diff --git a/cmd/doctor.go b/cmd/doctor.go
index 0795e8b..e20c956 100644
--- a/cmd/doctor.go
+++ b/cmd/doctor.go
@@ -11,6 +11,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/warrenronsiek/ctask/internal/config"
+	"github.com/warrenronsiek/ctask/internal/shell"
 	"github.com/warrenronsiek/ctask/internal/workspace"
 )
 
@@ -203,6 +204,9 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	// Check 8: CTASK_PROJECT_ROOT (v0.5).
 	checkProjectRoot(&passed, &failed)
 
+	// Check 9: tmux availability for persistent session mode (v0.5.3).
+	checkTmux(&passed, &failed)
+
 	// Summary
 	fmt.Println()
 	fmt.Printf("%d checks passed, %d failed\n", passed, failed)
@@ -254,3 +258,29 @@ func checkSeedDir(label, envValue, resolved, envName string, passed, failed *int
 	fmt.Printf("         Fix: create the directory or unset %s to use built-in defaults.\n", envName)
 	*failed++
 }
+
+// checkTmux reports the three-state tmux check (v0.5.3):
+//   - CTASK_SESSION_MODE != "persistent" -> INFO (direct mode, tmux optional)
+//   - persistent + tmux on PATH + version OK -> two INFO lines
+//   - persistent + tmux missing or too old -> FAIL with install/update hint
+func checkTmux(passed, failed *int) {
+	_ = passed
+	mode := config.ResolveSessionMode()
+	if mode != "persistent" {
+		fmt.Printf("  [INFO] Session mode: direct (tmux not required)\n")
+		return
+	}
+	fmt.Printf("  [INFO] Session mode: persistent\n")
+	tmuxPath, ver, err := shell.LookupTmux()
+	if err != nil {
+		fmt.Printf("  [FAIL] tmux not found on PATH or unsupported version: %v\n", err)
+		fmt.Printf("         Fix: install tmux 3.0+ (apt/brew/pacman/dnf), or unset CTASK_SESSION_MODE\n")
+		*failed++
+		return
+	}
+	rawVer := ver.Raw
+	if rawVer == "" {
+		rawVer = "unknown version"
+	}
+	fmt.Printf("  [INFO] tmux found: %s (%s)\n", rawVer, tmuxPath)
+}
diff --git a/cmd/doctor_test.go b/cmd/doctor_test.go
index 4b50c4e..054de11 100644
--- a/cmd/doctor_test.go
+++ b/cmd/doctor_test.go
@@ -2,12 +2,31 @@ package cmd
 
 import (
 	"bytes"
+	"io"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"strings"
 	"testing"
 )
 
+// captureStdout runs fn while capturing os.Stdout and returns the output.
+func captureStdout(t *testing.T, fn func()) string {
+	t.Helper()
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatalf("pipe: %v", err)
+	}
+	orig := os.Stdout
+	os.Stdout = w
+	defer func() { os.Stdout = orig }()
+
+	fn()
+	w.Close()
+	data, _ := io.ReadAll(r)
+	return string(data)
+}
+
 // This file swaps process-global os.Stdout and env vars. Do not call
 // t.Parallel() in this file.
 
@@ -161,3 +180,50 @@ func TestDoctorProjectRootSetButMissingFails(t *testing.T) {
 		t.Errorf("expected FAIL line, got:\n%s", out)
 	}
 }
+
+func TestCheckTmuxNotConfigured(t *testing.T) {
+	os.Unsetenv("CTASK_SESSION_MODE")
+	out := captureStdout(t, func() {
+		passed, failed := 0, 0
+		checkTmux(&passed, &failed)
+	})
+	if !strings.Contains(out, "Session mode: direct") {
+		t.Errorf("expected 'Session mode: direct' info line: %q", out)
+	}
+}
+
+func TestCheckTmuxConfiguredAndPresent(t *testing.T) {
+	if _, err := exec.LookPath("tmux"); err != nil {
+		t.Skip("tmux not on PATH")
+	}
+	os.Setenv("CTASK_SESSION_MODE", "persistent")
+	defer os.Unsetenv("CTASK_SESSION_MODE")
+	out := captureStdout(t, func() {
+		passed, failed := 0, 0
+		checkTmux(&passed, &failed)
+	})
+	if !strings.Contains(out, "Session mode: persistent") {
+		t.Errorf("expected 'Session mode: persistent': %q", out)
+	}
+	if !strings.Contains(out, "tmux found") {
+		t.Errorf("expected 'tmux found' info line: %q", out)
+	}
+}
+
+func TestCheckTmuxConfiguredAndMissing(t *testing.T) {
+	orig := os.Getenv("PATH")
+	defer os.Setenv("PATH", orig)
+	os.Setenv("PATH", "")
+	os.Setenv("CTASK_SESSION_MODE", "persistent")
+	defer os.Unsetenv("CTASK_SESSION_MODE")
+
+	failed := 0
+	passed := 0
+	out := captureStdout(t, func() { checkTmux(&passed, &failed) })
+	if failed != 1 {
+		t.Errorf("missing tmux must increment failed counter; got %d", failed)
+	}
+	if !strings.Contains(out, "tmux not found") {
+		t.Errorf("expected 'tmux not found' fail line: %q", out)
+	}
+}