diff --git a/internal/controllogic/value.go b/internal/controllogic/value.go
new file mode 100644
index 0000000..921743d
--- /dev/null
+++ b/internal/controllogic/value.go
@@ -0,0 +1,156 @@
+// Value model for control-logic locals/expressions. A Value is either a scalar
+// (float64; booleans are 1/0) or an array ([]Value). This is the Go port of
+// web/src/lib/arraypolicy.ts (sizing) plus the asNum/asArr/idx narrowing from
+// web/src/lib/expr.ts. Pure, dependency-free.
+package controllogic
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+// Value is a scalar (float64) or an array ([]Value).
+type Value = any
+
+// ARRAY_MAX is the global hard cap on dynamic array length (drops oldest).
+const ARRAY_MAX = 1_000_000
+
+// StateVar declares a graph-local variable. Mirrors web/src/lib/types.ts StateVar.
+type StateVar struct {
+	Name     string  `json:"name"`
+	Type     string  `json:"type,omitempty"` // number|bool|string|array (default number)
+	Initial  string  `json:"initial"`        // initial value, stored as a string
+	Unit     string  `json:"unit,omitempty"`
+	Low      float64 `json:"low,omitempty"`
+	High     float64 `json:"high,omitempty"`
+	Elem     string  `json:"elem,omitempty"`     // array-only: number|bool|array
+	Sizing   string  `json:"sizing,omitempty"`   // array-only: dynamic|capped|fixed
+	Capacity int     `json:"capacity,omitempty"` // array-only
+}
+
+func asNum(v Value) (float64, error) {
+	f, ok := v.(float64)
+	if !ok {
+		return 0, fmt.Errorf("expected a number, got an array")
+	}
+	return f, nil
+}
+
+func asArr(v Value) ([]Value, error) {
+	a, ok := v.([]Value)
+	if !ok {
+		return nil, fmt.Errorf("expected an array, got a number")
+	}
+	return a, nil
+}
+
+// idxResolve resolves a possibly-negative index against length; range-checked.
+func idxResolve(i float64, length int) (int, error) {
+	k := int(i) // truncates toward zero, matching Math.trunc
+	if k < 0 {
+		k = length + k
+	}
+	if k < 0 || k >= length {
+		return 0, fmt.Errorf("index %v out of range (len %d)", i, length)
+	}
+	return k, nil
+}
+
+// normalizeValue coerces an arbitrary decoded value (e.g. from JSON: float64,
+// bool, []interface{}) into a canonical Value (float64 leaves, []Value arrays).
+func normalizeValue(v any) Value {
+	switch t := v.(type) {
+	case float64:
+		return t
+	case float32:
+		return float64(t)
+	case int:
+		return float64(t)
+	case int64:
+		return float64(t)
+	case bool:
+		if t {
+			return 1.0
+		}
+		return 0.0
+	case []interface{}:
+		out := make([]Value, len(t))
+		for i, e := range t {
+			out[i] = normalizeValue(e)
+		}
+		return out
+	default:
+		return 0.0
+	}
+}
+
+func zeroFill(n int) []Value {
+	if n < 0 {
+		n = 0
+	}
+	out := make([]Value, n)
+	for i := range out {
+		out[i] = 0.0
+	}
+	return out
+}
+
+// parseInitialArray returns the starting contents of an array local. Mirrors
+// arraypolicy.ts parseInitialArray.
+func parseInitialArray(sv StateVar) []Value {
+	cap := sv.Capacity
+	raw := strings.TrimSpace(sv.Initial)
+	var parsed []Value
+	if raw != "" {
+		var j interface{}
+		if err := json.Unmarshal([]byte(raw), &j); err == nil {
+			if arr, ok := j.([]interface{}); ok {
+				parsed = normalizeValue(arr).([]Value)
+			}
+		}
+	}
+	if sv.Sizing == "fixed" {
+		if parsed == nil {
+			return zeroFill(cap)
+		}
+		out := make([]Value, 0, cap)
+		for i := 0; i < len(parsed) && i < cap; i++ {
+			out = append(out, parsed[i])
+		}
+		for len(out) < cap {
+			out = append(out, 0.0)
+		}
+		return out
+	}
+	if parsed == nil {
+		return []Value{}
+	}
+	return parsed
+}
+
+// applySizing clamps arr to the declared sizing policy. Mirrors arraypolicy.ts.
+func applySizing(arr []Value, sv StateVar) []Value {
+	cap := sv.Capacity
+	switch sv.Sizing {
+	case "fixed":
+		out := make([]Value, 0, cap)
+		for i := 0; i < len(arr) && i < cap; i++ {
+			out = append(out, arr[i])
+		}
+		for len(out) < cap {
+			out = append(out, 0.0)
+		}
+		return out
+	case "capped":
+		if len(arr) > cap {
+			return arr[len(arr)-cap:]
+		}
+		return arr
+	default:
+		if len(arr) > ARRAY_MAX {
+			return arr[len(arr)-ARRAY_MAX:]
+		}
+		return arr
+	}
+}
diff --git a/internal/controllogic/value_test.go b/internal/controllogic/value_test.go
new file mode 100644
index 0000000..5aae8be
--- /dev/null
+++ b/internal/controllogic/value_test.go
@@ -0,0 +1,67 @@
+package controllogic
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestAsNumAsArr(t *testing.T) {
+	if n, err := asNum(3.0); err != nil || n != 3 {
+		t.Fatalf("asNum(3)=%v,%v", n, err)
+	}
+	if _, err := asNum([]Value{1.0}); err == nil {
+		t.Fatal("asNum(array) should error")
+	}
+	if a, err := asArr([]Value{1.0, 2.0}); err != nil || len(a) != 2 {
+		t.Fatalf("asArr=%v,%v", a, err)
+	}
+	if _, err := asArr(3.0); err == nil {
+		t.Fatal("asArr(number) should error")
+	}
+}
+
+func TestIdxResolve(t *testing.T) {
+	if k, err := idxResolve(-1, 3); err != nil || k != 2 {
+		t.Fatalf("idx(-1,3)=%v,%v", k, err)
+	}
+	if _, err := idxResolve(3, 3); err == nil {
+		t.Fatal("idx(3,3) should be out of range")
+	}
+}
+
+func TestNormalizeValue(t *testing.T) {
+	got := normalizeValue([]interface{}{1.0, true, []interface{}{2.0}})
+	want := []Value{1.0, 1.0, []Value{2.0}}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("normalize=%#v want %#v", got, want)
+	}
+	if normalizeValue(5) != Value(5.0) {
+		t.Fatalf("normalize(int) = %#v", normalizeValue(5))
+	}
+}
+
+func TestParseInitialArray(t *testing.T) {
+	fixed := parseInitialArray(StateVar{Type: "array", Sizing: "fixed", Capacity: 3, Initial: "[1,2]"})
+	if !reflect.DeepEqual(fixed, []Value{1.0, 2.0, 0.0}) {
+		t.Fatalf("fixed init = %#v", fixed)
+	}
+	dyn := parseInitialArray(StateVar{Type: "array", Sizing: "dynamic", Initial: "[5,6,7]"})
+	if !reflect.DeepEqual(dyn, []Value{5.0, 6.0, 7.0}) {
+		t.Fatalf("dynamic init = %#v", dyn)
+	}
+	empty := parseInitialArray(StateVar{Type: "array", Sizing: "dynamic", Initial: ""})
+	if len(empty) != 0 {
+		t.Fatalf("empty init = %#v", empty)
+	}
+}
+
+func TestApplySizing(t *testing.T) {
+	capped := applySizing([]Value{1.0, 2.0, 3.0, 4.0}, StateVar{Sizing: "capped", Capacity: 2})
+	if !reflect.DeepEqual(capped, []Value{3.0, 4.0}) {
+		t.Fatalf("capped = %#v", capped)
+	}
+	fixed := applySizing([]Value{1.0}, StateVar{Sizing: "fixed", Capacity: 3})
+	if !reflect.DeepEqual(fixed, []Value{1.0, 0.0, 0.0}) {
+		t.Fatalf("fixed = %#v", fixed)
+	}
+}