# UNDF: UNDF-2026-000000101
--- a/include/haproxy/server-t.h
+++ b/include/haproxy/server-t.h
@@ -343,6 +343,7 @@ struct server {
 	int cklen;				/* the len of the cookie, to speed up checks */
 	unsigned int flags;			/* server permanent flags */
 	char *cookie;				/* the id set in the cookie */
+	struct ebpt_node cookie_node;		/* eb-tree node keyed by cookie string; used by proxy->cookies_tree */

--- a/include/haproxy/proxy-t.h
+++ b/include/haproxy/proxy-t.h
@@ -364,6 +364,7 @@ struct proxy {
 	struct server *srv, *defsrv;		/* known servers; default server configuration */
+	struct eb_root cookies_tree;		/* eb-tree of srv->cookie_node for O(1) cookie lookup; populated after config parse */

--- a/src/server.c
+++ b/src/server.c
@@ -XXX,0 +XXX,12 @@
+/*
+ * Build or rebuild the proxy's cookies_tree index.
+ * Called after srv_set_dyncookie() and after initial config parse.
+ * O(S log S) one-time cost; amortises the O(C×S) per-request cookie scan.
+ */
+void proxy_build_cookie_tree(struct proxy *px)
+{
+	struct server *srv;
+	px->cookies_tree = EB_ROOT_UNIQUE;
+	for (srv = px->srv; srv; srv = srv->next) {
+		if (srv->cookie) {
+			srv->cookie_node.key = srv->cookie;
+			ebis_insert(&px->cookies_tree, &srv->cookie_node);
+		}
+	}
+}

--- a/src/http_ana.c
+++ b/src/http_ana.c
@@ -3514,10 +3514,18 @@ static void http_manage_client_side_cookies(...)
 			if ((delim == val_beg) || (s->flags & (SF_IGNORE_PRST | SF_ASSIGNED)))
 				srv = NULL;

-			while (srv) {
-				if (srv->cookie && (srv->cklen == delim - val_beg) &&
-				    !memcmp(val_beg, srv->cookie, delim - val_beg)) {
-					/* ... */
-				}
-				srv = srv->next;
-			}
+			/* Replace O(S) linked-list walk with O(log S) eb-tree lookup.
+			 * proxy_build_cookie_tree() indexes all srv->cookie strings at
+			 * config-time; lookup is safe at runtime (cookies are immutable). */
+			if (srv) {
+				char cookie_key[delim - val_beg + 1];
+				memcpy(cookie_key, val_beg, delim - val_beg);
+				cookie_key[delim - val_beg] = '\0';
+
+				struct ebpt_node *node = ebis_lookup(&s->be->cookies_tree, cookie_key);
+				srv = node ? container_of(node, struct server, cookie_node) : NULL;
+				if (srv) {
+					if ((srv->cur_state != SRV_ST_STOPPED) ||
+					    (s->be->options & PR_O_PERSIST) ||
+					    (s->flags & SF_FORCE_PRST)) {
+						txn->flags &= ~TX_CK_MASK;
+						txn->flags |= (srv->cur_state != SRV_ST_STOPPED) ? TX_CK_VALID : TX_CK_DOWN;
+						s->flags |= SF_DIRECT | SF_ASSIGNED;
+						stream_set_srv_target(s, srv);
+					} else {
+						txn->flags &= ~TX_CK_MASK;
+						txn->flags |= TX_CK_DOWN;
+						srv = NULL;
+					}
+				}
+			}