Version 3 of the Zotero Web API introduces a new all-JSON response format and various other changes. While API v3 is mostly backwards compatible, existing clients may need to make a few small adjustments for full compatibility, depending on usage.

• New default all-JSON response format, format=json
  • Contains a single JSON object for single-object requests and an array of objects for multi-object requests
  • All individual objects contain top-level key and version properties and top-level library, links, and meta objects.
  • meta contains non-editable system-generated properties like createdByUser/ lastModifiedByUser (for group items), creatorSummary, and numChildren.
  • Other Atom-specific feed properties (title, author, published, updated) have been removed.
  • Clients sending application/atom+xml in the Accept header will continue to receive Atom responses if no other format is requested
• For format=json, include=data has replaced Atom's content=json and is now the default mode, with a top-level data object containing the editable fields. As with content, additional comma-separated types can be requested (e.g., include=data,bib). The requested types are provided as top-level properties. content=html remains the default in Atom.
• Multi-object writes now take an array of JSON objects directly, rather than an object with an items/collections/searches property containing an array.
• For write requests, the API now accepts either the editable JSON (data) or the full parent JSON object, with the server extracting the data object automatically. The latter allows for some editing tasks to be performed without any programming.
• The parsedDate property in the format=json meta object gives the full parsed date in YYYY-MM-DD form, so that clients don't need to replicate Zotero's date-parsing logic to get exact dates. In v3 Atom, zapi:parsedDate replaces zapi:year.
• zapi:numTags is removed in v3 Atom, since it's unnecessary with the tags array in the editable json.
• The API now returns 25 results per request instead of 50 if limit isn't provided.
• The total result count for multi-object responses is available in a new custom HTTP header, Total-Results. zapi:totalResults is removed in v3 Atom.
• rel=first/prev/next/last/alternate links for multi-object responses are now provided in the Link HTTP header.
• The API key can be provided in the Authorization request header instead of the key query parameter. Since API keys have never been included in the URLs provided in responses, previously all provided URLs had to be modified for key-based access.
• The API version can be provided as a query parameter (v=3) instead of the Zotero-API-Version header for easier debugging and sharing of requests, though both will remain supported.
• For formats other than Atom, dateModified descending is the default sort instead of dateAdded descending.
• itemKey/itemVersion (and similar properties on collections and searches) in the editable JSON are now just key and version for easier handling by clients. Clients that simply pass back the edited JSON without touching those properties shouldn't be affected. Clients that store the JSON will need to modify it before sending in v3.
• The version metadata field in the computerProgram item type is now versionNumber to avoid a conflict with the renamed object version property.
• dateAdded/dateModified are included in the 'data' object in ISO 8601 form. Previously these timestamps were provided only in the Atom published/updated elements, though in v2 they can be sent back in the JSON as dateAdded/dateModified in YYYY-MM-DD hh-mm-dd format, interpreted as UTC. In v3 write requests, either is accepted, though the previous form is deprecated.
• The accessDate field, which was also previously YYYY-MM-DD[ hh-mm-dd], is ISO 8601 in v3 (including in Atom) for both reading and writing. The previous form is accepted but deprecated.
• The pagination links (rel=self/first/prev/next/last) on multi-object responses can be used without modification by clients using the Authorization header. The rel=self links in individual objects are meant as base URIs and do not include any query parameters (e.g., include=data,bib). This is a change from the previous behavior, where the Atom entry rel=“self” links include all non-default provided parameters. But with the Authorization header and include=data as the new default, the base URI may be sufficient for most individual-object requests.
• The newer parameter is now since for clarity. newer is deprecated.
• The order parameter is now sort and sort is now direction. order=<field> and sort=<asc/desc> are deprecated.
• Requests for updated group metadata can now use format=versions instead of format=etags. format=etags is deprecated.
• pprint=1 has been removed, and all responses are now pretty-printed.
• '<', '>', and '&' are no longer unnecessarily escaped to \u…. in returned JSON data. In Atom, these characters are instead turned into XML numeric character references. Proper XML and JSON parsers shouldn't be affected by these changes.
• The HTTP Warning header may be used to send clients non-fatal warnings — such as deprecation warnings — that can be logged.

• Request format=atom explicitly, or send application/atom+xml in the Accept header
• Use zapi:parsedDate instead of zapi:year
• Use Total-Results HTTP header instead of zapi:totalResults
• Count the tags array in editable JSON instead of using zapi:numTags
• Use key/version instead of itemKey/itemVersion (and collection*/search*) in editable JSON
• Use versionNumber instead of version metadata field in computerProgram item type
• Use ISO 8601 dates for accessDate, dateAdded, and dateModified
• Use since parameter instead of newer
• Use sort parameter instead of order and direction instead of sort
• For writes, upload an array of JSON objects directly instead of an object containing an items/collections/searches array
• Optionally, use Authorization: Bearer <apiKey> instead of key parameter
• Optionally, use v parameter instead of Zotero-API-Version for debugging