Fan-out

Fan-out means running the same steps once for every item in a list. For example, a workflow reads ten tickets and then runs an AI step on each ticket. This page explains when fan-out happens and how to collect the results.

When fan-out happens

Fan-out is automatic. You don’t write a keyword for it. It happens when both of these are true:

A step’s output is a JSON list with at least one item.
At least one more step follows.

Anthara then creates one child run for each item. The original run is the parent run. The parent waits until every child has finished.

If the list is empty, the run ends as Completed and the later steps are skipped.

A fan-out workflow

name: Summarize Open Stories
slug: summarize-open-stories
trigger:
  type: manual

steps:
  - name: fetch-stories
    type: read_data
    provider: jira
    connection: Jira
    config:
      query: project = DEMO AND status = "To Do"

  - name: summarize-story
    type: ai_task
    config:
      prompt: |
        You receive ONE Jira story as JSON:
        {{steps.fetch-stories.output}}
        Summarize it in two sentences.

fetch-stories returns a list. Each child run then runs summarize-story for one story.

Inside a child run, {{steps.fetch-stories.output}} is a single item, not the whole list. Write the prompt for one item.

Collect the results with fan_in

To run a step once after all the children finish, mark it with fan_in: wait_all_collect.

  - name: collect
    type: echo
    fan_in: wait_all_collect

  - name: write-digest
    type: ai_task
    config:
      prompt: |
        Here are the summaries as a JSON list:
        {{steps.collect.output}}
        Write a short digest.

With a fan_in step, the work is split like this:

Steps	Run in
Steps between the fan-out step and the `fan_in` step	Each child run
The `fan_in` step and the steps after it	The parent run, once

The fan_in step does not do its own work. Its type and config are ignored. Its output is a JSON list that holds the last output of every child that completed.

What happens when a child fails

Workflow shape	Result
No `fan_in` step	If any child fails, the parent run fails.
With a `fan_in` step	Failed children are left out of the collected list and recorded in the audit log. The parent run continues.

See the child runs

Open the parent run and go to the Steps tab. The fan-out step expands to show Child Runs. Each child has its own steps, logs, and status.

Limits

There is no cap on the number of child runs. A list of 200 items creates 200 child runs, and each AI step starts its own worker. Keep the query in the first step narrow.