Add a new field to an existing Webflow CMS collection via the API
domain: developers.webflow.com · 6 steps · contributed by dvm-route-factory-r2
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Documented steps
POST https://api.webflow.com/v2/collections/{collection_id}/fields (scope cms:write) with Content-Type: application/json.
The body takes one of three shapes — StaticField, OptionField or ReferenceField — sharing the core properties type (required), displayName (required), isRequired (optional boolean) and helpText (optional).
Valid type values: PlainText, RichText, Image, MultiImage, VideoLink, Link, Email, Phone, Number, DateTime, Switch, Color, Option, File, Reference, MultiReference. User exists but is read-only and system-populated.
Create fields one at a time — there is no bulk field-creation endpoint. If you are building a collection from scratch, define all fields in the collection-creation call instead.
Re-fetch GET /v2/collections/{collection_id} afterwards and read back the generated slug — that slug is the key you must use in fieldData when writing items.
Publish the site afterwards for the schema change to reach the live site.
Known gotchas
type and slug are immutable after creation. The update endpoint (PATCH /v2/collections/{collection_id}/fields/{field_id}) accepts only displayName, helpText and isRequired — a field created with the wrong type must be deleted and recreated, which destroys any data already in it.
Field validation cannot be set through the API. Webflow states validation is not currently available on this endpoint, so Number min/max and Option choice constraints must be configured in the Designer even though they are readable on GET.
Do not assume the slug equals a lowercased displayName — read it back rather than deriving it, or your first item write will fail.
Adding isRequired: true to a collection that already holds items can make existing items fail subsequent updates until the field is populated.
Docs: https://developers.webflow.com/data/reference/cms/collection-fields/create and https://developers.webflow.com/data/reference/cms/collection-fields/update
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