JSON content type not being set when a single header is present

Start by reproducing the two POST commands in the issue with HTTPie 3.2.4 and compare their verbose and debug output. Trace how request headers are assembled when one versus two custom headers are supplied. Done means a single custom header still results in Content-Type: application/json being sent and reported by the request output and pie.dev response.

Checklist

• I've searched for similar issues.
• I'm using the latest version of HTTPie.

Minimal reproduction code and steps

1. $ https post pie.dev/post -v 'header1: xyz' x=1
2. $ https post pie.dev/post -v 'header1: xyz' 'header2: abc' x=1

Current result

output from step 1:

POST /post HTTP/1.1
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: pie.dev
header1: xyz

{"x": "1"}

HTTP/1.1 200 OK
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
CF-RAY: 95566dc8c8d1f7c9-LAX
Cf-Cache-Status: DYNAMIC
Connection: keep-alive
Content-Encoding: gzip
Content-Type: application/json
Date: Wed, 25 Jun 2025 18:11:41 GMT
Nel: {"report_to":"cf-nel","success_fraction":0.0,"max_age":604800}
Report-To: {"group":"cf-nel","max_age":604800,"endpoints":[{"url":"https://a.nel.cloudflare.com/report/v4?s=T5F5MLxiQSAvDrINeG1ku5IkxuGFXPCXZCppIMBEnShUajvjeC3%2BvZcwVBmdlCmAv4Nb9LuUyx2EnQd45Dnmkx%2FAojwaiPk%3D"}]}
Server: cloudflare
Transfer-Encoding: chunked
alt-svc: h3=":443"; ma=86400

{
    "args": {},
    "data": "{\"x\": \"1\"}",
    "files": {},
    "form": {},
    "headers": {
        "Accept-Encoding": "gzip, br",
        "Cdn-Loop": "cloudflare; loops=1",
        "Cf-Connecting-Ip": "<REDACTED>",
        "Cf-Ipcountry": "US",
        "Cf-Ray": "95566dc8c8d1f7c9-FRA",
        "Cf-Visitor": "{\"scheme\":\"https\"}",
        "Content-Length": "10",
        "Header1": "xyz",
        "Host": "pie.dev",
        "User-Agent": "python-urllib3/2.5.0"
    },
    "json": {
        "x": "1"
    },
    "origin": "<REDACTED>",
    "url": "https://pie.dev/post"
}

(Neither HTTPie's verbose output or pie.dev's response reports Content-Type: application/json)
…

Expected result

output from step 2:

POST /post HTTP/1.1
Accept-Encoding: gzip, deflate
Connection: keep-alive
Content-Type: application/json
header1: xyz
header2: abc
Host: pie.dev

{"x": "1"}

HTTP/1.1 200 OK
Date: Wed, 25 Jun 2025 18:13:03 GMT
Content-Type: application/json
Transfer-Encoding: chunked
Connection: keep-alive
Server: cloudflare
Nel: {"report_to":"cf-nel","success_fraction":0.0,"max_age":604800}
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true
Cf-Cache-Status: DYNAMIC
Report-To: {"group":"cf-nel","max_age":604800,"endpoints":[{"url":"https://a.nel.cloudflare.com/report/v4?s=rtMPq5%2BI8UInMXCz1Fm7mh3yvd5yUt%2BZrSaQMEqe68bvTDT0Aka8A3gc8wSEQP1ZNkbhXMV6QB%2FcyVjAgpW04b439a9nRYM%3D"}]}
Content-Encoding: gzip
CF-RAY: 95566fc8bb3c279a-LAX
alt-svc: h3=":443"; ma=86400

{
  "args": {}, 
  "data": "{\"x\": \"1\"}", 
  "files": {}, 
  "form": {}, 
  "headers": {
    "Accept-Encoding": "gzip, br", 
    "Cdn-Loop": "cloudflare; loops=1", 
    "Cf-Connecting-Ip": "<REDACTED>", 
    "Cf-Ipcountry": "US", 
    "Cf-Ray": "95566fc8bb3c279a-FRA", 
    "Cf-Visitor": "{\"scheme\":\"https\"}", 
    "Content-Length": "10", 
    "Content-Type": "application/json", 
    "Header1": "xyz", 
    "Header2": "abc", 
    "Host": "pie.dev", 
    "User-Agent": "python-urllib3/2.5.0"
  }, 
  "json": {
    "x": "1"
  }, 
  "origin": "<REDACTED>", 
  "url": "https://pie.dev/post"
}

(both HTTPie's verbose output and pie.dev's response report Content-Type: application/json)
…

Debug output

Please re-run the command with --debug, then copy the entire command & output and paste both below:

$ https post pie.dev/post --debug -v 'header1: xyz' x=1

HTTPie 3.2.4
Requests 2.32.4
Pygments 2.19.1
Python 3.13.5 (main, Jun 11 2025, 15:36:57) [Clang 17.0.0 (clang-1700.0.13.3)]
/opt/homebrew/Cellar/httpie/3.2.4_3/libexec/bin/python
Darwin 24.5.0

<Environment {'apply_warnings_filter': <function Environment.apply_warnings_filter at 0x101b88cc0>,
 'args': Namespace(),
 'as_silent': <function Environment.as_silent at 0x101b88b80>,
 'colors': 256,
 'config': {'default_options': []},
 'config_dir': PosixPath('/Users/andrewmuro/.config/httpie'),
 'devnull': <property object at 0x101b66a70>,
 'is_windows': False,
 'log_error': <function Environment.log_error at 0x101b88c20>,
 'program_name': 'https',
 'quiet': 0,
 'rich_console': <functools.cached_property object at 0x10103e690>,
 'rich_error_console': <functools.cached_property object at 0x101b5cc00>,
 'show_displays': True,
 'stderr': <_io.TextIOWrapper name='<stderr>' mode='w' encoding='utf-8'>,
 'stderr_isatty': True,
 'stdin': <_io.TextIOWrapper name='<stdin>' mode='r' encoding='utf-8'>,
 'stdin_encoding': 'utf-8',
 'stdin_isatty': True,
 'stdout': <_io.TextIOWrapper name='<stdout>' mode='w' encoding='utf-8'>,
 'stdout_encoding': 'utf-8',
 'stdout_isatty': True}>

<PluginManager {'adapters': [],
 'auth': [<class 'httpie.plugins.builtin.BasicAuthPlugin'>,
          <class 'httpie.plugins.builtin.DigestAuthPlugin'>,
          <class 'httpie.plugins.builtin.BearerAuthPlugin'>],
 'converters': [],
 'formatters': [<class 'httpie.output.formatters.headers.HeadersFormatter'>,
                <class 'httpie.output.formatters.json.JSONFormatter'>,
                <class 'httpie.output.formatters.xml.XMLFormatter'>,
                <class 'httpie.output.formatters.colors.ColorFormatter'>]}>

>>> requests.request(**{'auth': None,
 'data': b'{"x": "1"}',
 'headers': <HTTPHeadersDict('User-Agent': b'HTTPie/3.2.4', 'Accept': b'application/json, */*;q=0.5', 'Content-Type': b'application/json', 'header1': b'xyz')>,
 'method': 'post',
 'params': <generator object MultiValueOrderedDict.items at 0x101facf40>,
 'url': 'https://pie.dev/post'})

POST /post HTTP/1.1
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: pie.dev
header1: xyz

{"x": "1"}

HTTP/1.1 200 OK
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
CF-RAY: 95567851aa815263-LAX
Cf-Cache-Status: DYNAMIC
Connection: keep-alive
Content-Encoding: gzip
Content-Type: application/json
Date: Wed, 25 Jun 2025 18:18:53 GMT
Nel: {"report_to":"cf-nel","success_fraction":0.0,"max_age":604800}
Report-To: {"group":"cf-nel","max_age":604800,"endpoints":[{"url":"https://a.nel.cloudflare.com/report/v4?s=esCpq2VXduYA9W9gSJ1M5zi9BdGYBrkJ8fq8Jc8tIJf2QEO4REGWcuonYlkM5cAHzETL%2BPuzAcAuyUgokl6O74%2F7D53b"}]}
Server: cloudflare
Transfer-Encoding: chunked
alt-svc: h3=":443"; ma=86400

{
    "args": {},
    "data": "{\"x\": \"1\"}",
    "files": {},
    "form": {},
    "headers": {
        "Accept-Encoding": "gzip, br",
        "Cdn-Loop": "cloudflare; loops=1",
        "Cf-Connecting-Ip": "<REDACTED>",
        "Cf-Ipcountry": "US",
        "Cf-Ray": "95567851aa815263-FRA",
        "Cf-Visitor": "{\"scheme\":\"https\"}",
        "Content-Length": "10",
        "Header1": "xyz",
        "Host": "pie.dev",
        "User-Agent": "python-urllib3/2.5.0"
    },
    "json": {
        "x": "1"
    },
    "origin": "<REDACTED>",
    "url": "https://pie.dev/post"
}

Additional information, screenshots, or code examples

The underlying issue here is that when sending a request to a Serverless app (in my case, using only an Authorization header), API gateway will automatically encode the request body as base64 because the Content-Type header isn't set, which causes JSON parsing of the body to fail.
…