n8n expressions: $json vs $input vs $('Node').item — paired-item gotchas and webhook response modes

TL;DR

Use "Immediately" for fire-and-forget, or "Using 'Respond to Webhook' Node" to reply early (for example a quick 202 ack) and keep processing after it. Also use this when configuring how a Webhook node replies to the caller. Use $('Node Name').first().json.FIELD. Replace.item with.first(),.last(), or.all()[INDEX].

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

n8n Expressions: $json vs $input vs $('Node').item, Paired-Item Gotchas and Webhook Response Modes

When to use this

You are writing or debugging an n8n workflow and an expression returns undefined, the wrong row, or an error like Multiple matching items for expression or Info for expression missing from previous node. Also use this when configuring how a Webhook node replies to the caller.

The three reference styles

Expression-capable parameters accept ={{ ... }}. A plain string that does not start with = is literal and is never evaluated.

$json is the immediate predecessor's output, not "your data"

$json sees only what the node directly before this one emitted for this item. After a database node (Postgres insert, Supabase upsert, etc.), the item JSON is replaced by the DB row, fields produced further upstream become undefined.

Broken:

Set node "Set name from signup form":
  full_name = {{ $json.full_name }}

If a Postgres node ran between the signup Webhook node and the Set node, $json.full_name is undefined, the Postgres row has no full_name.

Fixed, reference the producing node explicitly:

full_name = {{ $('Webhook').first().json.full_name }}

Rule: once any transform, DB, HTTP, or aggregation node touches an item, reach back with $('Node Name').first() (literal first item) or .item (paired item) instead of $json.

$('Node').item follows paired-item lineage, and aggregation breaks it

Each item n8n emits carries pairing info (pairedItem) recording which input item produced it. $('Node Name').item uses that lineage, so in a loop it tracks the matching row rather than always grabbing item 0. Use .first() when you genuinely want the literal first item of the named node.

Gotcha 1: Code nodes break the lineage unless you return pairedItem

A Code node that returns brand-new items without pairing info breaks $('Node').item in downstream nodes with Info for expression missing from previous node. Fix it by returning which input item produced each output item:

// Code node, "Run Once for Each Item"
return {
  json: {
    ...$input.item.json,
    total: $input.item.json.price * $input.item.json.qty
  },
  pairedItem: $itemIndex
};

For a one-to-many expansion:

// Code node, "Run Once for All Items"
const out = [];
for (const [index, item] of $input.all().entries()) {
  for (const tag of item.json.tags) {
    out.push({ json: { tag }, pairedItem: index });
  }
}
return out;

Gotcha 2: Aggregations throw Multiple matching items for expression

Summarize, Aggregate, and Merge nodes combine several input items into one output item. After them, $('Upstream').item is ambiguous, so n8n errors with Multiple matching items for expression. Resolve it explicitly:

{{ $('Webhook').first().json.full_name }}        // literal first
{{ $('Webhook').last().json.full_name }}         // literal last
{{ $('Webhook').all()[2].json.full_name }}       // by index

Or reference a node whose output still has one-to-one lineage with the current item.

Gotcha 3: branches change what "current item" means

After an IF or Switch, $json in a downstream node only sees items that took that branch, and $input.all() returns only the items entering this node on this execution path. To compare across branches, use the Merge node to combine streams, or reference the named pre-branch node: {{ $('Set Inputs').first().json.threshold }}.

$input helpers

{{ $input.all().length }}        // how many items entered this node
{{ $input.first().json.id }}     // first item of immediate predecessor
{{ $input.last().json.id }}      // last item
{{ $input.params.operation }}     // the previous node's configured operation
{{ $input.context.noItemsLeft }} // Loop Over Items only: true when the loop is done

$json is NOT available in a Code node running "Run Once for All Items", use $input.all() or $input.first() there. It IS available when running "Run Once for Each Item".

Webhook node: who responds, and with what

The Webhook node's Respond setting decides timing; Response Data decides the body. Both are on the Webhook node itself, not on the workflow.

Respond = Immediately, replies as soon as the Webhook node executes. The body is the response code plus the message "Workflow got started". The rest of the workflow keeps running but the caller never sees its result.

Respond = When Last Node Finishes, holds the HTTP connection open until the workflow completes, then returns the response code plus data from the last executed node. Response Data (visible only for this mode) picks the body shape:

• All Entries: all output items of the last node, as an array.
• First Entry JSON: JSON of the first entry of the last node, as an object.
• First Entry Binary: binary data of the first entry, as a file.
• No Response Body: status code only.

Respond = Using 'Respond to Webhook' Node, replies when the flow reaches a Respond to Webhook node, using that node's configured status code, body, and headers. Full control, including redirects (status 301/302 plus a Location header) and custom error bodies.

Gotchas:

• If the workflow contains a Respond to Webhook node but the Webhook node is NOT set to "Using 'Respond to Webhook' Node", the execution fails with Unused Respond to Webhook node found in the workflow. The mode must match the node.
• Conversely, set to "Using 'Respond to Webhook' Node" with no such node in the flow means the caller hangs until it times out. Only one Respond to Webhook node may execute per run.
• Long synchronous mode: "When Last Node Finishes" keeps the caller's connection open for the whole run. Senders with short timeouts will see a timeout even though the workflow succeeded. Use "Immediately" for fire-and-forget, or "Using 'Respond to Webhook' Node" to reply early (for example a quick 202 ack) and keep processing after it.

Webhook output shape

Items leaving a Webhook trigger node look like this:

{
  "headers": { "content-type": "application/json" },
  "params": {},
  "query": { "campaign": "spring" },
  "body": { "full_name": "Ava", "email_address": "ava" }
}

So the posted payload is at {{ $json.body.full_name }}, not {{ $json.full_name }}. Query string values are at {{ $json.query.campaign }}.

Quick debug checklist

1. undefined on a field that clearly exists upstream → you are reading $json after a node that replaced the item. Use $('Node Name').first().json.FIELD.
2. Info for expression missing from previous node → a Code node upstream returned items without pairedItem. Add it.
3. Multiple matching items for expression → an aggregation or Merge is upstream. Replace .item with .first(), .last(), or .all()[INDEX].
4. Caller gets "Workflow got started" instead of real data → Webhook Respond is set to "Immediately"; switch to "When Last Node Finishes".
5. Caller hangs forever → Webhook expects a Respond to Webhook node that never executes, or the branch containing it did not run.
6. Test an expression safely: drop a Set node after the node in question and read {{ JSON.stringify($json) }} there before wiring the expression into the real node.

Grounded in official n8n documentation and community answers from the n8n team: $json as shorthand for $input.item.json (immediate predecessor only); $('NodeName').item as the linked/paired item with item linking concepts; item-linking error fixes for 'Info for expression missing from previous node' (Code nodes must return pairedItem) and 'Multiple matching items for expression' (Summarize/Aggregate/Merge — use .first()/.last()/.all()[index]); Code-node pairedItem example from n8n team member ria; Webhook node Respond options (Immediately / When Last Node Finishes / Using 'Respond to Webhook' Node) and Response Data options (All Entries / First Entry JSON / First Entry Binary / No Response Body); webhook trigger output shape with body/query/headers/params.