docs: clarify fetch and FormData pairing

README.md

@@ -154,6 +154,57 @@ const { statusCode, body } = await request('https://api.example.com/data');
 const data = await body.json();
 ```
 
+### Keep `fetch` and `FormData` together
+
+When you send a `FormData` body, keep `fetch` and `FormData` from the same
+implementation.
+
+Use one of these patterns:
+
+```js
+// Built-in globals
+const body = new FormData()
+body.set('name', 'some')
+await fetch('https://example.com', {
+  method: 'POST',
+  body
+})
+```
+
+```js
+// undici module imports
+import { fetch, FormData } from 'undici'
+
+const body = new FormData()
+body.set('name', 'some')
+await fetch('https://example.com', {
+  method: 'POST',
+  body
+})
+```
+
+If you want the installed `undici` package to provide the globals, call
+`install()` first:
+
+```js
+import { install } from 'undici'
+
+install()
+
+const body = new FormData()
+body.set('name', 'some')
+await fetch('https://example.com', {
+  method: 'POST',
+  body
+})
+```
+
+`install()` replaces the global `fetch`, `Headers`, `Response`, `Request`, and
+`FormData` implementations with undici's versions, so they all match.
+
+Avoid mixing a global `FormData` with `undici.fetch()`, or `undici.FormData`
+with the built-in global `fetch()`.
+
 ### Version Compatibility
 
 You can check which version of undici is bundled with your Node.js version:
@@ -263,6 +314,11 @@ The `install()` function adds the following classes to `globalThis`:
 - `CloseEvent`, `ErrorEvent`, `MessageEvent` - WebSocket events
 - `EventSource` - Server-sent events client
 
+When you call `install()`, these globals come from the same undici
+implementation. For example, global `fetch` and global `FormData` will both be
+undici's versions, which is the recommended setup if you want to use undici
+through globals.
+
 This is useful for:
 - Polyfilling environments that don't have fetch
 - Ensuring consistent fetch behavior across different Node.js versions

docs/docs/api/Fetch.md

@@ -10,6 +10,14 @@ This API is implemented as per the standard, you can find documentation on [MDN]
 
 If any parameters are passed to the FormData constructor other than `undefined`, an error will be thrown. Other parameters are ignored.
 
+When you use `FormData` as a request body, keep `fetch` and `FormData` from the
+same implementation. Use the built-in global `FormData` with the built-in
+global `fetch()`, and use `undici`'s `FormData` with `undici.fetch()`.
+
+If you want the installed `undici` package to provide the globals, call
+[`install()`](/docs/api/GlobalInstallation.md) so `fetch`, `Headers`,
+`Response`, `Request`, and `FormData` are installed together as a matching set.
+
 ## Response
 
 This API is implemented as per the standard, you can find documentation on [MDN](https://developer.mozilla.org/en-US/docs/Web/API/Response)

docs/docs/api/GlobalInstallation.md

@@ -43,6 +43,54 @@ The `install()` function adds the following classes to `globalThis`:
 | `MessageEvent` | WebSocket message event |
 | `EventSource` | Server-sent events client |
 
+## Using `FormData` with `fetch`
+
+If you send a `FormData` body, use matching implementations for `fetch` and
+`FormData`.
+
+These two patterns are safe:
+
+```js
+// Built-in globals from Node.js
+const body = new FormData()
+await fetch('https://example.com', {
+  method: 'POST',
+  body
+})
+```
+
+```js
+// Globals installed from the undici package
+import { install } from 'undici'
+
+install()
+
+const body = new FormData()
+await fetch('https://example.com', {
+  method: 'POST',
+  body
+})
+```
+
+After `install()`, `fetch`, `Headers`, `Response`, `Request`, and `FormData`
+all come from the installed `undici` package, so they work as a matching set.
+
+If you do not want to install globals, import both from `undici` instead:
+
+```js
+import { fetch, FormData } from 'undici'
+
+const body = new FormData()
+await fetch('https://example.com', {
+  method: 'POST',
+  body
+})
+```
+
+Avoid mixing a global `FormData` with `undici.fetch()`, or `undici.FormData`
+with the built-in global `fetch()`. Keeping them paired avoids surprising
+multipart behavior across Node.js and undici versions.
+
 ## Use Cases
 
 Global installation is useful for:

docs/docs/best-practices/undici-vs-builtin-fetch.md

@@ -19,6 +19,93 @@ When you install undici from npm, you get the full library with all of its
 additional APIs, and potentially a newer release than what your Node.js version
 bundles.
 
+## Keep `fetch` and `FormData` from the same implementation
+
+When you send a `FormData` body, keep `fetch` and `FormData` together from the
+same implementation.
+
+Use one of these patterns:
+
+### Built-in globals
+
+```js
+const body = new FormData()
+body.set('name', 'some')
+body.set('someOtherProperty', '8000')
+
+await fetch('https://example.com', {
+  method: 'POST',
+  body
+})
+```
+
+### `undici` module imports
+
+```js
+import { fetch, FormData } from 'undici'
+
+const body = new FormData()
+body.set('name', 'some')
+body.set('someOtherProperty', '8000')
+
+await fetch('https://example.com', {
+  method: 'POST',
+  body
+})
+```
+
+### `undici.install()` globals
+
+If you want the installed `undici` package to provide the globals, call
+[`install()`](/docs/api/GlobalInstallation.md):
+
+```js
+import { install } from 'undici'
+
+install()
+
+const body = new FormData()
+body.set('name', 'some')
+body.set('someOtherProperty', '8000')
+
+await fetch('https://example.com', {
+  method: 'POST',
+  body
+})
+```
+
+`install()` replaces the global `fetch`, `Headers`, `Response`, `Request`, and
+`FormData` implementations with undici's versions, and also installs undici's
+`WebSocket`, `CloseEvent`, `ErrorEvent`, `MessageEvent`, and `EventSource`
+globals.
+
+Avoid mixing implementations in the same request, for example:
+
+```js
+import { fetch } from 'undici'
+
+const body = new FormData()
+
+await fetch('https://example.com', {
+  method: 'POST',
+  body
+})
+```
+
+```js
+import { FormData } from 'undici'
+
+const body = new FormData()
+
+await fetch('https://example.com', {
+  method: 'POST',
+  body
+})
+```
+
+Those combinations may behave differently across Node.js and undici versions.
+Using matching pairs keeps multipart handling predictable.
+
 ## When you do NOT need to install undici
 
 If all of the following are true, you can rely on the built-in globals and skip
@@ -119,12 +206,12 @@ You can always check the exact bundled version at runtime with
 `process.versions.undici`.
 
 Installing undici from npm does not replace the built-in globals. If you want
-your installed version to override the global `fetch`, use
-[`setGlobalDispatcher`](/docs/api/GlobalInstallation.md) or import `fetch`
+your installed version to replace the global `fetch` and related classes, use
+[`install()`](/docs/api/GlobalInstallation.md). Otherwise, import `fetch`
 directly from `'undici'`:
 
 ```js
-import { fetch } from 'undici'; // uses your installed version, not the built-in
+import { fetch } from 'undici' // uses your installed version, not the built-in
 ```
 
 ## Further reading