diff --git a/cmd/doctor.go b/cmd/doctor.go
index ef4f1e2..0795e8b 100644
--- a/cmd/doctor.go
+++ b/cmd/doctor.go
@@ -200,6 +200,9 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 		&failed,
 	)
 
+	// Check 8: CTASK_PROJECT_ROOT (v0.5).
+	checkProjectRoot(&passed, &failed)
+
 	// Summary
 	fmt.Println()
 	fmt.Printf("%d checks passed, %d failed\n", passed, failed)
@@ -210,6 +213,28 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	return nil
 }
 
+// checkProjectRoot prints the three-state CTASK_PROJECT_ROOT doctor line:
+//   - unset           -> INFO pointing at the default location under CTASK_ROOT
+//   - set and exists  -> INFO with custom-root advisory
+//   - set and missing -> FAIL (increments failed counter)
+func checkProjectRoot(passed, failed *int) {
+	_ = passed // reserved for future symmetry with checkSeedDir
+	envValue := os.Getenv("CTASK_PROJECT_ROOT")
+	if envValue == "" {
+		defaultProj := filepath.Join(config.ResolveRoot(), "projects")
+		fmt.Printf("  [INFO] CTASK_PROJECT_ROOT: not set (projects discovered under %s)\n", defaultProj)
+		return
+	}
+	resolved := config.ResolveProjectRoot()
+	if info, err := os.Stat(resolved); err == nil && info.IsDir() {
+		fmt.Printf("  [INFO] CTASK_PROJECT_ROOT: %s (custom -- recommended: set at user scope so all terminals can discover these workspaces)\n", resolved)
+		return
+	}
+	fmt.Printf("  [FAIL] CTASK_PROJECT_ROOT configured but not found: %s\n", resolved)
+	fmt.Printf("         Fix: create the directory or unset CTASK_PROJECT_ROOT.\n")
+	*failed++
+}
+
 // checkSeedDir prints one of the three seed-directory doctor states.
 // envValue is the raw value of the CTASK_SEED_* variable (empty means unset).
 // resolved is the absolute path returned by the config resolver (used only
diff --git a/cmd/doctor_test.go b/cmd/doctor_test.go
index ec2bbb3..4b50c4e 100644
--- a/cmd/doctor_test.go
+++ b/cmd/doctor_test.go
@@ -3,6 +3,7 @@ package cmd
 import (
 	"bytes"
 	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 )
@@ -98,3 +99,65 @@ func TestDoctorSeedDirConfiguredButMissingFails(t *testing.T) {
 		t.Errorf("expected FAIL line for missing seed dir, got:\n%s", out)
 	}
 }
+
+func TestDoctorProjectRootNotSetShowsDefault(t *testing.T) {
+	root := t.TempDir()
+	prevProj := os.Getenv("CTASK_PROJECT_ROOT")
+	os.Unsetenv("CTASK_PROJECT_ROOT")
+	defer func() {
+		if prevProj != "" {
+			os.Setenv("CTASK_PROJECT_ROOT", prevProj)
+		}
+	}()
+
+	out, _ := runDoctorCapture(t, root, "", "", false, false)
+	if !strings.Contains(out, "[INFO] CTASK_PROJECT_ROOT: not set") {
+		t.Errorf("expected 'not set' INFO line, got:\n%s", out)
+	}
+}
+
+func TestDoctorProjectRootSetAndExistsShowsCustom(t *testing.T) {
+	root := t.TempDir()
+	projRoot := t.TempDir()
+
+	prevProj := os.Getenv("CTASK_PROJECT_ROOT")
+	os.Setenv("CTASK_PROJECT_ROOT", projRoot)
+	defer func() {
+		if prevProj == "" {
+			os.Unsetenv("CTASK_PROJECT_ROOT")
+		} else {
+			os.Setenv("CTASK_PROJECT_ROOT", prevProj)
+		}
+	}()
+
+	out, _ := runDoctorCapture(t, root, "", "", false, false)
+	if !strings.Contains(out, "[INFO] CTASK_PROJECT_ROOT: "+projRoot) {
+		t.Errorf("expected custom INFO line, got:\n%s", out)
+	}
+	if !strings.Contains(out, "user scope") {
+		t.Errorf("expected user-scope advice, got:\n%s", out)
+	}
+}
+
+func TestDoctorProjectRootSetButMissingFails(t *testing.T) {
+	root := t.TempDir()
+	missing := filepath.Join(root, "no-such-project-root")
+
+	prevProj := os.Getenv("CTASK_PROJECT_ROOT")
+	os.Setenv("CTASK_PROJECT_ROOT", missing)
+	defer func() {
+		if prevProj == "" {
+			os.Unsetenv("CTASK_PROJECT_ROOT")
+		} else {
+			os.Setenv("CTASK_PROJECT_ROOT", prevProj)
+		}
+	}()
+
+	out, err := runDoctorCapture(t, root, "", "", false, false)
+	if err == nil {
+		t.Fatal("expected doctor to fail when CTASK_PROJECT_ROOT set but missing")
+	}
+	if !strings.Contains(out, "[FAIL] CTASK_PROJECT_ROOT") {
+		t.Errorf("expected FAIL line, got:\n%s", out)
+	}
+}