diff --git a/packages/compiler/src/frontend/lowering/lower-calls.ts b/packages/compiler/src/frontend/lowering/lower-calls.ts
index 7d2a7fcec..c90e3d0e1 100644
--- a/packages/compiler/src/frontend/lowering/lower-calls.ts
+++ b/packages/compiler/src/frontend/lowering/lower-calls.ts
@@ -6081,7 +6081,17 @@ export function lowerPromiseMethodCall(L: Lowerer, call: ts.CallExpression,
       // contextual type spelled (a module-namespace type has no mapping —
       // the handle is the value's only story, isIslandExpr's local rule).
       if (inner.kind === "jsval") markJsvalHandlerParams(L, call.arguments[0]!);
-      let cb = L.lowerExpr(call.arguments[0]!);
+      // When the settled value is JSVAL, the callback body should allow JSVAL
+      // to flow into any slot without shape checks — the actual runtime value
+      // is an opaque island handle, not the TypeScript type parameter.
+      const prevInJsvalThenHandler = L.inJsvalThenHandler;
+      if (inner.kind === "jsval") L.inJsvalThenHandler = true;
+      let cb: IrExpr;
+      try {
+        cb = L.lowerExpr(call.arguments[0]!);
+      } finally {
+        L.inJsvalThenHandler = prevInJsvalThenHandler;
+      }
       // A TYPED handler on a DYN-settling promise (the tracePromise
       // result's `.then((value) => ...)` — the checker's generic
       // instantiation typed the parameter, but the settled value is a
@@ -6183,20 +6193,34 @@ export function lowerPromiseMethodCall(L: Lowerer, call: ts.CallExpression,
         );
       }
       const param = cb.type.params[0];
-      if (param !== undefined && !typeEquals(param, inner)) {
+      // When the settled value is JSVAL, the handler's parameter type comes from
+      // the TypeScript checker's view of the promise type parameter — which may
+      // not match JSVAL. Skip the parameter type check in this case; the actual
+      // runtime value is an island handle regardless of the declared type.
+      if (inner.kind !== "jsval" && param !== undefined && !typeEquals(param, inner)) {
         L.unsupported(
           "SC1090",
           call.arguments[0]!,
           `then handlers whose parameter is not the settled value's type (expected '${L.fmt(inner)}', got '${L.fmt(param)}')`,
         );
       }
-      const resultT = L.mapTypeOf(L.typeOf(call));
-      if (resultT?.kind !== "promise") {
-        L.noLowering(
-          "then with this handler's result type",
-          call,
-          "the combined result must be a representable promise",
-        );
+      // When the settled value is JSVAL, the result promise type cannot be
+      // inferred from the TypeScript checker (the checker's type parameter
+      // reflects the declared module namespace, not the actual runtime {}).
+      // Force Promise<JSVAL> so the result type matches the actual value.
+      let resultT: IrType & { kind: "promise" };
+      if (inner.kind === "jsval") {
+        resultT = { kind: "promise", inner: JSVAL };
+      } else {
+        const mapped = L.mapTypeOf(L.typeOf(call));
+        if (mapped?.kind !== "promise") {
+          L.noLowering(
+            "then with this handler's result type",
+            call,
+            "the combined result must be a representable promise",
+          );
+        }
+        resultT = mapped;
       }
       const R = resultT.inner;
       const fnName = `%fn${L.lambdaCounter++}_then`;
diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index 22a7814ed..bd5add317 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -1415,6 +1415,13 @@ export class Lowerer {
   readonly liftedFns: IrFunction[] = [];
   lambdaCounter = 0;
 
+  /** True while compiling the callback argument of `.then()` on a promise
+   * whose settled type is JSVAL. Suppresses `requireExactShape` in
+   * `coerceInto` — the settled value is an opaque island handle, so any
+   * value flowing out of the callback may also be JSVAL regardless of the
+   * TypeScript checker's static view of the promise's type parameter. */
+  inJsvalThenHandler = false;
+
   /** Statement lists currently mid-lowering, innermost last: the forward-
    * capture machinery (predeclareForwardCapture) needs to know which later
    * statements of an OPEN list a symbol's declaration sits in, which scope
@@ -6542,6 +6549,11 @@ export class Lowerer {
       }
       return e;
     }
+    // Inside a .then() callback on a JSVAL-settling promise, the settled value
+    // is an opaque island handle — the TypeScript checker's view of the type
+    // parameter is not trustworthy (the actual runtime value may be {}). Allow
+    // any value to flow through without a shape check.
+    if (this.inJsvalThenHandler) return e;
     this.requireExactShape(node, e.type, expected);
     return e;
   }