diff --git a/src/foreground-plugin.ts b/src/foreground-plugin.ts
index 22f66e7..b734d21 100644
--- a/src/foreground-plugin.ts
+++ b/src/foreground-plugin.ts
@@ -1,5 +1,5 @@
 import { CallContext, RESET, GET_BLOCK, BEGIN, END, ENV, STORE } from './call-context.ts';
-import { PluginOutput, type InternalConfig } from './interfaces.ts';
+import { PluginOutput, type InternalConfig, InternalWasi } from './interfaces.ts';
 import { loadWasi } from './polyfills/deno-wasi.ts';
 
 export const EXTISM_ENV = 'extism:host/env';
@@ -11,11 +11,13 @@ export class ForegroundPlugin {
   #modules: InstantiatedModule[];
   #names: string[];
   #active: boolean = false;
+  #wasi: InternalWasi | null;
 
-  constructor(context: CallContext, names: string[], modules: InstantiatedModule[]) {
+  constructor(context: CallContext, names: string[], modules: InstantiatedModule[], wasi: InternalWasi | null) {
     this.#context = context;
     this.#names = names;
     this.#modules = modules;
+    this.#wasi = wasi;
   }
 
   async reset(): Promise<boolean> {
@@ -144,7 +146,10 @@ export class ForegroundPlugin {
   }
 
   async close(): Promise<void> {
-    // noop
+    if (this.#wasi) {
+      await this.#wasi.close();
+      this.#wasi = null;
+    }
   }
 }
 
@@ -191,5 +196,5 @@ export async function createForegroundPlugin(
     }),
   );
 
-  return new ForegroundPlugin(context, names, instances);
+  return new ForegroundPlugin(context, names, instances, wasi);
 }
diff --git a/src/interfaces.ts b/src/interfaces.ts
index 7e4e61e..88d70c4 100644
--- a/src/interfaces.ts
+++ b/src/interfaces.ts
@@ -135,7 +135,11 @@ export interface ExtismPluginOptions {
   /**
    * Whether or not to run the Wasm module in a Worker thread. Requires
    * {@link Capabilities#hasWorkerCapability | `CAPABILITIES.hasWorkerCapability`} to
-   * be true.
+   * be true. Defaults to false.
+   *
+   * This feature is marked experimental as we work out [a bug](https://github.com/extism/js-sdk/issues/46).
+   *
+   * @experimental
    */
   runInWorker?: boolean;
 
@@ -190,6 +194,7 @@ export interface InternalConfig {
 export interface InternalWasi {
   importObject(): Promise<Record<string, WebAssembly.ImportValue>>;
   initialize(instance: WebAssembly.Instance): Promise<void>;
+  close(): Promise<void>;
 }
 
 /**
diff --git a/src/mod.ts b/src/mod.ts
index 0da7a24..ace467b 100644
--- a/src/mod.ts
+++ b/src/mod.ts
@@ -77,7 +77,8 @@ export async function createPlugin(
   opts.config ??= {};
   opts.fetch ??= fetch;
 
-  opts.runInWorker ??= CAPABILITIES.hasWorkerCapability;
+  // TODO(chrisdickinson): reset this to `CAPABILITIES.hasWorkerCapability` once we've fixed https://github.com/extism/js-sdk/issues/46.
+  opts.runInWorker ??= false;
   if (opts.runInWorker && !CAPABILITIES.hasWorkerCapability) {
     throw new Error(
       'Cannot enable off-thread wasm; current context is not `crossOriginIsolated` (see https://mdn.io/crossOriginIsolated)',
diff --git a/src/polyfills/browser-wasi.ts b/src/polyfills/browser-wasi.ts
index 5de63e9..e2d4db3 100644
--- a/src/polyfills/browser-wasi.ts
+++ b/src/polyfills/browser-wasi.ts
@@ -49,6 +49,10 @@ export async function loadWasi(
       return context.wasiImport;
     },
 
+    async close() {
+      // noop
+    },
+
     async initialize(instance: WebAssembly.Instance) {
       const memory = instance.exports.memory as WebAssembly.Memory;
 
diff --git a/src/polyfills/deno-wasi.ts b/src/polyfills/deno-wasi.ts
index 91fed35..c9e57c0 100644
--- a/src/polyfills/deno-wasi.ts
+++ b/src/polyfills/deno-wasi.ts
@@ -6,10 +6,10 @@ import { closeSync } from 'node:fs';
 
 async function createDevNullFDs() {
   const [stdin, stdout] = await Promise.all([open(devNull, 'r'), open(devNull, 'w')]);
-
+  let needsClose = true;
   const fr = new globalThis.FinalizationRegistry((held: number) => {
     try {
-      closeSync(held);
+      if (needsClose) closeSync(held);
     } catch {
       // The fd may already be closed.
     }
@@ -17,14 +17,23 @@ async function createDevNullFDs() {
   fr.register(stdin, stdin.fd);
   fr.register(stdout, stdout.fd);
 
-  return [stdin.fd, stdout.fd, stdout.fd];
+  return {
+    async close() {
+      needsClose = false;
+      await Promise.all([stdin.close(), stdout.close()]).catch(() => {});
+    },
+    fds: [stdin.fd, stdout.fd, stdout.fd],
+  };
 }
 
 export async function loadWasi(
   allowedPaths: { [from: string]: string },
   enableWasiOutput: boolean,
 ): Promise<InternalWasi> {
-  const [stdin, stdout, stderr] = enableWasiOutput ? [0, 1, 2] : await createDevNullFDs();
+  const {
+    close,
+    fds: [stdin, stdout, stderr],
+  } = enableWasiOutput ? { async close() {}, fds: [0, 1, 2] } : await createDevNullFDs();
   const context = new Context({
     preopens: allowedPaths,
     exitOnReturn: false,
@@ -38,6 +47,10 @@ export async function loadWasi(
       return context.exports;
     },
 
+    async close() {
+      await close();
+    },
+
     async initialize(instance: WebAssembly.Instance) {
       const memory = instance.exports.memory as WebAssembly.Memory;
 
diff --git a/src/polyfills/node-wasi.ts b/src/polyfills/node-wasi.ts
index 21833ff..512515a 100644
--- a/src/polyfills/node-wasi.ts
+++ b/src/polyfills/node-wasi.ts
@@ -6,25 +6,44 @@ import { closeSync } from 'node:fs';
 
 async function createDevNullFDs() {
   const [stdin, stdout] = await Promise.all([open(devNull, 'r'), open(devNull, 'w')]);
+  let needsClose = true;
+  // TODO: make this check always run when bun fixes [1], so `fs.promises.open()` returns a `FileHandle` as expected.
+  // [1]: https://github.com/oven-sh/bun/issues/5918
+  let close = async () => {
+    closeSync(stdin as any);
+    closeSync(stdout as any);
+  };
+  if (typeof stdin !== 'number') {
+    const fr = new globalThis.FinalizationRegistry((held: number) => {
+      try {
+        if (needsClose) closeSync(held);
+      } catch {
+        // The fd may already be closed.
+      }
+    });
 
-  const fr = new globalThis.FinalizationRegistry((held: number) => {
-    try {
-      closeSync(held);
-    } catch {
-      // The fd may already be closed.
-    }
-  });
-  fr.register(stdin, stdin.fd);
-  fr.register(stdout, stdout.fd);
+    fr.register(stdin, stdin.fd);
+    fr.register(stdout, stdout.fd);
+    close = async () => {
+      needsClose = false;
+      await Promise.all([stdin.close(), stdout.close()]).catch(() => {});
+    };
+  }
 
-  return [stdin.fd, stdout.fd, stdout.fd];
+  return {
+    close,
+    fds: [stdin.fd, stdout.fd, stdout.fd],
+  };
 }
 
 export async function loadWasi(
   allowedPaths: { [from: string]: string },
   enableWasiOutput: boolean,
 ): Promise<InternalWasi> {
-  const [stdin, stdout, stderr] = enableWasiOutput ? [0, 1, 2] : await createDevNullFDs();
+  const {
+    close,
+    fds: [stdin, stdout, stderr],
+  } = enableWasiOutput ? { async close() {}, fds: [0, 1, 2] } : await createDevNullFDs();
 
   const context = new WASI({
     version: 'preview1',
@@ -39,6 +58,10 @@ export async function loadWasi(
       return context.wasiImport;
     },
 
+    async close() {
+      await close();
+    },
+
     async initialize(instance: WebAssembly.Instance) {
       const memory = instance.exports.memory as WebAssembly.Memory;