# curl Data Flags and the Content-Type Trap

> curl has several ways to attach a body, and they differ in encoding and default Content-Type. The big surprise is that -d defaults to form encoding, not JSON, so a JSON body can be mislabeled and rejected.

Source: https://ronutz.com/en/learn/curl-data-flags-and-content-type  
Updated: 2026-07-01  
Related tools: https://ronutz.com/en/tools/http-request-translator

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curl can attach a request body in several ways, and they are not interchangeable.

`-d` / `--data` sends the value and, importantly, sets `Content-Type: application/x-www-form-urlencoded` by default. This is the most common curl surprise: `curl -d '{"name":"Alice"}'` sends JSON text but labels it as a form, so a JSON API may reject it. The fix is to state the type: add `-H "Content-Type: application/json"`.

`--data-urlencode` percent-encodes its value, which is what you want for form fields that contain spaces or symbols. `--data-binary` sends the bytes exactly, without the newline stripping that `-d` performs. Multiple `-d` flags are joined with `&`.

`-F` / `--form` builds a `multipart/form-data` body. Each `-F name=value` is a field, and `-F name=@file` attaches a file. curl chooses the multipart boundary for you, so you never set it by hand.

When you translate a curl command to another language, carry the Content-Type curl actually sends, not the one you would guess from the body's shape. That single detail is where most hand conversions go wrong.