diff --git a/lib/datalog/db.sx b/lib/datalog/db.sx
index 7930fdda..798fcfa7 100644
--- a/lib/datalog/db.sx
+++ b/lib/datalog/db.sx
@@ -182,6 +182,20 @@
       ((facts (get db :facts)))
       (if (has-key? facts rel-key) (get facts rel-key) (list)))))
 
+;; Reserved relation names: built-in / aggregate / negation / arrow.
+;; Rules and facts may not have these as their head's relation, since
+;; the saturator treats them specially or they are not relation names
+;; at all.
+(define
+  dl-reserved-rel-names
+  (list "not" "count" "sum" "min" "max" "findall" "is"
+        "<" "<=" ">" ">=" "=" "!=" "+" "-" "*" "/" ":-" "?-"))
+
+(define
+  dl-reserved-rel?
+  (fn
+    (name) (dl-member-string? name dl-reserved-rel-names)))
+
 (define
   dl-add-fact!
   (fn
@@ -189,6 +203,9 @@
     (cond
       ((not (and (list? lit) (> (len lit) 0)))
         (error (str "dl-add-fact!: expected literal list, got " lit)))
+      ((dl-reserved-rel? (dl-rel-name lit))
+        (error (str "dl-add-fact!: '" (dl-rel-name lit)
+                    "' is a reserved name (built-in / aggregate / negation)")))
       ((not (dl-ground? lit (dl-empty-subst)))
         (error (str "dl-add-fact!: expected ground literal, got " lit)))
       (else
@@ -301,6 +318,9 @@
         (error (str "dl-add-rule!: expected rule dict, got " rule)))
       ((not (has-key? rule :head))
         (error (str "dl-add-rule!: rule missing :head, got " rule)))
+      ((dl-reserved-rel? (dl-rel-name (get rule :head)))
+        (error (str "dl-add-rule!: '" (dl-rel-name (get rule :head))
+                    "' is a reserved name (built-in / aggregate / negation)")))
       (else
         (let ((rule (dl-rename-anon-rule rule)))
           (let
diff --git a/lib/datalog/scoreboard.json b/lib/datalog/scoreboard.json
index 182ca67b..b67356f8 100644
--- a/lib/datalog/scoreboard.json
+++ b/lib/datalog/scoreboard.json
@@ -1,13 +1,13 @@
 {
   "lang": "datalog",
-  "total_passed": 242,
+  "total_passed": 246,
   "total_failed": 0,
-  "total": 242,
+  "total": 246,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":20,"failed":0,"total":20},
     {"name":"unify","passed":28,"failed":0,"total":28},
-    {"name":"eval","passed":32,"failed":0,"total":32},
+    {"name":"eval","passed":36,"failed":0,"total":36},
     {"name":"builtins","passed":23,"failed":0,"total":23},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":34,"failed":0,"total":34},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-09T13:11:25+00:00"
+  "generated": "2026-05-10T20:51:33+00:00"
 }
diff --git a/lib/datalog/scoreboard.md b/lib/datalog/scoreboard.md
index f82a3b30..7c5196ef 100644
--- a/lib/datalog/scoreboard.md
+++ b/lib/datalog/scoreboard.md
@@ -1,13 +1,13 @@
 # datalog scoreboard
 
-**242 / 242 passing** (0 failure(s)).
+**246 / 246 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | tokenize | 26 | 26 | ok |
 | parse | 20 | 20 | ok |
 | unify | 28 | 28 | ok |
-| eval | 32 | 32 | ok |
+| eval | 36 | 36 | ok |
 | builtins | 23 | 23 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |
diff --git a/lib/datalog/tests/eval.sx b/lib/datalog/tests/eval.sx
index 822e58fb..75541282 100644
--- a/lib/datalog/tests/eval.sx
+++ b/lib/datalog/tests/eval.sx
@@ -165,6 +165,28 @@
           ((db (dl-program "edge(1, 2). edge(2, 3). edge(3, 1).\n                  reach(X, Y) :- edge(X, Y).\n                  reach(X, Z) :- edge(X, Y), reach(Y, Z).")))
           (do (dl-saturate! db) (len (dl-relation db "reach"))))
         9)
+      ;; Reserved relation names rejected as rule/fact heads.
+      (dl-et-test!
+        "reserved name `not` as head rejected"
+        (dl-et-throws? (fn () (dl-program "not(X) :- p(X).")))
+        true)
+
+      (dl-et-test!
+        "reserved name `count` as head rejected"
+        (dl-et-throws?
+          (fn () (dl-program "count(N, X, p(X)) :- p(X).")))
+        true)
+
+      (dl-et-test!
+        "reserved name `<` as head rejected"
+        (dl-et-throws? (fn () (dl-program "<(X, 5) :- p(X).")))
+        true)
+
+      (dl-et-test!
+        "reserved name `is` as head rejected"
+        (dl-et-throws? (fn () (dl-program "is(N, +(1, 2)) :- p(N).")))
+        true)
+
       (dl-et-test!
         "unsafe head var"
         (dl-et-throws? (fn () (dl-program "p(X, Y) :- q(X).")))