diff --git a/internal/state/store.go b/internal/state/store.go
index ce1c5f6..e918f47 100644
--- a/internal/state/store.go
+++ b/internal/state/store.go
@@ -2,12 +2,16 @@ package state
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"math"
 	"os"
 	"path/filepath"
 	"sort"
+	"strconv"
+	"strings"
 	"sync"
+	"syscall"
 	"time"
 )
 
@@ -42,15 +46,75 @@ func AcquireLock(path string) (func(), error) {
 	if err := os.MkdirAll(filepath.Dir(path), 0o750); err != nil {
 		return nil, err
 	}
-	f, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0o600)
-	if err != nil {
+
+	create := func() (func(), error) {
+		f, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0o600)
+		if err != nil {
+			return nil, err
+		}
+		_, _ = f.WriteString(fmt.Sprintf("pid=%d started=%s\n", os.Getpid(), time.Now().Format(time.RFC3339)))
+		_ = f.Close()
+		return func() {
+			_ = os.Remove(path)
+		}, nil
+	}
+
+	unlock, err := create()
+	if err == nil {
+		return unlock, nil
+	}
+	if !errors.Is(err, os.ErrExist) {
 		return nil, fmt.Errorf("acquire lock %s: %w", path, err)
 	}
-	_, _ = f.WriteString(fmt.Sprintf("pid=%d started=%s\n", os.Getpid(), time.Now().Format(time.RFC3339)))
-	_ = f.Close()
-	return func() {
-		_ = os.Remove(path)
-	}, nil
+
+	stale, staleErr := staleLock(path)
+	if staleErr != nil {
+		return nil, fmt.Errorf("acquire lock %s: existing lock check failed: %w", path, staleErr)
+	}
+	if !stale {
+		return nil, fmt.Errorf("acquire lock %s: lock is held by active process", path)
+	}
+	if rmErr := os.Remove(path); rmErr != nil {
+		return nil, fmt.Errorf("acquire lock %s: remove stale lock: %w", path, rmErr)
+	}
+	unlock, err = create()
+	if err != nil {
+		return nil, fmt.Errorf("acquire lock %s: recreate after stale lock removal: %w", path, err)
+	}
+	return unlock, nil
+}
+
+func staleLock(path string) (bool, error) {
+	b, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return true, nil
+		}
+		return false, err
+	}
+	lines := strings.Split(string(b), "\n")
+	var pid int
+	for _, line := range lines {
+		line = strings.TrimSpace(line)
+		if strings.HasPrefix(line, "pid=") {
+			v := strings.TrimPrefix(line, "pid=")
+			parsed, parseErr := strconv.Atoi(v)
+			if parseErr != nil {
+				return true, nil
+			}
+			pid = parsed
+			break
+		}
+	}
+	if pid <= 0 {
+		return true, nil
+	}
+	if err := syscall.Kill(pid, 0); err != nil {
+		if errors.Is(err, syscall.ESRCH) {
+			return true, nil
+		}
+	}
+	return false, nil
 }
 
 func (s *Store) Append(record RunRecord) error {
diff --git a/internal/state/store_test.go b/internal/state/store_test.go
new file mode 100644
index 0000000..5a7a826
--- /dev/null
+++ b/internal/state/store_test.go
@@ -0,0 +1,57 @@
+package state
+
+import (
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestAcquireLockLifecycle(t *testing.T) {
+	lockPath := filepath.Join(t.TempDir(), "hecate.lock")
+	unlock, err := AcquireLock(lockPath)
+	if err != nil {
+		t.Fatalf("acquire lock: %v", err)
+	}
+	if _, err := os.Stat(lockPath); err != nil {
+		t.Fatalf("expected lock file to exist: %v", err)
+	}
+	unlock()
+	if _, err := os.Stat(lockPath); !os.IsNotExist(err) {
+		t.Fatalf("expected lock file to be removed, got: %v", err)
+	}
+}
+
+func TestAcquireLockReclaimsStaleLock(t *testing.T) {
+	lockPath := filepath.Join(t.TempDir(), "hecate.lock")
+	if err := os.WriteFile(lockPath, []byte("pid=999999\n"), 0o600); err != nil {
+		t.Fatalf("write stale lock: %v", err)
+	}
+
+	unlock, err := AcquireLock(lockPath)
+	if err != nil {
+		t.Fatalf("acquire lock with stale predecessor: %v", err)
+	}
+	defer unlock()
+
+	b, err := os.ReadFile(lockPath)
+	if err != nil {
+		t.Fatalf("read lock: %v", err)
+	}
+	if !strings.Contains(string(b), "pid=") {
+		t.Fatalf("expected lock content to contain pid, got %q", string(b))
+	}
+}
+
+func TestAcquireLockRejectsActiveLock(t *testing.T) {
+	lockPath := filepath.Join(t.TempDir(), "hecate.lock")
+	active := "pid=" + strconv.Itoa(os.Getpid()) + "\n"
+	if err := os.WriteFile(lockPath, []byte(active), 0o600); err != nil {
+		t.Fatalf("write active lock: %v", err)
+	}
+
+	if _, err := AcquireLock(lockPath); err == nil {
+		t.Fatalf("expected acquire lock to fail when active pid holds lock")
+	}
+}