diff --git a/boundaries2_test.go b/boundaries2_test.go
index d614912..14f63a1 100644
--- a/boundaries2_test.go
+++ b/boundaries2_test.go
@@ -661,14 +661,30 @@ func TestAdversarial_NegativeIndex(t *testing.T) {
 	require.Error(t, err)
 }
 
-// --- 39. Map index with int idx but the idx is float64 (1.0) ---
+// --- 39. Slice index with float64 ---
 
+// Integer-valued floats (1.0) are accepted as indices, matching the
+// "ints and floats are fungible when integral" rule used by
+// arithmetic. JSON-derived numbers arrive as float64 so accepting
+// them removes a CLI papercut. Non-integer floats still error.
 func TestAdversarial_FloatIndexOnSlice(t *testing.T) {
-	env := map[string]any{"xs": []any{int64(1), int64(2), int64(3)}}
+	env := map[string]any{"xs": []any{int64(10), int64(20), int64(30)}}
+
 	got, err := evalExpr(t.Context(), "xs[1.0]", env)
-	t.Logf("xs[1.0] => %v, err=%v", got, err)
-	// toInt64 doesn't accept float; should error rather than silently work.
-	require.Error(t, err)
+	require.NoError(t, err)
+	require.Equal(t, int64(20), got)
+
+	_, err = evalExpr(t.Context(), "xs[1.5]", env)
+	require.ErrorIs(t, err, ErrEvaluate)
+	require.Contains(t, err.Error(), "index must be integer")
+
+	got, err = evalExpr(t.Context(), `s[2.0]`, map[string]any{"s": "hello"})
+	require.NoError(t, err)
+	require.Equal(t, "l", got)
+
+	_, err = evalExpr(t.Context(), `s[2.5]`, map[string]any{"s": "hello"})
+	require.ErrorIs(t, err, ErrEvaluate)
+	require.Contains(t, err.Error(), "index must be integer")
 }
 
 // --- 40. Nested template inside composite literal ---
diff --git a/docs/reference/spec.md b/docs/reference/spec.md
index 7914e59..6c2df2a 100644
--- a/docs/reference/spec.md
+++ b/docs/reference/spec.md
@@ -218,12 +218,14 @@ limited by [evaluation depth](#limits-and-safety).
 - `map[string]any`: `i` must be a string. Missing key → `ErrEvaluate`.
 - Other maps: `i` is converted to the map's key type if assignable or
   convertible. A nil index on a typed map is an error (not a panic).
-- Slice, array: `i` must be an integer. Negative indices and indices
-  `>= len(x)` return `ErrEvaluate` (expr does not support Python-style
-  negative indexing).
+- Slice, array: `i` must be an integer or an integer-valued float
+  (`xs[1.0]` works, `xs[1.5]` is an error). Negative indices and
+  indices `>= len(x)` return `ErrEvaluate` (expr does not support
+  Python-style negative indexing).
 - String: `i` selects the `i`-th **rune** (Unicode code point) and
   returns it as a one-rune string. `len(s)` is also in runes, so indexing
-  and length stay consistent for non-ASCII strings.
+  and length stay consistent for non-ASCII strings. Integer-valued
+  floats are accepted here as well.
 - Anything else → `ErrEvaluate`.
 
 Slice expressions (`x[a:b]`), full slices (`x[a:b:c]`), and type
diff --git a/llms.txt b/llms.txt
index 2646e53..b09f2bb 100644
--- a/llms.txt
+++ b/llms.txt
@@ -271,6 +271,10 @@ See `docs/guides/examples.md` for worked multi-line expressions.
   `xs || []`, `count || 0`. Wrap with `bool(...)` when you need a
   strict bool.
 - **String indexing** counts runes, matching `len` on strings.
+- **Slice / string indices** accept integer-valued floats (`xs[1.0]`
+  works, `xs[1.5]` errors), matching the "ints and floats fungible
+  when integral" arithmetic rule. JSON-derived indices arrive as
+  float64, so this removes a CLI papercut.
 - **Selectors and index errors:** missing map keys, missing struct
   fields, nil selectors, and out-of-bounds indexes all return
   `ErrEvaluate` — expr never panics on well-formed env input.
diff --git a/program.go b/program.go
index f474502..cf678f6 100644
--- a/program.go
+++ b/program.go
@@ -994,18 +994,18 @@ func indexValue(recv, idx any) (any, error) {
 	rv := reflect.ValueOf(recv)
 	switch rv.Kind() {
 	case reflect.Slice, reflect.Array:
-		i, ok := toInt64(idx)
-		if !ok {
-			return nil, fmt.Errorf("%w: index must be integer, got %T", ErrEvaluate, idx)
+		i, err := toIndexInt(idx)
+		if err != nil {
+			return nil, err
 		}
 		if i < 0 || i >= int64(rv.Len()) {
 			return nil, fmt.Errorf("%w: index %d out of range [0, %d)", ErrEvaluate, i, rv.Len())
 		}
 		return rv.Index(int(i)).Interface(), nil
 	case reflect.String:
-		i, ok := toInt64(idx)
-		if !ok {
-			return nil, fmt.Errorf("%w: index must be integer, got %T", ErrEvaluate, idx)
+		i, err := toIndexInt(idx)
+		if err != nil {
+			return nil, err
 		}
 		runes := []rune(rv.String())
 		if i < 0 || i >= int64(len(runes)) {
diff --git a/reflect.go b/reflect.go
index baf49a8..6e15cb9 100644
--- a/reflect.go
+++ b/reflect.go
@@ -351,6 +351,32 @@ func truncFloatToInt64(f float64, target string) (int64, error) {
 	return int64(t), nil
 }
 
+// toIndexInt converts an evaluated index value to int64. Integers of
+// any kind pass through. Floats are accepted only when their value is
+// finite and exactly integral, matching expr's "ints and floats are
+// fungible when integral" arithmetic rule. Non-integer or non-numeric
+// indices return an ErrEvaluate-wrapped error suitable for surfacing
+// directly to callers.
+func toIndexInt(v any) (int64, error) {
+	if i, ok := toInt64(v); ok {
+		return i, nil
+	}
+	if f, ok := toFloat64(v); ok {
+		if math.IsNaN(f) || math.IsInf(f, 0) {
+			return 0, fmt.Errorf("%w: index must be integer, got %v", ErrEvaluate, f)
+		}
+		t := math.Trunc(f)
+		if t != f {
+			return 0, fmt.Errorf("%w: index must be integer, got %v", ErrEvaluate, f)
+		}
+		if t >= int64LimitFloat || t < -int64LimitFloat {
+			return 0, fmt.Errorf("%w: index %v out of int64 range", ErrEvaluate, f)
+		}
+		return int64(t), nil
+	}
+	return 0, fmt.Errorf("%w: index must be integer, got %T", ErrEvaluate, v)
+}
+
 func mapStringKey(keyType reflect.Type, key string) reflect.Value {
 	kv := reflect.ValueOf(key)
 	if kv.Type().AssignableTo(keyType) {