diff --git a/packages/base-engine/src/expressions/evaluator.ts b/packages/base-engine/src/expressions/evaluator.ts
index bcf00cf..2d98c5f 100644
--- a/packages/base-engine/src/expressions/evaluator.ts
+++ b/packages/base-engine/src/expressions/evaluator.ts
@@ -27,6 +27,7 @@ export type EvaluationContext = {
   byPath: Map<string, BaseFileInspection>;
   byBasename: Map<string, BaseFileInspection>;
   value?: unknown;
+  index?: number;
   acc?: unknown;
   values?: unknown[];
 };
@@ -81,8 +82,10 @@ export function evaluateExpression(
         );
         const predicate = expr.args[0];
         return predicate
-          ? source.filter((value) =>
-              truthy(evaluateExpression(predicate, { ...context, value })),
+          ? source.filter((value, index) =>
+              truthy(
+                evaluateExpression(predicate, { ...context, value, index }),
+              ),
             )
           : source;
       }
@@ -176,6 +179,10 @@ function readIdentifier(name: string, context: EvaluationContext): unknown {
     return context.value ?? null;
   }
 
+  if (name === "index") {
+    return context.index ?? null;
+  }
+
   if (name === "acc") {
     return context.acc ?? null;
   }
diff --git a/packages/base-engine/test/query.test.ts b/packages/base-engine/test/query.test.ts
index 48f8d2f..8160401 100644
--- a/packages/base-engine/test/query.test.ts
+++ b/packages/base-engine/test/query.test.ts
@@ -296,6 +296,28 @@ describe("Base query engine", () => {
     expect(b?.formulas.HasAll).toBe(false);
   });
 
+  // Regression: filter must bind index as the 0-based position of the item.
+  it("binds index inside list filter", () => {
+    const base = parseBase(
+      "bases/FilterIndex.base",
+      [
+        "formulas:",
+        '  Kept: "[1, 2, 3].filter(index > 0)"',
+        '  All: "[1, 2, 3].filter(index >= 0)"',
+        "views:",
+        "  - name: FilterIndex",
+        "    order:",
+        "      - formula.Kept",
+        "      - formula.All",
+      ].join("\n"),
+    );
+    const result = queryBase(base, [file("A.md", "A", [])]);
+
+    const a = result.rows.find((r) => r.file.path === "A.md");
+    expect(a?.formulas.Kept).toEqual([2, 3]);
+    expect(a?.formulas.All).toEqual([1, 2, 3]);
+  });
+
   it("evaluates string matches and date format methods", () => {
     const base = parseBase(
       "bases/Fmt.base",