From 1504ac3d19e2225f7713eabc4c20c8d543a91bfa Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Tue, 21 Apr 2026 14:32:29 +0200
Subject: [PATCH] [core] Replace strnlen in buf_append_str for Zephyr
 compatibility (#15892)

---
 esphome/core/helpers.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/esphome/core/helpers.h b/esphome/core/helpers.h
index 78476bf596..6b71916cd2 100644
--- a/esphome/core/helpers.h
+++ b/esphome/core/helpers.h
@@ -1117,7 +1117,10 @@ inline size_t buf_append_str(char *buf, size_t size, size_t pos, const char *str
     return size;
   }
   size_t remaining = size - pos - 1;  // reserve space for null terminator
-  size_t len = strnlen(str, remaining);
+  size_t len = 0;
+  while (len < remaining && str[len] != '\0') {
+    len++;
+  }
   memcpy(buf + pos, str, len);
   pos += len;
   buf[pos] = '\0';