diff --git a/devel/cbmc/Makefile b/devel/cbmc/Makefile
index dd76cebffe07..9dab943a9d27 100644
--- a/devel/cbmc/Makefile
+++ b/devel/cbmc/Makefile
@@ -1,39 +1,50 @@
 PORTNAME=	cbmc
 DISTVERSIONPREFIX=	cbmc-
 DISTVERSION=	6.9.0
 PORTREVISION=	1
 CATEGORIES=	devel
 MASTER_SITES=	DEBIAN/pool/main/m/minisat2:minisat
 DISTFILES=	minisat2_2.2.1.orig.tar.gz:minisat
 
 MAINTAINER=	olivier@FreeBSD.org
 COMMENT=	Bounded Model Checker for C and C++ programs
 WWW=		https://github.com/diffblue/cbmc
 
 LICENSE=	BSD4CLAUSE
 LICENSE_FILE=	${WRKSRC}/LICENSE
 
 BUILD_DEPENDS=	flex:textproc/flex \
 		bash:shells/bash \
 		git:devel/git@lite \
 		jq:textproc/jq
 RUN_DEPENDS=	cvc5:math/cvc5 \
 		z3:math/z3
 
 USES=		cmake bison python shebangfix perl5
 USE_PERL5=	build
 
 USE_GITHUB=	yes
 GH_ACCOUNT=	diffblue
 SHEBANG_FILES=	${WRKSRC}/scripts/ls_parse.py
 
 CMAKE_OFF=	WITH_JBMC
 
+.include <bsd.port.pre.mk>
+
+# Pre-llvm-21.1.8 clang flags bundled catch.hpp's zero-arg variadic macros
+# under -Wc++20-extensions; project's -Wpedantic -Werror in CMakeLists comes
+# after CMAKE_CXX_FLAGS, so patch the silencer in there directly.
+.if ${OSVERSION} < 1600017
+post-patch:
+	${REINPLACE_CMD} -e 's|-Wno-deprecated-declarations")|-Wno-deprecated-declarations -Wno-c++20-extensions")|' \
+		${WRKSRC}/CMakeLists.txt
+.endif
+
 pre-extract:
 	${MKDIR} ${BUILD_WRKSRC}/minisat2-download/minisat2-download-prefix/src
 	${CP} ${DISTDIR}/minisat2_2.2.1.orig.tar.gz ${BUILD_WRKSRC}/minisat2-download/minisat2-download-prefix/src/
 
 do-test:
 	cd ${BUILD_WRKSRC} && ctest . -V -L CORE
 
-.include <bsd.port.mk>
+.include <bsd.port.post.mk>