Reduce

The Reduce node aggregates the output of an upstream fanned-out node (a Generate Image, Generate Video, etc. driven by a List/Loop) into a single value.

Without Reduce, the pattern “generate N variants, pick the best, continue” requires custom downstream logic. Reduce closes the loop in one node.

Position in the canvas

List ──▶ Generate Image ──▶ Reduce ──▶ DownstreamNode
        (fanned out N×)     (runs 1×)

Strategies

Pick one strategy in the config panel. Each behaves differently:

Strategy	Use when	Credits
Pick best (LLM judge)	You want Claude Sonnet to rank N candidates against your criteria	3 cr
Concatenate	You want to join all survivors with a separator	0 cr
First non-empty	You want the first non-empty survivor	0 cr
Count	You want the number of survivors	0 cr
Majority vote	You want the most common survivor (ties → first)	0 cr
Merge JSON	You want to deep- or shallow-merge JSON objects	0 cr

Pick best (LLM judge)

Sends survivors to Claude Sonnet with your criteria. Sonnet replies with chosen_index + a one-sentence reason.

Config:

Criteria — what to optimize for. Example: “Pick the sharpest image with no artifacts.”
Input kind — text (default) or image-url (Sonnet sees the images via URL).

Worked example:

Upstream: List of 5 prompts → Generate Image → Reduce(pick-best-llm, criteria=”brightest colors”, inputKind=”image-url”)
Cost: 5 image generations + 3 cr for pick-best = e.g. 5×2 + 3 = 13 cr.

Concatenate, First non-empty, Count, Majority vote, Merge JSON

These are pure functions (0 cr). All strategies first filter empty strings from the dense input array — empty strings are how upstream failures appear in listResults. Count and Concat operate on survivors only, not attempts.

Worked example (count):

Upstream: List of 10 → Generate Image (3 fail) → Reduce(count) returns 7, not 10.

Behavior on failures

If upstream fails on all N iterations (every survivor is empty / whitespace), the strategy decides what happens:

Strategy	All-empty behavior
`concat`	Returns `""` with `summary: "Joined 0 of N inputs"`. No error.
`count`	Returns `0` with `summary: "Counted 0 of N inputs"`. No error.
`first-non-empty`	Fails with HTTP 400 `no_valid_inputs`.
`vote`	Fails with HTTP 400 `no_valid_inputs`.
`merge-json`	Fails with HTTP 400 `no_valid_inputs`.
`pick-best-llm`	Fails with HTTP 400 `no_valid_inputs`.

The error message is "All upstream iterations failed; nothing to reduce." Configure upstream nodes to default to a placeholder if you want the workflow to keep running on empty fan-in.

Output

Single value, type depends on strategy. Downstream nodes can consume it as text (URL for image strategies, JSON string for merge-json, stringified number for count).

Limits (v1)

Single source supported. Multi-source merging happens by concatenation (multiple incoming edges’ results are appended).
No nested fan-out. A Reduce cannot itself drive a new fan-out chain unless downstream uses a Split-Text or List node.
Sequential fan-out. Upstream nodes still run sequentially per item. Parallel fan-out is a separate Phase 2 feature.

Dedup-bypass within a workflow run

Reduce routes opt out of the standard 10-second input-fingerprint dedup guard ({ dedup: false }). This is what protects loop-iteration / retry collisions within ONE workflow run from silently collapsing into a single job — when an upstream fan-out runs Reduce N times in quick succession with identical bodies (same strategy, same inputs), each iteration gets its own Reduce job and its own credit reservation.

(Human-paced re-runs — clicking Run again a minute later — wouldn’t hit the dedup window anyway. The opt-out only matters for fast intra-run repetition.)

Renamed from “Collect”

This node was previously called Collect. The name was changed because “Collect” semantically suggests gathering / aggregating items, but the node actually reduces N values into one — which fits the canonical functional-programming term. Saved workflows referencing the old "collect" type are auto-migrated on load (via migration 151 + a one-line backward-compat shim in the orchestrator); the public API now exposes POST /v1/reduce, MCP tool reduce, and SDK client.reduce.