@jv.bianchi I found a way

Make sure to make an Inertia request before, so that Inertia::getVersion() won't return null. Then you make a second request with all the headers as follower.

Note that the response is a json, it is not an inertiaResponse anymore.

test('deferred props', function () {
    $user = User::factory()->has(Patient::factory()->hasEvaluations())->create();
    $evaluation = $user->evaluations->firstOrFail();

    actingAs($user)->get(route('evaluations.show', $evaluation));
    $response = actingAs($user)->get(route('evaluations.show', $evaluation), [
        'X-Inertia' => true,
        'X-Inertia-Version' => Inertia::getVersion(),
        'X-Inertia-Partial-Component' => 'Evaluations/Show',
        'X-Inertia-Partial-Data' => 'evaluationQuestionnaires',
    ])->assertOk();

    $response->assertJson([
        'props' => [
            'evaluationQuestionnaires' => [],
        ],
    ]);
});

Deferred Props