diff --git a/lib/prolog/conformance.sh b/lib/prolog/conformance.sh
index e30af481..0a963778 100755
--- a/lib/prolog/conformance.sh
+++ b/lib/prolog/conformance.sh
@@ -39,6 +39,7 @@ SUITES=(
   "char_predicates:lib/prolog/tests/char_predicates.sx:pl-char-predicates-tests-run!"
   "io_predicates:lib/prolog/tests/io_predicates.sx:pl-io-predicates-tests-run!"
   "assert_rules:lib/prolog/tests/assert_rules.sx:pl-assert-rules-tests-run!"
+  "string_agg:lib/prolog/tests/string_agg.sx:pl-string-agg-tests-run!"
 )
 
 SCRIPT='(epoch 1)
diff --git a/lib/prolog/runtime.sx b/lib/prolog/runtime.sx
index 4e6f77a7..74581361 100644
--- a/lib/prolog/runtime.sx
+++ b/lib/prolog/runtime.sx
@@ -1517,6 +1517,139 @@
         false))))
 
 
+(define
+  pl-substring
+  (fn (s start sublen) (substring s start (+ start sublen))))
+
+(define
+  pl-sub-atom-try-one!
+  (fn
+    (s start sublen total-len before-arg len-arg after-arg sub-arg trail k)
+    (let
+      ((mark (pl-trail-mark trail))
+        (after-val (- total-len (+ start sublen)))
+        (sub (pl-substring s start sublen)))
+      (if
+        (and
+          (pl-unify! before-arg (list "num" start) trail)
+          (pl-unify! len-arg (list "num" sublen) trail)
+          (pl-unify! after-arg (list "num" after-val) trail)
+          (pl-unify! sub-arg (list "atom" sub) trail))
+        (let
+          ((kresult (k)))
+          (if kresult kresult (begin (pl-trail-undo-to! trail mark) false)))
+        (begin (pl-trail-undo-to! trail mark) false)))))
+
+(define
+  pl-sub-atom-loop!
+  (fn
+    (s total-len start sublen before-arg len-arg after-arg sub-arg trail k)
+    (cond
+      ((> start total-len) false)
+      ((> sublen (- total-len start))
+        (pl-sub-atom-loop!
+          s
+          total-len
+          (+ start 1)
+          0
+          before-arg
+          len-arg
+          after-arg
+          sub-arg
+          trail
+          k))
+      (true
+        (let
+          ((one-result (pl-sub-atom-try-one! s start sublen total-len before-arg len-arg after-arg sub-arg trail k)))
+          (if
+            one-result
+            one-result
+            (pl-sub-atom-loop!
+              s
+              total-len
+              start
+              (+ sublen 1)
+              before-arg
+              len-arg
+              after-arg
+              sub-arg
+              trail
+              k)))))))
+
+(define
+  pl-solve-aggregate-all!
+  (fn
+    (db tmpl goal result trail k)
+    (let
+      ((tmpl-walked (pl-walk-deep tmpl)))
+      (cond
+        ((and (pl-atom? tmpl-walked) (= (pl-atom-name tmpl-walked) "count"))
+          (let
+            ((solutions (pl-collect-solutions db (list "atom" "true") goal trail)))
+            (if
+              (pl-unify! result (list "num" (len solutions)) trail)
+              (k)
+              false)))
+        ((and (pl-compound? tmpl-walked) (= (pl-fun tmpl-walked) "bag") (= (len (pl-args tmpl-walked)) 1))
+          (let
+            ((template (nth (pl-args tmpl-walked) 0)))
+            (let
+              ((solutions (pl-collect-solutions db template goal trail)))
+              (let
+                ((prolog-list (pl-mk-list-term solutions (pl-nil-term))))
+                (if (pl-unify! result prolog-list trail) (k) false)))))
+        ((and (pl-compound? tmpl-walked) (= (pl-fun tmpl-walked) "sum") (= (len (pl-args tmpl-walked)) 1))
+          (let
+            ((template (nth (pl-args tmpl-walked) 0)))
+            (let
+              ((solutions (pl-collect-solutions db template goal trail)))
+              (let
+                ((total (reduce (fn (acc sol) (+ acc (pl-eval-arith sol))) 0 solutions)))
+                (if (pl-unify! result (list "num" total) trail) (k) false)))))
+        ((and (pl-compound? tmpl-walked) (= (pl-fun tmpl-walked) "max") (= (len (pl-args tmpl-walked)) 1))
+          (let
+            ((template (nth (pl-args tmpl-walked) 0)))
+            (let
+              ((solutions (pl-collect-solutions db template goal trail)))
+              (if
+                (empty? solutions)
+                false
+                (let
+                  ((vals (map pl-eval-arith solutions)))
+                  (let
+                    ((mx (reduce (fn (a b) (if (> a b) a b)) (first vals) (rest vals))))
+                    (if (pl-unify! result (list "num" mx) trail) (k) false)))))))
+        ((and (pl-compound? tmpl-walked) (= (pl-fun tmpl-walked) "min") (= (len (pl-args tmpl-walked)) 1))
+          (let
+            ((template (nth (pl-args tmpl-walked) 0)))
+            (let
+              ((solutions (pl-collect-solutions db template goal trail)))
+              (if
+                (empty? solutions)
+                false
+                (let
+                  ((vals (map pl-eval-arith solutions)))
+                  (let
+                    ((mn (reduce (fn (a b) (if (< a b) a b)) (first vals) (rest vals))))
+                    (if (pl-unify! result (list "num" mn) trail) (k) false)))))))
+        ((and (pl-compound? tmpl-walked) (= (pl-fun tmpl-walked) "set") (= (len (pl-args tmpl-walked)) 1))
+          (let
+            ((template (nth (pl-args tmpl-walked) 0)))
+            (let
+              ((solutions (pl-collect-solutions db template goal trail)))
+              (let
+                ((deduped (pl-list-to-set-sx solutions (list))))
+                (let
+                  ((keyed (map (fn (t) (list (pl-format-term t) t)) deduped)))
+                  (let
+                    ((sorted (sort keyed)))
+                    (let
+                      ((sorted-terms (map (fn (pair) (nth pair 1)) sorted)))
+                      (let
+                        ((prolog-list (pl-mk-list-term sorted-terms (pl-nil-term))))
+                        (if (pl-unify! result prolog-list trail) (k) false)))))))))
+        (true false)))))
+
 (define
   pl-solve!
   (fn
@@ -2240,7 +2373,40 @@
         ((and (pl-compound? g) (= (pl-fun g) "format") (= (len (pl-args g)) 1))
           (pl-solve-format-1! (nth (pl-args g) 0) k))
         ((and (pl-compound? g) (= (pl-fun g) "format") (= (len (pl-args g)) 2))
-          (pl-solve-format-2! db (nth (pl-args g) 0) (nth (pl-args g) 1) trail k))
+          (pl-solve-format-2!
+            db
+            (nth (pl-args g) 0)
+            (nth (pl-args g) 1)
+            trail
+            k))
+        ((and (pl-compound? g) (= (pl-fun g) "sub_atom") (= (len (pl-args g)) 5))
+          (let
+            ((atom-term (pl-walk-deep (nth (pl-args g) 0))))
+            (if
+              (pl-atom? atom-term)
+              (let
+                ((s (pl-atom-name atom-term))
+                  (total-len (len (pl-atom-name atom-term))))
+                (pl-sub-atom-loop!
+                  s
+                  total-len
+                  0
+                  0
+                  (pl-walk (nth (pl-args g) 1))
+                  (pl-walk (nth (pl-args g) 2))
+                  (pl-walk (nth (pl-args g) 3))
+                  (pl-walk (nth (pl-args g) 4))
+                  trail
+                  k))
+              false)))
+        ((and (pl-compound? g) (= (pl-fun g) "aggregate_all") (= (len (pl-args g)) 3))
+          (pl-solve-aggregate-all!
+            db
+            (pl-walk (nth (pl-args g) 0))
+            (pl-walk (nth (pl-args g) 1))
+            (pl-walk (nth (pl-args g) 2))
+            trail
+            k))
         (true (pl-solve-user! db g trail cut-box k))))))
 
 (define
diff --git a/lib/prolog/scoreboard.json b/lib/prolog/scoreboard.json
index 04d96af5..d8032461 100644
--- a/lib/prolog/scoreboard.json
+++ b/lib/prolog/scoreboard.json
@@ -1,7 +1,7 @@
 {
-  "total_passed": 471,
+  "total_passed": 496,
   "total_failed": 0,
-  "total": 471,
-  "suites": {"parse":{"passed":25,"total":25,"failed":0},"unify":{"passed":47,"total":47,"failed":0},"clausedb":{"passed":14,"total":14,"failed":0},"solve":{"passed":62,"total":62,"failed":0},"operators":{"passed":19,"total":19,"failed":0},"dynamic":{"passed":11,"total":11,"failed":0},"findall":{"passed":11,"total":11,"failed":0},"term_inspect":{"passed":14,"total":14,"failed":0},"append":{"passed":6,"total":6,"failed":0},"reverse":{"passed":6,"total":6,"failed":0},"member":{"passed":7,"total":7,"failed":0},"nqueens":{"passed":6,"total":6,"failed":0},"family":{"passed":10,"total":10,"failed":0},"atoms":{"passed":34,"total":34,"failed":0},"query_api":{"passed":16,"total":16,"failed":0},"iso_predicates":{"passed":29,"total":29,"failed":0},"meta_predicates":{"passed":25,"total":25,"failed":0},"list_predicates":{"passed":33,"total":33,"failed":0},"meta_call":{"passed":15,"total":15,"failed":0},"set_predicates":{"passed":15,"total":15,"failed":0},"char_predicates":{"passed":27,"total":27,"failed":0},"io_predicates":{"passed":24,"total":24,"failed":0},"assert_rules":{"passed":15,"total":15,"failed":0}},
-  "generated": "2026-04-25T13:21:37+00:00"
+  "total": 496,
+  "suites": {"parse":{"passed":25,"total":25,"failed":0},"unify":{"passed":47,"total":47,"failed":0},"clausedb":{"passed":14,"total":14,"failed":0},"solve":{"passed":62,"total":62,"failed":0},"operators":{"passed":19,"total":19,"failed":0},"dynamic":{"passed":11,"total":11,"failed":0},"findall":{"passed":11,"total":11,"failed":0},"term_inspect":{"passed":14,"total":14,"failed":0},"append":{"passed":6,"total":6,"failed":0},"reverse":{"passed":6,"total":6,"failed":0},"member":{"passed":7,"total":7,"failed":0},"nqueens":{"passed":6,"total":6,"failed":0},"family":{"passed":10,"total":10,"failed":0},"atoms":{"passed":34,"total":34,"failed":0},"query_api":{"passed":16,"total":16,"failed":0},"iso_predicates":{"passed":29,"total":29,"failed":0},"meta_predicates":{"passed":25,"total":25,"failed":0},"list_predicates":{"passed":33,"total":33,"failed":0},"meta_call":{"passed":15,"total":15,"failed":0},"set_predicates":{"passed":15,"total":15,"failed":0},"char_predicates":{"passed":27,"total":27,"failed":0},"io_predicates":{"passed":24,"total":24,"failed":0},"assert_rules":{"passed":15,"total":15,"failed":0},"string_agg":{"passed":25,"total":25,"failed":0}},
+  "generated": "2026-04-25T13:49:43+00:00"
 }
diff --git a/lib/prolog/scoreboard.md b/lib/prolog/scoreboard.md
index ec0c2b07..4f2ad17d 100644
--- a/lib/prolog/scoreboard.md
+++ b/lib/prolog/scoreboard.md
@@ -1,7 +1,7 @@
 # Prolog scoreboard
 
-**471 / 471 passing** (0 failure(s)).
-Generated 2026-04-25T13:21:37+00:00.
+**496 / 496 passing** (0 failure(s)).
+Generated 2026-04-25T13:49:43+00:00.
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -28,6 +28,7 @@ Generated 2026-04-25T13:21:37+00:00.
 | char_predicates | 27 | 27 | ok |
 | io_predicates | 24 | 24 | ok |
 | assert_rules | 15 | 15 | ok |
+| string_agg | 25 | 25 | ok |
 
 Run `bash lib/prolog/conformance.sh` to refresh. Override the binary
 with `SX_SERVER=path/to/sx_server.exe bash …`.
diff --git a/lib/prolog/tests/string_agg.sx b/lib/prolog/tests/string_agg.sx
new file mode 100644
index 00000000..3ec3b2f6
--- /dev/null
+++ b/lib/prolog/tests/string_agg.sx
@@ -0,0 +1,273 @@
+;; lib/prolog/tests/string_agg.sx -- sub_atom/5 + aggregate_all/3
+
+(define pl-sa-test-count 0)
+(define pl-sa-test-pass 0)
+(define pl-sa-test-fail 0)
+(define pl-sa-test-failures (list))
+
+(define
+  pl-sa-test!
+  (fn
+    (name got expected)
+    (begin
+      (set! pl-sa-test-count (+ pl-sa-test-count 1))
+      (if
+        (= got expected)
+        (set! pl-sa-test-pass (+ pl-sa-test-pass 1))
+        (begin
+          (set! pl-sa-test-fail (+ pl-sa-test-fail 1))
+          (append!
+            pl-sa-test-failures
+            (str name "\n    expected: " expected "\n    got:      " got)))))))
+
+(define
+  pl-sa-goal
+  (fn
+    (src env)
+    (pl-instantiate (nth (first (pl-parse (str "g :- " src "."))) 2) env)))
+
+(define pl-sa-db (pl-mk-db))
+
+(define
+  pl-sa-num-val
+  (fn (env key) (pl-num-val (pl-walk-deep (dict-get env key)))))
+
+(define
+  pl-sa-list-to-atoms
+  (fn
+    (t)
+    (let
+      ((w (pl-walk-deep t)))
+      (cond
+        ((and (pl-atom? w) (= (pl-atom-name w) "[]")) (list))
+        ((and (pl-compound? w) (= (pl-fun w) ".") (= (len (pl-args w)) 2))
+          (cons
+            (pl-atom-name (first (pl-args w)))
+            (pl-sa-list-to-atoms (nth (pl-args w) 1))))
+        (true (list))))))
+
+(define pl-sa-prog-src "member(X, [X|_]). member(X, [_|T]) :- member(X, T).")
+(pl-db-load! pl-sa-db (pl-parse pl-sa-prog-src))
+
+;; -- sub_atom/5 --
+
+(pl-sa-test!
+  "sub_atom ground: sub_atom(abcde,0,3,2,abc)"
+  (pl-solve-once!
+    pl-sa-db
+    (pl-sa-goal "sub_atom(abcde, 0, 3, 2, abc)" {})
+    (pl-mk-trail))
+  true)
+
+(pl-sa-test!
+  "sub_atom ground: sub_atom(abcde,2,2,1,cd)"
+  (pl-solve-once!
+    pl-sa-db
+    (pl-sa-goal "sub_atom(abcde, 2, 2, 1, cd)" {})
+    (pl-mk-trail))
+  true)
+
+(pl-sa-test!
+  "sub_atom ground mismatch fails"
+  (pl-solve-once!
+    pl-sa-db
+    (pl-sa-goal "sub_atom(abcde, 0, 2, 3, cd)" {})
+    (pl-mk-trail))
+  false)
+
+(pl-sa-test!
+  "sub_atom empty sub at start"
+  (pl-solve-once!
+    pl-sa-db
+    (pl-sa-goal "sub_atom(abcde, 0, 0, 5, '')" {})
+    (pl-mk-trail))
+  true)
+
+(pl-sa-test!
+  "sub_atom whole string"
+  (pl-solve-once!
+    pl-sa-db
+    (pl-sa-goal "sub_atom(hello, 0, 5, 0, hello)" {})
+    (pl-mk-trail))
+  true)
+
+(define pl-sa-env-b1 {})
+(pl-solve-once!
+  pl-sa-db
+  (pl-sa-goal "sub_atom(abcde, B, 2, A, cd)" pl-sa-env-b1)
+  (pl-mk-trail))
+(pl-sa-test!
+  "sub_atom bound SubAtom gives B=2"
+  (pl-sa-num-val pl-sa-env-b1 "B")
+  2)
+(pl-sa-test!
+  "sub_atom bound SubAtom gives A=1"
+  (pl-sa-num-val pl-sa-env-b1 "A")
+  1)
+
+(define pl-sa-env-b2 {})
+(pl-solve-once!
+  pl-sa-db
+  (pl-sa-goal "sub_atom(hello, B, L, A, ello)" pl-sa-env-b2)
+  (pl-mk-trail))
+(pl-sa-test! "sub_atom ello: B=1" (pl-sa-num-val pl-sa-env-b2 "B") 1)
+(pl-sa-test! "sub_atom ello: L=4" (pl-sa-num-val pl-sa-env-b2 "L") 4)
+(pl-sa-test! "sub_atom ello: A=0" (pl-sa-num-val pl-sa-env-b2 "A") 0)
+
+(pl-sa-test!
+  "sub_atom ab: 6 total solutions"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-sa-db
+      (pl-sa-goal "aggregate_all(count, sub_atom(ab, _, _, _, _), N)" env)
+      (pl-mk-trail))
+    (pl-sa-num-val env "N"))
+  6)
+
+(pl-sa-test!
+  "sub_atom a: 3 total solutions"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-sa-db
+      (pl-sa-goal "aggregate_all(count, sub_atom(a, _, _, _, _), N)" env)
+      (pl-mk-trail))
+    (pl-sa-num-val env "N"))
+  3)
+
+;; -- aggregate_all/3 --
+
+(pl-sa-test!
+  "aggregate_all count member [a,b,c] = 3"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-sa-db
+      (pl-sa-goal "aggregate_all(count, member(_, [a,b,c]), N)" env)
+      (pl-mk-trail))
+    (pl-sa-num-val env "N"))
+  3)
+
+(pl-sa-test!
+  "aggregate_all count fail = 0"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-sa-db
+      (pl-sa-goal "aggregate_all(count, fail, N)" env)
+      (pl-mk-trail))
+    (pl-sa-num-val env "N"))
+  0)
+
+(pl-sa-test!
+  "aggregate_all count always succeeds"
+  (pl-solve-once!
+    pl-sa-db
+    (pl-sa-goal "aggregate_all(count, fail, _)" {})
+    (pl-mk-trail))
+  true)
+
+(define pl-sa-env-bag1 {})
+(pl-solve-once!
+  pl-sa-db
+  (pl-sa-goal "aggregate_all(bag(X), member(X, [a,b,c]), L)" pl-sa-env-bag1)
+  (pl-mk-trail))
+(pl-sa-test!
+  "aggregate_all bag [a,b,c]"
+  (pl-sa-list-to-atoms (dict-get pl-sa-env-bag1 "L"))
+  (list "a" "b" "c"))
+
+(define pl-sa-env-bag2 {})
+(pl-solve-once!
+  pl-sa-db
+  (pl-sa-goal "aggregate_all(bag(X), member(X, []), L)" pl-sa-env-bag2)
+  (pl-mk-trail))
+(pl-sa-test!
+  "aggregate_all bag empty goal = []"
+  (pl-sa-list-to-atoms (dict-get pl-sa-env-bag2 "L"))
+  (list))
+
+(pl-sa-test!
+  "aggregate_all sum [1,2,3,4] = 10"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-sa-db
+      (pl-sa-goal "aggregate_all(sum(X), member(X, [1,2,3,4]), S)" env)
+      (pl-mk-trail))
+    (pl-sa-num-val env "S"))
+  10)
+
+(pl-sa-test!
+  "aggregate_all max [3,1,4,1,5,9,2,6] = 9"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-sa-db
+      (pl-sa-goal "aggregate_all(max(X), member(X, [3,1,4,1,5,9,2,6]), M)" env)
+      (pl-mk-trail))
+    (pl-sa-num-val env "M"))
+  9)
+
+(pl-sa-test!
+  "aggregate_all max empty fails"
+  (pl-solve-once!
+    pl-sa-db
+    (pl-sa-goal "aggregate_all(max(X), member(X, []), M)" {})
+    (pl-mk-trail))
+  false)
+
+(pl-sa-test!
+  "aggregate_all min [3,1,4,1,5,9,2,6] = 1"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-sa-db
+      (pl-sa-goal "aggregate_all(min(X), member(X, [3,1,4,1,5,9,2,6]), M)" env)
+      (pl-mk-trail))
+    (pl-sa-num-val env "M"))
+  1)
+
+(pl-sa-test!
+  "aggregate_all min empty fails"
+  (pl-solve-once!
+    pl-sa-db
+    (pl-sa-goal "aggregate_all(min(X), member(X, []), M)" {})
+    (pl-mk-trail))
+  false)
+
+(define pl-sa-env-set1 {})
+(pl-solve-once!
+  pl-sa-db
+  (pl-sa-goal
+    "aggregate_all(set(X), member(X, [b,a,c,a,b]), S)"
+    pl-sa-env-set1)
+  (pl-mk-trail))
+(pl-sa-test!
+  "aggregate_all set [b,a,c,a,b] = [a,b,c]"
+  (pl-sa-list-to-atoms (dict-get pl-sa-env-set1 "S"))
+  (list "a" "b" "c"))
+
+(define pl-sa-env-set2 {})
+(pl-solve-once!
+  pl-sa-db
+  (pl-sa-goal "aggregate_all(set(X), fail, S)" pl-sa-env-set2)
+  (pl-mk-trail))
+(pl-sa-test!
+  "aggregate_all set fail = []"
+  (pl-sa-list-to-atoms (dict-get pl-sa-env-set2 "S"))
+  (list))
+
+(pl-sa-test!
+  "aggregate_all sum empty = 0"
+  (let
+    ((env {}))
+    (pl-solve-once!
+      pl-sa-db
+      (pl-sa-goal "aggregate_all(sum(X), fail, S)" env)
+      (pl-mk-trail))
+    (pl-sa-num-val env "S"))
+  0)
+
+(define pl-string-agg-tests-run! (fn () {:failed pl-sa-test-fail :passed pl-sa-test-pass :total pl-sa-test-count :failures pl-sa-test-failures}))