A Structure Type is a named, reusable data shape you define once and use in many places in a thunk. Instead of describing the same object — a contact, an address, a line item — separately on every column and tool that handles it, you define it one time, give it a name, and then refer to it by that name wherever a structured value is needed.
Structure Types build on the idea in Schematized State: the more specific the shape of a property, the better an AI agent can read and fill it in. A Structure Type is the most specific shape you can give a structured value, and it is shared, so every place that uses it stays consistent.
What a Structure Type is
A Structure Type is an object shape with a fixed set of fields. For example, a Contact type might have name, email, and company fields. Each field has a name, a description, and a type of its own.
You define and manage these types in the Type Library tab, alongside the thunk's Properties. A type defined there is available to the whole thunk.
Defining a type
In the Type Library, add a type, give it a meaningful name, and add its fields. For each field you choose a type, just as you do for a work item column:
a basic type such as Text, Number, Yes/No, Date, URL, File, Image, Address, Location, or Country;
a fixed list of allowed values (an enumeration) on a Text field, so the agent can only record one of the choices you listed;
another Structure Type, so you can compose larger shapes out of smaller ones; or
a list of any of the above — for example a list of Text, or a list of another type.
Every field of a Structure Type is required. Structure Types are strict by design: when the shape is fixed and complete, an AI agent cannot leave a field out or invent an extra one, which is what makes the type reliable to read and write. Give each field a clear description for the same reason you would on a column — it tells the agent exactly what belongs there.
Referring to another type
A field can itself be a Structure Type, so you can compose larger shapes out of smaller ones. A Contact can have a company field whose type is an Organization; an Order can have an items field that is a list of LineItem.
Two rules keep these references sound:
Define the referenced type first. A field can only use a type that already exists.
References cannot form a loop. If
ContactusesOrganization, thenOrganizationcannot useContactback, directly or through other types.
Using a Structure Type
Once a type exists, it appears as a first-class entry in the type picker — alongside the basic types — everywhere a structured value is chosen. You can use it as:
the type of a work item column (the column then reads as the type's name, such as
Contact, orList of Contact);the input or output type of a custom tool, so a tool can take or return a whole structured object instead of loose values (see Custom Tools); and
the type of a field inside another Structure Type.
For how lists of values — including lists of a Structure Type — are shown and configured, see Format a list property.
Reuse, rename, and delete
The point of a named type is that there is one definition and one place to change it:
Edit once. When you change a type's fields, every column, tool, and other type that uses it picks up the change.
Rename safely. Renaming a type updates every place that refers to it, so nothing is left pointing at the old name.
Delete only when unused. You cannot delete a type while a work item column or another type still uses it. Remove those uses first, then delete the type.
Structure Type or JSON Object property?
Thunk.AI has two ways to hold a structured value, and they suit different situations:
A JSON Object property is a one-off shape defined inline on a single column, often from a pasted example. It is anonymous and not shared — two columns that hold the same kind of object have two separate shapes that can drift apart — and a custom tool cannot take or return it.
A Structure Type is the named, shared version of that idea: define it once in the Type Library and reuse it across columns, tools, and other types.
Reach for a Structure Type when the same shape appears in more than one place, when you want a custom tool to take or return it, or when you want a single definition to keep everything consistent. A JSON Object property is fine when the shape is used in exactly one place and nothing else needs to share it.