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Atom 1.0

<id>

A permanent, universally unique IRI for the feed or an entry. Required on both, and the element Atom is strictest about.

An Atom id must be an IRI, must be unique, and must never change once assigned — not when the entry is edited, not when the site moves domain. A tag: URI or a UUID is the safe choice precisely because a URL is tempting to change.

Readers use it exactly as RSS readers use guid, with the same consequence for getting it wrong: reuse it and two entries collapse into one, regenerate it and every subscriber sees the whole feed as new.

Format
Atom 1.0
Namespace
Atom Syndication Format 1.0
Parent
<feed><entry>
How many
Exactly one per <feed> and per <entry>.

What it contains

Nothing — this element holds a value rather than other elements. Its content is text, and the constraints below are what that text has to be.

Cardinality and constraints

Exactly one per <feed> and per <entry>.

  • An IRI, and the specification means it: a bare number or slug is not valid.
  • Unique and permanent. Never regenerate one for an existing entry.
  • tag: URIs and urn:uuid: values are the usual choices, because neither invites editing.

What our validator does

Presence is required on the feed and on every entry, and duplicate values across entries are reported as an error.

Broken and fixed

Both documents below are well-formed XML — that is the point of this corpus — and both are run through the validator on every test run. The verdicts here are what it returned, not what we expect it to.

broken
<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
  <id>urn:uuid:60a76c80-d399-11d9-b93c-0003939e0af6</id>
  <title>Example</title>
  <updated>2026-08-10T09:00:00Z</updated>
  <author><name>Ada Lovelace</name></author>
  <entry>
    <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
    <title>Hen lays egg</title>
    <updated>2026-08-10T09:00:00Z</updated>
  </entry>
  <entry>
    <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
    <title>Hen lays another</title>
    <updated>2026-08-10T10:00:00Z</updated>
  </entry>
</feed>
1 duplicate <id> value. An Atom id must be unique and permanent.
fixed
<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
  <id>urn:uuid:60a76c80-d399-11d9-b93c-0003939e0af6</id>
  <title>Example</title>
  <updated>2026-08-10T09:00:00Z</updated>
  <link rel="self" href="https://example.com/feed.xml"/>
  <author><name>Ada Lovelace</name></author>
  <entry>
    <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
    <title>Hen lays egg</title>
    <updated>2026-08-10T09:00:00Z</updated>
  </entry>
</feed>
No errors. Warnings may remain — a conforming feed can still be missing a self link — but nothing here is wrong.

Where it reaches

Related elements

All 34 elements

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