diff --git a/esphome/components/web_server/web_server.cpp b/esphome/components/web_server/web_server.cpp
index 0c22c2f08d..6870a1dc87 100644
--- a/esphome/components/web_server/web_server.cpp
+++ b/esphome/components/web_server/web_server.cpp
@@ -1042,7 +1042,13 @@ std::string WebServer::date_json(datetime::DateEntity *obj, JsonDetail start_con
   json::JsonBuilder builder;
   JsonObject root = builder.root();
 
-  std::string value = str_sprintf("%d-%02d-%02d", obj->year, obj->month, obj->day);
+  // Format: YYYY-MM-DD (max 10 chars + null)
+  char value[12];
+#ifdef USE_ESP8266
+  snprintf_P(value, sizeof(value), PSTR("%d-%02d-%02d"), obj->year, obj->month, obj->day);
+#else
+  snprintf(value, sizeof(value), "%d-%02d-%02d", obj->year, obj->month, obj->day);
+#endif
   set_json_icon_state_value(root, obj, "date", value, value, start_config);
   if (start_config == DETAIL_ALL) {
     this->add_sorting_info_(root, obj);
@@ -1098,7 +1104,13 @@ std::string WebServer::time_json(datetime::TimeEntity *obj, JsonDetail start_con
   json::JsonBuilder builder;
   JsonObject root = builder.root();
 
-  std::string value = str_sprintf("%02d:%02d:%02d", obj->hour, obj->minute, obj->second);
+  // Format: HH:MM:SS (8 chars + null)
+  char value[12];
+#ifdef USE_ESP8266
+  snprintf_P(value, sizeof(value), PSTR("%02d:%02d:%02d"), obj->hour, obj->minute, obj->second);
+#else
+  snprintf(value, sizeof(value), "%02d:%02d:%02d", obj->hour, obj->minute, obj->second);
+#endif
   set_json_icon_state_value(root, obj, "time", value, value, start_config);
   if (start_config == DETAIL_ALL) {
     this->add_sorting_info_(root, obj);
@@ -1154,8 +1166,15 @@ std::string WebServer::datetime_json(datetime::DateTimeEntity *obj, JsonDetail s
   json::JsonBuilder builder;
   JsonObject root = builder.root();
 
-  std::string value =
-      str_sprintf("%d-%02d-%02d %02d:%02d:%02d", obj->year, obj->month, obj->day, obj->hour, obj->minute, obj->second);
+  // Format: YYYY-MM-DD HH:MM:SS (max 19 chars + null)
+  char value[24];
+#ifdef USE_ESP8266
+  snprintf_P(value, sizeof(value), PSTR("%d-%02d-%02d %02d:%02d:%02d"), obj->year, obj->month, obj->day, obj->hour,
+             obj->minute, obj->second);
+#else
+  snprintf(value, sizeof(value), "%d-%02d-%02d %02d:%02d:%02d", obj->year, obj->month, obj->day, obj->hour, obj->minute,
+           obj->second);
+#endif
   set_json_icon_state_value(root, obj, "datetime", value, value, start_config);
   if (start_config == DETAIL_ALL) {
     this->add_sorting_info_(root, obj);
diff --git a/esphome/components/web_server_idf/web_server_idf.cpp b/esphome/components/web_server_idf/web_server_idf.cpp
index 8c3ad288c0..3d76b86a14 100644
--- a/esphome/components/web_server_idf/web_server_idf.cpp
+++ b/esphome/components/web_server_idf/web_server_idf.cpp
@@ -343,8 +343,9 @@ bool AsyncWebServerRequest::authenticate(const char *username, const char *passw
 
 void AsyncWebServerRequest::requestAuthentication(const char *realm) const {
   httpd_resp_set_hdr(*this, "Connection", "keep-alive");
-  auto auth_val = str_sprintf("Basic realm=\"%s\"", realm ? realm : "Login Required");
-  httpd_resp_set_hdr(*this, "WWW-Authenticate", auth_val.c_str());
+  // Note: realm is never configured in ESPHome, always nullptr -> "Login Required"
+  (void) realm;  // Unused - always use default
+  httpd_resp_set_hdr(*this, "WWW-Authenticate", "Basic realm=\"Login Required\"");
   httpd_resp_send_err(*this, HTTPD_401_UNAUTHORIZED, nullptr);
 }
 #endif