() time.Time {
+	return time.Time{}
+}
+
+// Elapsed returns 0. Not meaningful for preview sessions.
+func (s *PreviewSession) Elapsed() time.Duration {
+	return 0
+}
+
+// MarkComplete sets the session as completed. Called by the tweak
+// coordinator when a preview traversal finishes normally. On Ctrl-C
+// the context cancels the traversal and MarkComplete is never called,
+// so IsComplete remains false.
+func (s *PreviewSession) MarkComplete() {
+	s.done.Store(true)
+}
+
+// compile-time check: *PreviewSession satisfies core.Session.
+var _ core.Session = (*PreviewSession)(nil)
diff --git a/src/app/controller/preview-session_test.go b/src/app/controller/preview-session_test.go
new file mode 100644
index 0000000..c7c2eea
--- /dev/null
+++ b/src/app/controller/preview-session_test.go
@@ -0,0 +1,63 @@
+package controller_test
+
+import (
+	"time"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+	"github.com/snivilised/jaywalk/src/agenor/core"
+	jac "github.com/snivilised/jaywalk/src/app/controller"
+)
+
+var _ = Describe("PreviewSession", func() {
+	var session *jac.PreviewSession
+
+	BeforeEach(func() {
+		session = jac.NewPreviewSession()
+	})
+
+	Describe("NewPreviewSession", func() {
+		It("starts with IsComplete == false", func() {
+			Expect(session.IsComplete()).To(BeFalse())
+		})
+	})
+
+	Describe("MarkComplete", func() {
+		It("sets IsComplete to true", func() {
+			session.MarkComplete()
+			Expect(session.IsComplete()).To(BeTrue())
+		})
+
+		It("can be called multiple times (idempotent)", func() {
+			session.MarkComplete()
+			session.MarkComplete()
+			Expect(session.IsComplete()).To(BeTrue())
+		})
+	})
+
+	Describe("StartedAt", func() {
+		It("returns the zero time", func() {
+			Expect(session.StartedAt()).To(Equal(time.Time{}))
+		})
+	})
+
+	Describe("Elapsed", func() {
+		It("returns 0", func() {
+			Expect(session.Elapsed()).To(Equal(time.Duration(0)))
+		})
+	})
+
+	Describe("core.Session implementation", func() {
+		It("satisfies the core.Session interface", func() {
+			var s core.Session = session
+			Expect(s).NotTo(BeNil())
+		})
+
+		It("can be assigned to a core.Session variable", func() {
+			var s core.Session = session
+			Expect(s.IsComplete()).To(BeFalse())
+			session.MarkComplete()
+			Expect(s.IsComplete()).To(BeTrue())
+		})
+	})
+})
diff --git a/src/prism/contract/messages.go b/src/prism/contract/messages.go
new file mode 100644
index 0000000..23a775c
--- /dev/null
+++ b/src/prism/contract/messages.go
@@ -0,0 +1,9 @@
+package contract
+
+// ThemeUpdateMsg carries a freshly built Theme to the active view
+// model. Sent by tweak whenever a working-state palette change occurs.
+// Each view model's Update method gains a ThemeUpdateMsg case that
+// stores the new theme. The next render uses it automatically.
+type ThemeUpdateMsg struct {
+	Theme Theme
+}