Problem

The module-level capability probe in the fetch adapter (lib/adapters/fetch.js) creates a new ReadableStream() as a Request body to test for streaming support, but never cancels it. The Request constructor sets up an internal pull pipeline on the stream body; since the stream is never consumed or cancelled, the [[pullAlgorithm]] Promise remains pending indefinitely, causing an async resource leak.

How to reproduce

Run any test file that imports axios (with the default fetch adapter, which has been the default since v1.7) using Vitest with async leak detection:

vitest --detect-async-leaks

This reports 1 PROMISE leak per test file due to the uncancelled stream from the probe.

Root cause

In the factory() function, the supportsRequestStream probe creates a ReadableStream inline:

const hasContentType = new Request(platform.origin, {
  body: new ReadableStream(),  // ← never cancelled
  method: 'POST',
  // ...
}).headers.has('Content-Type');

The Request constructor enqueues an internal pull on the stream. Without cancellation, this pull promise persists for the lifetime of the process.

Fix

Extract the ReadableStream to a variable and call body.cancel() after the probe completes:

const body = new ReadableStream();

const hasContentType = new Request(platform.origin, {
  body,
  method: 'POST',
  // ...
}).headers.has('Content-Type');

body.cancel();

Impact

Affects every project using the axios fetch adapter (default since v1.7) under test tooling with async leak detection (Vitest --detect-async-leaks, Jest --detectOpenHandles, Node.js async_hooks). The fix is a single cancel() call with no behavioral change to the probe logic.

Test

Added a unit test that spies on ReadableStream.prototype.cancel during a fresh getFetch() call (via a unique env to bypass the seed cache) and asserts the cancel method is invoked.

Summary by cubic

Fixes an async resource leak in the fetch adapter by cancelling the ReadableStream used in the request-stream capability probe. Adds a unit test to verify the stream is cancelled.

Bug Fixes

• Cancel the probe ReadableStream in lib/adapters/fetch.js after creating the Request.
• Prevents a pending pull promise and async leaks seen in vitest --detect-async-leaks, Jest open handles, and Node async_hooks.
• No behavior change to request-stream support detection.

Testing

• Added a unit test that spies on ReadableStream.prototype.cancel during a fresh getFetch() run; asserts it is called.
• No other tests changed.

^{Written for commit 69588e8. Summary will update on new commits.}