What are the top-level structured field types?

RFC 8941 defines three: an Item (a single bare item with optional parameters), a List (a comma-separated sequence of items or inner-lists), and a Dictionary (comma-separated key=value pairs). A header's specification fixes which of the three it uses.

What bare-item types exist?

Integer, Decimal, String (double-quoted), Token (unquoted, starts with a letter or *), Byte Sequence (base64 wrapped in colons), and Boolean (?1 for true, ?0 for false). Each member of a list or dictionary is one of these, optionally with parameters.

How are parameters written?

Parameters are appended to an item or inner-list member with a semicolon, as key or key=value, e.g. text/html;q=0.9. A bare parameter key with no value means a Boolean true. Multiple parameters chain with more semicolons.

What is an inner list?

An inner list is a space-separated sequence of items wrapped in parentheses, used as a single member of an outer List or Dictionary, e.g. ("a" "b");param. It lets one list slot hold several values plus its own parameters.

Does this tool send my header anywhere?

No. Parsing and classification happen entirely in your browser. Nothing is uploaded or stored.

HTTP Structured Fields Reference

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A typed grammar for HTTP headers

Historically every HTTP header invented its own ad-hoc syntax, making parsers fragile. RFC 8941 Structured Field Values defines a small, typed grammar so new headers can be parsed uniformly. This reference explains the three top-level types, the bare-item types and parameter syntax, with a live parser that classifies any value you paste.

How it works

A header’s spec declares it as one of three top-level types:

Item:       Content-Length: 42
List:       Accept-Encoding: gzip, br;q=0.8, *;q=0.1
Dictionary: Cache-Status: ExampleCache; hit, ExampleCache2; fwd=uri-miss

Each member is a bare item of a fixed type — Integer, Decimal, String (double-quoted), Token (unquoted), Byte Sequence (:base64:) or Boolean (?1/?0) — optionally carrying ;key=value parameters. An inner list wraps several items in parentheses as one member. The parser below detects the top-level type heuristically (a top-level = outside quotes implies a Dictionary; a comma implies a List; otherwise an Item) and reports each member.

Tips and notes

Strings are double-quoted and may only contain printable ASCII; Tokens are unquoted.
A bare parameter key (;sni) is shorthand for ;sni=?1 (Boolean true).
Byte sequences are base64 wrapped in colons: :cHF=:.
Decimals have at most three fractional digits; larger precision needs a String.
Use Structured Fields for new headers so generic libraries can parse them safely.