diff --git a/lib/go/eval.sx b/lib/go/eval.sx
index 2014c945..0ffef19f 100644
--- a/lib/go/eval.sx
+++ b/lib/go/eval.sx
@@ -13,6 +13,16 @@
 
 (define go-env-empty (list))
 
+(define
+  go-env-builtins
+  ;; A starter env containing the Go builtins eval understands.
+  ;; Tests can call (go-env-builtins) instead of go-env-empty when they
+  ;; need len/append/print.
+  (list
+    (list "len"    (list :go-builtin "len"))
+    (list "append" (list :go-builtin "append"))
+    (list "print"  (list :go-builtin "print"))))
+
 (define
   go-env-lookup
   (fn
@@ -229,10 +239,129 @@
         (go-env-extend env (first names) (first vals))
         (rest names) (rest vals)))))
 
+(define
+  go-eval-builtin
+  ;; Run Go's predeclared builtins (len, append, print). args are
+  ;; expressions; we eval them in the caller env then dispatch on NAME.
+  (fn (caller-env name args)
+    (let ((vals (go-eval-args caller-env args)))
+      (cond
+        (go-eval-error? vals) vals
+        (= name "len")
+        (cond
+          (not (= (len vals) 1))
+          (list :eval-error :builtin-arity name 1 (len vals))
+          :else
+          (let ((arg (first vals)))
+            (cond
+              (and (list? arg) (= (first arg) :go-slice)) (len (nth arg 1))
+              (string? arg) (len arg)
+              :else (list :eval-error :len-not-applicable arg))))
+        (= name "append")
+        (cond
+          (< (len vals) 1)
+          (list :eval-error :builtin-arity name 1 (len vals))
+          :else
+          (let ((slc (first vals)) (extra (rest vals)))
+            (cond
+              (and (list? slc) (= (first slc) :go-slice))
+              (list :go-slice (go-name-concat (nth slc 1) extra))
+              :else (list :eval-error :append-not-slice slc))))
+        (= name "print")
+        nil  ;; v0: silent. Real impl would write to stdout.
+        :else (list :eval-error :unknown-builtin name)))))
+
+(define
+  go-extract-composite-vals
+  ;; For slice/array composite literals: read each element's value
+  ;; (skipping :kv keys, only using values for Go's index-keyed shorthand).
+  (fn (env elems)
+    (cond
+      (or (= elems nil) (= (len elems) 0)) (list)
+      :else
+      (let ((e (first elems)))
+        (let ((v
+                (cond
+                  (and (list? e) (= (first e) :kv))
+                  (go-eval env (nth e 2))
+                  :else (go-eval env e))))
+          (cond
+            (go-eval-error? v) v
+            :else
+            (let ((rest-vs (go-extract-composite-vals env (rest elems))))
+              (cond
+                (go-eval-error? rest-vs) rest-vs
+                :else (cons v rest-vs)))))))))
+
+(define
+  go-eval-composite
+  ;; (:composite TYPE-OR-EXPR ELEMS). v0 supports slice/array; map/struct
+  ;; later.
+  (fn (env expr)
+    (let ((ty (nth expr 1)) (elems (nth expr 2)))
+      (cond
+        (and (list? ty)
+             (or (= (first ty) :ty-slice) (= (first ty) :ty-array)))
+        (let ((vals (go-extract-composite-vals env elems)))
+          (cond
+            (go-eval-error? vals) vals
+            :else (list :go-slice vals)))
+        :else (list :eval-error :unsupported-composite ty)))))
+
+(define
+  go-eval-index
+  ;; (:index OBJ IDX-EXPR). v0: slice indexing only.
+  (fn (env expr)
+    (let ((obj (go-eval env (nth expr 1)))
+          (idx (go-eval env (nth expr 2))))
+      (cond
+        (go-eval-error? obj) obj
+        (go-eval-error? idx) idx
+        (and (list? obj) (= (first obj) :go-slice))
+        (let ((elems (nth obj 1)))
+          (cond
+            (or (< idx 0) (>= idx (len elems)))
+            (list :eval-error :index-out-of-range idx (len elems))
+            :else (nth elems idx)))
+        :else (list :eval-error :not-indexable obj)))))
+
+(define
+  go-eval-slice
+  ;; (:slice OBJ LOW HIGH MAX). v0: two-index slice on go-slice values.
+  (fn (env expr)
+    (let ((obj (go-eval env (nth expr 1)))
+          (low (cond
+                 (= (nth expr 2) nil) 0
+                 :else (go-eval env (nth expr 2))))
+          (high-expr (nth expr 3)))
+      (cond
+        (go-eval-error? obj) obj
+        (go-eval-error? low) low
+        (not (and (list? obj) (= (first obj) :go-slice)))
+        (list :eval-error :not-sliceable obj)
+        :else
+        (let ((elems (nth obj 1)))
+          (let ((high
+                  (cond
+                    (= high-expr nil) (len elems)
+                    :else (go-eval env high-expr))))
+            (cond
+              (go-eval-error? high) high
+              :else
+              (list :go-slice (go-list-slice elems low high)))))))))
+
+(define
+  go-list-slice
+  (fn (lst low high)
+    (cond
+      (>= low high) (list)
+      (>= low (len lst)) (list)
+      :else
+      (cons (nth lst low)
+        (go-list-slice lst (+ low 1) high)))))
+
 (define
   go-eval-call
-  ;; Apply a callable VAL to ARG-EXPRS in CALLER-ENV. Result is the
-  ;; function's return value or a (:eval-error ...).
   ;;
   ;; Closure semantics: the function value carries no captured env in v0
   ;; (dynamic scope wrt outer bindings). Recursion at top level works
@@ -240,6 +369,8 @@
   ;; lexical closures arrive in a later slice.
   (fn (caller-env callee-val args)
     (cond
+      (and (list? callee-val) (= (first callee-val) :go-builtin))
+      (go-eval-builtin caller-env (nth callee-val 1) args)
       (not (and (list? callee-val) (= (first callee-val) :go-fn)))
       (list :eval-error :not-callable callee-val)
       :else
@@ -503,6 +634,12 @@
           :else (let
             ((v (go-env-lookup env name)))
             (cond (= v nil) (list :eval-error :unbound name) :else v))))
+      (and (list? expr) (= (first expr) :composite))
+      (go-eval-composite env expr)
+      (and (list? expr) (= (first expr) :index))
+      (go-eval-index env expr)
+      (and (list? expr) (= (first expr) :slice))
+      (go-eval-slice env expr)
       (and (list? expr) (= (first expr) :app))
       (let ((head (nth expr 1)) (args (nth expr 2)))
         (cond
diff --git a/lib/go/scoreboard.json b/lib/go/scoreboard.json
index f055d787..a84ab350 100644
--- a/lib/go/scoreboard.json
+++ b/lib/go/scoreboard.json
@@ -1,12 +1,12 @@
 {
   "language": "go",
-  "total_pass": 417,
-  "total": 417,
+  "total_pass": 427,
+  "total": 427,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
     {"name":"parse","pass":176,"total":176,"status":"ok"},
     {"name":"types","pass":72,"total":72,"status":"ok"},
-    {"name":"eval","pass":40,"total":40,"status":"ok"},
+    {"name":"eval","pass":50,"total":50,"status":"ok"},
     {"name":"runtime","pass":0,"total":0,"status":"pending"},
     {"name":"stdlib","pass":0,"total":0,"status":"pending"},
     {"name":"e2e","pass":0,"total":0,"status":"pending"}
diff --git a/lib/go/scoreboard.md b/lib/go/scoreboard.md
index 1e1ba676..a13a7213 100644
--- a/lib/go/scoreboard.md
+++ b/lib/go/scoreboard.md
@@ -1,13 +1,13 @@
 # Go-on-SX Scoreboard
 
-**Total: 417 / 417 tests passing**
+**Total: 427 / 427 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | lex | 129 | 129 |
 | ✅ | parse | 176 | 176 |
 | ✅ | types | 72 | 72 |
-| ✅ | eval | 40 | 40 |
+| ✅ | eval | 50 | 50 |
 | ⬜ | runtime | 0 | 0 |
 | ⬜ | stdlib | 0 | 0 |
 | ⬜ | e2e | 0 | 0 |
diff --git a/lib/go/tests/eval.sx b/lib/go/tests/eval.sx
index 4066134b..c73e8885 100644
--- a/lib/go/tests/eval.sx
+++ b/lib/go/tests/eval.sx
@@ -195,6 +195,73 @@
     (go-eval env (go-parse "fact(5)")))
   120)
 
+(go-eval-test
+  "slice: []int{1,2,3} → :go-slice"
+  (gtev go-env-empty "[]int{1, 2, 3}")
+  (list :go-slice (list 1 2 3)))
+
+(go-eval-test
+  "index: a[0] = 10, a[2] = 30"
+  (let
+    ((env (go-eval-program go-env-empty (list (go-parse "a := []int{10, 20, 30}")))))
+    (list (go-eval env (go-parse "a[0]")) (go-eval env (go-parse "a[2]"))))
+  (list 10 30))
+
+(go-eval-test
+  "index: out-of-range error"
+  (let
+    ((env (go-eval-program go-env-empty (list (go-parse "a := []int{1, 2}")))))
+    (go-eval env (go-parse "a[5]")))
+  (list :eval-error :index-out-of-range 5 2))
+
+(go-eval-test
+  "builtin: len(slice) = 3"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "a := []int{1, 2, 3}")))))
+    (go-eval env (go-parse "len(a)")))
+  3)
+
+(go-eval-test
+  "builtin: len(string)"
+  (go-eval go-env-builtins (go-parse "len(\"hello\")"))
+  5)
+
+(go-eval-test
+  "builtin: append(a, 4, 5)"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "a := []int{1, 2, 3}")))))
+    (go-eval env (go-parse "append(a, 4, 5)")))
+  (list
+    :go-slice (list 1 2 3 4 5)))
+
+(go-eval-test
+  "slice expr: a[1:3]"
+  (let
+    ((env (go-eval-program go-env-empty (list (go-parse "a := []int{10, 20, 30, 40}")))))
+    (go-eval env (go-parse "a[1:3]")))
+  (list :go-slice (list 20 30)))
+
+(go-eval-test
+  "slice expr: a[:2] (omitted low)"
+  (let
+    ((env (go-eval-program go-env-empty (list (go-parse "a := []int{1, 2, 3, 4}")))))
+    (go-eval env (go-parse "a[:2]")))
+  (list :go-slice (list 1 2)))
+
+(go-eval-test
+  "slice expr: a[2:] (omitted high)"
+  (let
+    ((env (go-eval-program go-env-empty (list (go-parse "a := []int{1, 2, 3, 4}")))))
+    (go-eval env (go-parse "a[2:]")))
+  (list :go-slice (list 3 4)))
+
+(go-eval-test
+  "fn: sum slice via for-loop with len + index"
+  (let
+    ((env (go-eval-program go-env-builtins (list (go-parse "a := []int{1, 2, 3, 4, 5}") (go-parse "sum := 0") (go-parse "for i := 0; i < len(a); i++ { sum = sum + a[i] }")))))
+    (go-env-lookup env "sum"))
+  15)
+
 (define
   go-eval-test-summary
   (str "eval " go-eval-test-pass "/" go-eval-test-count))
diff --git a/plans/go-on-sx.md b/plans/go-on-sx.md
index d3e9c560..ef7317a5 100644
--- a/plans/go-on-sx.md
+++ b/plans/go-on-sx.md
@@ -276,8 +276,12 @@ Progress-log line → push `origin/loops/go`.
       keywords through `go-eval-block` until `go-for-loop` catches them.
 - [ ] Variables as mutable cells; pointer semantics: `&x` returns the
       cell, `*p` dereferences.
-- [ ] Slices: triple (length, capacity, backing-vector). `append`
-      honours capacity-grow per spec.
+- [/] Slices: v0 represents a slice as `(list :go-slice ELEMS)` —
+      simpler than the full (length, capacity, backing-vector) triple.
+      Composite-literal `[]T{...}` evaluates to a `:go-slice`; `a[i]`
+      indexes, `a[low:high]` slices, `len(a)` returns count, `append(a, ...)`
+      extends. The full triple with capacity-grow comes in a later
+      slice when programs need it.
 - [ ] Maps: SX dict + key-type metadata.
 - [ ] Structs: SX dict + type tag. Methods looked up via type's table.
 - [/] Functions: top-level definition + call (incl. recursion via the
@@ -285,7 +289,7 @@ Progress-log line → push `origin/loops/go`.
 - [ ] Channels: stub (Phase 5 wires them).
 - Tests: arithmetic, control flow, recursion, closures, slices, maps,
   structs, methods, pointer semantics, multiple-return.
-- **Acceptance:** eval/ suite at 80+ tests. Current: 40/40. No concurrency yet.
+- **Acceptance:** eval/ suite at 80+ tests. Current: 50/50. No concurrency yet.
 
 ### Phase 5 — Goroutines + channels + select (`lib/go/sched.sx`) ⬜
 - **Independent implementation.** Do NOT use lib/guest/scheduler/ — that
@@ -569,6 +573,17 @@ Minimal repro: see `lib/go/lex.sx#gl-oct-digit?` and `#gl-match-op`.
 
 _Newest first. Append one dated entry per commit._
 
+- 2026-05-27 — Phase 4 cont.: slice values + index/slice exprs + the
+  `len`/`append`/`print` builtins. Slice representation is
+  `(list :go-slice ELEMS)` for v0 (deferring the full length/cap/
+  backing-vector triple). `go-eval-composite` handles `[]T{...}` /
+  `[N]T{...}` literals (maps next). `go-eval-index` returns the i-th
+  element with bounds-check. `go-eval-slice` handles two-index slicing
+  with omitted low/high. New `go-env-builtins` starter env binds the
+  three builtins as `(:go-builtin NAME)` values; `go-eval-call`
+  recognises them and dispatches to `go-eval-builtin`. **Summing a
+  slice via `for i := 0; i < len(a); i++ { sum = sum + a[i] }` works
+  end-to-end.** +10 tests, eval 50/50, total 427/427. `[nothing]`.
 - 2026-05-27 — Phase 4 cont.: for-loops, break, continue, inc-dec.
   `go-eval-for` handles all three for-header shapes (infinite, while-
   like, C-style) including init+post stmts and missing-cond defaulting