This is the “magic number” that starts a PNG file, except the first byte with a value of 0x89 has instead been replaced with the Unicode replacement character (\uFFFD).

The data was already corrupted by the time it got to whatever printed that log line; you’ll need to look earlier in the call stack for where things are going wrong.

It will help troubleshoot if you can capture exactly the bytes that the API is replying with - my suspicion is that it’s sending something like:

"html":"<BYTE 0x89>PNG<BYTE 0x0D><BYTE 0x0A>...etc...

which is problematic from both a “valid UTF8” and a “valid JSON” perspective.

That “html” is not HTML, but a PNG, jason wants to serialize that into a string, which needs to be valid UTF-8. This String isn’t.

And as far as I read the Jason sources, Jason shouldn’t care at all.

$ nix shell nixpkgs#elixir -c iex
Erlang/OTP 25 [erts-13.2.2.1] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [jit:ns]

Interactive Elixir (1.14.5) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> s = ~S|{"server": "Microsoft-IIS/10.0", "headers_hash": 1111111111, "host": "127.0.0.1", "html": "\ufffdPNG\r\n\u001a\n\u0000\u0000\u0000\rIHDR\u0000\u0000\u0003\u0000\u0000\u0000\u0002f\b\u0006\u0000\u0000\u0000\ufffd[\ufffd}\u0000\u0000\u0000\u0001sRGB\u0000\ufffd\ufffd\u001c\ufffd\u0000\u0000\u0000\u0004gAMA\u0000\u0000\ufffd\ufffd\u000b\ufffda\u0005\u0000\u0000\u0000\tpHYs\u0000\u0000\u000e\ufffd\u0000\u0000\u000e\ufffd\u0001\ufffd"}|
"{\"server\": \"Microsoft-IIS/10.0\", \"headers_hash\": 1111111111, \"host\": \"127.0.0.1\", \"html\": \"\\ufffdPNG\\r\\n\\u001a\\n\\u0000\\u0000\\u0000\\rIHDR\\u0000\\u0000\\u0003\\u0000\\u0000\\u0000\\u0002f\\b\\u0006\\u0000\\u0000\\u0000\\ufffd[\\ufffd}\\u0000\\u0000\\u0000\\u0001sRGB\\u0000\\ufffd\\ufffd\\u001c\\ufffd\\u0000\\u0000\\u0000\\u0004gAMA\\u0000\\u0000\\ufffd\\ufffd\\u000b\\ufffda\\u0005\\u0000\\u0000\\u0000\\tpHYs\\u0000\\u0000\\u000e\\ufffd\\u0000\\u0000\\u000e\\ufffd\\u0001\\ufffd\"}"
iex(2)> Mix.install([{:jason, "~> 1.4"}])
Resolving Hex dependencies...
Dependency resolution completed:
New:
  jason 1.4.0
==> jason
Compiling 10 files (.ex)
Generated jason app
:ok
iex(3)> Jason.decode(s)
{:ok,
 %{
   "headers_hash" => 1111111111,
   "host" => "127.0.0.1",
   "html" => <<239, 191, 189, 80, 78, 71, 13, 10, 26, 10, 0, 0, 0, 13, 73, 72,
     68, 82, 0, 0, 3, 0, 0, 0, 2, 102, 8, 6, 0, 0, 0, 239, 191, 189, 91, 239,
     191, 189, 125, 0, 0, 0, 1, 115, 82, ...>>,
   "server" => "Microsoft-IIS/10.0"
 }}

As you can see, Jason happily decodes the blob into a binary.

Thanks for the very informative answer. I think you’re right about the Python approach.