diff --git a/src/log.c b/src/log.c
index a58c6fc3c..fd8be525f 100644
--- a/src/log.c
+++ b/src/log.c
@@ -989,20 +989,23 @@ static int lf_expr_postcheck_node_opt(struct lf_expr *lf_expr, struct logformat_
 /* Performs a postparsing check on logformat expression <expr> for a given <px>
  * proxy. The function will behave differently depending on the proxy state
  * (during parsing we will try to adapt proxy configuration to make it
- * compatible with logformat expression, but once the proxy is checked, we fail
- * as soon as we face incompatibilities)
+ * compatible with logformat expression, but once the proxy is checked, we
+ * cannot help anymore so all we can do is raise a diag warning and hope that
+ * the conditions will be met when executing the logformat expression)
  *
- * If the proxy is a default section, then allow the postcheck to succeed:
- * the logformat expression may or may not work properly depending on the
- * actual proxy that effectively runs it during runtime, but we have to stay
- * permissive since we cannot assume it won't work.
+ * If the proxy is a default section, then allow the postcheck to succeed
+ * without errors nor warnings: the logformat expression may or may not work
+ * properly depending on the actual proxy that effectively runs it during
+ * runtime, but we have to stay permissive since we cannot assume it won't work.
  *
- * It returns 1 on success and 0 on error, <err> will be set in case of error.
+ * It returns 1 on success (although diag_warnings may have been emitted) and 0
+ * on error (cannot be recovered from), <err> will be set in case of error.
  */
 int lf_expr_postcheck(struct lf_expr *lf_expr, struct proxy *px, char **err)
 {
 	struct logformat_node *lf;
 	int default_px = (px->cap & PR_CAP_DEF);
+	uint8_t http_mode = (px->mode == PR_MODE_HTTP || (px->options & PR_O_HTTP_UPG));
 
 	if (!(px->flags & PR_FL_CHECKED))
 		px->to_log |= LW_INIT;
@@ -1015,13 +1018,18 @@ int lf_expr_postcheck(struct lf_expr *lf_expr, struct proxy *px, char **err)
 			struct sample_expr *expr = lf->expr;
 			uint8_t http_needed = !!(expr->fetch->use & SMP_USE_HTTP_ANY);
 
+			if (!default_px && !http_mode && http_needed)
+				ha_diag_warning("parsing [%s:%d]: sample fetch '%s' used from %s '%s' may not work as expected (item depends on HTTP proxy mode).\n",
+				                lf_expr->conf.file, lf_expr->conf.line,
+				                expr->fetch->kw,
+				                proxy_type_str(px), px->id);
+
 			if ((px->flags & PR_FL_CHECKED)) {
-				/* fail as soon as proxy properties are not compatible */
-				if (http_needed && !px->http_needed) {
-					memprintf(err, "sample fetch '%s' requires HTTP enabled proxy which is not available here",
-					          expr->fetch->kw);
-					goto fail;
-				}
+				if (http_needed && !px->http_needed)
+					ha_diag_warning("parsing [%s:%d]: sample fetch '%s' used from %s '%s' requires HTTP-specific proxy attributes, but the current proxy lacks them.\n",
+					                lf_expr->conf.file, lf_expr->conf.line,
+					                expr->fetch->kw,
+			                                proxy_type_str(px), px->id);
 				goto next_node;
 			}
 			/* check if we need to allocate an http_txn struct for HTTP parsing */
@@ -1037,15 +1045,17 @@ int lf_expr_postcheck(struct lf_expr *lf_expr, struct proxy *px, char **err)
 				px->to_log |= LW_REQ;
 		}
 		else if (lf->type == LOG_FMT_ALIAS) {
-			if (lf->alias->mode == PR_MODE_HTTP &&
-			    !default_px && px->mode != PR_MODE_HTTP) {
-				memprintf(err, "format alias '%s' is reserved for HTTP mode",
-				          lf->alias->name);
-				goto fail;
-			}
+			if (!default_px && !http_mode &&
+			    (lf->alias->mode == PR_MODE_HTTP ||
+			    (lf->alias->lw & ((LW_REQ | LW_RESP)))))
+				ha_diag_warning("parsing [%s:%d]: format alias '%s' used from %s '%s' may not work as expected (item depends on HTTP proxy mode).\n",
+		                                lf_expr->conf.file, lf_expr->conf.line,
+				                lf->alias->name,
+				                proxy_type_str(px), px->id);
+
 			if (lf->alias->config_callback &&
 			    !lf->alias->config_callback(lf, px)) {
-				memprintf(err, "cannot configure format alias '%s' in this context",
+				memprintf(err, "error while configuring format alias '%s'",
 				          lf->alias->name);
 				goto fail;
 			}
@@ -1057,11 +1067,6 @@ int lf_expr_postcheck(struct lf_expr *lf_expr, struct proxy *px, char **err)
 		if (!lf_expr_postcheck_node_opt(lf_expr, lf, err))
 			goto fail;
 	}
-	if (!default_px && (px->to_log & (LW_REQ | LW_RESP)) &&
-	    (px->mode != PR_MODE_HTTP && !(px->options & PR_O_HTTP_UPG))) {
-		memprintf(err, "logformat expression not usable here (at least one item depends on HTTP mode)");
-		goto fail;
-	}
 
 	return 1;
  fail: