diff --git a/packages/compiler/src/frontend/lowering/lower-exprs.ts b/packages/compiler/src/frontend/lowering/lower-exprs.ts
index b0f1f0973..36d2c0987 100644
--- a/packages/compiler/src/frontend/lowering/lower-exprs.ts
+++ b/packages/compiler/src/frontend/lowering/lower-exprs.ts
@@ -961,6 +961,12 @@ function lowerExprInner(L: Lowerer, expr: ts.Expression): IrExpr {
             const canonical = stdlibGlobalNameOf(L, expr) ?? expr.text;
             return { kind: "strLit", value: `[builtin ${canonical}]`, type: STRING, loc };
           }
+          // In --dynamic mode, `globalThis` is available as a JSVAL via the
+          // engine's global object. Property accesses (globalThis.window,
+          // globalThis.__vibeXxx) then flow through as JSVAL member reads.
+          if (L.dynamic && expr.text === "globalThis") {
+            return { kind: "jsOp", op: "globalGet", name: "globalThis", args: [], type: JSVAL, loc };
+          }
           // The families with a WHY: each hint states what makes the
           // surface genuinely non-static (or what to use instead).
           const globalHints: Record<string, string | undefined> = {
@@ -1844,6 +1850,13 @@ function lowerExprInner(L: Lowerer, expr: ts.Expression): IrExpr {
           expr,
         );
       }
+      // The lowered receiver is JSVAL (e.g. `(await import("excluded-module")).tools`
+      // where the dynamic import was stubbed to Promise.resolve({}) by lowerOwnModuleImport,
+      // or any other case where a TypeScript-typed expression lowered to the JS engine island).
+      // Property access on JSVAL is a native engine getProp — always valid.
+      if (recvLowered.type.kind === "jsval") {
+        return { kind: "jsOp", op: "getProp", name: expr.name.text, args: [recvLowered], type: JSVAL, loc };
+      }
       // The lowered receiver is a RECORD the checker spelled wider —
       // `s.match(re).groups.key` in a JS file: the checker says
       // `{ [key: string]: string } | undefined`, but the groups
@@ -9292,6 +9305,16 @@ export function lowerBinary(L: Lowerer, expr: ts.BinaryExpression): IrExpr {
     if (recv.type.kind === "dyn") {
       return { kind: "dynHasKey", key, value: recv, type: BOOL, loc };
     }
+    // `"key" in globalThis` (or any JSVAL receiver) — the globalThis object is a JSVAL in
+    // --dynamic mode. Emit a JS-engine property existence check: getProp(recv, key) !== undefined.
+    // This correctly handles `"window" in globalThis` (false on server, true on browser),
+    // `"document" in globalThis`, and similar feature-detection patterns.
+    if (recv.type.kind === "jsval") {
+      const prop: IrExpr = { kind: "jsOp", op: "getProp", name: key, args: [recv], type: JSVAL, loc };
+      const undef: IrExpr = { kind: "unitLit", unit: "undefined", type: UNDEFINED_T, loc };
+      const marshaledUndef: IrExpr = { kind: "jsMarshal", value: undef, type: JSVAL, loc };
+      return { kind: "jsOp", op: "neq", args: [prop, marshaledUndef], type: BOOL, loc };
+    }
     // `"k" in u` over a UNION whose arms are FIXED record shapes: every
     // arm answers membership STATICALLY (a declared non-optional field is
     // always present, an undeclared name never is), so the whole test