---
title: Wrangler 4.117 Drops containerEngine From Miniflare
description: Wrangler 4.117 removes containerEngine from unstable_getMiniflareWorkerOptions. Custom local-testing harnesses reading it now get undefined, silently.
date: 2026-08-06T00:00:00.000Z
category: dev-tools
tags: wrangler, miniflare, cloudflare-workers, testing
---

## Quick Answer

Wrangler 4.117.0 removes `containerEngine` from the object `unstable_getMiniflareWorkerOptions` returns. If a custom local-testing harness reads `containerEngine` off that returned options object, it now gets `undefined` instead of a real value, with no error thrown. `containerEngine` needs to be set at the Miniflare instance level instead, not read from per-worker options.

## Who this actually affects

`unstable_getMiniflareWorkerOptions` is part of Wrangler's unstable Miniflare integration API: code that programmatically builds a Miniflare-backed local Worker instance, most commonly a custom Vitest or testing-harness setup that needs finer control than plain `wrangler dev` gives. `containerEngine` configures which local container runtime backs Cloudflare's Containers feature when a Worker uses container bindings.

The change itself, from `cloudflare/workers-sdk`'s `packages/wrangler/CHANGELOG.md`:

> "Remove `containerEngine` from the worker options returned by `unstable_getMiniflareWorkerOptions`"

This shipped in [`wrangler@4.117.0`](https://github.com/cloudflare/workers-sdk/releases/tag/wrangler%404.117.0), released 2026-07-31, as part of the broader Miniflare v5 upgrade bundled into that release.

## Structural Comparison Matrix

| Operational Aspect                | Before (Miniflare v4-era)                                                               | After (v4.117.0+)                                   |
| :-------------------------------- | :-------------------------------------------------------------------------------------- | :-------------------------------------------------- |
| **Where `containerEngine` lives** | Read off the per-worker options object returned by `unstable_getMiniflareWorkerOptions` | Set at the Miniflare instance level, not per-worker |
| **Failure mode for old code**     | Field present, real value                                                               | Field absent; reads as `undefined`, no error thrown |
| **Who is affected**               | Anyone using the plain `wrangler dev` flow: not affected                                | Custom harnesses calling the unstable API directly  |

## Fix it: move the setting to the Miniflare instance

<Callout type="danger" title="This fails silently, not loudly">
  There is no thrown error, no deprecation warning printed at runtime, and no
  type error unless your harness has strict typing on the returned shape. A
  harness that destructures `containerEngine` off the worker options and passes
  it along gets `undefined` and keeps running, which can mean local
  container-backed tests silently stop using the engine you intended.
</Callout>

### Before: reading it off worker options

```ts title="custom test harness (broken on 4.117+)"
const workerOptions = await unstable_getMiniflareWorkerOptions(config);

// containerEngine is now undefined here, not thrown
const engine = workerOptions.containerEngine;
```

### After: set it on the Miniflare instance directly

```ts title="custom test harness (fixed)"
const workerOptions = await unstable_getMiniflareWorkerOptions(config);

const mf = new Miniflare({
  ...workerOptions,
  containerEngine: "docker", // set at the instance level now
});
```

The exact shape of the instance-level option depends on which Miniflare version your harness pins, so confirm the current constructor signature against your installed `miniflare` package version rather than copying this verbatim; the point that matters is which layer owns the setting, not the exact call shape.

## Confirmed version

Sourced directly from `cloudflare/workers-sdk`'s `packages/wrangler/CHANGELOG.md`, `wrangler@4.117.0`, released 2026-07-31. This one wasn't independently reproduced against a live Containers-backed harness, since that requires a container runtime and a bound Worker beyond what this post's scope covers; the changelog entry and the API's documented shape are the source. If your own harness pins an older Wrangler and hasn't hit this yet, check your `wrangler` and `miniflare` versions before upgrading past 4.117.0. Browse more posts like this in the [Dev Tools](/dev-tools) archive.