diff --git a/docs/REFACTORING.md b/docs/REFACTORING.md
index fa77079..47015da 100644
--- a/docs/REFACTORING.md
+++ b/docs/REFACTORING.md
@@ -260,7 +260,7 @@ Track progress here. Mark tasks complete as they land and pass review.
 - [x] T2.4 — Update `storage`: load/save Job only; move runtime init _(landed with T2.3)_
 
 ### Phase 3 — Application service layer
-- [ ] T3.1 — Create `src/app/service.go`; owns state behind mutex
+- [x] T3.1 — Create `src/app/service.go`; owns state behind mutex
 - [ ] T3.2 — Add `src/app/events.go`; Event types + Observer
 - [ ] T3.3 — Add state-mutating operations to service
 - [ ] T3.4 — Convert `scheduler` to use service; inject Clock
diff --git a/src/app/service.go b/src/app/service.go
new file mode 100644
index 0000000..196eaed
--- /dev/null
+++ b/src/app/service.go
@@ -0,0 +1,84 @@
+package app
+
+import (
+	"sync"
+
+	"gitea.mixdep.ru/mix/gosentry/src/domain"
+	"gitea.mixdep.ru/mix/gosentry/src/storage"
+)
+
+// Service is the application-service layer: the single owner of GoSentry's
+// in-memory state. It holds the durable jobs slice, the transient runtime map
+// keyed by Job.ID, and a reference to the store that persists them. All access
+// to that state goes through a mutex so the GUI and the scheduler can no longer
+// race on a shared *[]Job.
+//
+// This is the first slice of the layer (T3.1): it establishes ownership and the
+// locking contract. State-mutating intents (CreateJob, RunNow, SetGlobalPause,
+// ...) and the event/observer machinery are added in later tasks; for now the
+// Service only owns state and exposes read snapshots.
+//
+// Locking contract: mu is a plain, non-reentrant mutex. Exported methods take
+// it; unexported helpers ending in "Locked" assume the caller already holds it.
+// The Service must never call back into the UI (or any code that might re-enter
+// the Service) while holding mu.
+type Service struct {
+	mu       sync.Mutex
+	store    *storage.Store
+	jobs     []domain.Job
+	runtimes map[int]*domain.JobRuntime
+}
+
+// NewService wires the Service to a loaded store and its jobs. It builds the
+// initial runtime map from the durable jobs so every job has transient state
+// from the moment the Service exists. The store is the Service's sole channel
+// to persistence.
+func NewService(store *storage.Store, jobs []domain.Job) *Service {
+	return &Service{
+		store:    store,
+		jobs:     jobs,
+		runtimes: domain.NewRuntimes(jobs),
+	}
+}
+
+// Open loads the store and constructs a Service from it in one step. It is the
+// convenience entry point for the application; tests inject a pre-built store
+// via NewService instead.
+func Open() (*Service, error) {
+	store, jobs, err := storage.OpenStore()
+	if err != nil {
+		return nil, err
+	}
+	return NewService(store, jobs), nil
+}
+
+// Store returns the underlying store. It is exposed so callers that still need
+// resolved paths and config (the GUI, during the transition) can reach them;
+// later phases narrow this surface.
+func (s *Service) Store() *storage.Store {
+	return s.store
+}
+
+// Jobs returns a copy of the durable jobs slice. Returning a copy keeps callers
+// from mutating Service-owned state behind its back: the Service stays the sole
+// writer.
+func (s *Service) Jobs() []domain.Job {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	jobs := make([]domain.Job, len(s.jobs))
+	copy(jobs, s.jobs)
+	return jobs
+}
+
+// Runtime returns the transient runtime state for a job ID, or nil if no job
+// with that ID is loaded. The returned pointer is the live runtime; reads of it
+// are only safe while no concurrent mutation is in flight, which holds during
+// the current single-threaded transition and is tightened as the scheduler
+// moves behind the Service in T3.4.
+func (s *Service) Runtime(id int) *domain.JobRuntime {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	return s.runtimes[id]
+}
diff --git a/src/app/service_test.go b/src/app/service_test.go
new file mode 100644
index 0000000..b43bc55
--- /dev/null
+++ b/src/app/service_test.go
@@ -0,0 +1,57 @@
+package app
+
+import (
+	"testing"
+
+	"gitea.mixdep.ru/mix/gosentry/src/domain"
+	"gitea.mixdep.ru/mix/gosentry/src/storage"
+)
+
+func newTestService(jobs []domain.Job) *Service {
+	return NewService(&storage.Store{}, jobs)
+}
+
+func TestNewServiceBuildsRuntimePerJob(t *testing.T) {
+	jobs := []domain.Job{
+		{ID: 1, Name: "Enabled", Enabled: true},
+		{ID: 2, Name: "Disabled", Enabled: false},
+	}
+	svc := newTestService(jobs)
+
+	if got := svc.Runtime(1); got == nil {
+		t.Fatal("expected runtime for enabled job 1")
+	} else if got.LastState != "Ready" {
+		t.Errorf("enabled job runtime state = %q, want %q", got.LastState, "Ready")
+	}
+	if got := svc.Runtime(2); got == nil {
+		t.Fatal("expected runtime for disabled job 2")
+	} else if got.LastState != "Paused" {
+		t.Errorf("disabled job runtime state = %q, want %q", got.LastState, "Paused")
+	}
+	if got := svc.Runtime(99); got != nil {
+		t.Errorf("expected nil runtime for unknown job, got %+v", got)
+	}
+}
+
+func TestJobsReturnsCopy(t *testing.T) {
+	jobs := []domain.Job{{ID: 1, Name: "Original"}}
+	svc := newTestService(jobs)
+
+	snapshot := svc.Jobs()
+	if len(snapshot) != 1 {
+		t.Fatalf("Jobs() len = %d, want 1", len(snapshot))
+	}
+	// Mutating the returned slice must not affect Service-owned state.
+	snapshot[0].Name = "Mutated"
+	if again := svc.Jobs(); again[0].Name != "Original" {
+		t.Errorf("Service state leaked through Jobs(): name = %q, want %q", again[0].Name, "Original")
+	}
+}
+
+func TestStoreReturnsWiredStore(t *testing.T) {
+	store := &storage.Store{}
+	svc := NewService(store, nil)
+	if svc.Store() != store {
+		t.Error("Store() did not return the wired store")
+	}
+}