Edit a json file with batch file or command prompt [closed]

I have a json file for some conversation with the user and an ai assistant,

{ "model": "meta-llama/llama-4-scout-17b-16e-instruct", "messages": [     { "role": "system", "content": "You are Julia, and you are a helpful ai.  Here is your previous memory: "     },     { "role": "user", "content": "hi"     }   ] } 

I wanted to edit the json directly from a batch file using some command line interface to add another text like the following in the messages array:

{ "role": "assistant", "content": "hi, how's your day going?" }

I tried many different ways but I am encounter error on every attempt

What I have tried:

1. Powershell:

$json = Get-Content -Raw -Path "body.json" | ConvertFrom-Json$newMessage = [PSCustomObject]@{   role = "assistant"   content = "hello, how can i help you"}$json.messages +=$newMessage$json | ConvertTo-Json -Depth 10 | Set-Content -Path "body.json"

Result:

{"model":  "meta-llama/llama-4-scout-17b-16e-instruct","messages":  [                     {"role":  "system","content":  "You are Julia, and you are a helpful ai.  Here is your previous memory: "                     },                     {"role":  "user","content":  "hi"                     },                     {"role":  "assistant","content":  "hello, how can i help you"                     }                 ]}

The problem here is that it has to much gap between start and the code2. JQ command prompt interface

jq '.messages +=[{"role": "assistant", "content": "hi, how's your day going?"}]' body.json

Result:

jq: error: syntax error, unexpected INVALID_CHARACTER, expecting end of file (Windows cmd shell quoting issues?) at <top-level>, line 1:'.messagesjq: 1 compile error

3. JJ Stream command prompt interface

type body.json | jj -v '{"role": "assistant", "content": "hi, how's your day going?"}' messages+

Result:

jj - JSON Stream Editor 1.9.2unknown option argument: "assistant,"

It can be a Powershell script inside a batch file but I don't want a Powershell script file to run outside the batch program.

NOTE: I am a beginner in this topic so please don't blame me for silly mistakes

NOTE: I know batch file is not the best method to edit json files but that what I am trying to accomplish