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

Comienza reproduciendo los dos comandos POST del issue con HTTPie 3.2.4 y compara sus salidas verbose y de depuración. Traza cómo se ensamblan los headers de la solicitud cuando se proporciona uno o dos headers personalizados. La tarea estará terminada cuando un solo header personalizado siga dando como resultado que se envíe Content-Type: application/json y que aparezca tanto en la salida de la solicitud como en la respuesta de pie.dev.

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.
…