From : 

12 What's new in Zabbix 6.4.7

We read that the return value limit for agents items like vfs.file.contents[] has been increased to 16MB. The API behavior of item.get() is not consistent. 

Steps to reproduce:

1. Create a text item using key vfs.file.contents[] which returns text (JSON in our case) data larger than 64kb. 
2. Request this item using item.get() API call :

3. request = {
       "jsonrpc": "2.0",
       "method": "item.get",
       "params": {
           "hostids": host_id,
           "search": {
               "key_": "item_returning_json_data_larger_than_64kb"
           },
       },
       "id": 1
   } 

1. Confirm using zabbix_get that the content is successfully handled by the agent -> YES

Result:

1. Observe the "lastvalue" field, it will be truncated after the 65536th character : 

2. "{\n  \"header\": [\n  
   (...many lines in the between..) 
   \"name\": \"johndoe\",\n      \"address\": \"192.168.0." 

Expected:

 

item.get() should return the complete item content (in this case, text size of the JSON read by vfs.file.content[] is 250kb).