diff -up ./gtests/ssl_gtest/manifest.mn.disable_ech ./gtests/ssl_gtest/manifest.mn
--- ./gtests/ssl_gtest/manifest.mn.disable_ech	2024-06-12 13:29:17.162207862 -0700
+++ ./gtests/ssl_gtest/manifest.mn	2024-06-12 13:30:25.699047788 -0700
@@ -59,7 +59,6 @@ CPPSRCS = \
       tls_protect.cc \
       tls_psk_unittest.cc \
       tls_subcerts_unittest.cc \
-      tls_ech_unittest.cc \
       tls_xyber_unittest.cc \
       $(SSLKEYLOGFILE_FILES) \
       $(NULL)
diff -up ./lib/ssl/sslsock.c.disable_ech ./lib/ssl/sslsock.c
--- ./lib/ssl/sslsock.c.disable_ech	2024-06-07 09:26:03.000000000 -0700
+++ ./lib/ssl/sslsock.c	2024-06-12 13:29:17.162207862 -0700
@@ -4415,17 +4415,23 @@ ssl_ClearPRCList(PRCList *list, void (*f
 SECStatus
 SSLExp_EnableTls13GreaseEch(PRFileDesc *fd, PRBool enabled)
 {
+#ifdef notdef
     sslSocket *ss = ssl_FindSocket(fd);
     if (!ss) {
         return SECFailure;
     }
     ss->opt.enableTls13GreaseEch = enabled;
     return SECSuccess;
+#else
+    PORT_SetError(SSL_ERROR_UNSUPPORTED_EXPERIMENTAL_API);
+    return SECFailure;
+#endif
 }
 
 SECStatus
 SSLExp_SetTls13GreaseEchSize(PRFileDesc *fd, PRUint8 size)
 {
+#ifdef notdef
     sslSocket *ss = ssl_FindSocket(fd);
     if (!ss || size == 0) {
         return SECFailure;
@@ -4439,28 +4445,42 @@ SSLExp_SetTls13GreaseEchSize(PRFileDesc
     ssl_Release1stHandshakeLock(ss);
 
     return SECSuccess;
+#else
+    PORT_SetError(SSL_ERROR_UNSUPPORTED_EXPERIMENTAL_API);
+    return SECFailure;
+#endif
 }
 
 SECStatus
 SSLExp_EnableTls13BackendEch(PRFileDesc *fd, PRBool enabled)
 {
+#ifdef notdef
     sslSocket *ss = ssl_FindSocket(fd);
     if (!ss) {
         return SECFailure;
     }
     ss->opt.enableTls13BackendEch = enabled;
     return SECSuccess;
+#else
+    PORT_SetError(SSL_ERROR_UNSUPPORTED_EXPERIMENTAL_API);
+    return SECFailure;
+#endif
 }
 
 SECStatus
 SSLExp_CallExtensionWriterOnEchInner(PRFileDesc *fd, PRBool enabled)
 {
+#ifdef notdef
     sslSocket *ss = ssl_FindSocket(fd);
     if (!ss) {
         return SECFailure;
     }
     ss->opt.callExtensionWriterOnEchInner = enabled;
     return SECSuccess;
+#else
+    PORT_SetError(SSL_ERROR_UNSUPPORTED_EXPERIMENTAL_API);
+    return SECFailure;
+#endif
 }
 
 SECStatus