diff --git a/packages/compiler/src/executable/prepare.ts b/packages/compiler/src/executable/prepare.ts
index 8a5d11caa..2abf95ffb 100644
--- a/packages/compiler/src/executable/prepare.ts
+++ b/packages/compiler/src/executable/prepare.ts
@@ -156,18 +156,20 @@ export function prepareExecutableModule(
         );
       }
       // A failed program whose compiled packages need the runtime or a
-      // mutable object model (NI-2's report): try them as islands, and
-      // report this failure if that build fails too.
+      // mutable object model (NI-2's report): try them as islands. If that
+      // build fails too, its failure is the one reported. This build typed
+      // those packages from their JavaScript, where the types they only
+      // declare (interfaces, aliases such as `Static<T>`) are `any`, so its
+      // diagnostics describe a program the island build does not compile:
+      // there the packages' declarations type the user code.
       const marked = retryFences
         ? fenceIslands(entryPath, opts, fe, lowered, lowerIslands, false)
         : null;
       if (marked !== null) {
-        const diagnostics = lowered.diagnostics;
-        const texts = fe.sourceTexts();
         fe.dispose();
         frontendDisposed();
         disposed = true;
-        const islanded = prepareExecutableModule(
+        return prepareExecutableModule(
           entryPath,
           opts,
           ffi,
@@ -177,8 +179,6 @@ export function prepareExecutableModule(
           marked,
           false,
         );
-        if (islanded.ok) return islanded;
-        return { ok: false, diagnostics, sourceTexts: texts };
       }
       return fail(lowered.diagnostics);
     }
diff --git a/packages/compiler/src/frontend/lowering/lower-node-island.ts b/packages/compiler/src/frontend/lowering/lower-node-island.ts
index f1e705aae..5d5e934fe 100644
--- a/packages/compiler/src/frontend/lowering/lower-node-island.ts
+++ b/packages/compiler/src/frontend/lowering/lower-node-island.ts
@@ -929,11 +929,20 @@ function toIsland(lowerer: Lowerer, node: ts.Expression): IrExpr {
       // NI-5: the runtime exports dyn objects by reference and copies only
       // primitives, strings and (for now) arrays.
       return lowerer.coerceToExpected(lowerer.lowerExpr(node), DYN);
-    case "refused":
+    case "refused": {
+      const type = lowerer.typeOf(node);
+      // A type parameter the instance binds to dyn (a generic function
+      // over island types, `<T extends TSchema>`): its values are dyn
+      // values, and they cross the way `any` values do.
+      if (
+        (type.flags & ts.TypeFlags.TypeParameter) !== 0 &&
+        lowerer.typeParamResolver(type)?.kind === "dyn"
+      )
+        return lowerer.coerceToExpected(lowerer.lowerExpr(node), DYN);
       // NI-5: closures, promises, records and class instances cross by
       // reference (scr_node_island_export.c): their dyn boxes keep the
       // native referent, and the closure's thunk checks its arguments.
-      if (exportedByReference(lowerer, lowerer.typeOf(node))) {
+      if (exportedByReference(lowerer, type)) {
         const lowered = lowerer.lowerExpr(node);
         const value = lowerer.coerceToExpected(lowered, DYN);
         return lowered.type.kind === "object"
@@ -942,9 +951,10 @@ function toIsland(lowerer: Lowerer, node: ts.Expression): IrExpr {
       }
       return boundaryFail(
         lowerer,
-        `a '${lowerer.checker.typeToString(lowerer.typeOf(node))}' value (a native object would cross as a copy and lose its identity)`,
+        `a '${lowerer.checker.typeToString(type)}' value (a native object would cross as a copy and lose its identity)`,
         node,
       );
+    }
   }
 }
 
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6829-node-island-declared-types/main.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6829-node-island-declared-types/main.ts.json
new file mode 100644
index 000000000..8a467a09c
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6829-node-island-declared-types/main.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6829-node-island-declared-types/main.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/6829-node-island-declared-types/main.ts b/tests/corpus/6829-node-island-declared-types/main.ts
new file mode 100644
index 000000000..52c2e5c40
--- /dev/null
+++ b/tests/corpus/6829-node-island-declared-types/main.ts
@@ -0,0 +1,39 @@
+// @requires-libnode
+// Typed code names an island package's declared interfaces and generic
+// aliases: the package's declarations type it, while its values cross the
+// boundary as handles.
+import { Arr, Num, Str, check, type Static, type TArray, type TNumber, type TSchema, type TString } from "schemakit";
+
+interface Tool<T extends TSchema = TSchema> {
+  name: string;
+  parameters: T;
+  execute(id: string, params: Static<T>): string;
+}
+
+const sum: Tool<TArray<TNumber>> = {
+  name: "sum",
+  parameters: Arr(Num()),
+  execute: (id, params) => `${id}: ${params.reduce((a, b) => a + b, 0)}`,
+};
+const shout: Tool<TString> = {
+  name: "shout",
+  parameters: Str(),
+  execute: (id, params) => `${id}: ${params.toUpperCase()}`,
+};
+
+function run<T extends TSchema>(tool: Tool<T>, input: unknown): string {
+  if (!check(tool.parameters, input)) return `${tool.name}: rejected ${JSON.stringify(input)}`;
+  return tool.execute(tool.name, input);
+}
+
+function describeAll(schemas: TSchema[]): string[] {
+  return schemas.map((s) => `${s.kind} ${s.describe()}`);
+}
+
+console.log(run(sum, [1, 2, 3]));
+console.log(run(sum, [1, "2"]));
+console.log(run(shout, "quiet"));
+console.log(run(shout, 7));
+console.log(describeAll([sum.parameters, shout.parameters, Arr(Arr(Str()))]).join(" | "));
+const tools: Tool[] = [sum, shout];
+console.log(tools.map((t) => t.name).join(","));
diff --git a/tests/corpus/6829-node-island-declared-types/node_modules/schemakit/index.d.ts b/tests/corpus/6829-node-island-declared-types/node_modules/schemakit/index.d.ts
new file mode 100644
index 000000000..d64602f56
--- /dev/null
+++ b/tests/corpus/6829-node-island-declared-types/node_modules/schemakit/index.d.ts
@@ -0,0 +1,27 @@
+// Types the package only declares: its JavaScript has no interfaces and no
+// conditional aliases, so code typed from the JavaScript sees `any` here.
+export interface TSchema {
+  readonly kind: string;
+  describe(): string;
+}
+export interface TNumber extends TSchema {
+  readonly kind: "number";
+}
+export interface TString extends TSchema {
+  readonly kind: "string";
+}
+export interface TArray<T extends TSchema = TSchema> extends TSchema {
+  readonly kind: "array";
+  readonly items: T;
+}
+export type Static<T extends TSchema> = T extends TNumber
+  ? number
+  : T extends TString
+    ? string
+    : T extends TArray<infer I>
+      ? Static<I>[]
+      : unknown;
+export declare function Num(): TNumber;
+export declare function Str(): TString;
+export declare function Arr<T extends TSchema>(items: T): TArray<T>;
+export declare function check<T extends TSchema>(schema: T, value: unknown): value is Static<T>;
diff --git a/tests/corpus/6829-node-island-declared-types/node_modules/schemakit/index.js b/tests/corpus/6829-node-island-declared-types/node_modules/schemakit/index.js
new file mode 100644
index 000000000..9fb64f5c3
--- /dev/null
+++ b/tests/corpus/6829-node-island-declared-types/node_modules/schemakit/index.js
@@ -0,0 +1,37 @@
+"use strict";
+// The esbuild CommonJS bundle prologue: getters over live `let` bindings
+// and a constructor function with prototype methods. scriptc compiles it
+// natively only with runtime fences, so it runs in a Node island.
+var __defProp = Object.defineProperty;
+var __export = (target, all) => {
+  for (var name in all) __defProp(target, name, { get: all[name], enumerable: true });
+};
+var src_exports = {};
+__export(src_exports, {
+  Num: () => Num,
+  Str: () => Str,
+  Arr: () => Arr,
+  check: () => check,
+});
+module.exports = src_exports;
+function Schema(kind, items) {
+  this.kind = kind;
+  if (items !== undefined) this.items = items;
+}
+Schema.prototype.describe = function () {
+  return this.items === undefined ? this.kind : this.kind + "<" + this.items.describe() + ">";
+};
+function Num() {
+  return new Schema("number");
+}
+function Str() {
+  return new Schema("string");
+}
+function Arr(items) {
+  return new Schema("array", items);
+}
+function check(schema, value) {
+  if (schema.kind === "array") return Array.isArray(value) && value.every((v) => check(schema.items, v));
+  return typeof value === schema.kind;
+}
+0 && (module.exports = { Num, Str, Arr, check });
diff --git a/tests/corpus/6829-node-island-declared-types/node_modules/schemakit/package.json b/tests/corpus/6829-node-island-declared-types/node_modules/schemakit/package.json
new file mode 100644
index 000000000..2ccb6f961
--- /dev/null
+++ b/tests/corpus/6829-node-island-declared-types/node_modules/schemakit/package.json
@@ -0,0 +1 @@
+{ "name": "schemakit", "version": "1.0.0", "main": "index.js", "types": "index.d.ts" }
diff --git a/tests/fixtures/node-island/declared-types.ts b/tests/fixtures/node-island/declared-types.ts
new file mode 100644
index 000000000..05835c751
--- /dev/null
+++ b/tests/fixtures/node-island/declared-types.ts
@@ -0,0 +1,27 @@
+// A failed build whose package runs in a Node island reports the island
+// build's errors: there the package's declarations type the program. The
+// test builds it beside corpus 6829's schemakit package.
+import { Num, check, type Static, type TNumber, type TSchema } from "schemakit";
+
+interface Tool<T extends TSchema = TSchema> {
+  name: string;
+  parameters: T;
+  execute(id: string, params: Static<T>): string;
+}
+
+const double: Tool<TNumber> = {
+  name: "double",
+  parameters: Num(),
+  execute: (id, params) => `${id}: ${params * 2}`,
+};
+const input: unknown = 4;
+if (check(double.parameters, input)) console.log(double.execute(double.name, input));
+// The build's own error: member decorators have no native lowering.
+function logged(method: () => number, context: ClassMethodDecoratorContext): void {}
+class Counter {
+  @logged
+  next(): number {
+    return 1;
+  }
+}
+console.log(new Counter().next());
diff --git a/tests/harness/corpus-costs/6829-node-island-declared-types/main.ts.json b/tests/harness/corpus-costs/6829-node-island-declared-types/main.ts.json
new file mode 100644
index 000000000..c2f7906fb
--- /dev/null
+++ b/tests/harness/corpus-costs/6829-node-island-declared-types/main.ts.json
@@ -0,0 +1 @@
+{ "plain": 3400, "san": 62000 }
diff --git a/tests/harness/node-island.test.ts b/tests/harness/node-island.test.ts
index cbb9daf66..85774285e 100644
--- a/tests/harness/node-island.test.ts
+++ b/tests/harness/node-island.test.ts
@@ -354,3 +354,28 @@ test("recursive JSON types cross into Node", async () => {
   const ir = JSON.stringify(deserializeModule(readFileSync(outPath, "utf8")));
   expect(ir).toContain("nodeIsland.callSync");
 }, 300_000);
+
+// When a build fails and the packages it compiled with runtime fences then
+// fail as islands too, the island build's errors are reported. That build
+// types the user code from the packages' declarations; the native one
+// typed it from their JavaScript, where declared-only types (`Static<T>`)
+// are `any`. Needs no libnode pack: both builds fail before linking.
+test("a failed build reports the island build's errors", async () => {
+  const dir = join(outDir, "declared-types");
+  rmSync(dir, { recursive: true, force: true });
+  cpSync(
+    join(repoRoot, "tests/corpus/6829-node-island-declared-types/node_modules"),
+    join(dir, "node_modules"),
+    { recursive: true },
+  );
+  copyFileSync(
+    join(repoRoot, "tests/fixtures/node-island/declared-types.ts"),
+    join(dir, "main.ts"),
+  );
+  const result = await compile(join(dir, "main.ts"), { outPath: join(dir, "main"), outDir: dir });
+  expect(result.ok).toBe(false);
+  if (result.ok) return;
+  const messages = result.diagnostics.map((d) => `${d.code}: ${d.message}`);
+  expect(messages.some((m) => m.startsWith("SC1090: method decorators"))).toBe(true);
+  expect(messages.filter((m) => /Tool|Static|'check'/.test(m))).toEqual([]);
+}, 300_000);