Add return_keys parameter to Send

[nick-youngblut]

Feature request

It would be helpful to filter the returned state from Send to certain state variables (e.g., just messages) versus having to return the entire state for each sub-workflow called by Send.

This merging of the returned values from Send into the parent graph state can lead to errors such as:

• langgraph.errors.InvalidUpdateError: At key 'MY_KEY': Can receive only one value per step. Use an Annotated key to handle multiple values.
• ...and TypeError: can only concatenate list (not "str") to list if MY_KEY: Annotated[List[str], operator.add] is used in the parent graph state
  • For this error, I seem to either have to make MY_KEY a string, and then I will just get a concatenated string (not optimal), or I have to edit all of my sub-workflow code so that MY_KEY is a list of strings instead of just a string

Motivation

While there is probably an existing way to filter the state returned from each Send call, the docs do not describe this.
Including such a filtering parameter directly in Send would make it easy for anyone to quickly filter the returned state from each sub-workflow.