diff --git a/wolfx509/certgen.go b/wolfx509/certgen.go
index bd37eae..6e525ac 100644
--- a/wolfx509/certgen.go
+++ b/wolfx509/certgen.go
@@ -83,6 +83,7 @@ func CreateCertificate(template, parent *Certificate, pubKey, signer KeyHandle)
 		DNSNames:    template.DNSNames,
 		IPAddresses: template.IPAddresses,
 		AcmeKeyAuth: template.AcmeKeyAuth,
+		BasicConstraintsValid: template.BasicConstraintsValid,
 	}
 
 	if template == parent && pubKey == signer {
diff --git a/wolfx509/certgen_test.go b/wolfx509/certgen_test.go
index 16cf2a7..d4b0a05 100644
--- a/wolfx509/certgen_test.go
+++ b/wolfx509/certgen_test.go
@@ -22,6 +22,8 @@ package wolfx509
 
 import (
 	"bytes"
+	"encoding/asn1"
+	"encoding/hex"
 	"math/big"
 	"net"
 	"testing"
@@ -448,3 +450,74 @@ func TestCreateCertificateValidDays(t *testing.T) {
 		t.Errorf("NotAfter = %s, want within [%s, %s]", parsed.NotAfter, naLo, naHi)
 	}
 }
+
+// oidBasicConstraints is id-ce-basicConstraints (RFC 5280 4.2.1.9).
+var oidBasicConstraints = asn1.ObjectIdentifier{2, 5, 29, 19}
+
+// TestBasicConstraintsMatrix pins the BasicConstraints extension that is
+// emitted for a given template:
+//   - emitted only when BasicConstraintsValid is set,
+//   - always critical,
+//   - present but with the cA boolean omitted (an empty SEQUENCE) when IsCA
+//     is false.
+func TestBasicConstraintsMatrix(t *testing.T) {
+	for _, tc := range []struct {
+		name        string
+		bcValid     bool
+		isCA        bool
+		wantPresent bool
+		wantValue   string // hex of the extension value
+	}{
+		{"valid+CA", true, true, true, "30030101ff"},
+		{"valid+notCA", true, false, true, "3000"},
+		{"notValid+CA", false, true, false, ""},
+		{"notValid+notCA", false, false, false, ""},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			k, err := GenerateP256Key()
+			if err != nil {
+				t.Fatalf("GenerateP256Key: %v", err)
+			}
+			defer k.Free()
+
+			tmpl := &Certificate{
+				SerialNumber:          big.NewInt(1),
+				Subject:               Name{CommonName: "bc test"},
+				NotBefore:             time.Now().Add(-time.Hour),
+				NotAfter:              time.Now().Add(24 * time.Hour),
+				BasicConstraintsValid: tc.bcValid,
+				IsCA:                  tc.isCA,
+			}
+			der, err := CreateCertificate(tmpl, tmpl, k, k)
+			if err != nil {
+				t.Fatalf("CreateCertificate: %v", err)
+			}
+			parsed, err := stdlibParseCert(der)
+			if err != nil {
+				t.Fatalf("stdlibParseCert: %v", err)
+			}
+
+			for _, ext := range parsed.Extensions {
+				if !ext.Id.Equal(oidBasicConstraints) {
+					continue
+				}
+				if !tc.wantPresent {
+					t.Fatalf("BasicConstraints emitted for BasicConstraintsValid=false")
+				}
+				if !ext.Critical {
+					t.Error("BasicConstraints is not critical; RFC 5280 4.2.1.9 requires it of CA certs and crypto/x509 always marks it")
+				}
+				if got := hex.EncodeToString(ext.Value); got != tc.wantValue {
+					t.Errorf("BasicConstraints value = %s, want %s", got, tc.wantValue)
+				}
+				if parsed.IsCA != tc.isCA {
+					t.Errorf("parsed IsCA = %v, want %v", parsed.IsCA, tc.isCA)
+				}
+				return
+			}
+			if tc.wantPresent {
+				t.Fatal("BasicConstraints extension missing")
+			}
+		})
+	}
+}
diff --git a/wolfx509/certgen_wolfcrypt.go b/wolfx509/certgen_wolfcrypt.go
index b7389fe..a12302b 100644
--- a/wolfx509/certgen_wolfcrypt.go
+++ b/wolfx509/certgen_wolfcrypt.go
@@ -51,6 +51,12 @@ package wolfx509
 //     return -174;
 // }
 // #endif
+//
+// /* Cert.basicConstSet is a C bitfield, which cgo cannot address
+//  * without a setter. */
+// static void wolfx509_set_basic_const(Cert* cert, int toggle) {
+//     cert->basicConstSet = (toggle != 0);
+// }
 import "C"
 import (
 	"encoding/asn1"
@@ -120,6 +126,9 @@ type certBuildOpts struct {
 	DNSNames    []string  // SubjectAltName dNSName entries
 	IPAddresses []net.IP  // SubjectAltName iPAddress entries
 
+	// BasicConstraintsValid requests the BasicConstraints extension.
+	BasicConstraintsValid bool
+
 	// AcmeKeyAuth, if non-empty, sets the RFC 8737 id-pe-acmeIdentifier
 	// (1.3.6.1.5.5.7.1.31) extension on the cert. Pass the raw keyAuth
 	// bytes (token "." JWK_thumbprint per RFC 8555 §8.1); wolfCrypt
@@ -182,8 +191,16 @@ func buildAndSignCert(opts certBuildOpts, parentDER []byte, pubKey, signerKey Ke
 	}
 	cert.version = 2
 	cert.sigType = C.int(sigType)
-	if opts.IsCA {
-		cert.isCA = 1
+	// BasicConstraints: emitted only when BasicConstraintsValid is set, has to
+	// be marked CRITICAL if set.
+	// When IsCA is TRUE, wolfSSL will automatically encode the extension.
+	if opts.BasicConstraintsValid {
+		cert.basicConstCrit = 1
+		if opts.IsCA {
+			cert.isCA = 1
+		} else {
+			C.wolfx509_set_basic_const(&cert, 1)
+		}
 	}
 	cert.keyUsage = C.word16(opts.KeyUsage)
 	cert.extKeyUsage = C.byte(opts.ExtKeyUsage)