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Use the fluent .branchParallel() helper on WorkflowBuilder whenever you want to fan out into multiple branch sequences (each with their own then, while, or conditions) while keeping a single linear definition. It creates a synthetic ParallelWorkflowStep that orchestrates the branches, aggregates their outputs, and forwards the result to the next step. 
import { createStep, createWorkflow } from "@ai_kit/core";

const workflow = createWorkflow({
  id: "create-workflow",
})
  .then(prepareEnvironmentStep)
  .branchParallel("prepare-infra", parallel =>
    parallel
      .branch("provisioning", branch =>
        branch
          .then(createClusterStep)
          .then(configureIngressStep),
      )
      .branch("observability", branch =>
        branch
          .then(setupGrafanaStep)
          .then(configureAlertsStep),
      )
      .onError("wait-all"),
  )
  .then(summarizeStep)
  .commit();

​
API surface

  • .branchParallel(id, configure, options?) registers a new parallel block. options lets you provide description, inputSchema, or outputSchema for the synthetic step.
  • configure receives a ParallelWorkflowBuilder instance:
    • .branch(name, build => build.then(step).then(...)) defines each branch sequence. Branches reuse the workflow input and have read-only access to ctx.
    • .aggregate(fn) changes the resulting value. Without it, the block returns { [branchName]: branchOutput }.
    • .onError("fail-fast" | "wait-all") controls how branch failures propagate. Fail-fast aborts siblings immediately; wait-all waits for every branch and surfaces a combined WorkflowExecutionError containing parallelErrors.

​
Constraints & behaviour

  • Nested .branchParallel() blocks are rejected in the current release to keep the graph simple.
  • Human steps are not allowed inside branches. Use .branchParallel() to orchestrate automatic work, then chain a human step afterwards if needed.
  • Branches cannot mutate the workflow runtime context; any stepRuntime.updateCtx() call from a branch throws. Emit events or return outputs instead.
  • Each branch emits the standard step:* events and snapshots decorated with parallelGroupId / parallelBranchId. This makes watchers, telemetry traces, and history dumps easy to filter.

​
Observability tips

  • run.watch() will surface step:start, step:success, step:error, and step:branch events tagged with the parallel identifiers so you can chart branch-level timings.
  • OpenTelemetry spans inherit the same attributes: ai_kit.workflow.step.parallel_group_id and ai_kit.workflow.step.parallel_branch_id.
  • When using "wait-all", inspect the parallelErrors array on the thrown error to list failing branches and bubble up richer status reports.

​
Relation to createParallelStep

  • createParallelStep is a single step composed of substeps that all consume the same input, ideal for chunk-level analytics or when used inside createForEachStep.
  • .branchParallel() works at the workflow-builder level: each branch can hold full sequences (including other builders like while or conditions), and the block returns a single aggregated value to the next workflow step.
  • Keep using createParallelStep for localised concurrent work, and reach for .branchParallel() when you need to fan out entire sections of your workflow graph.