n8n-connections

n8n Connections

n8n's SDK has a small connection grammar. Two of its shapes silently produce broken workflows that pass validation. This skill is mostly about not falling into those traps.

The non-negotiable: the .to() trap

.to() must go inside .add(), not after.

.add(node.output(0)).to(target)      // ❌ connection silently dropped
.add(node.output(0).to(target))      // ✅

validate_workflow does not catch this. The workflow validates, publishes, and runs without the wire. The bug looks identical to a misconfigured or "not firing" node.

If you've written .add(...).to(...) outside the parens, you have a bug. No exceptions.

The universal connection pattern

One shape covers IF/Switch branches, error outputs, merge inputs, and any generic multi-IO:

.add(source.output(n).to(target))

• source.output(n): pick the output (0-indexed)
• .to(target): pick the target (default input 0)
• .to(target.input(m)): pick a specific input on the target (0-indexed)

Call .add() once per wire. To fan out, repeat .add().

Decision tree

Wiring a connection?
├── Linear (one source → one target, single output, single input)?
│   └── .add(source).to(target). The simple case; .to() outside is fine here
│       because there's no .output(n) selector inside .add()
│
├── Selector involved (.output(n) or composite handlers)?
│   └── .to() MUST go inside .add(). See "the trap" above
│
├── Targeting a specific input slot on a multi-input node (Merge)?
│   └── .add(source.output(n).to(target.input(m)))
│       AND check useDataOfInput. See references/MERGE_INDEX_RULES.md
│
├── Error branch?
│   └── .add(node.output(1).to(handler))
│       AND set onError: 'continueErrorOutput' on the node config.
│       See references/ERROR_OUTPUTS.md
│
└── Fan-out (one source → many targets) or fan-in (many sources → one target)?
    └── See references/FAN_OUT_FAN_IN.md

Composite handlers (.onTrue, .onFalse, .onCase, .onError)

The SDK provides convenience handlers on IF, Switch, and any node with an error output:

.add(ifNode.onTrue(targetA))     // same as .add(ifNode.output(0).to(targetA))
.add(ifNode.onFalse(targetB))    // same as .add(ifNode.output(1).to(targetB))
.add(sw.onCase(2, target))       // same as .add(sw.output(2).to(target))
.add(node.onError(handler))      // same as .add(node.output(1).to(handler))

These compose with .output(n) calls without conflict.

.add(ifNode.onTrue(targetA))
.add(ifNode.output(0).to(targetB))
// Result: BOTH targetA and targetB on IF's main[0]. Composite + .output(n) merge.

The only shape to avoid is the .add(selector).to(target) trap above. That's always wrong, regardless of whether a composite handler ran earlier.

After every create or update: verify

validate_workflow reports valid even when wires are missing. After create_workflow_from_code or update_workflow, pull with get_workflow_details and check the connections object:

• Each main[i] has the expected set of targets (fan-outs preserved, not collapsed).
• Merge inputs land on the right indices, and useDataOfInput matches the wiring.
• Error-output nodes have onError: 'continueErrorOutput' AND main[1] wired to a handler.

If any check fails, the workflow is broken despite passing validation. Fix and re-update.

See references/VERIFICATION.md for the full post-create checklist.

Reference files

Read the file that matches the situation. Don't read all of them:

File	Read when
references/FAN_OUT_FAN_IN.md	One source → many targets, or many sources → one target
references/MERGE_INDEX_RULES.md	Wiring a Merge node, or you see useDataOfInput in node config
references/ERROR_OUTPUTS.md	Adding error handling on an individual node (not error workflow, that's n8n-error-handling)
references/VERIFICATION.md	Just created or updated a workflow with non-trivial connections

Anti-patterns

Anti-pattern	What goes wrong	Fix
.add(node.output(0)).to(target)	Wire silently dropped, validation passes	Move .to() inside .add()
Mixing useDataOfInput: "2" with .input(2)	Off-by-one, wire feeds the wrong input	Use .input(N - 1) when useDataOfInput: "N". See MERGE_INDEX_RULES.md
Error branch wired without onError: 'continueErrorOutput'	Branch is unreachable, and node fails the whole workflow on error	Set onError: 'continueErrorOutput' on the node config
Skipping get_workflow_details after create	Silently broken workflows ship	Always pull and inspect after create/update
Reading all four reference files before wiring one connection	Wasted context	Read only the file matching the situation