Temporarily adopting this point of view (of the *external developer*) can help you feel like it's more obvious where to put the parameters, the Pydantic model for the body, for the response, etc. for that *external API*.

### Create a callback `APIRouter`

!!! warning
    These are very technical details about the standards **JSON Schema** and **OpenAPI**.

    If the ideas above already work for you, that might be enough, and you probably don't need these details, feel free to skip them.

Before OpenAPI 3.1.0, OpenAPI used an older and modified version of **JSON Schema**.

JSON Schema didn't have `examples`, so OpenAPI added it's own `example` field to its own modified version.

This is because FastAPI uses this `root_path` to create the default `server` in OpenAPI with the URL provided by `root_path`.

## Additional servers

!!! warning
    This is a more advanced use case. Feel free to skip it.

By default, **FastAPI** will create a `server` in the OpenAPI schema with the URL for the `root_path`.

For this reason it's common to provide them in environment variables that are read by the application.

## Environment Variables

!!! tip
    If you already know what "environment variables" are and how to use them, feel free to skip to the next section below.

		return
	}

	// TODO: This causes allocations, but escape analysis doesn't really show the cause.
	// If another faithful engineer wants to take a stab, feel free.
	go func(m message) {
		var start time.Time
		if m.DeadlineMS > 0 {
			start = time.Now()
		}
		var b []byte
		var err *RemoteErr
		func() {
			defer func() {

         * a key that appears once in `right` might appear multiple times in `left`). We don't
         * guarantee behavior in that case, anyway, and the current behavior is likely undesirable.
         * So that's either a reason to feel free to change it or a reason to not bother thinking
         * further about this.
         */
        V rightValue = uncheckedCastNullableTToT(onlyOnRight.remove(leftKey));

Heungsub Lee, Hoeseong Kim, I-Hong Jhuo, Ilango R, Innovimax, Irene Dea, Jacky
Ko, Jakub Lipinski, Jason Zaman, jcf94, Jeffrey Poznanovic, Jens Elofsson,
Jeroen BéDorf, Jia Qingtong, Jiankang, Joe Q, Joe Quadrino, Joeran Beel, Jonas
Rauber, Jonathan, Jonathan Kyl, Joppe Geluykens, Joseph Friedman, jtressle, jwu,
K Yasaswi Sri Chandra Gandhi, K. Hodges, Kaixi Hou, Karl Lessard, Karl