curl -v -X POST -H "Content-Type: application/json" -d "5" http://192.168.4.1/json2
Note: Unnecessary use of -X or --request, POST is already inferred.
*   Trying xxx
* Connected to xxx port 80
> POST /json2 HTTP/1.1
> Host: xxxx
> User-Agent: curl/8.9.1
> Accept: */*
> Content-Type: application/json
> Content-Length: 1
>
* upload completely sent off: 1 bytes
< HTTP/1.1 400 Bad Request
< connection: close
< accept-ranges: none
< content-length: 0
<
* shutting down connection #

C:\Users\arik>curl -v -X POST -H "Content-Type: application/json" -d "5" http://192.168.4.1/json2
Note: Unnecessary use of -X or --request, POST is already inferred.
*   Trying xxxxx
* Connected to xxxx port 80
> POST /json2 HTTP/1.1
> Host: xxxx
> User-Agent: curl/8.9.1
> Accept: */*
> Content-Type: application/json
> Content-Length: 1
>
* upload completely sent off: 1 bytes
< HTTP/1.1 200 OK
< connection: close
< accept-ranges: none
< content-length: 3
< content-type: text/plain
<
int* shutting down connection #0

12:01:59.418 > . Json:
12:04:39.267 > 5
12:04:39.267 > Got an int

// based on https://github.com/ESP32Async/ESPAsyncWebServer/blob/main/examples/Json/Json.ino
// SPDX-License-Identifier: LGPL-3.0-or-later
// Copyright 2016-2025 Hristo Gochkov, Mathieu Carbou, Emil Muratov

#include <Arduino.h>
#include <AsyncTCP.h>
#include <WiFi.h>

#include <ESPAsyncWebServer.h>

#include <ArduinoJson.h>
#include <AsyncJson.h>
#include <AsyncMessagePack.h>

static AsyncWebServer server(80);

static AsyncCallbackJsonWebHandler *handler = new AsyncCallbackJsonWebHandler("/json2");

void setup()
{
  Serial.begin(115200);

  // setup wifi
  // AP
  // WiFi.mode(WIFI_AP);
  // WiFi.softAP("esp-captive");
  // client
  WiFi.mode(WIFI_STA); // for STA mode
  WiFi.begin("x", "y");
  while (WiFi.status() != WL_CONNECTED)
  {
    Serial.print(". ");
    delay(500);
  }

  // test cases
  // curl -v -X POST -H 'Content-Type: application/json' -d '{' http://192.168.4.1/json2
  // curl -v -X POST -H 'Content-Type: application/json' -d '5' http://192.168.4.1/json2
  handler->setMethod(HTTP_POST);
  handler->onRequest([](AsyncWebServerRequest *request, JsonVariant &json)
                     {
    Serial.println("Json:");
    serializeJson(json, Serial);
    Serial.println();
    if(json.is<int>())
    {
      Serial.println("Got an int");
    }
    // response
    request->send(200, "text/plain", json.is<int>() ? "int" : "not int"); });

  server.addHandler(handler);

  server.begin();
}

// not needed
void loop()
{
  delay(100);
}