Parallel Steps

Run independent steps concurrently with .parallel(). Each branch is a named handler that runs at the same time as its siblings; the next step receives a merged record of all branch outputs as prev.

const pipeline = createWorkflow({ name: 'pipeline', input: z.object({ url: z.string() }) })
  .step('fetch', async ({ input }) => ({ body: await fetchPage(input.url) }))
  .parallel({
    summary: async ({ prev }) => ({ text: await summarize(prev.body) }),
    keywords: async ({ prev }) => ({ tags: await extractKeywords(prev.body) }),
    images: async ({ prev }) => ({ urls: await extractImages(prev.body) }),
  })
  .step('save', async ({ prev }) => {
    // prev is fully typed:
    // { summary: { text }, keywords: { tags }, images: { urls } }
    await save(prev.summary.text, prev.keywords.tags, prev.images.urls)
  })

Per-branch retry and timeout

Branches accept the same { retry, timeoutMs, handler } config form as .step(), so each branch can have its own policy:

.parallel({
  flaky: {
    retry: { maxAttempts: 3, backoff: 'exponential', initialDelayMs: 100 },
    handler: async () => await callFlakyApi(),
  },
  stable: async () => await callStableApi(),
})

Semantics

• Fail-fast. When one branch fails (after exhausting its own retries), siblings receive signal.abort() and the run is marked failed. onRunFailed and onFailure report the branch that actually caused the failure, not a sibling aborted by the propagation.
• Crash recovery is per-branch. If the engine crashes after some branches persisted their results, recovery skips those and re-runs only the missing ones. Side effects in already-completed branches do not fire twice.
• No complete() inside a branch. Calling complete() from a parallel branch throws ParallelCompleteError — early completion is only meaningful in sequential context.
• Each branch must be idempotent. Like sequential steps, a branch may run multiple times across crash recoveries before its result is persisted.
• steps is a frozen snapshot. All sibling branches see the same steps view, taken before the group started; they cannot observe each other's outputs mid-flight.

Naming

Branch names share the same namespace as .step() names — a duplicate anywhere in the workflow throws DuplicateStepError. In the next step's prev and in steps, each branch appears under its own key.