From bf5265ad110596c2c4acf0548d5dc9162b4ddc24 Mon Sep 17 00:00:00 2001
From: Daniel Stenberg <daniel@haxx.se>
Date: Wed, 6 Aug 2025 00:52:04 +0200
Subject: [PATCH] dynbuf: assert on curlx_dyn_addf use with just "%s"

Because it is wasteful and should be done better.

Closes #18194
---
 lib/curlx/dynbuf.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/curlx/dynbuf.c b/lib/curlx/dynbuf.c
index cd4f4635a6..447203e42a 100644
--- a/lib/curlx/dynbuf.c
+++ b/lib/curlx/dynbuf.c
@@ -231,6 +231,7 @@ CURLcode curlx_dyn_addf(struct dynbuf *s, const char *fmt, ...)
   DEBUGASSERT(s);
   DEBUGASSERT(s->init == DYNINIT);
   DEBUGASSERT(!s->leng || s->bufr);
+  DEBUGASSERT(strcmp(fmt, "%s")); /* use curlx_dyn_add instead */
   va_start(ap, fmt);
   result = curlx_dyn_vaddf(s, fmt, ap);
   va_end(ap);