State and references¶

The state schema¶

Declare the state under State schema in the left panel. Each field has a name and a type:

Type	JSON value	Reducers
`string`	text	`replace`
`number`	float	`replace`, `add`
`integer`	whole number	`replace`, `add`
`boolean`	`true` / `false`	`replace`
`object`	JSON object	`replace`, `merge`
`list`	JSON array	`replace`, `append`

The run input is the initial state.

Reducers¶

A reducer decides how a write combines with the value already in the field. replace (the default) overwrites it. The others matter when several nodes write in the same superstep, or when a loop collects results:

Reducer	Effect
`append`	Adds the new item (or items) to the list.
`merge`	Merges the new keys into the object.
`add`	Adds the number.

Parallel branches that write the same field without a reducer fail validation, because LangGraph cannot know which write wins.

References¶

Prompts, Human questions, HTTP Tool URLs and bodies, Retriever queries, Send email fields and MCP arguments are templates. A reference is {{state.field}}, with an optional path and filters:

{{state.order.items.0.sku}}        walk objects by key and lists by index
{{state.name | trim | upper}}      filters apply left to right
{{state.tags | join(", ")}}        filters can take one argument
{{state.note | default("none")}}   a fallback for an empty value

A missing step in a path gives an empty value.

Filter	Result
`upper`, `lower`, `trim`	Text case and whitespace
`json`	The value as JSON
`length`	Length of a list, object or text
`first`, `last`	First or last item of a list
`default("text")`	`text` when the value is empty
`join(", ")`	List items joined by the separator
`truncate(80)`	Text cut to 80 characters

An MCP argument (and a Decision text) that is exactly one reference keeps the value's type, so {{state.items | length}} passes a number.

In the editor¶

References show as tags. Type {{ for state fields, and | inside a reference for filters. Double-click a tag to edit it as text. A field missing from the state, or an unknown filter, is red, and Save lists it. A run refuses an invalid reference with TEMPLATE_REF_INVALID.