diff --git a/lib/prolog/conformance.sh b/lib/prolog/conformance.sh
index d42bc0dc..afe54227 100755
--- a/lib/prolog/conformance.sh
+++ b/lib/prolog/conformance.sh
@@ -29,6 +29,7 @@ SUITES=(
   "member:lib/prolog/tests/programs/member.sx:pl-member-tests-run!"
   "nqueens:lib/prolog/tests/programs/nqueens.sx:pl-nqueens-tests-run!"
   "family:lib/prolog/tests/programs/family.sx:pl-family-tests-run!"
+  "atoms:lib/prolog/tests/atoms.sx:pl-atom-tests-run!"
 )
 
 SCRIPT='(epoch 1)
diff --git a/lib/prolog/runtime.sx b/lib/prolog/runtime.sx
index 6f565a02..8455c3fb 100644
--- a/lib/prolog/runtime.sx
+++ b/lib/prolog/runtime.sx
@@ -595,6 +595,182 @@
   pl-cut?
   (fn (t) (and (list? t) (not (empty? t)) (= (first t) "cut"))))
 
+(define
+  pl-list-to-prolog
+  (fn
+    (xs)
+    (if
+      (empty? xs)
+      (list "atom" "[]")
+      (list "compound" "." (list (first xs) (pl-list-to-prolog (rest xs)))))))
+
+(define
+  pl-proper-list?
+  (fn
+    (t)
+    (let
+      ((w (pl-walk t)))
+      (cond
+        ((and (pl-atom? w) (= (pl-atom-name w) "[]")) true)
+        ((and (pl-compound? w) (= (pl-fun w) ".") (= (len (pl-args w)) 2))
+          (pl-proper-list? (nth (pl-args w) 1)))
+        (true false)))))
+
+(define
+  pl-prolog-list-to-sx
+  (fn
+    (t)
+    (let
+      ((w (pl-walk 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-walk (first (pl-args w)))
+            (pl-prolog-list-to-sx (nth (pl-args w) 1))))
+        (true (list))))))
+
+(define
+  pl-solve-atom-concat!
+  (fn
+    (a1-rt a2-rt a3-rt trail k)
+    (let
+      ((a1 (pl-walk a1-rt)) (a2 (pl-walk a2-rt)) (a3 (pl-walk a3-rt)))
+      (cond
+        ((and (pl-atom? a1) (pl-atom? a2))
+          (pl-solve-eq!
+            a3-rt
+            (list "atom" (str (pl-atom-name a1) (pl-atom-name a2)))
+            trail
+            k))
+        ((and (pl-atom? a3) (pl-atom? a1))
+          (let
+            ((s3 (pl-atom-name a3)) (s1 (pl-atom-name a1)))
+            (if
+              (starts-with? s3 s1)
+              (pl-solve-eq!
+                a2-rt
+                (list "atom" (substring s3 (len s1) (len s3)))
+                trail
+                k)
+              false)))
+        ((and (pl-atom? a3) (pl-atom? a2))
+          (let
+            ((s3 (pl-atom-name a3)) (s2 (pl-atom-name a2)))
+            (if
+              (ends-with? s3 s2)
+              (pl-solve-eq!
+                a1-rt
+                (list "atom" (substring s3 0 (- (len s3) (len s2))))
+                trail
+                k)
+              false)))
+        (true false)))))
+
+(define
+  pl-solve-atom-chars!
+  (fn
+    (atom-rt chars-rt trail k)
+    (let
+      ((a (pl-walk atom-rt)))
+      (cond
+        ((pl-atom? a)
+          (pl-solve-eq!
+            chars-rt
+            (pl-list-to-prolog
+              (map (fn (c) (list "atom" c)) (split (pl-atom-name a) "")))
+            trail
+            k))
+        ((pl-num? a)
+          (pl-solve-eq!
+            chars-rt
+            (pl-list-to-prolog
+              (map
+                (fn (c) (list "atom" c))
+                (split (str (pl-num-val a)) "")))
+            trail
+            k))
+        ((pl-var? a)
+          (if
+            (pl-proper-list? chars-rt)
+            (let
+              ((char-terms (pl-prolog-list-to-sx chars-rt)))
+              (pl-solve-eq!
+                atom-rt
+                (list
+                  "atom"
+                  (join "" (map (fn (t) (pl-atom-name t)) char-terms)))
+                trail
+                k))
+            false))
+        (true false)))))
+
+(define
+  pl-solve-atom-codes!
+  (fn
+    (atom-rt codes-rt trail k)
+    (let
+      ((a (pl-walk atom-rt)))
+      (cond
+        ((pl-atom? a)
+          (pl-solve-eq!
+            codes-rt
+            (pl-list-to-prolog
+              (map
+                (fn (c) (list "num" (char-code c)))
+                (split (pl-atom-name a) "")))
+            trail
+            k))
+        ((pl-num? a)
+          (pl-solve-eq!
+            codes-rt
+            (pl-list-to-prolog
+              (map
+                (fn (c) (list "num" (char-code c)))
+                (split (str (pl-num-val a)) "")))
+            trail
+            k))
+        ((pl-var? a)
+          (if
+            (pl-proper-list? codes-rt)
+            (let
+              ((code-terms (pl-prolog-list-to-sx codes-rt)))
+              (pl-solve-eq!
+                atom-rt
+                (list
+                  "atom"
+                  (join
+                    ""
+                    (map
+                      (fn (t) (char-from-code (pl-num-val t)))
+                      code-terms)))
+                trail
+                k))
+            false))
+        (true false)))))
+
+(define
+  pl-solve-char-code!
+  (fn
+    (char-rt code-rt trail k)
+    (let
+      ((c (pl-walk char-rt)) (n (pl-walk code-rt)))
+      (cond
+        ((pl-atom? c)
+          (let
+            ((s (pl-atom-name c)))
+            (if
+              (= (len s) 1)
+              (pl-solve-eq! code-rt (list "num" (char-code s)) trail k)
+              false)))
+        ((pl-num? n)
+          (pl-solve-eq!
+            char-rt
+            (list "atom" (char-from-code (pl-num-val n)))
+            trail
+            k))
+        (true false)))))
+
 (define
   pl-solve!
   (fn
@@ -726,6 +902,106 @@
             (nth (pl-args g) 2)
             trail
             k))
+        ((and (pl-compound? g) (= (pl-fun g) "var") (= (len (pl-args g)) 1))
+          (let
+            ((a (pl-walk (first (pl-args g)))))
+            (if (pl-var? a) (k) false)))
+        ((and (pl-compound? g) (= (pl-fun g) "nonvar") (= (len (pl-args g)) 1))
+          (let
+            ((a (pl-walk (first (pl-args g)))))
+            (if (not (pl-var? a)) (k) false)))
+        ((and (pl-compound? g) (= (pl-fun g) "atom") (= (len (pl-args g)) 1))
+          (let
+            ((a (pl-walk (first (pl-args g)))))
+            (if (pl-atom? a) (k) false)))
+        ((and (pl-compound? g) (= (pl-fun g) "number") (= (len (pl-args g)) 1))
+          (let
+            ((a (pl-walk (first (pl-args g)))))
+            (if (pl-num? a) (k) false)))
+        ((and (pl-compound? g) (= (pl-fun g) "integer") (= (len (pl-args g)) 1))
+          (let
+            ((a (pl-walk (first (pl-args g)))))
+            (if (pl-num? a) (k) false)))
+        ((and (pl-compound? g) (= (pl-fun g) "float") (= (len (pl-args g)) 1))
+          false)
+        ((and (pl-compound? g) (= (pl-fun g) "compound") (= (len (pl-args g)) 1))
+          (let
+            ((a (pl-walk (first (pl-args g)))))
+            (if (pl-compound? a) (k) false)))
+        ((and (pl-compound? g) (= (pl-fun g) "callable") (= (len (pl-args g)) 1))
+          (let
+            ((a (pl-walk (first (pl-args g)))))
+            (if (or (pl-atom? a) (pl-compound? a)) (k) false)))
+        ((and (pl-compound? g) (= (pl-fun g) "atomic") (= (len (pl-args g)) 1))
+          (let
+            ((a (pl-walk (first (pl-args g)))))
+            (if (or (pl-atom? a) (or (pl-num? a) (pl-str? a))) (k) false)))
+        ((and (pl-compound? g) (= (pl-fun g) "is_list") (= (len (pl-args g)) 1))
+          (if (pl-proper-list? (first (pl-args g))) (k) false))
+        ((and (pl-compound? g) (= (pl-fun g) "atom_length") (= (len (pl-args g)) 2))
+          (let
+            ((a (pl-walk (first (pl-args g)))))
+            (if
+              (pl-atom? a)
+              (pl-solve-eq!
+                (nth (pl-args g) 1)
+                (list "num" (len (pl-atom-name a)))
+                trail
+                k)
+              false)))
+        ((and (pl-compound? g) (= (pl-fun g) "atom_concat") (= (len (pl-args g)) 3))
+          (pl-solve-atom-concat!
+            (first (pl-args g))
+            (nth (pl-args g) 1)
+            (nth (pl-args g) 2)
+            trail
+            k))
+        ((and (pl-compound? g) (= (pl-fun g) "atom_chars") (= (len (pl-args g)) 2))
+          (pl-solve-atom-chars!
+            (first (pl-args g))
+            (nth (pl-args g) 1)
+            trail
+            k))
+        ((and (pl-compound? g) (= (pl-fun g) "atom_codes") (= (len (pl-args g)) 2))
+          (pl-solve-atom-codes!
+            (first (pl-args g))
+            (nth (pl-args g) 1)
+            trail
+            k))
+        ((and (pl-compound? g) (= (pl-fun g) "char_code") (= (len (pl-args g)) 2))
+          (pl-solve-char-code!
+            (first (pl-args g))
+            (nth (pl-args g) 1)
+            trail
+            k))
+        ((and (pl-compound? g) (= (pl-fun g) "number_codes") (= (len (pl-args g)) 2))
+          (let
+            ((a (pl-walk (first (pl-args g)))))
+            (if
+              (pl-num? a)
+              (pl-solve-eq!
+                (nth (pl-args g) 1)
+                (pl-list-to-prolog
+                  (map
+                    (fn (c) (list "num" (char-code c)))
+                    (split (str (pl-num-val a)) "")))
+                trail
+                k)
+              false)))
+        ((and (pl-compound? g) (= (pl-fun g) "number_chars") (= (len (pl-args g)) 2))
+          (let
+            ((a (pl-walk (first (pl-args g)))))
+            (if
+              (pl-num? a)
+              (pl-solve-eq!
+                (nth (pl-args g) 1)
+                (pl-list-to-prolog
+                  (map
+                    (fn (c) (list "atom" c))
+                    (split (str (pl-num-val a)) "")))
+                trail
+                k)
+              false)))
         (true (pl-solve-user! db g trail cut-box k))))))
 
 (define
diff --git a/lib/prolog/scoreboard.json b/lib/prolog/scoreboard.json
index 6c2ea2a6..92369b64 100644
--- a/lib/prolog/scoreboard.json
+++ b/lib/prolog/scoreboard.json
@@ -1,7 +1,7 @@
 {
-  "total_passed": 238,
+  "total_passed": 272,
   "total_failed": 0,
-  "total": 238,
-  "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}},
-  "generated": "2026-04-25T08:39:07+00:00"
+  "total": 272,
+  "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}},
+  "generated": "2026-04-25T09:26:33+00:00"
 }
diff --git a/lib/prolog/scoreboard.md b/lib/prolog/scoreboard.md
index ef861cb1..6797f516 100644
--- a/lib/prolog/scoreboard.md
+++ b/lib/prolog/scoreboard.md
@@ -1,7 +1,7 @@
 # Prolog scoreboard
 
-**238 / 238 passing** (0 failure(s)).
-Generated 2026-04-25T08:39:07+00:00.
+**272 / 272 passing** (0 failure(s)).
+Generated 2026-04-25T09:26:33+00:00.
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -18,6 +18,7 @@ Generated 2026-04-25T08:39:07+00:00.
 | member | 7 | 7 | ok |
 | nqueens | 6 | 6 | ok |
 | family | 10 | 10 | ok |
+| atoms | 34 | 34 | 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/atoms.sx b/lib/prolog/tests/atoms.sx
new file mode 100644
index 00000000..e1b09bae
--- /dev/null
+++ b/lib/prolog/tests/atoms.sx
@@ -0,0 +1,305 @@
+;; lib/prolog/tests/atoms.sx — type predicates + string/atom built-ins
+
+(define pl-at-test-count 0)
+(define pl-at-test-pass 0)
+(define pl-at-test-fail 0)
+(define pl-at-test-failures (list))
+
+(define
+  pl-at-test!
+  (fn
+    (name got expected)
+    (begin
+      (set! pl-at-test-count (+ pl-at-test-count 1))
+      (if
+        (= got expected)
+        (set! pl-at-test-pass (+ pl-at-test-pass 1))
+        (begin
+          (set! pl-at-test-fail (+ pl-at-test-fail 1))
+          (append!
+            pl-at-test-failures
+            (str name "\n    expected: " expected "\n    got:      " got)))))))
+
+(define
+  pl-at-goal
+  (fn
+    (src env)
+    (pl-instantiate (nth (first (pl-parse (str "g :- " src "."))) 2) env)))
+
+(define pl-at-db (pl-mk-db))
+
+;; ── var/1 + nonvar/1 ──
+
+(pl-at-test!
+  "var(X) for unbound var"
+  (pl-solve-once! pl-at-db (pl-at-goal "var(X)" {}) (pl-mk-trail))
+  true)
+(pl-at-test!
+  "var(foo) fails"
+  (pl-solve-once! pl-at-db (pl-at-goal "var(foo)" {}) (pl-mk-trail))
+  false)
+(pl-at-test!
+  "nonvar(foo) succeeds"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "nonvar(foo)" {})
+    (pl-mk-trail))
+  true)
+(pl-at-test!
+  "nonvar(X) for unbound var fails"
+  (pl-solve-once! pl-at-db (pl-at-goal "nonvar(X)" {}) (pl-mk-trail))
+  false)
+
+;; ── atom/1 ──
+
+(pl-at-test!
+  "atom(foo) succeeds"
+  (pl-solve-once! pl-at-db (pl-at-goal "atom(foo)" {}) (pl-mk-trail))
+  true)
+(pl-at-test!
+  "atom([]) succeeds"
+  (pl-solve-once! pl-at-db (pl-at-goal "atom([])" {}) (pl-mk-trail))
+  true)
+(pl-at-test!
+  "atom(42) fails"
+  (pl-solve-once! pl-at-db (pl-at-goal "atom(42)" {}) (pl-mk-trail))
+  false)
+(pl-at-test!
+  "atom(f(x)) fails"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "atom(f(x))" {})
+    (pl-mk-trail))
+  false)
+
+;; ── number/1 + integer/1 ──
+
+(pl-at-test!
+  "number(42) succeeds"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "number(42)" {})
+    (pl-mk-trail))
+  true)
+(pl-at-test!
+  "number(foo) fails"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "number(foo)" {})
+    (pl-mk-trail))
+  false)
+(pl-at-test!
+  "integer(7) succeeds"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "integer(7)" {})
+    (pl-mk-trail))
+  true)
+
+;; ── compound/1 + callable/1 + atomic/1 ──
+
+(pl-at-test!
+  "compound(f(x)) succeeds"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "compound(f(x))" {})
+    (pl-mk-trail))
+  true)
+(pl-at-test!
+  "compound(foo) fails"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "compound(foo)" {})
+    (pl-mk-trail))
+  false)
+(pl-at-test!
+  "callable(foo) succeeds"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "callable(foo)" {})
+    (pl-mk-trail))
+  true)
+(pl-at-test!
+  "callable(f(x)) succeeds"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "callable(f(x))" {})
+    (pl-mk-trail))
+  true)
+(pl-at-test!
+  "callable(42) fails"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "callable(42)" {})
+    (pl-mk-trail))
+  false)
+(pl-at-test!
+  "atomic(foo) succeeds"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "atomic(foo)" {})
+    (pl-mk-trail))
+  true)
+(pl-at-test!
+  "atomic(42) succeeds"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "atomic(42)" {})
+    (pl-mk-trail))
+  true)
+(pl-at-test!
+  "atomic(f(x)) fails"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "atomic(f(x))" {})
+    (pl-mk-trail))
+  false)
+
+;; ── is_list/1 ──
+
+(pl-at-test!
+  "is_list([]) succeeds"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "is_list([])" {})
+    (pl-mk-trail))
+  true)
+(pl-at-test!
+  "is_list([1,2,3]) succeeds"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "is_list([1,2,3])" {})
+    (pl-mk-trail))
+  true)
+(pl-at-test!
+  "is_list(foo) fails"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "is_list(foo)" {})
+    (pl-mk-trail))
+  false)
+
+;; ── atom_length/2 ──
+
+(define pl-at-env-al {})
+(pl-solve-once!
+  pl-at-db
+  (pl-at-goal "atom_length(hello, N)" pl-at-env-al)
+  (pl-mk-trail))
+(pl-at-test!
+  "atom_length(hello, N) -> N=5"
+  (pl-num-val (pl-walk-deep (dict-get pl-at-env-al "N")))
+  5)
+(pl-at-test!
+  "atom_length empty atom"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "atom_length('', 0)" {})
+    (pl-mk-trail))
+  true)
+
+;; ── atom_concat/3 ──
+
+(define pl-at-env-ac {})
+(pl-solve-once!
+  pl-at-db
+  (pl-at-goal "atom_concat(foo, bar, X)" pl-at-env-ac)
+  (pl-mk-trail))
+(pl-at-test!
+  "atom_concat(foo, bar, X) -> X=foobar"
+  (pl-atom-name (pl-walk-deep (dict-get pl-at-env-ac "X")))
+  "foobar")
+
+(pl-at-test!
+  "atom_concat(foo, bar, foobar) check"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "atom_concat(foo, bar, foobar)" {})
+    (pl-mk-trail))
+  true)
+(pl-at-test!
+  "atom_concat(foo, bar, foobaz) fails"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "atom_concat(foo, bar, foobaz)" {})
+    (pl-mk-trail))
+  false)
+
+(define pl-at-env-ac2 {})
+(pl-solve-once!
+  pl-at-db
+  (pl-at-goal "atom_concat(foo, Y, foobar)" pl-at-env-ac2)
+  (pl-mk-trail))
+(pl-at-test!
+  "atom_concat(foo, Y, foobar) -> Y=bar"
+  (pl-atom-name (pl-walk-deep (dict-get pl-at-env-ac2 "Y")))
+  "bar")
+
+;; ── atom_chars/2 ──
+
+(define pl-at-env-ach {})
+(pl-solve-once!
+  pl-at-db
+  (pl-at-goal "atom_chars(cat, Cs)" pl-at-env-ach)
+  (pl-mk-trail))
+(pl-at-test!
+  "atom_chars(cat, Cs) -> Cs=[c,a,t]"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "atom_chars(cat, [c,a,t])" {})
+    (pl-mk-trail))
+  true)
+
+(define pl-at-env-ach2 {})
+(pl-solve-once!
+  pl-at-db
+  (pl-at-goal "atom_chars(A, [h,i])" pl-at-env-ach2)
+  (pl-mk-trail))
+(pl-at-test!
+  "atom_chars(A, [h,i]) -> A=hi"
+  (pl-atom-name (pl-walk-deep (dict-get pl-at-env-ach2 "A")))
+  "hi")
+
+;; ── char_code/2 ──
+
+(define pl-at-env-cc {})
+(pl-solve-once!
+  pl-at-db
+  (pl-at-goal "char_code(a, N)" pl-at-env-cc)
+  (pl-mk-trail))
+(pl-at-test!
+  "char_code(a, N) -> N=97"
+  (pl-num-val (pl-walk-deep (dict-get pl-at-env-cc "N")))
+  97)
+
+(define pl-at-env-cc2 {})
+(pl-solve-once!
+  pl-at-db
+  (pl-at-goal "char_code(C, 65)" pl-at-env-cc2)
+  (pl-mk-trail))
+(pl-at-test!
+  "char_code(C, 65) -> C='A'"
+  (pl-atom-name (pl-walk-deep (dict-get pl-at-env-cc2 "C")))
+  "A")
+
+;; ── number_codes/2 ──
+
+(pl-at-test!
+  "number_codes(42, [52,50])"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "number_codes(42, [52,50])" {})
+    (pl-mk-trail))
+  true)
+
+;; ── number_chars/2 ──
+
+(pl-at-test!
+  "number_chars(42, ['4','2'])"
+  (pl-solve-once!
+    pl-at-db
+    (pl-at-goal "number_chars(42, ['4','2'])" {})
+    (pl-mk-trail))
+  true)
+
+(define pl-atom-tests-run! (fn () {:failed pl-at-test-fail :passed pl-at-test-pass :total pl-at-test-count :failures pl-at-test-failures}))
diff --git a/plans/prolog-on-sx.md b/plans/prolog-on-sx.md
index 909ada79..091e4498 100644
--- a/plans/prolog-on-sx.md
+++ b/plans/prolog-on-sx.md
@@ -69,7 +69,7 @@ Representation choices (finalise in phase 1, document here):
 - [x] `assert/1`, `asserta/1`, `assertz/1`, `retract/1` — `assert` aliases `assertz`. Helpers `pl-rt-to-ast` (deep-walk + replace runtime vars with `_G<id>` parse markers) + `pl-build-clause` (detect `:-` head). `assertz` uses `pl-db-add!`; `asserta` uses new `pl-db-prepend!`. `retract` walks goal, looks up by functor/arity, tries each clause via unification, removes first match by index (`pl-list-without`). 11 tests in `tests/dynamic.sx`. Rule-asserts deferred — `:-` not in op table yet, so only fact-shaped clauses for now.
 - [x] `findall/3`, `bagof/3`, `setof/3` — shared `pl-collect-solutions` runs the goal in a fresh cut-box, deep-copies the template (via `pl-deep-copy` with var-map for shared-var preservation) on each success, returns false to backtrack, then restores trail. `findall` always succeeds with a (possibly empty) list. `bagof` fails on empty. `setof` builds a string-keyed dict via `pl-format-term` for sort+dedupe (via `keys` + `sort`), fails on empty. Existential `^` deferred (operator). 11 tests in `tests/findall.sx`.
 - [x] `copy_term/2`, `functor/3`, `arg/3`, `=../2` — `copy_term/2` reuses `pl-deep-copy` with a fresh var-map (preserves source aliasing). `functor/3` handles 4 modes: compound→{name, arity}, atom→{atom, 0}, num→{num, 0}, var with ground name+arity→constructed term (`pl-make-fresh-args` for compound case). `arg/3` extracts 1-indexed arg from compound. **`=../2` deferred** — the tokenizer treats `.` as the clause terminator unconditionally, so `=..` lexes as `=` + `.` + `.`; needs special-case lex (or surface syntax via a different name). 14 tests in `tests/term_inspect.sx`.
-- [ ] String/atom predicates
+- [x] String/atom predicates
 
 ### Phase 5 — Hyperscript integration
 - [ ] `prolog-query` primitive callable from SX/Hyperscript
@@ -88,6 +88,7 @@ Representation choices (finalise in phase 1, document here):
 
 _Newest first. Agent appends on every commit._
 
+- 2026-04-25 — String/atom predicates. Type-test predicates: `var/1`, `nonvar/1`, `atom/1`, `number/1`, `integer/1`, `float/1` (always-fail), `compound/1`, `callable/1`, `atomic/1`, `is_list/1`. String/atom operations: `atom_length/2`, `atom_concat/3` (3 modes: both-ground, result+first, result+second), `atom_chars/2` (bidirectional), `atom_codes/2` (bidirectional), `char_code/2` (bidirectional), `number_codes/2`, `number_chars/2`. 7 helper functions in runtime.sx (`pl-list-to-prolog`, `pl-proper-list?`, `pl-prolog-list-to-sx`, `pl-solve-atom-concat!`, `pl-solve-atom-chars!`, `pl-solve-atom-codes!`, `pl-solve-char-code!`). 34 tests in `tests/atoms.sx`. Total **272** (+34).
 - 2026-04-25 — `copy_term/2` + `functor/3` + `arg/3` (term inspection). `copy_term` is a one-line dispatch to existing `pl-deep-copy`. `functor/3` is bidirectional — decomposes a bound compound/atom/num into name+arity OR constructs from ground name+arity (atom+positive-arity → compound with N anonymous fresh args via `pl-make-fresh-args`; arity 0 → atom/num). `arg/3` extracts 1-indexed arg with bounds-fail. New helper `pl-solve-eq2!` for paired-unification with shared trail-undo. 14 tests in `tests/term_inspect.sx`. Total **238** (+14). `=..` deferred — `.` always tokenizes as clause terminator; needs special lexer case.
 - 2026-04-25 — `findall/3` + `bagof/3` + `setof/3`. Shared collector `pl-collect-solutions` runs the goal in a fresh cut-box, deep-copies the template per success (`pl-deep-copy` walks term, allocates fresh runtime vars via shared var-map so co-occurrences keep aliasing), returns false to keep backtracking, then `pl-trail-undo-to!` to clean up. `findall` always builds a list. `bagof` fails on empty. `setof` uses a `pl-format-term`-keyed dict + SX `sort` for dedupe + ordering. New `tests/findall.sx` 11 tests. Total **224** (+11). Existential `^` deferred — needs operator.
 - 2026-04-25 — Dynamic clauses: `assert/1`, `assertz/1`, `asserta/1`, `retract/1`. New helpers `pl-rt-to-ast` (deep-walk runtime term → parse-AST, mapping unbound runtime vars to `_G<id>` markers so `pl-instantiate-fresh` produces fresh vars per call) + `pl-build-clause` + `pl-db-prepend!` + `pl-list-without`. `retract` keeps runtime vars (so the caller's vars get bound), walks head for the functor/arity key, tries each stored clause via `pl-unify!`, removes the first match by index. 11 tests in `tests/dynamic.sx`; conformance script gained dynamic row. Total **213** (+11). Rule-form asserts (`(H :- B)`) deferred until `:-` is in the op table.