From 52be8b8e47141220081d5f953ad45e2ee65dc801 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Carlos=20Flor=C3=AAncio?= <carlosmflorencio@gmail.com>
Date: Sun, 9 Aug 2026 21:44:52 +0100
Subject: [PATCH] fix: guard backend runtime-state ownership (#2468)

---
 .github/workflows/backend-tests.yml           |   2 +-
 apps/backend/Makefile                         |   2 +-
 apps/backend/internal/backendapp/main.go      |  27 ++-
 apps/backend/internal/backendapp/main_test.go |  57 +++++
 .../internal/backendapp/ownershiplock/lock.go | 100 ++++++++
 .../backendapp/ownershiplock/lock_test.go     | 226 ++++++++++++++++++
 .../backendapp/ownershiplock/lock_unix.go     |  25 ++
 .../backendapp/ownershiplock/lock_windows.go  |  32 +++
 .../backendapp/ownershiplock/targets.go       | 117 +++++++++
 .../backendapp/ownershiplock/targets_test.go  |  89 +++++++
 .../orchestrator/reconcile_restart_test.go    |  69 ++++++
 apps/backend/internal/orchestrator/service.go |  18 +-
 .../task/statussummary/projector_test.go      |  53 ++++
 ...08-09-exclusive-runtime-state-ownership.md |  68 ++++++
 docs/decisions/INDEX.md                       |   1 +
 .../backend-runtime-state-ownership/plan.md   | 179 ++++++++++++++
 .../task-01-runtime-state-ownership.md        |  83 +++++++
 .../task-02-startup-recovery-publication.md   |  75 ++++++
 .../task-03-runtime-ownership-docs.md         |  66 +++++
 docs/public/backend-development.md            |   2 +
 docs/public/configuration.md                  |   2 +
 docs/public/contributing.md                   |   2 +
 .../platform/bounded-task-status-delivery.md  |  11 +-
 docs/specs/port-collision-safety/spec.md      |  58 ++++-
 .../specs/tasks/interrupted-task-indicator.md |   6 +
 25 files changed, 1356 insertions(+), 14 deletions(-)
 create mode 100644 apps/backend/internal/backendapp/main_test.go
 create mode 100644 apps/backend/internal/backendapp/ownershiplock/lock.go
 create mode 100644 apps/backend/internal/backendapp/ownershiplock/lock_test.go
 create mode 100644 apps/backend/internal/backendapp/ownershiplock/lock_unix.go
 create mode 100644 apps/backend/internal/backendapp/ownershiplock/lock_windows.go
 create mode 100644 apps/backend/internal/backendapp/ownershiplock/targets.go
 create mode 100644 apps/backend/internal/backendapp/ownershiplock/targets_test.go
 create mode 100644 docs/decisions/2026-08-09-exclusive-runtime-state-ownership.md
 create mode 100644 docs/plans/backend-runtime-state-ownership/plan.md
 create mode 100644 docs/plans/backend-runtime-state-ownership/task-01-runtime-state-ownership.md
 create mode 100644 docs/plans/backend-runtime-state-ownership/task-02-startup-recovery-publication.md
 create mode 100644 docs/plans/backend-runtime-state-ownership/task-03-runtime-ownership-docs.md

diff --git a/.github/workflows/backend-tests.yml b/.github/workflows/backend-tests.yml
index 01bed54b2e..9da7cee6e5 100644
--- a/.github/workflows/backend-tests.yml
+++ b/.github/workflows/backend-tests.yml
@@ -378,7 +378,7 @@ jobs:
       # a self-hosting Go helper (see testmain_test.go) so they run on
       # every platform.
       - name: Test windows-sensitive packages
-        run: go test -race -v ./internal/agentctl/server/process/... ./internal/agent/runtime/agentctl/launcher/... ./internal/common/ptyexec/...
+        run: go test -race -v ./internal/agentctl/server/process/... ./internal/agent/runtime/agentctl/launcher/... ./internal/common/ptyexec/... ./internal/backendapp/ownershiplock/...
 
       # Keep port-collision coverage targeted: the instance package contains
       # broader stdio MCP fixture tests that depend on Unix command behavior.
diff --git a/apps/backend/Makefile b/apps/backend/Makefile
index 919dc67c9a..1e56cf8e62 100644
--- a/apps/backend/Makefile
+++ b/apps/backend/Makefile
@@ -286,7 +286,7 @@ test:
 ## runtime.GOOS guards in source.
 test-windows:
 	@echo "Running Windows-sensitive packages..."
-	$(CGO_PREFIX) $(GO) test -tags fts5 -v ./internal/agentctl/server/process/... ./internal/agent/runtime/agentctl/launcher/...
+	$(CGO_PREFIX) $(GO) test -tags fts5 -v ./internal/agentctl/server/process/... ./internal/agent/runtime/agentctl/launcher/... ./internal/backendapp/ownershiplock/...
 	@echo "Running Windows port-collision tests..."
 	$(CGO_PREFIX) $(GO) test -tags fts5 -v ./internal/common/netutil -run TestIsAddrInUse
 	$(CGO_PREFIX) $(GO) test -tags fts5 -v ./internal/agentctl/server/instance -run TestAllocatePortAndListenerRetriesAddressInUse
diff --git a/apps/backend/internal/backendapp/main.go b/apps/backend/internal/backendapp/main.go
index 457d1cd957..2251a86cfe 100644
--- a/apps/backend/internal/backendapp/main.go
+++ b/apps/backend/internal/backendapp/main.go
@@ -25,6 +25,7 @@ import (
 	"go.uber.org/zap"
 
 	// Common packages
+	"github.com/kandev/kandev/internal/backendapp/ownershiplock"
 	"github.com/kandev/kandev/internal/common/config"
 	"github.com/kandev/kandev/internal/common/logger"
 
@@ -208,7 +209,23 @@ func Run(args []string, build BuildInfo) int {
 		cfg.Logging.Level = parsedFlags.LogLevel
 	}
 
-	// 2. Initialize logger
+	// Acquire runtime-state ownership before any shared-state initialization.
+	// The lock remains held through logger and service cleanup, so a second
+	// backend cannot reconcile or migrate the live home before its bind fails.
+	owner, err := acquireRuntimeStateOwnership(cfg)
+	if err != nil {
+		fmt.Fprintf(os.Stderr,
+			"Failed to acquire backend runtime-state ownership: %v; use a separate KANDEV_HOME_DIR for an intentional second instance\n",
+			err)
+		return 1
+	}
+	defer func() {
+		if closeErr := owner.Close(); closeErr != nil {
+			fmt.Fprintf(os.Stderr, "Failed to release backend runtime-state ownership: %v\n", closeErr)
+		}
+	}()
+
+	// Initialize logger only after runtime-state ownership is secured.
 	log, err := logger.NewBackendLogger(logger.BackendLoggingConfig{
 		HomeDir:      cfg.ResolvedHomeDir(),
 		Level:        cfg.Logging.Level,
@@ -252,6 +269,14 @@ func Run(args []string, build BuildInfo) int {
 	return 0
 }
 
+func acquireRuntimeStateOwnership(cfg *config.Config) (*ownershiplock.Owner, error) {
+	targets, err := ownershiplock.Targets(cfg.ResolvedHomeDir(), cfg.Database.Driver, cfg.Database.Path)
+	if err != nil {
+		return nil, fmt.Errorf("resolve backend runtime-state ownership: %w", err)
+	}
+	return ownershiplock.Acquire(targets)
+}
+
 func setBuildInfo(build BuildInfo) {
 	if build.Version != "" {
 		Version = build.Version
diff --git a/apps/backend/internal/backendapp/main_test.go b/apps/backend/internal/backendapp/main_test.go
new file mode 100644
index 0000000000..2d9d5a19bd
--- /dev/null
+++ b/apps/backend/internal/backendapp/main_test.go
@@ -0,0 +1,57 @@
+package backendapp
+
+import (
+	"bytes"
+	"errors"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/kandev/kandev/internal/backendapp/ownershiplock"
+)
+
+func TestBackendStartupConflictStopsBeforeSharedStateInitialization(t *testing.T) {
+	home := t.TempDir()
+	targets, err := ownershiplock.Targets(home, "sqlite", "")
+	if err != nil {
+		t.Fatalf("Targets: %v", err)
+	}
+	owner, err := ownershiplock.Acquire(targets)
+	if err != nil {
+		t.Fatalf("Acquire primary owner: %v", err)
+	}
+	t.Cleanup(func() { _ = owner.Close() })
+
+	var stderr bytes.Buffer
+	cmd := exec.Command(os.Args[0], "-test.run", "^TestBackendStartupConflictHelper$")
+	cmd.Env = append(os.Environ(),
+		"KANDEV_BACKEND_OWNERSHIP_HELPER=1",
+		"KANDEV_HOME_DIR="+home,
+	)
+	cmd.Stderr = &stderr
+	err = cmd.Run()
+	var exitErr *exec.ExitError
+	if !errors.As(err, &exitErr) || exitErr.ExitCode() != 1 {
+		t.Fatalf("backend helper error = %v, want exit code 1; stderr=%s", err, stderr.String())
+	}
+	if !strings.Contains(stderr.String(), home) || !strings.Contains(stderr.String(), "separate KANDEV_HOME_DIR") {
+		t.Fatalf("conflict stderr = %q, want target and isolation guidance", stderr.String())
+	}
+	for _, path := range []string{
+		filepath.Join(home, "logs"),
+		filepath.Join(home, "data", "kandev.db"),
+	} {
+		if _, statErr := os.Stat(path); !errors.Is(statErr, os.ErrNotExist) {
+			t.Fatalf("shared-state path %q exists or failed unexpectedly: %v", path, statErr)
+		}
+	}
+}
+
+func TestBackendStartupConflictHelper(t *testing.T) {
+	if os.Getenv("KANDEV_BACKEND_OWNERSHIP_HELPER") != "1" {
+		return
+	}
+	os.Exit(Run(nil, BuildInfo{Version: "test"}))
+}
diff --git a/apps/backend/internal/backendapp/ownershiplock/lock.go b/apps/backend/internal/backendapp/ownershiplock/lock.go
new file mode 100644
index 0000000000..e203e12b9f
--- /dev/null
+++ b/apps/backend/internal/backendapp/ownershiplock/lock.go
@@ -0,0 +1,100 @@
+package ownershiplock
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync"
+)
+
+// ErrConflict means another process currently owns a required runtime target.
+var ErrConflict = errors.New("runtime state is already owned")
+
+// ConflictError identifies the target that prevented startup.
+type ConflictError struct {
+	Target Target
+}
+
+func (e *ConflictError) Error() string {
+	return fmt.Sprintf("%s target %q is already owned by another backend", e.Target.Kind, e.Target.ResourcePath)
+}
+
+func (e *ConflictError) Unwrap() error { return ErrConflict }
+
+type heldLock struct {
+	target Target
+	file   *os.File
+}
+
+// Owner holds all locks acquired for one backend process. Close is idempotent
+// and never removes the sidecar files; the OS lock is the ownership proof.
+type Owner struct {
+	mu     sync.Mutex
+	held   []heldLock
+	closed bool
+}
+
+// Acquire obtains every target without waiting. If any target is unavailable,
+// previously acquired targets are released before the error is returned.
+func Acquire(targets []Target) (*Owner, error) {
+	owner := &Owner{}
+	seen := make(map[string]struct{}, len(targets))
+	for _, target := range targets {
+		if _, exists := seen[target.LockPath]; exists {
+			continue
+		}
+		seen[target.LockPath] = struct{}{}
+
+		file, err := openLockFile(target.LockPath)
+		if err != nil {
+			_ = owner.Close()
+			return nil, fmt.Errorf("open %s ownership lock %q: %w", target.Kind, target.LockPath, err)
+		}
+		if err := lockFile(file); err != nil {
+			_ = file.Close()
+			_ = owner.Close()
+			if errors.Is(err, ErrConflict) {
+				return nil, &ConflictError{Target: target}
+			}
+			return nil, fmt.Errorf("acquire %s ownership lock %q: %w", target.Kind, target.LockPath, err)
+		}
+		owner.held = append(owner.held, heldLock{target: target, file: file})
+	}
+	return owner, nil
+}
+
+func openLockFile(path string) (*os.File, error) {
+	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
+		return nil, err
+	}
+	return os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0o600)
+}
+
+// Close releases every held OS lock in reverse acquisition order. The first
+// release or close error is returned, while all handles are still drained.
+func (o *Owner) Close() error {
+	if o == nil {
+		return nil
+	}
+	o.mu.Lock()
+	if o.closed {
+		o.mu.Unlock()
+		return nil
+	}
+	o.closed = true
+	held := o.held
+	o.held = nil
+	o.mu.Unlock()
+
+	var firstErr error
+	for i := len(held) - 1; i >= 0; i-- {
+		if err := unlockFile(held[i].file); err != nil && firstErr == nil {
+			firstErr = fmt.Errorf("release %s ownership lock %q: %w", held[i].target.Kind, held[i].target.LockPath, err)
+		}
+		if err := held[i].file.Close(); err != nil && firstErr == nil {
+			firstErr = fmt.Errorf("close %s ownership lock %q: %w", held[i].target.Kind, held[i].target.LockPath, err)
+		}
+	}
+	return firstErr
+}
diff --git a/apps/backend/internal/backendapp/ownershiplock/lock_test.go b/apps/backend/internal/backendapp/ownershiplock/lock_test.go
new file mode 100644
index 0000000000..3a9d1c5885
--- /dev/null
+++ b/apps/backend/internal/backendapp/ownershiplock/lock_test.go
@@ -0,0 +1,226 @@
+package ownershiplock
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"go.uber.org/goleak"
+)
+
+func TestMain(m *testing.M) {
+	if os.Getenv("KANDEV_OWNERSHIP_LOCK_HELPER") == "1" {
+		os.Exit(m.Run())
+	}
+	goleak.VerifyTestMain(m)
+}
+
+func TestOwnershipLockRejectsSecondProcessAndReleasesAfterNormalExit(t *testing.T) {
+	home := t.TempDir()
+	targets, err := Targets(home, "sqlite", "")
+	if err != nil {
+		t.Fatalf("Targets: %v", err)
+	}
+	owner, err := Acquire(targets)
+	if err != nil {
+		t.Fatalf("Acquire primary owner: %v", err)
+	}
+	t.Cleanup(func() { _ = owner.Close() })
+
+	conflict := startLockHelper(t, home, "conflict")
+	if line := readHelperLine(t, conflict); line != "conflict" {
+		t.Fatalf("helper result = %q, want conflict", line)
+	}
+	if err := conflict.cmd.Wait(); err != nil {
+		t.Fatalf("conflict helper: %v; stderr=%s", err, conflict.stderr.String())
+	}
+
+	_ = owner.Close()
+	released := startLockHelper(t, home, "normal")
+	if line := readHelperLine(t, released); line != "ready" {
+		t.Fatalf("normal helper result = %q, want ready", line)
+	}
+	if err := released.cmd.Wait(); err != nil {
+		t.Fatalf("normal helper: %v; stderr=%s", err, released.stderr.String())
+	}
+
+	second, err := Acquire(targets)
+	if err != nil {
+		t.Fatalf("Acquire after normal exit: %v", err)
+	}
+	if err := second.Close(); err != nil {
+		t.Fatalf("release after normal exit: %v", err)
+	}
+}
+
+func TestOwnershipLockReleasesAfterForcedProcessExit(t *testing.T) {
+	home := t.TempDir()
+	helper := startLockHelper(t, home, "hold")
+	if line := readHelperLine(t, helper); line != "ready" {
+		t.Fatalf("holding helper result = %q, want ready", line)
+	}
+
+	if err := helper.cmd.Process.Kill(); err != nil {
+		t.Fatalf("kill holding helper: %v", err)
+	}
+	if err := helper.cmd.Wait(); err == nil {
+		t.Fatal("holding helper exited successfully after forced termination")
+	}
+
+	targets, err := Targets(home, "sqlite", "")
+	if err != nil {
+		t.Fatalf("Targets after forced exit: %v", err)
+	}
+	owner, err := Acquire(targets)
+	if err != nil {
+		t.Fatalf("Acquire after forced exit: %v", err)
+	}
+	if err := owner.Close(); err != nil {
+		t.Fatalf("release after forced exit: %v", err)
+	}
+}
+
+func TestOwnershipLockAllowsIndependentHomes(t *testing.T) {
+	firstHome := t.TempDir()
+	firstTargets, err := Targets(firstHome, "sqlite", "")
+	if err != nil {
+		t.Fatalf("first Targets: %v", err)
+	}
+	first, err := Acquire(firstTargets)
+	if err != nil {
+		t.Fatalf("Acquire first home: %v", err)
+	}
+	t.Cleanup(func() { _ = first.Close() })
+
+	secondHome := t.TempDir()
+	second := startLockHelper(t, secondHome, "normal")
+	if line := readHelperLine(t, second); line != "ready" {
+		t.Fatalf("independent helper result = %q, want ready", line)
+	}
+	if err := second.cmd.Wait(); err != nil {
+		t.Fatalf("independent helper: %v; stderr=%s", err, second.stderr.String())
+	}
+}
+
+func TestOwnershipLockHelper(t *testing.T) {
+	if os.Getenv("KANDEV_OWNERSHIP_LOCK_HELPER") != "1" {
+		return
+	}
+	home := os.Getenv("KANDEV_OWNERSHIP_HOME")
+	targets, err := Targets(home, "sqlite", "")
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Targets: %v\n", err)
+		os.Exit(2)
+	}
+	owner, err := Acquire(targets)
+	if err != nil {
+		if errors.Is(err, ErrConflict) {
+			_, _ = fmt.Fprintln(os.Stdout, "conflict")
+			return
+		}
+		fmt.Fprintf(os.Stderr, "Acquire: %v\n", err)
+		os.Exit(2)
+	}
+	defer func() { _ = owner.Close() }()
+	_, _ = fmt.Fprintln(os.Stdout, "ready")
+	if os.Getenv("KANDEV_OWNERSHIP_MODE") == "hold" {
+		timer := time.NewTimer(5 * time.Minute)
+		defer timer.Stop()
+		<-timer.C
+	}
+}
+
+type lockHelper struct {
+	cmd    *exec.Cmd
+	stdout *os.File
+	stderr *strings.Builder
+}
+
+func startLockHelper(t *testing.T, home, mode string) *lockHelper {
+	t.Helper()
+	stdout, stdin, err := os.Pipe()
+	if err != nil {
+		t.Fatalf("create helper stdout pipe: %v", err)
+	}
+	cmd := exec.Command(os.Args[0], "-test.run", "^TestOwnershipLockHelper$")
+	cmd.Env = append(os.Environ(),
+		"KANDEV_OWNERSHIP_LOCK_HELPER=1",
+		"KANDEV_OWNERSHIP_HOME="+home,
+		"KANDEV_OWNERSHIP_MODE="+mode,
+	)
+	cmd.Stdout = stdin
+	stderr := new(strings.Builder)
+	cmd.Stderr = stderr
+	if err := cmd.Start(); err != nil {
+		_ = stdout.Close()
+		_ = stdin.Close()
+		t.Fatalf("start lock helper: %v", err)
+	}
+	t.Cleanup(func() {
+		if cmd.ProcessState == nil && cmd.Process != nil {
+			_ = cmd.Process.Kill()
+		}
+		_ = cmd.Wait()
+		_ = stdout.Close()
+		_ = stdin.Close()
+	})
+	return &lockHelper{cmd: cmd, stdout: stdout, stderr: stderr}
+}
+
+func readHelperLine(t *testing.T, helper *lockHelper) string {
+	t.Helper()
+	lines := make(chan string, 1)
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		scanner := bufio.NewScanner(helper.stdout)
+		if scanner.Scan() {
+			lines <- scanner.Text()
+			return
+		}
+		lines <- ""
+	}()
+	timer := time.NewTimer(5 * time.Second)
+	defer timer.Stop()
+	select {
+	case line := <-lines:
+		<-done
+		return line
+	case <-timer.C:
+		_ = helper.stdout.Close()
+		<-done
+		t.Fatal("timed out waiting for lock helper")
+		return ""
+	}
+}
+
+func TestOwnershipLockConflictErrorIncludesTarget(t *testing.T) {
+	home := filepath.Join(t.TempDir(), "home")
+	targets, err := Targets(home, "sqlite", "")
+	if err != nil {
+		t.Fatalf("Targets: %v", err)
+	}
+	owner, err := Acquire(targets)
+	if err != nil {
+		t.Fatalf("Acquire: %v", err)
+	}
+	t.Cleanup(func() { _ = owner.Close() })
+
+	_, err = Acquire(targets)
+	if !errors.Is(err, ErrConflict) {
+		t.Fatalf("second Acquire error = %v, want ErrConflict", err)
+	}
+	var conflict *ConflictError
+	if !errors.As(err, &conflict) {
+		t.Fatalf("second Acquire error = %T, want ConflictError", err)
+	}
+	if conflict.Target.ResourcePath != targets[0].ResourcePath {
+		t.Fatalf("conflict target = %q, want canonical home path %q", conflict.Target.ResourcePath, targets[0].ResourcePath)
+	}
+}
diff --git a/apps/backend/internal/backendapp/ownershiplock/lock_unix.go b/apps/backend/internal/backendapp/ownershiplock/lock_unix.go
new file mode 100644
index 0000000000..3994b80d51
--- /dev/null
+++ b/apps/backend/internal/backendapp/ownershiplock/lock_unix.go
@@ -0,0 +1,25 @@
+//go:build !windows
+
+package ownershiplock
+
+import (
+	"errors"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+func lockFile(file *os.File) error {
+	err := unix.Flock(int(file.Fd()), unix.LOCK_EX|unix.LOCK_NB)
+	if err == nil {
+		return nil
+	}
+	if errors.Is(err, unix.EWOULDBLOCK) || errors.Is(err, unix.EAGAIN) {
+		return ErrConflict
+	}
+	return err
+}
+
+func unlockFile(file *os.File) error {
+	return unix.Flock(int(file.Fd()), unix.LOCK_UN)
+}
diff --git a/apps/backend/internal/backendapp/ownershiplock/lock_windows.go b/apps/backend/internal/backendapp/ownershiplock/lock_windows.go
new file mode 100644
index 0000000000..6b19ae4dca
--- /dev/null
+++ b/apps/backend/internal/backendapp/ownershiplock/lock_windows.go
@@ -0,0 +1,32 @@
+//go:build windows
+
+package ownershiplock
+
+import (
+	"errors"
+	"os"
+
+	"golang.org/x/sys/windows"
+)
+
+func lockFile(file *os.File) error {
+	err := windows.LockFileEx(
+		windows.Handle(file.Fd()),
+		windows.LOCKFILE_EXCLUSIVE_LOCK|windows.LOCKFILE_FAIL_IMMEDIATELY,
+		0,
+		1,
+		0,
+		&windows.Overlapped{},
+	)
+	if err == nil {
+		return nil
+	}
+	if errors.Is(err, windows.ERROR_LOCK_VIOLATION) {
+		return ErrConflict
+	}
+	return err
+}
+
+func unlockFile(file *os.File) error {
+	return windows.UnlockFileEx(windows.Handle(file.Fd()), 0, 1, 0, &windows.Overlapped{})
+}
diff --git a/apps/backend/internal/backendapp/ownershiplock/targets.go b/apps/backend/internal/backendapp/ownershiplock/targets.go
new file mode 100644
index 0000000000..841d570512
--- /dev/null
+++ b/apps/backend/internal/backendapp/ownershiplock/targets.go
@@ -0,0 +1,117 @@
+package ownershiplock
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+// TargetKind identifies the persistent resource protected by an ownership lock.
+type TargetKind string
+
+const (
+	TargetHome     TargetKind = "home"
+	TargetDatabase TargetKind = "database"
+)
+
+// Target describes the canonical resource path and the stable sidecar file used
+// to hold its operating-system lock.
+type Target struct {
+	Kind         TargetKind
+	ResourcePath string
+	LockPath     string
+}
+
+// Targets returns the persistent resources a backend must own before startup.
+// The Kandev home owns the default SQLite database, so an explicit database
+// target is needed only when SQLite points outside that home.
+func Targets(homeDir, databaseDriver, databasePath string) ([]Target, error) {
+	home, err := canonicalPath(homeDir)
+	if err != nil {
+		return nil, fmt.Errorf("canonicalize Kandev home %q: %w", homeDir, err)
+	}
+	if home == "" {
+		return nil, errors.New("kandev home cannot be empty")
+	}
+
+	targets := []Target{{
+		Kind:         TargetHome,
+		ResourcePath: home,
+		LockPath:     filepath.Join(home, ".kandev-backend.lock"),
+	}}
+	if !strings.EqualFold(databaseDriver, "sqlite") {
+		return targets, nil
+	}
+	if strings.TrimSpace(databasePath) == "" {
+		// Empty path uses the in-home SQLite database, already covered by the
+		// home lock.
+		return targets, nil
+	}
+
+	database, err := canonicalPath(databasePath)
+	if err != nil {
+		return nil, fmt.Errorf("canonicalize SQLite database %q: %w", databasePath, err)
+	}
+	if pathWithin(home, database) {
+		return targets, nil
+	}
+
+	return append(targets, Target{
+		Kind:         TargetDatabase,
+		ResourcePath: database,
+		LockPath:     database + ".lock",
+	}), nil
+}
+
+func canonicalPath(path string) (string, error) {
+	path = strings.TrimSpace(path)
+	if path == "" {
+		return "", nil
+	}
+	abs, err := filepath.Abs(path)
+	if err != nil {
+		return "", err
+	}
+	abs = filepath.Clean(abs)
+	if resolved, err := filepath.EvalSymlinks(abs); err == nil {
+		return filepath.Clean(resolved), nil
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return "", err
+	}
+
+	// The resource itself may not exist yet, but every existing ancestor still
+	// needs to resolve to the same lock identity as its real path. This also
+	// normalizes Windows short-name aliases when a database directory is new.
+	for parent := filepath.Dir(abs); ; parent = filepath.Dir(parent) {
+		resolvedParent, err := filepath.EvalSymlinks(parent)
+		if err == nil {
+			relative, relErr := filepath.Rel(parent, abs)
+			if relErr != nil {
+				return "", relErr
+			}
+			return filepath.Clean(filepath.Join(resolvedParent, relative)), nil
+		}
+		if !errors.Is(err, os.ErrNotExist) {
+			return "", err
+		}
+		next := filepath.Dir(parent)
+		if next == parent {
+			return abs, nil
+		}
+	}
+}
+
+func pathWithin(base, candidate string) bool {
+	if runtime.GOOS == "windows" {
+		base = strings.ToLower(base)
+		candidate = strings.ToLower(candidate)
+	}
+	relative, err := filepath.Rel(base, candidate)
+	if err != nil {
+		return false
+	}
+	return relative == "." || (relative != ".." && !strings.HasPrefix(relative, ".."+string(filepath.Separator)))
+}
diff --git a/apps/backend/internal/backendapp/ownershiplock/targets_test.go b/apps/backend/internal/backendapp/ownershiplock/targets_test.go
new file mode 100644
index 0000000000..adad5895fb
--- /dev/null
+++ b/apps/backend/internal/backendapp/ownershiplock/targets_test.go
@@ -0,0 +1,89 @@
+package ownershiplock
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestTargetsDefaultSQLiteUsesHomeOwnershipOnly(t *testing.T) {
+	root := evalTempDir(t)
+	separator := string(filepath.Separator)
+	home := root + separator + "home" + separator + "." + separator + "nested" + separator + ".."
+
+	targets, err := Targets(home, "sqlite", "")
+	if err != nil {
+		t.Fatalf("Targets: %v", err)
+	}
+	if len(targets) != 1 {
+		t.Fatalf("targets = %#v, want one home target", targets)
+	}
+	if targets[0].Kind != TargetHome {
+		t.Fatalf("target kind = %q, want %q", targets[0].Kind, TargetHome)
+	}
+	if targets[0].ResourcePath != filepath.Join(root, "home") {
+		t.Fatalf("resource path = %q, want canonical home", targets[0].ResourcePath)
+	}
+}
+
+func TestTargetsExternalSQLiteAddsCanonicalDatabaseTarget(t *testing.T) {
+	root := evalTempDir(t)
+	home := filepath.Join(root, "home")
+	databaseDir := filepath.Join(root, "database")
+	if err := os.MkdirAll(databaseDir, 0o700); err != nil {
+		t.Fatalf("create database dir: %v", err)
+	}
+	separator := string(filepath.Separator)
+	databasePath := databaseDir + separator + ".." + separator + "database" + separator + "kandev.db"
+
+	targets, err := Targets(home, "sqlite", databasePath)
+	if err != nil {
+		t.Fatalf("Targets: %v", err)
+	}
+	if len(targets) != 2 {
+		t.Fatalf("targets = %#v, want home and database targets", targets)
+	}
+	if targets[1].Kind != TargetDatabase {
+		t.Fatalf("database target kind = %q, want %q", targets[1].Kind, TargetDatabase)
+	}
+	wantDatabase := filepath.Join(databaseDir, "kandev.db")
+	if targets[1].ResourcePath != wantDatabase {
+		t.Fatalf("database resource path = %q, want %q", targets[1].ResourcePath, wantDatabase)
+	}
+	if targets[1].LockPath != wantDatabase+".lock" {
+		t.Fatalf("database lock path = %q, want %q", targets[1].LockPath, wantDatabase+".lock")
+	}
+}
+
+func TestTargetsDoesNotDuplicateDatabaseInsideHome(t *testing.T) {
+	home := evalTempDir(t)
+	separator := string(filepath.Separator)
+	databasePath := home + separator + "data" + separator + ".." + separator + "data" + separator + "kandev.db"
+
+	targets, err := Targets(filepath.Join(home, "."), "sqlite", databasePath)
+	if err != nil {
+		t.Fatalf("Targets: %v", err)
+	}
+	if len(targets) != 1 {
+		t.Fatalf("targets = %#v, want one canonical home target", targets)
+	}
+}
+
+func TestTargetsPostgresLocksOnlyHome(t *testing.T) {
+	targets, err := Targets(t.TempDir(), "postgres", filepath.Join(t.TempDir(), "external.db"))
+	if err != nil {
+		t.Fatalf("Targets: %v", err)
+	}
+	if len(targets) != 1 || targets[0].Kind != TargetHome {
+		t.Fatalf("targets = %#v, want only home target", targets)
+	}
+}
+
+func evalTempDir(t *testing.T) string {
+	t.Helper()
+	dir, err := filepath.EvalSymlinks(t.TempDir())
+	if err != nil {
+		t.Fatalf("resolve temp dir: %v", err)
+	}
+	return dir
+}
diff --git a/apps/backend/internal/orchestrator/reconcile_restart_test.go b/apps/backend/internal/orchestrator/reconcile_restart_test.go
index cc9f9bf52f..13d0cd3593 100644
--- a/apps/backend/internal/orchestrator/reconcile_restart_test.go
+++ b/apps/backend/internal/orchestrator/reconcile_restart_test.go
@@ -9,6 +9,7 @@ import (
 
 	runtimeapi "github.com/kandev/kandev/internal/agent/runtime"
 	"github.com/kandev/kandev/internal/agentruntime"
+	"github.com/kandev/kandev/internal/events"
 	"github.com/kandev/kandev/internal/task/models"
 )
 
@@ -16,6 +17,74 @@ import (
 // stop failure never causes a confirmed-dead row to be pruned.
 var errWorkTransient = errors.New("runtime stop transient failure")
 
+func TestReconcileSessionsOnStartupPublishesSettledSessionState(t *testing.T) {
+	for _, previousState := range []models.TaskSessionState{
+		models.TaskSessionStateStarting,
+		models.TaskSessionStateRunning,
+	} {
+		t.Run(string(previousState), func(t *testing.T) {
+			repo := setupTestRepo(t)
+			ctx := context.Background()
+			seedTaskAndSession(t, repo, "task-startup-event", "session-startup-event", previousState)
+			if err := repo.SetSessionPrimary(ctx, "session-startup-event"); err != nil {
+				t.Fatalf("set primary session: %v", err)
+			}
+			if err := repo.UpsertExecutorRunning(ctx, &models.ExecutorRunning{
+				ID:        "session-startup-event",
+				SessionID: "session-startup-event",
+				TaskID:    "task-startup-event",
+				Runtime:   agentruntime.RuntimeStandalone,
+				Status:    models.ExecutorRunningStatusRunning,
+				LocalPID:  4242,
+				CreatedAt: time.Now().UTC(),
+				UpdatedAt: time.Now().UTC(),
+			}); err != nil {
+				t.Fatalf("upsert executor row: %v", err)
+			}
+
+			eventBus := &recordingEventBus{}
+			svc := createTestServiceWithAgent(repo, newMockStepGetter(), newMockTaskRepo(), &mockAgentManager{})
+			svc.eventBus = eventBus
+			svc.reconcileSessionsOnStartup(ctx)
+
+			var stateEvent *recordedEvent
+			for i := range eventBus.events {
+				if eventBus.events[i].subject == events.TaskSessionStateChanged {
+					stateEvent = &eventBus.events[i]
+					break
+				}
+			}
+			if stateEvent == nil {
+				t.Fatalf("startup reconciliation published no %s event; events=%+v", events.TaskSessionStateChanged, eventBus.events)
+			}
+
+			data, ok := stateEvent.event.Data.(map[string]interface{})
+			if !ok {
+				t.Fatalf("session state event data = %T, want map[string]interface{}", stateEvent.event.Data)
+			}
+			if got := data["old_state"]; got != string(previousState) {
+				t.Errorf("old_state = %v, want %q", got, previousState)
+			}
+			if got := data["new_state"]; got != string(models.TaskSessionStateWaitingForInput) {
+				t.Errorf("new_state = %v, want %q", got, models.TaskSessionStateWaitingForInput)
+			}
+			if got := data["is_primary"]; got != true {
+				t.Errorf("is_primary = %v, want true", got)
+			}
+			foregroundActivity, present := data["foreground_activity"]
+			if !present {
+				t.Fatal("foreground_activity is missing from startup state event")
+			}
+			if foregroundActivity != nil {
+				t.Errorf("foreground_activity = %v, want explicit nil", foregroundActivity)
+			}
+			if got := data["active_subagent_count"]; got != 0 {
+				t.Errorf("active_subagent_count = %v, want 0", got)
+			}
+		})
+	}
+}
+
 // TestReconcileSessionsOnStartupMakesRowsTrue is the restart-reconciliation
 // integration test for #1597 startup reconciliation (the backlog stops
 // growing, rows are made true, resumability is never lost). After a restart,
diff --git a/apps/backend/internal/orchestrator/service.go b/apps/backend/internal/orchestrator/service.go
index 35aea70b33..f5a5d19421 100644
--- a/apps/backend/internal/orchestrator/service.go
+++ b/apps/backend/internal/orchestrator/service.go
@@ -1819,7 +1819,7 @@ func (s *Service) reconcileOneSessionOnStartup(ctx context.Context, running *mod
 		return
 	}
 
-	s.reconcileActiveSessionOnStartup(ctx, running, sessionID, previousState)
+	s.reconcileActiveSessionOnStartup(ctx, running, sessionID, previousState, session)
 }
 
 func (s *Service) reconcileActiveSessionOnStartup(
@@ -1827,15 +1827,21 @@ func (s *Service) reconcileActiveSessionOnStartup(
 	running *models.ExecutorRunning,
 	sessionID string,
 	previousState models.TaskSessionState,
+	session *models.TaskSession,
 ) {
 	// Active states: STARTING, RUNNING, WAITING_FOR_INPUT
 	// Set session to WAITING_FOR_INPUT (idle, ready for lazy resume when user opens it)
 	if previousState != models.TaskSessionStateWaitingForInput {
-		if err := s.repo.UpdateTaskSessionState(ctx, sessionID, models.TaskSessionStateWaitingForInput, ""); err != nil {
-			s.logger.Warn("failed to set session to WAITING_FOR_INPUT on startup",
-				zap.String("session_id", sessionID),
-				zap.Error(err))
-		}
+		s.updateTaskSessionStateWithHook(
+			ctx,
+			running.TaskID,
+			sessionID,
+			models.TaskSessionStateWaitingForInput,
+			"",
+			false,
+			nil,
+			session,
+		)
 	}
 	s.abandonOpenTurnsOnStartup(ctx, sessionID, "active session reconciled to waiting")
 
diff --git a/apps/backend/internal/task/statussummary/projector_test.go b/apps/backend/internal/task/statussummary/projector_test.go
index be366fbb7f..440a5ff74d 100644
--- a/apps/backend/internal/task/statussummary/projector_test.go
+++ b/apps/backend/internal/task/statussummary/projector_test.go
@@ -176,6 +176,59 @@ func TestProjectorDoesNotSubscribeToRawStreamsOrStreamingMessageAppends(t *testi
 	_ = ctx
 }
 
+func TestProjectorRepairsStoredRunningSummaryAfterStartupRecovery(t *testing.T) {
+	store := newProjectorTestStore()
+	storedAt := time.Date(2026, 8, 1, 17, 59, 0, 0, time.UTC)
+	store.rows["task-startup-recovery"] = &StoredTaskStatusSummary{
+		TaskID:      "task-startup-recovery",
+		WorkspaceID: "workspace-1",
+		Summary: TaskStatusSummary{
+			Revision:            7,
+			UpdatedAt:           storedAt,
+			PrimarySession:      &PrimarySessionSummary{ID: "session-startup-recovery", State: "RUNNING"},
+			ForegroundActivity:  "generating",
+			ActiveSubagentCount: 2,
+		},
+	}
+
+	projector := NewProjector(ProjectorConfig{
+		Store: store,
+		ResolveWorkspace: func(context.Context, string) (string, error) {
+			return "workspace-1", nil
+		},
+		Now: func() time.Time { return storedAt.Add(time.Minute) },
+	})
+
+	if err := projector.HandleEvent(context.Background(), bus.NewEvent(events.TaskSessionStateChanged, "test", map[string]interface{}{
+		"task_id":               "task-startup-recovery",
+		"workspace_id":          "workspace-1",
+		"session_id":            "session-startup-recovery",
+		"new_state":             "WAITING_FOR_INPUT",
+		"is_primary":            true,
+		"foreground_activity":   nil,
+		"active_subagent_count": 0,
+	})); err != nil {
+		t.Fatalf("apply startup recovery event: %v", err)
+	}
+
+	got := store.summary("task-startup-recovery")
+	if got == nil || got.PrimarySession == nil {
+		t.Fatalf("repaired summary = %+v, want primary session", got)
+	}
+	if got.Revision <= 7 {
+		t.Fatalf("repaired summary revision = %d, want newer than 7", got.Revision)
+	}
+	if got.PrimarySession.State != "WAITING_FOR_INPUT" {
+		t.Errorf("primary session state = %q, want WAITING_FOR_INPUT", got.PrimarySession.State)
+	}
+	if got.ForegroundActivity != "" {
+		t.Errorf("foreground activity = %q, want empty", got.ForegroundActivity)
+	}
+	if got.ActiveSubagentCount != 0 {
+		t.Errorf("active subagent count = %d, want 0", got.ActiveSubagentCount)
+	}
+}
+
 func TestProjectorDerivesBoundedStatusAcrossSources(t *testing.T) {
 	_, store, eventBus, updates, _ := newProjectorTest(t)
 	const taskID = "task-status-sources"
diff --git a/docs/decisions/2026-08-09-exclusive-runtime-state-ownership.md b/docs/decisions/2026-08-09-exclusive-runtime-state-ownership.md
new file mode 100644
index 0000000000..e6d485b332
--- /dev/null
+++ b/docs/decisions/2026-08-09-exclusive-runtime-state-ownership.md
@@ -0,0 +1,68 @@
+# ADR-2026-08-09-exclusive-runtime-state-ownership: Lock Runtime State Before Backend Startup
+
+**Status:** accepted
+**Date:** 2026-08-09
+**Area:** backend, infra
+
+## Context
+
+Backend startup changes shared state before it binds the HTTP port. It opens logs, creates database
+backups, applies migrations, and reconciles active sessions. Launcher port checks do not protect
+direct backend commands or a bind race.
+
+A development command started a second backend against the live Kandev home. The process changed
+session state and then failed to bind the occupied HTTP port. SQLite serialized individual writes,
+but it did not protect the ownership of the runtime state.
+
+## Decision
+
+Each backend process must acquire non-blocking operating-system advisory locks before shared-state
+initialization. The process locks the canonical Kandev home. It also locks a custom SQLite database
+when that database is outside the home.
+
+The backend acquires all required locks after it loads configuration and before it initializes the
+backend logger. It holds the locks until backend cleanup is complete. Lock acquisition errors stop
+startup before logs, backups, migrations, session recovery, agentctl, or HTTP services start.
+
+The implementation uses file-handle locks on Unix and Windows. A crash releases these locks. The
+implementation keeps lock files after release because removing a locked file can create two lock
+identities.
+
+The lock scope is independent of ports. Intentional local instances use separate Kandev homes,
+separate SQLite databases, and separate ports. The launcher supervisor stops the old backend before
+it starts the replacement, so a planned restart preserves this ownership rule.
+
+## Consequences
+
+A stray direct backend command cannot change a live Kandev database or session state. The failure
+appears before the process creates its normal backend log, so the launcher or terminal must show a
+clear stderr message.
+
+The lock adds platform-specific code and native Windows coverage. Custom SQLite paths outside the
+home need a second lock target. Postgres backends still lock their local home, but this decision does
+not define active-active Postgres operation.
+
+An operator can find an old lock file after a crash. The file is harmless because the operating
+system lock, not the file, proves ownership.
+
+## Alternatives Considered
+
+### Bind the HTTP port first
+
+Rejected. A different port can still open the same home or SQLite database. Port ownership does not
+protect logs, secrets, backups, worktrees, or migrations.
+
+### Keep ownership checks in launchers
+
+Rejected. Direct `__backend`, `make dev-backend`, service managers, and bind races can bypass a
+launcher check. The backend owns persistent initialization and must enforce the boundary.
+
+### Use a PID file
+
+Rejected. A crash leaves stale PID data, and operating systems can reuse a PID. A PID file does not
+provide atomic ownership.
+
+### Depend on SQLite write locking
+
+Rejected. SQLite protects transactions, not the semantic ownership of startup recovery. It also
+does not protect other files in the Kandev home.
diff --git a/docs/decisions/INDEX.md b/docs/decisions/INDEX.md
index 3fe2b8c03c..3d0146319f 100644
--- a/docs/decisions/INDEX.md
+++ b/docs/decisions/INDEX.md
@@ -140,3 +140,4 @@ Read individual ADRs for full context. Create new ones via `/record decision` or
 | 2026-08-08-workspace-scoped-task-create-workflow-memory | [Remember Task-Create Workflows Per Workspace](2026-08-08-workspace-scoped-task-create-workflow-memory.md) | accepted | backend, frontend | 2026-08-08 |
 | 2026-08-08-owned-temp-artifact-cleanup | [Clean only registered Kandev temporary artifacts](2026-08-08-owned-temp-artifact-cleanup.md) | accepted | backend, frontend, infra, security | 2026-08-08 |
 | 2026-08-08-task-owned-worktree-lifetime | [Keep Worktree Ownership at the Task Lifecycle](2026-08-08-task-owned-worktree-lifetime.md) | accepted | backend, frontend | 2026-08-08 |
+| 2026-08-09-exclusive-runtime-state-ownership | [Lock Runtime State Before Backend Startup](2026-08-09-exclusive-runtime-state-ownership.md) | accepted | backend, infra | 2026-08-09 |
diff --git a/docs/plans/backend-runtime-state-ownership/plan.md b/docs/plans/backend-runtime-state-ownership/plan.md
new file mode 100644
index 0000000000..b1eace2206
--- /dev/null
+++ b/docs/plans/backend-runtime-state-ownership/plan.md
@@ -0,0 +1,179 @@
+---
+spec: docs/specs/port-collision-safety/spec.md
+created: 2026-08-09
+status: completed
+---
+
+# Implementation Plan: Backend Runtime-State Ownership
+
+## Overview
+
+This repair prevents a second backend from changing the runtime state of a live Kandev instance.
+It also makes startup session recovery publish authoritative state before backend readiness.
+
+The confirmed reproduction used this command in a task worktree while another backend was active:
+
+```bash
+make -C apps/backend dev
+```
+
+The second process opened the shared database and reconciled sessions. It then failed to bind the
+occupied HTTP port. The first backend later saw an already-settled session, so it did not clear the
+stored `generating` task summary.
+
+This plan implements
+[ADR-2026-08-09-exclusive-runtime-state-ownership](../../decisions/2026-08-09-exclusive-runtime-state-ownership.md).
+It also updates the status-delivery and interrupted-task contracts.
+
+## Backend
+
+### Runtime-state ownership locks
+
+Add `apps/backend/internal/backendapp/ownershiplock/` with a small cross-platform lock API. The
+package owns target selection, canonical paths, lock acquisition, and lock release.
+
+- `lock.go` defines the owner type, conflict error, target metadata, and all-or-nothing acquisition.
+- `lock_unix.go` uses a non-blocking exclusive file-handle lock.
+- `lock_windows.go` uses the equivalent non-blocking Windows file lock through
+  `golang.org/x/sys/windows`, which is already a backend dependency.
+- `targets.go` selects the Kandev-home lock. It adds a database lock when SQLite uses a path outside
+  the canonical home.
+- Lock files remain after release. The open file handle owns the lock.
+
+Update `apps/backend/internal/backendapp/main.go`. Acquire ownership after `config.Load()` and flag
+overrides, but before `logger.NewBackendLogger()`. Hold ownership until `Run` completes all service
+cleanup. On any acquisition error, write one clear stderr error and return exit code 1.
+
+The rejected process must not create backend logs, open a database, take a backup, apply a
+migration, launch agentctl, reconcile a session, or start an HTTP listener.
+
+Add the native lock package to `apps/backend/Makefile` and
+`.github/workflows/backend-tests.yml` Windows-sensitive package lists.
+
+### Startup state publication
+
+Update `apps/backend/internal/orchestrator/service.go`. Route a successful startup transition from
+`STARTING` or `RUNNING` to `WAITING_FOR_INPUT` through the existing semantic session-state
+publication path.
+
+The transition must keep these existing startup behaviors:
+
+- Abandon an orphan turn.
+- Move an eligible task from `IN_PROGRESS` to `REVIEW`.
+- Add the interrupted-task marker.
+- Preserve the executor row and resume data.
+- Repair confirmed-dead local process data.
+
+The semantic state event must contain the authoritative primary-session state and an explicit empty
+foreground-activity value. The existing task activity publisher remains the settle fallback for a
+`RUNNING` session. A session that is already `WAITING_FOR_INPUT` remains a no-op.
+
+No status-summary schema or frontend change is necessary. The current projector consumes semantic
+session events and explicit task activity values. The current sidebar already shows the interrupted
+icon when `generating` is absent.
+
+## Public Documentation
+
+Update these public pages:
+
+- `docs/public/backend-development.md`
+- `docs/public/contributing.md`
+- `docs/public/configuration.md`
+
+State that one backend owns a Kandev home at a time. Explain that raw backend commands use the
+normal home unless `KANDEV_HOME_DIR` is set. Keep the existing isolated command example.
+
+Do not document lock-file removal as a recovery step. A stale file does not hold an operating-system
+lock.
+
+## Tests
+
+### Runtime-state ownership
+
+- **What:** A second process cannot acquire the same Kandev-home lock.
+- **File:** `apps/backend/internal/backendapp/ownershiplock/lock_test.go`
+- **How:** Use a subprocess helper and temporary directories. Assert conflict, release after normal
+  exit, release after forced exit, and independent-home success.
+
+- **What:** Separate homes cannot open the same external SQLite database.
+- **File:** `apps/backend/internal/backendapp/ownershiplock/targets_test.go`
+- **How:** Build target sets for default and custom database paths. Assert canonical deduplication
+  and external-database ownership.
+
+- **What:** Backend startup stops before shared-state initialization when ownership is unavailable.
+- **File:** `apps/backend/internal/backendapp/main_test.go`
+- **How:** Hold a temporary-home lock, invoke the backend entry seam, and assert exit code 1. Assert
+  that no database, backup, backend log, agentctl child, or listener appears.
+
+### Startup state publication
+
+- **What:** Startup recovery publishes `RUNNING -> WAITING_FOR_INPUT` through the semantic session
+  event path.
+- **File:** `apps/backend/internal/orchestrator/reconcile_restart_test.go`
+- **How:** Seed a running session, task, orphan turn, executor row, and recording event bus. This
+  regression must fail before the production change because the state event is absent.
+
+- **What:** A stale `RUNNING` and `generating` summary converges after startup recovery.
+- **File:** `apps/backend/internal/task/statussummary/projector_test.go`
+- **How:** Seed a stored summary, send the authoritative recovery event, and assert a newer revision
+  with `WAITING_FOR_INPUT`, empty foreground activity, and zero active subagents.
+
+- **What:** Existing task review, interruption, resume, and executor-row behavior does not change.
+- **File:** `apps/backend/internal/orchestrator/reconcile_restart_test.go` and
+  `apps/backend/internal/orchestrator/turn_lifecycle_test.go`
+- **How:** Extend the current startup-reconciliation cases. Assert the existing database outcomes
+  with the new event assertions.
+
+### Public docs
+
+- **What:** Published docs describe one-home ownership and isolated direct backend commands.
+- **Files:** `docs/public/backend-development.md`, `docs/public/contributing.md`, and
+  `docs/public/configuration.md`
+- **How:** Run the public-doc tests and validator.
+
+## Verification Results
+
+- Focused ownership tests: `go test -tags fts5 ./internal/backendapp/ownershiplock ./internal/backendapp` — 304 tests passed.
+- Startup recovery, task service, and projector tests: `go test -tags fts5 ./internal/orchestrator ./internal/task/service ./internal/task/statussummary` — 2,658 tests passed.
+- Public documentation tests: `node --test scripts/validate-public-docs.test.mjs` — 58 tests passed; `node scripts/validate-public-docs.mjs` — 41 pages validated.
+
+## Implementation Waves And Parallel Candidates
+
+Wave 1 contains two parallel candidates. User authorization is required before delegation.
+
+- [x] [Task 01: Enforce runtime-state ownership](task-01-runtime-state-ownership.md) —
+  `parallel-safe`
+- [x] [Task 02: Publish startup recovery state](task-02-startup-recovery-publication.md) —
+  `parallel-safe`
+
+The tasks use disjoint source packages. Neither task changes a schema, generated contract,
+dependency file, or shared package configuration.
+
+Wave 2:
+
+- [x] [Task 03: Document runtime ownership](task-03-runtime-ownership-docs.md) — depends on Task 01
+
+Implementation remains sequential in the primary conversation unless the user explicitly
+authorizes subagents.
+
+## Risks
+
+- File-lock behavior differs between Unix and Windows. Native Windows coverage is required.
+- A process test can leak a child and retain a lock after an early test error. Each test must
+  register cleanup before its first assertion.
+- A launcher restart must wait for the old backend to release ownership. Existing supervisors
+  already stop and join the old process before replacement startup. Regression coverage must keep
+  this behavior.
+- Network filesystems can have different advisory-lock guarantees. This repair targets supported
+  local Kandev homes and local SQLite files.
+
+## Out of Scope
+
+- Automatic `KANDEV_HOME_DIR` selection for raw backend commands.
+- Active-active backend support for Postgres or NATS.
+- A sidebar or debug-panel redesign.
+- A database migration or status-summary schema change.
+
+## Open Questions
+
+None.
diff --git a/docs/plans/backend-runtime-state-ownership/task-01-runtime-state-ownership.md b/docs/plans/backend-runtime-state-ownership/task-01-runtime-state-ownership.md
new file mode 100644
index 0000000000..f2acf72291
--- /dev/null
+++ b/docs/plans/backend-runtime-state-ownership/task-01-runtime-state-ownership.md
@@ -0,0 +1,83 @@
+---
+id: "01-runtime-state-ownership"
+title: "Enforce runtime-state ownership"
+status: completed
+wave: 1
+depends_on: []
+plan: "plan.md"
+spec: "../../specs/port-collision-safety/spec.md"
+---
+
+# Task 01: Enforce Runtime-State Ownership
+
+Add a cross-platform ownership lock before any backend shared-state initialization.
+
+## Acceptance
+
+- A second backend that uses the same canonical Kandev home exits with code 1 before persistent
+  backend initialization.
+- Separate homes that reference one external SQLite database cannot open that database at the same
+  time.
+- The operating system releases ownership after normal process exit or forced process exit. No
+  lock-file removal is necessary.
+
+## Verification
+
+Run these commands from the repository root:
+
+```bash
+cd apps/backend && go test -tags fts5 ./internal/backendapp/ownershiplock ./internal/backendapp
+make -C apps/backend test-windows
+git diff --check
+```
+
+The first regression test must fail before the production change because no backend-home lock
+exists.
+
+## Files Likely Touched
+
+- `apps/backend/internal/backendapp/main.go`
+- `apps/backend/internal/backendapp/main_test.go`
+- `apps/backend/internal/backendapp/ownershiplock/lock.go`
+- `apps/backend/internal/backendapp/ownershiplock/lock_unix.go`
+- `apps/backend/internal/backendapp/ownershiplock/lock_windows.go`
+- `apps/backend/internal/backendapp/ownershiplock/lock_test.go`
+- `apps/backend/internal/backendapp/ownershiplock/targets.go`
+- `apps/backend/internal/backendapp/ownershiplock/targets_test.go`
+- `apps/backend/Makefile`
+- `.github/workflows/backend-tests.yml`
+
+## Dependencies
+
+None.
+
+## Parallelism
+
+`parallel-safe` with Task 02. This task owns `internal/backendapp`, the new lock package, and Windows
+test-package configuration. Task 02 owns orchestrator and status-summary files.
+
+## Inputs
+
+- Spec section **Exclusive runtime-state ownership**.
+- ADR `2026-08-09-exclusive-runtime-state-ownership`.
+- Plan section **Runtime-state ownership locks**.
+- Existing supervisor stop-before-start behavior in `internal/launcher/supervisor.go` and
+  `apps/cli/src/supervisor/child.ts`.
+
+## Risks
+
+- Do not erase a lock file during release. Another process can then lock a different file identity.
+- Register subprocess cleanup before assertions so a failed test cannot retain ownership.
+- Acquire every required target as one operation. Release earlier targets if a later target fails.
+
+## Output Contract
+
+Report the behavior, changed files, exact test results, blockers, and risks. Update this task and
+`plan.md` in the same conversation.
+
+## Results
+
+- Added cross-platform ownership locks for the canonical Kandev home and external SQLite paths.
+- Acquired ownership before backend logging and shared-state initialization; conflicts return exit code 1.
+- Added subprocess, target-selection, startup-order, and Windows-sensitive package coverage.
+- Verification: `go test -tags fts5 ./internal/backendapp/ownershiplock ./internal/backendapp` passed (304 tests).
diff --git a/docs/plans/backend-runtime-state-ownership/task-02-startup-recovery-publication.md b/docs/plans/backend-runtime-state-ownership/task-02-startup-recovery-publication.md
new file mode 100644
index 0000000000..171cba476f
--- /dev/null
+++ b/docs/plans/backend-runtime-state-ownership/task-02-startup-recovery-publication.md
@@ -0,0 +1,75 @@
+---
+id: "02-startup-recovery-publication"
+title: "Publish startup recovery state"
+status: completed
+wave: 1
+depends_on: []
+plan: "plan.md"
+spec: "../../specs/platform/bounded-task-status-delivery.md"
+---
+
+# Task 02: Publish Startup Recovery State
+
+Publish authoritative session and activity state when startup recovery settles an active session.
+
+## Acceptance
+
+- A successful startup transition from `STARTING` or `RUNNING` to `WAITING_FOR_INPUT` emits the
+  semantic session-state event with current primary-session and activity fields.
+- A stored `RUNNING` and `generating` summary advances to `WAITING_FOR_INPUT` with no generating
+  activity before backend readiness.
+- Existing review-state, interrupted-marker, orphan-turn, executor-row, and resume behavior stays
+  unchanged. An already-waiting session remains a publication no-op.
+
+## Verification
+
+Run these commands from the repository root:
+
+```bash
+cd apps/backend && go test -tags fts5 ./internal/orchestrator ./internal/task/service ./internal/task/statussummary
+git diff --check
+```
+
+The orchestrator regression must fail before the production change because startup recovery writes
+the session row directly and emits no session-state event.
+
+## Files Likely Touched
+
+- `apps/backend/internal/orchestrator/service.go`
+- `apps/backend/internal/orchestrator/reconcile_restart_test.go`
+- `apps/backend/internal/orchestrator/turn_lifecycle_test.go`
+- `apps/backend/internal/task/statussummary/projector_test.go`
+
+## Dependencies
+
+None.
+
+## Parallelism
+
+`parallel-safe` with Task 01. This task owns orchestrator and status-summary files. Task 01 owns
+backend startup, the new lock package, and Windows test-package configuration.
+
+## Inputs
+
+- Spec scenario for stale startup summaries in `bounded-task-status-delivery.md`.
+- Spec scenario for the interrupted icon in `tasks/interrupted-task-indicator.md`.
+- Plan section **Startup state publication**.
+- The existing `updateTaskSessionStateWithHook` and `republishTaskActivityOnSettle` paths.
+
+## Risks
+
+- Do not publish a settled state when the conditional state update fails.
+- Keep startup recovery free of agent launches and prompt dispatch.
+- Preserve per-task event order so the summary cannot apply an older generating value last.
+
+## Output Contract
+
+Report the behavior, changed files, exact test results, blockers, and risks. Update this task and
+`plan.md` in the same conversation.
+
+## Results
+
+- Routed startup session settlement through `updateTaskSessionStateWithHook`.
+- Startup recovery now publishes authoritative session state with explicit empty activity and preserves the existing task-activity settle fallback.
+- Added `STARTING`/`RUNNING` orchestrator regressions and stale-summary projector coverage.
+- Verification: `go test -tags fts5 ./internal/orchestrator ./internal/task/service ./internal/task/statussummary` passed (2,658 tests).
diff --git a/docs/plans/backend-runtime-state-ownership/task-03-runtime-ownership-docs.md b/docs/plans/backend-runtime-state-ownership/task-03-runtime-ownership-docs.md
new file mode 100644
index 0000000000..bc77002145
--- /dev/null
+++ b/docs/plans/backend-runtime-state-ownership/task-03-runtime-ownership-docs.md
@@ -0,0 +1,66 @@
+---
+id: "03-runtime-ownership-docs"
+title: "Document runtime ownership"
+status: completed
+wave: 2
+depends_on: ["01-runtime-state-ownership"]
+plan: "plan.md"
+spec: "../../specs/port-collision-safety/spec.md"
+---
+
+# Task 03: Document Runtime Ownership
+
+Document the one-home ownership rule and the safe command for an isolated backend-only instance.
+
+## Acceptance
+
+- Public development docs state that one backend owns a Kandev home at a time.
+- Raw backend commands explain their default-home behavior and show the existing isolated
+  `KANDEV_HOME_DIR` command.
+- The docs do not tell users to remove a stale lock file.
+
+## Verification
+
+Run these commands from the repository root:
+
+```bash
+node --test scripts/validate-public-docs.test.mjs
+node scripts/validate-public-docs.mjs
+git diff --check
+```
+
+## Files Likely Touched
+
+- `docs/public/backend-development.md`
+- `docs/public/contributing.md`
+- `docs/public/configuration.md`
+
+## Dependencies
+
+Task 01.
+
+## Parallelism
+
+`sequential`. The final error behavior and ownership scope must match Task 01.
+
+## Inputs
+
+- Spec section **Exclusive runtime-state ownership**.
+- Plan section **Public Documentation**.
+- Docs-maintainer guidance for development and configuration pages.
+
+## Risks
+
+- Keep `make dev` distinct from raw `make dev-backend` and `make -C apps/backend dev` behavior.
+- Do not expose the lock file as a user-managed control.
+
+## Output Contract
+
+Report the changed pages, their primary Diataxis type, exact test results, blockers, and risks.
+Update this task and `plan.md` in the same conversation.
+
+## Results
+
+- Documented one-backend-per-home ownership in backend development, contribution, and configuration guidance.
+- Documented separate home, database, and port requirements for intentional second instances.
+- Verification: `node --test scripts/validate-public-docs.test.mjs` passed (58 tests); `node scripts/validate-public-docs.mjs` validated 41 published pages.
diff --git a/docs/public/backend-development.md b/docs/public/backend-development.md
index fa93724c9b..60157cef53 100644
--- a/docs/public/backend-development.md
+++ b/docs/public/backend-development.md
@@ -18,6 +18,8 @@ The Go module in `apps/backend/` builds both the unified `kandev` binary and the
 
 Use `make dev` for normal full-stack work: it isolates state, starts Vite, and configures the backend proxy. A raw `make dev-backend` uses the normal application home unless you set an isolated `KANDEV_HOME_DIR`; see [Contributing](contributing.md).
 
+Only one backend can own a Kandev home at a time. A second raw backend using that home exits before it opens the database, writes logs, or performs startup recovery. For an intentional second backend, set `KANDEV_HOME_DIR` to a different directory and use a different port and database. The operating system releases ownership when the backend exits; no manual lock-file cleanup is required.
+
 From the repository root:
 
 ```bash
diff --git a/docs/public/configuration.md b/docs/public/configuration.md
index 8caca1e880..7f83d33ec6 100644
--- a/docs/public/configuration.md
+++ b/docs/public/configuration.md
@@ -81,6 +81,8 @@ SQLite is the supported default and enables WAL mode. PostgreSQL deployments mus
 
 `database.path` is an advanced override. Persistence honors it, but the current **Settings → System → Database** and **Backups** services derive their displayed path, WAL files, backup source, and restore destination from `<home>/data/kandev.db`. Those UI operations are therefore not reliable for a custom SQLite location. Use operator-managed backup/restore for a custom path and verify the actual database path printed at startup.
 
+One backend owns a Kandev home at a time. When SQLite uses a custom path outside that home, the backend also owns that database path, so separate homes alone do not permit concurrent backends against one SQLite file. Use a separate home and database for an intentional second instance. Ownership is released when the backend exits.
+
 Database-only snapshots also omit `<home>/data/master.key`, the AES-256 key used to decrypt stored secrets. Preserve that owner-only key with an independently secured home/data backup; restoring the database without its matching key leaves encrypted credentials unreadable. See [Operations](./operations.md).
 
 ### Event bus and NATS
diff --git a/docs/public/contributing.md b/docs/public/contributing.md
index e0b17b8ffd..1239cc78cd 100644
--- a/docs/public/contributing.md
+++ b/docs/public/contributing.md
@@ -46,6 +46,8 @@ make dev PORT=38430 WEB_PORT=37430
 KANDEV_HOME_DIR="$PWD/.kandev-dev" KANDEV_DEBUG_DEV_MODE=true make dev-backend
 ```
 
+One backend owns a Kandev home at a time. Raw backend commands use the normal home by default, so a second backend with that home stops before it changes shared state. For an intentional second backend, use a separate `KANDEV_HOME_DIR`, database, and port.
+
 Use `make build` for a production build. Use `make start` for a production-shaped local start; it installs dependencies, builds and synchronizes the embedded web application, then launches Kandev and writes `<resolved-home>/logs/backend-logs.log`.
 
 ## Find the owner
diff --git a/docs/specs/platform/bounded-task-status-delivery.md b/docs/specs/platform/bounded-task-status-delivery.md
index 667e86a0f3..31316339b6 100644
--- a/docs/specs/platform/bounded-task-status-delivery.md
+++ b/docs/specs/platform/bounded-task-status-delivery.md
@@ -1,7 +1,7 @@
 ---
 status: approved
 created: 2026-08-01
-updated: 2026-08-02
+updated: 2026-08-09
 owner: kandev
 ---
 
@@ -41,6 +41,8 @@ detail surface.
 - `message.add` uses a stable client-generated message ID so an uncertain send
   can be reconciled or retried without creating or dispatching a duplicate.
 - Existing desktop and mobile status/icon precedence remains unchanged.
+- Startup recovery publishes every repaired session state through the semantic task event path.
+  Persisted summaries converge before the backend reports readiness.
 
 ## Task status summary
 
@@ -128,6 +130,8 @@ remain during migration, but switchers use the summary when present.
   events from publishing duplicate revisions or losing a newer value.
 - Missing rows are rebuilt from authoritative records. List and boot loaders
   batch summary reads and may batch repairs; they do not perform an N+1 query.
+- Existing rows are repaired after startup recovery changes authoritative session state. Missing-row
+  hydration is not the only recovery path for a stale summary.
 - Live Git observations remain coalesced before persistence/publication.
   Running executions maintain the slow monitoring baseline independently of
   browser subscribers; active focus may request fast monitoring. Settled tasks
@@ -256,6 +260,9 @@ intermediate replacement.
 - **GIVEN** a phone viewport, **WHEN** task summaries change, **THEN** the
   existing task-switcher sheet shows the same badges and precedence as the
   desktop sidebar without new navigation, scroll, or touch behavior.
+- **GIVEN** a stored summary reports a `RUNNING` primary session with `generating` activity,
+  **WHEN** startup recovery changes that session to `WAITING_FOR_INPUT`, **THEN** a newer summary
+  reports the waiting state and no generating activity before the backend reports readiness.
 
 ## Out of scope
 
@@ -280,3 +287,5 @@ intermediate replacement.
   [`../../plans/bounded-task-status-delivery/plan.md`](../../plans/bounded-task-status-delivery/plan.md)
 - Stream overload repair:
   [`../../plans/session-stream-overload-isolation/plan.md`](../../plans/session-stream-overload-isolation/plan.md)
+- Startup-summary repair:
+  [`../../plans/backend-runtime-state-ownership/plan.md`](../../plans/backend-runtime-state-ownership/plan.md)
diff --git a/docs/specs/port-collision-safety/spec.md b/docs/specs/port-collision-safety/spec.md
index 8400758c2d..44edc74d97 100644
--- a/docs/specs/port-collision-safety/spec.md
+++ b/docs/specs/port-collision-safety/spec.md
@@ -1,9 +1,10 @@
 ---
 status: building
 created: 2026-08-07
+updated: 2026-08-09
 ---
 
-# Port collision safety and launcher readiness
+# Port collision and backend ownership safety
 
 ## Why
 
@@ -17,6 +18,10 @@ On Windows, the agentctl instance allocator has a separate failure: the operatin
 WSAEADDRINUSE (10048), which does not match the synthetic Go syscall.EADDRINUSE value used by the
 current retry check. The allocator therefore stops instead of trying the next port.
 
+A direct backend command can bypass launcher checks. A second backend can then open the same
+Kandev home before its HTTP bind fails. This sequence can migrate the live database and reconcile
+active sessions while the first backend still owns them.
+
 This repair covers GitHub issues
 [#2370](https://github.com/kdlbs/kandev/issues/2370),
 [#2372](https://github.com/kdlbs/kandev/issues/2372), and
@@ -70,6 +75,33 @@ Go syscall value and x/sys/windows.WSAEADDRINUSE on Windows.
 - Non-address-in-use bind errors still release the candidate and fail immediately.
 - String matching on the English error text is not part of the contract.
 
+### Exclusive runtime-state ownership
+
+Every backend process must acquire exclusive ownership before it initializes the backend logger or
+opens a persistent store. The ownership boundary covers these targets:
+
+- The canonical Kandev home, because it owns logs, secrets, worktrees, supervisor files, and the
+  default SQLite database.
+- A custom SQLite database outside that home, because separate homes can still reference the same
+  database file.
+
+The backend holds each operating-system advisory lock until all backend cleanup is complete. A
+crash releases the lock. The lock file can remain after exit because file existence does not prove
+ownership.
+
+If another process holds a required lock, startup exits non-zero. It names the conflicting home or
+database path and tells the operator to use a separate `KANDEV_HOME_DIR` for an intentional second
+instance. The rejected process must not initialize file logging, create a database backup, apply a
+migration, reconcile a session, launch agentctl, or start an HTTP server.
+
+The backend fails closed when it cannot create, open, or acquire a required lock. Launcher port
+preflight and health-token ownership remain useful readiness checks, but they are not persistent
+state ownership checks.
+
+Intentional local instances need separate Kandev homes and separate SQLite databases. A backend
+that uses Postgres still locks its local Kandev home. Active-active Postgres deployment behavior is
+not part of this contract.
+
 ## Scenarios
 
 ### Issue #2370: explicit port collisions
@@ -102,6 +134,19 @@ Go syscall value and x/sys/windows.WSAEADDRINUSE on Windows.
 3. Given a tunnel port is occupied on Windows, when a tunnel is requested, then the caller gets
    the existing clear “port is already in use” error.
 
+### Concurrent backend startup
+
+1. Given one backend owns a Kandev home, when another backend uses the same home, then the second
+   process exits before any persistent backend initialization.
+2. Given two homes reference the same external SQLite database, when both backends start, then
+   only one process opens or changes that database.
+3. Given a backend process stops or crashes, when its successor starts with the same home, then the
+   successor acquires ownership without manual lock-file removal.
+4. Given a live backend uses the default home, when a developer runs a direct backend target
+   against that home, then the direct command fails without changing task or session state.
+5. Given two distinct homes, databases, and ports, when two local backends start, then both can run
+   as independent instances.
+
 ## Out of scope
 
 - Changing default ports, fallback ranges, or automatic-port selection policy.
@@ -113,11 +158,16 @@ Go syscall value and x/sys/windows.WSAEADDRINUSE on Windows.
 - Renaming KANDEV_DESKTOP_HEALTH_TOKEN to a neutral variable; that would be a separate contract
   migration.
 - Changing service-install handling of KANDEV_SERVER_PORT, which is a separate installer issue.
-- UI changes, database migrations, or new public authentication semantics.
+- Database-schema changes or new public authentication semantics.
+- Automatic home selection for direct backend commands.
+- Active-active backend support for one Postgres database or one event namespace.
+- New UI diagnostics for ownership or task-summary fields.
 
 ## Contract notes
 
 The existing desktop health-token contract in
 docs/specs/desktop-tauri-app/spec.md is the authority for the environment variable and response
-header. This repair extends its use to CLI/native launcher ownership checks without changing the
-backend route contract, so a new architecture decision record is not required.
+header. Backend runtime-state ownership follows
+[ADR-2026-08-09-exclusive-runtime-state-ownership](../../decisions/2026-08-09-exclusive-runtime-state-ownership.md).
+The repair plan is
+[Backend runtime-state ownership](../../plans/backend-runtime-state-ownership/plan.md).
diff --git a/docs/specs/tasks/interrupted-task-indicator.md b/docs/specs/tasks/interrupted-task-indicator.md
index 5810d4464d..1bfe1cd509 100644
--- a/docs/specs/tasks/interrupted-task-indicator.md
+++ b/docs/specs/tasks/interrupted-task-indicator.md
@@ -1,5 +1,6 @@
 ---
 status: complete
+updated: 2026-08-09
 created: 2026-08-02
 owner: kandev
 ---
@@ -134,6 +135,9 @@ no effect beyond the icon.
   metadata contains `interrupted_at`, the task DTO reports `interrupted: true`,
   and the sidebar row, board card, graph node, rich list row, and mobile
   switcher row show the red alert icon.
+- **GIVEN** that task also has a stored `generating` task summary, **WHEN** reconciliation changes
+  the primary session to `WAITING_FOR_INPUT`, **THEN** the summary clears `generating` and the red
+  alert icon is not hidden by a stale running spinner.
 - **GIVEN** a task whose session was `WAITING_FOR_INPUT` when the backend
   died, **WHEN** reconciliation runs, **THEN** the task is not marked and shows
   no red icon.
@@ -167,3 +171,5 @@ no effect beyond the icon.
   the marker hooks into.
 - [Sidebar Task Completion Icons](../ui/sidebar-task-completion-icons.md) —
   the shared task-row icon precedence this feature extends.
+- [Backend Runtime-State Ownership](../../plans/backend-runtime-state-ownership/plan.md) —
+  startup publication that clears stale generating activity.