diff --git a/packages/compiler/src/frontend/lowering/lower-node-island.ts b/packages/compiler/src/frontend/lowering/lower-node-island.ts
index 207347e31..080a6862c 100644
--- a/packages/compiler/src/frontend/lowering/lower-node-island.ts
+++ b/packages/compiler/src/frontend/lowering/lower-node-island.ts
@@ -22,7 +22,9 @@ import { DYN, STRING, type IrExpr, type IrType } from "../../ir/ir.js";
 const JSON_HINT =
   "only JSON data (numbers, strings, booleans, null, arrays, plain objects of those, and unknown) crosses into Node.js in this version";
 
-/** The first part of `t` that is not JSON data, or null. */
+/** The first part of `t` that is not JSON data, or null. `seen` holds the
+ * records and unions already being checked, so a recursive type (a JSON
+ * value type that contains arrays or records of itself) ends there. */
 function nonJsonPart(lowerer: Lowerer, t: IrType, seen: Set<string>): IrType | null {
   switch (t.kind) {
     case "f64":
@@ -35,8 +37,8 @@ function nonJsonPart(lowerer: Lowerer, t: IrType, seen: Set<string>): IrType | n
     case "array":
       return nonJsonPart(lowerer, t.elem, seen);
     case "record": {
-      if (seen.has(t.shapeId)) return null;
-      seen.add(t.shapeId);
+      if (seen.has(`record:${t.shapeId}`)) return null;
+      seen.add(`record:${t.shapeId}`);
       const shape = lowerer.shapes.get(t.shapeId);
       if (shape === undefined) return t;
       for (const field of shape.fields) {
@@ -46,6 +48,8 @@ function nonJsonPart(lowerer: Lowerer, t: IrType, seen: Set<string>): IrType | n
       return shape.indexValue === undefined ? null : nonJsonPart(lowerer, shape.indexValue, seen);
     }
     case "union": {
+      if (seen.has(`union:${t.unionId}`)) return null;
+      seen.add(`union:${t.unionId}`);
       const def = lowerer.unions.get(t.unionId);
       if (def === undefined) return t;
       for (const arm of def.arms) {
diff --git a/tests/fixtures/node-island/helper.cjs b/tests/fixtures/node-island/helper.cjs
index 317a5b65b..903532b0c 100644
--- a/tests/fixtures/node-island/helper.cjs
+++ b/tests/fixtures/node-island/helper.cjs
@@ -1,2 +1,3 @@
 "use strict";
 exports.answer = () => 42;
+exports.echo = (value) => value;
diff --git a/tests/fixtures/node-island/lifecycle.ts b/tests/fixtures/node-island/lifecycle.ts
index 1185e0674..f18ec640d 100644
--- a/tests/fixtures/node-island/lifecycle.ts
+++ b/tests/fixtures/node-island/lifecycle.ts
@@ -4,8 +4,12 @@ import { readFileSync } from "node:fs";
 import { callSync } from "@scriptc/node";
 
 const mode = process.argv[2] ?? "call";
-const mapped = (): boolean =>
-  process.platform !== "linux" || readFileSync("/proc/self/maps", "utf8").includes("libnode");
+// Whether libnode is mapped into the process, from /proc/self/maps; other
+// platforms report "unknown".
+const mapped = (): boolean | string =>
+  process.platform === "linux"
+    ? readFileSync("/proc/self/maps", "utf8").includes("libnode")
+    : "unknown";
 console.log("mapped before", mapped());
 if (mode !== "never") {
   console.log(callSync<number>("./helper.cjs", "answer"));
diff --git a/tests/fixtures/node-island/recursive-json.ts b/tests/fixtures/node-island/recursive-json.ts
new file mode 100644
index 000000000..90d9c757f
--- /dev/null
+++ b/tests/fixtures/node-island/recursive-json.ts
@@ -0,0 +1,9 @@
+// A recursive JSON value type as an argument and a result of a Node island
+// call (tests/harness/node-island.test.ts).
+import { callSync } from "@scriptc/node";
+
+type Json = number | string | boolean | null | Json[] | { [key: string]: Json };
+
+const input: Json = { list: [1, "two", [true, null]] };
+const output = callSync<Json>("./helper.cjs", "echo", input);
+console.log(JSON.stringify(output));
diff --git a/tests/harness/node-island.test.ts b/tests/harness/node-island.test.ts
index 0ee5c0fa7..47a68e290 100644
--- a/tests/harness/node-island.test.ts
+++ b/tests/harness/node-island.test.ts
@@ -6,11 +6,11 @@
  * (6800-6803) pins the call semantics against Node. Needs a libnode pack
  * in SCRIPTC_LIBNODE; required in the Linux x64 CI lanes (libnode.ts). */
 import { execFile } from "node:child_process";
-import { existsSync, mkdirSync, writeFileSync } from "node:fs";
+import { existsSync, mkdirSync, readFileSync, writeFileSync } from "node:fs";
 import { join } from "node:path";
 import { promisify } from "node:util";
 import { beforeAll, describe, expect, test } from "vitest";
-import { compile } from "@scriptc/compiler";
+import { compile, deserializeModule } from "@scriptc/compiler";
 import { definedFunctionNames, nodeIslandSymbols } from "../../scripts/zero-cost-compare.mjs";
 import { libnodePack } from "./libnode.js";
 
@@ -31,6 +31,11 @@ interface Run {
 
 let binary = "";
 
+// The fixture reads whether libnode is mapped from /proc/self/maps; other
+// platforms print "unknown".
+const before = `mapped before ${process.platform === "linux" ? "false" : "unknown"}`;
+const after = `mapped after ${process.platform === "linux" ? "true" : "unknown"}`;
+
 async function run(
   path: string,
   args: string[],
@@ -103,12 +108,12 @@ describe.skipIf(skip)("Node islands in-process", () => {
 
   test("loads Node on the first call, and only then", async () => {
     expect(await run(binary, [])).toEqual({
-      stdout: "mapped before false\n42\nmapped after true\ndone\n",
+      stdout: `${before}\n42\n${after}\ndone\n`,
       stderr: "",
       code: 0,
     });
     expect(await run(binary, ["never"])).toEqual({
-      stdout: "mapped before false\ndone\n",
+      stdout: `${before}\ndone\n`,
       stderr: "",
       code: 0,
     });
@@ -117,7 +122,7 @@ describe.skipIf(skip)("Node islands in-process", () => {
   test("process.exit after loading Node keeps its code", async () => {
     const result = await run(binary, ["exit"]);
     expect(result.code).toBe(5);
-    expect(result.stdout).toBe("mapped before false\n42\nmapped after true\n");
+    expect(result.stdout).toBe(`${before}\n42\n${after}\n`);
   });
 
   test("beside packaging carries its own pack", async () => {
@@ -126,14 +131,14 @@ describe.skipIf(skip)("Node islands in-process", () => {
     const env = { ...process.env };
     delete env["SCRIPTC_LIBNODE"];
     const result = await run(beside, [], env);
-    expect(result.stdout).toBe("mapped before false\n42\nmapped after true\ndone\n");
+    expect(result.stdout).toBe(`${before}\n42\n${after}\ndone\n`);
     expect(result.code).toBe(0);
   }, 300_000);
 
   test("a missing pack fails the first call", async () => {
     const result = await run(binary, [], { ...process.env, SCRIPTC_LIBNODE: "/nonexistent/pack" });
     expect(result.code).toBe(1);
-    expect(result.stdout).toBe("mapped before false\n");
+    expect(result.stdout).toBe(`${before}\n`);
     expect(result.stderr).toMatch(
       /^scriptc: cannot load Node\.js for this program's Node islands: .*\/nonexistent\/pack\/libscr_node_bridge\.(so|dylib).*\n$/,
     );
@@ -184,3 +189,20 @@ describe.skipIf(skip)("Node islands in-process", () => {
     expect(conflicting).toEqual([]);
   });
 });
+
+// Lowering checks every argument and result type for JSON data; IR output
+// needs no libnode pack.
+test("recursive JSON types cross into Node", async () => {
+  const dir = join(outDir, "recursive-json");
+  mkdirSync(dir, { recursive: true });
+  const outPath = join(dir, "main.ir.json");
+  const result = await compile(join(repoRoot, "tests/fixtures/node-island/recursive-json.ts"), {
+    outPath,
+    outDir: dir,
+    outputKind: "ir",
+  });
+  if (!result.ok)
+    throw new Error(result.diagnostics.map((d) => `${d.code}: ${d.message}`).join("\n"));
+  const ir = JSON.stringify(deserializeModule(readFileSync(outPath, "utf8")));
+  expect(ir).toContain("nodeIsland.callSync");
+}, 300_000);