diff --git a/internal/cli/connection/core/sync/state.go b/internal/cli/connection/core/sync/state.go
index f7ab4de97..36095e72f 100644
--- a/internal/cli/connection/core/sync/state.go
+++ b/internal/cli/connection/core/sync/state.go
@@ -41,18 +41,20 @@ func loadState() (state, func(), error) {
 		return s, nil, mkErr
 	}
 
-	// Acquire lock: fail if another sync is running.
-	if _, statErr := os.Stat(lockPath); statErr == nil {
-		return s, nil, os.ErrExist
+	// Acquire lock: fail if another sync is running. The
+	// O_CREATE|O_EXCL create-or-fail is a single syscall, so
+	// there is no check-to-write window for a concurrent sync
+	// to slip through.
+	acquired, lockErr := io.SafeTryLock(lockPath, fs.PermFile)
+	if lockErr != nil {
+		return s, nil, lockErr
 	}
-	if writeErr := io.SafeWriteFile(
-		lockPath, []byte(cfgHub.LockSentinel), fs.PermFile,
-	); writeErr != nil {
-		return s, nil, writeErr
+	if !acquired {
+		return s, nil, os.ErrExist
 	}
 
 	release := func() {
-		if rmErr := os.Remove(lockPath); rmErr != nil {
+		if rmErr := io.SafeUnlock(lockPath); rmErr != nil {
 			logWarn.Warn(cfgWarn.Remove, lockPath, rmErr)
 		}
 	}
diff --git a/internal/cli/connection/core/sync/state_test.go b/internal/cli/connection/core/sync/state_test.go
new file mode 100644
index 000000000..dedafe601
--- /dev/null
+++ b/internal/cli/connection/core/sync/state_test.go
@@ -0,0 +1,153 @@
+//   /    ctx:                         https://ctx.ist
+// ,'`./    do you remember?
+// `.,'\
+//   \    Copyright 2026-present Context contributors.
+//                 SPDX-License-Identifier: Apache-2.0
+
+package sync
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/ActiveMemory/ctx/internal/config/fs"
+	cfgHub "github.com/ActiveMemory/ctx/internal/config/hub"
+	"github.com/ActiveMemory/ctx/internal/io"
+	"github.com/ActiveMemory/ctx/internal/testutil/testctx"
+)
+
+// declareContext positions the test in a temp project with a
+// materialized .context/ directory and returns its path.
+func declareContext(t *testing.T) string {
+	t.Helper()
+	dir := t.TempDir()
+	ctxDir := testctx.Declare(t, dir)
+	if mkErr := os.MkdirAll(ctxDir, fs.PermExec); mkErr != nil {
+		t.Fatal(mkErr)
+	}
+	return ctxDir
+}
+
+func TestLoadState_RejectsConcurrentSyncs(t *testing.T) {
+	declareContext(t)
+
+	const attempts = 16
+
+	type outcome struct {
+		release func()
+		err     error
+	}
+
+	start := make(chan struct{})
+	results := make(chan outcome, attempts)
+	for i := 0; i < attempts; i++ {
+		go func() {
+			<-start
+			_, release, lockErr := loadState()
+			results <- outcome{release: release, err: lockErr}
+		}()
+	}
+	close(start)
+
+	// Winners must not release until every attempt has
+	// finished, or a late goroutine could legitimately
+	// re-acquire the freed lock and skew the count.
+	var winners []func()
+	var contended int
+	for i := 0; i < attempts; i++ {
+		got := <-results
+		switch {
+		case got.err == nil:
+			winners = append(winners, got.release)
+		case errors.Is(got.err, os.ErrExist):
+			contended++
+		default:
+			t.Fatalf("unexpected error: %v", got.err)
+		}
+	}
+
+	if len(winners) != 1 {
+		t.Errorf("winners = %d, want exactly 1", len(winners))
+	}
+	if contended != attempts-1 {
+		t.Errorf(
+			"contended = %d, want %d", contended, attempts-1,
+		)
+	}
+	for _, release := range winners {
+		release()
+	}
+}
+
+func TestLoadState_ReleaseRemovesLock(t *testing.T) {
+	ctxDir := declareContext(t)
+	lockPath := filepath.Join(
+		ctxDir, cfgHub.DirHub, cfgHub.FileSyncLock,
+	)
+
+	_, release, lockErr := loadState()
+	if lockErr != nil {
+		t.Fatalf("first loadState: %v", lockErr)
+	}
+	if _, statErr := os.Stat(lockPath); statErr != nil {
+		t.Fatalf("lock file should exist while held: %v", statErr)
+	}
+
+	release()
+
+	if _, statErr := os.Stat(lockPath); !os.IsNotExist(statErr) {
+		t.Errorf(
+			"lock file should be gone after release: %v", statErr,
+		)
+	}
+
+	// The next sync must be able to proceed.
+	_, release, lockErr = loadState()
+	if lockErr != nil {
+		t.Fatalf("loadState after release: %v", lockErr)
+	}
+	release()
+}
+
+func TestLoadState_ReleasesLockOnCorruptState(t *testing.T) {
+	ctxDir := declareContext(t)
+	hubDir := filepath.Join(ctxDir, cfgHub.DirHub)
+	if mkErr := io.SafeMkdirAll(hubDir, fs.PermKeyDir); mkErr != nil {
+		t.Fatal(mkErr)
+	}
+	statePath := filepath.Join(hubDir, cfgHub.FileSyncState)
+	if writeErr := os.WriteFile(
+		statePath, []byte("{not json"), fs.PermFile,
+	); writeErr != nil {
+		t.Fatal(writeErr)
+	}
+
+	_, _, loadErr := loadState()
+	if loadErr == nil {
+		t.Fatal("loadState should fail on corrupt state")
+	}
+
+	lockPath := filepath.Join(hubDir, cfgHub.FileSyncLock)
+	if _, statErr := os.Stat(lockPath); !os.IsNotExist(statErr) {
+		t.Errorf(
+			"lock must not leak after a failed load: %v", statErr,
+		)
+	}
+}
+
+func TestLoadState_LockFileLocation(t *testing.T) {
+	ctxDir := declareContext(t)
+
+	_, release, lockErr := loadState()
+	if lockErr != nil {
+		t.Fatalf("loadState: %v", lockErr)
+	}
+	defer release()
+
+	want := filepath.Join(ctxDir, cfgHub.DirHub, cfgHub.FileSyncLock)
+	if _, statErr := os.Stat(want); statErr != nil {
+		t.Errorf("lock file not at %s: %v", want, statErr)
+	}
+}
diff --git a/internal/config/hub/doc.go b/internal/config/hub/doc.go
index 0eb87a683..d22c657ab 100644
--- a/internal/config/hub/doc.go
+++ b/internal/config/hub/doc.go
@@ -59,7 +59,7 @@
 //     connection config files
 //   - FilePID ("hub.pid"), FileAdminToken,
 //     DirHubData: daemon management files
-//   - JSONIndent, LockSentinel, SuffixPluralMD:
+//   - JSONIndent, SuffixPluralMD:
 //     formatting and naming helpers
 //
 // # Raft Cluster Configuration
diff --git a/internal/config/hub/hub.go b/internal/config/hub/hub.go
index 0d8fe4e70..16e7cc15d 100644
--- a/internal/config/hub/hub.go
+++ b/internal/config/hub/hub.go
@@ -167,8 +167,6 @@ const (
 	FileConnect = ".connect.enc"
 	// JSONIndent is the indentation string for JSON marshaling.
 	JSONIndent = "  "
-	// LockSentinel is the content written to lock files.
-	LockSentinel = "lock"
 	// SuffixPluralMD is the suffix for typed hub markdown
 	// filenames (e.g. "decisions.md").
 	SuffixPluralMD = "s.md"
diff --git a/specs/fix-connect-sync-lock-toctou.md b/specs/fix-connect-sync-lock-toctou.md
new file mode 100644
index 000000000..021f9edd5
--- /dev/null
+++ b/specs/fix-connect-sync-lock-toctou.md
@@ -0,0 +1,87 @@
+# Fix Connect Sync Lock TOCTOU Race
+
+`internal/cli/connection/core/sync.loadState` guarded
+`ctx connect sync` against concurrent execution with a
+check-then-write pattern (`os.Stat` followed by
+`io.SafeWriteFile`), not a real lock. Upstream issue:
+[ActiveMemory/ctx#93](https://github.com/ActiveMemory/ctx/issues/93).
+
+## Problem
+
+The window between the existence check and the write is
+unbounded, and the pattern is racy by construction:
+
+```go
+// Acquire lock: fail if another sync is running.
+if _, statErr := os.Stat(lockPath); statErr == nil {
+    return s, nil, os.ErrExist
+}
+if writeErr := io.SafeWriteFile(
+    lockPath, []byte(cfgHub.LockSentinel), fs.PermFile,
+); writeErr != nil {
+    return s, nil, writeErr
+}
+```
+
+Two processes racing past the `os.Stat` (hook-triggered plus
+manual invocation; cron plus interactive run; two terminals)
+can both decide the lock is free, both write the lock file,
+both load the same `LastSequence`, both fetch the same entries
+from the hub, and both write duplicate content into
+`.context/hub/`. (Issue #93 says `.context/shared/`; that is
+the pre-rename path — the renderer joins `cfgHub.DirHub` at
+`internal/cli/connection/core/render/render.go:32`.)
+
+The doc comment ("Acquires a lock file to prevent concurrent
+access.") overstated what the code did.
+
+The package had no test coverage at all: no test exercised
+the contention path, the release path, or the lock location.
+
+## Solution
+
+Replace the stat-then-write with the atomic create-or-fail
+primitive that already exists for exactly this purpose:
+`io.SafeTryLock` (`O_CREATE|O_EXCL` in a single syscall) and
+its counterpart `io.SafeUnlock`. Both landed with the dream
+consolidator and have prior art at
+`internal/cli/dream/core/pass/pass.go:62-70`.
+
+1. `internal/cli/connection/core/sync/state.go` — swap the
+   `os.Stat` / `io.SafeWriteFile` pair for one
+   `io.SafeTryLock` call. `acquired == false` maps to the
+   existing `os.ErrExist` contract so `Run`'s caller-facing
+   behavior is unchanged. The `release` closure delegates to
+   `io.SafeUnlock`, keeping the warn-on-failure logging.
+2. `internal/config/hub/hub.go` — delete `LockSentinel`. Its
+   only consumer was the racy write; `SafeTryLock` creates an
+   empty lock file (the lock is the file's existence, not its
+   content), so the constant is dead. Leaving it would be a
+   broken window.
+3. `internal/config/hub/doc.go` — drop `LockSentinel` from
+   the package-doc constant inventory.
+4. `internal/cli/connection/core/sync/state_test.go` — new;
+   regression-pins the contract:
+   - `TestLoadState_RejectsConcurrentSyncs`: N goroutines
+     race `loadState`; exactly one acquires, the rest observe
+     `os.ErrExist`.
+   - `TestLoadState_ReleaseRemovesLock`: after `release()`,
+     the lock file is gone and a subsequent `loadState`
+     succeeds.
+   - `TestLoadState_ReleasesLockOnCorruptState`: a corrupt
+     `.sync-state.json` makes `loadState` fail *after*
+     acquisition; the lock must not leak.
+   - `TestLoadState_LockFileLocation`: the lock lives at
+     `<ctxDir>/hub/.sync.lock` (pins `DirHub` +
+     `FileSyncLock` composition).
+
+## Out of Scope
+
+- Stale-lock detection (PID-in-lockfile, age-based cleanup).
+  A crashed process still leaves a stale lock; the issue
+  explicitly defers this to a follow-up.
+- `flock`-based locking (issue's Option B). Rejected for now:
+  `syscall.Flock` is Unix-only and `SafeTryLock` already
+  matches the existing lockfile-as-sentinel model.
+- The hubsync hook's silent error suppression
+  (ActiveMemory/ctx#100) — adjacent code, separate issue.