czlonkowski/n8n-skills/n8n-error-handling

n8n Error Handling

By default, when an n8n node throws, the whole workflow halts. For an interactive run you're watching, that's fine — you see the red node and fix it. For anything unattended (a webhook API, a cron job, a queue worker, an agent tool), it's the wrong default: the caller gets a timeout or an empty 500, the operator gets no alert, and the symptom is "the integration just stopped working" with no log and no clue.

This skill is about making failures loud, structured, and recoverable — and, best case, self-healing so transient blips never reach a human at all.

The two ideas that prevent most silent failures:

• Per-node error outputs — a node's failure routes down a second output you control, instead of killing the run.
• A workflow-level error workflow — a catch-all that fires for anything that escapes per-node handling (timeouts, crashes between nodes, unwired failures).

When you actually need this

The dividing line: if anyone other than you sees the output — a downstream system, an end user, an on-call engineer — the failure has to be handled, not swallowed. If you're the only watcher and the cost of failure is "I notice and re-run", looser is fine.

The #1 silent trap: per-node error output is a TWO-step setup

This is the single most common way an n8n workflow "handles" errors while actually swallowing them. Routing a node's failure to a handler takes two changes, and doing only one looks complete but misbehaves:

1. Set `onError: "continueErrorOutput"` on the node. This is what creates the second output. Without it, main[1] doesn't exist no matter what you wire.
2. Wire that error output (connections.<node>.main[1], i.e. sourceIndex: 1) to a real handler. Without a target, the error data is emitted into the void.

Get one without the other and you hit a failure mode:

Doing both with n8n_update_partial_workflow

// 1) Turn on the error output (creates main[1])
{ type: "updateNode", nodeName: "HTTP Request",
  changes: { onError: "continueErrorOutput" } }
// 2) Wire the error output to a handler. sourceIndex: 1 = the error output.
{ type: "addConnection",
  source: "HTTP Request",
  target: "Handle Error",
  sourceIndex: 1 }

sourceIndex: 0 is the success path, sourceIndex: 1 is the error path. (For IF nodes the aliases branch: "true"/"false" map to index 0/1; for a generic fallible node, use the explicit sourceIndex: 1.)

Then verify. This trap doesn't surface in validate_workflow — a half-wired error output validates clean. Pull the workflow with n8n_get_workflow and confirm both halves:

• The node's onError is "continueErrorOutput".
• connections["HTTP Request"].main[1] contains your handler.

Valid onError values:

Full failure-mode catalog, fan-in/fan-out shapes, and verification: NODE_ERROR_OUTPUTS.md.

Failures the error output never sees

A correctly wired error output still misses whole classes of failure. Verified on n8n 2.38.5:

So an error branch alone can't guard these:

• Guard the data, not just the node. After a transform whose output matters, check that the

field you produced exists (IF/Filter on {{ $json.total !== undefined && $json.total !== null }}) and send the miss down the error path yourself.

• Test-run and read node statuses and output values. The Python cases show a red node inside a

green execution. The expression cases show nothing but nulls.

• Root causes and fixes: n8n-expression-syntax (Debugging) and n8n-code-python (Errors and onError).

Self-healing first: retryOnFail before you wire error paths

Before you build error branches, absorb the transient failures so they never reach those branches. On any node that calls a network service — HTTP Request, comms (Gmail/Slack/Discord), databases, AI nodes, third-party integrations — set node-level retry:

{ type: "updateNode", nodeName: "HTTP Request",
  changes: {
    retryOnFail: true,
    maxTries: 3,
    waitBetweenTries: 5000   // ms
  } }

Why this comes first: a 429 or a brief upstream hiccup will retry and usually succeed on its own. The error output then fires only on real, persistent failures — so your 5xx responses and on-call alerts reflect actual problems instead of noise.

Engine limits to know: retry fires on any error (there's no per-status-code filter), maxTries caps at 5, and waitBetweenTries caps at 5000ms — so 5000 is both the max and a sensible default. See n8n-node-configuration (NODE_FAMILY_GOTCHAS.md) for node-specific notes.

API workflows: the canonical shape

A webhook-triggered workflow that responds to its caller has one rule that overrides everything else: no hanging branches. Every path — success and every error — must end at a Respond to Webhook, or the caller sits there until it times out.

Webhook (responseMode: "responseNode")
  ├── validate input → process → Respond (200, body)
  └── (any fallible node's error output → sourceIndex 1)
            → Respond (4xx/5xx, structured error body)
            → optional: log full error privately / notify

Three things make this work:

1. Fan-in to one error responder. Many fallible nodes can route their main[1] to a single Respond node. Keeps the graph readable.
2. Validation failures (4xx) are checked *upstream*, not via error outputs. A missing field isn't a node crashing — it's an expected outcome with a known response. Branch on it with IF/Switch (or the per-field validator below) and return 400/401/403/404 directly. Error outputs are for unexpected failures (5xx).
3. `options.responseCode` defaults to 200 — even on error branches. This is its own silent trap (see RESPONSE_SHAPES.md and n8n-node-configuration NODE_FAMILY_GOTCHAS.md): an error branch that returns 200 with an error body looks like success to the caller's HTTP client, so their error handling never fires. Set responseCode explicitly on every Respond node.

Input validation: conditions on the canvas

Put the checks in one IF node: one boolean condition per rule (is true), combinator and. True → your logic (200); false → a 400 Respond with a static validation_error body that echoes the expected schema, so the caller can self-correct. When the caller needs per-field messages (an LLM client, a public API), replace the IF with one Code node that loops over a schema list and returns { valid, validationError, details, requiredSchema }, with the note Why Code: IF considered — per-field messages need a loop over the schema. Both patterns, the constraint cookbook and the escaping rules are in API_WORKFLOWS.md.

Response shapes: map cause → status code

A 5xx with text/plain "Internal Server Error" is technically an error response and practically useless. And not every failure is a 5xx. Match the status code to *why* the request failed, because the caller branches on it: their monitoring alerts on 5xx (your fault) but not 4xx (their fault), and 5xx suggests "retry" while 4xx suggests "don't".

The common mistake: wiring everything — including bad input — to one Respond that returns 500 internal_error. Now the caller can't tell their bug from your outage, and your error rates can't separate real incidents from client noise.

So there are two distinct flows: 4xx is decided before the work (IF/Switch + dedicated Respond), 5xx comes out of error outputs ("we tried, it broke").

Route by cause with a Switch; one static Respond per status. Picking a status code from the error is a routing job. A Switch on the error message classifies it, and each output goes to a Respond with a fixed options.responseCode and a fixed body:

error outputs (main[1]) ─→ Switch "Classify error" (on $json.error.message)
                             ├─ contains "INVALID_ID"     → Respond 400 validation_error
                             ├─ contains "429"            → Respond 429 rate_limit_exceeded
                             ├─ contains "timeout"        → Respond 504 upstream_timeout
                             └─ fallback (extra output)   → Respond 500 internal_error

Every status is visible on the canvas, and adding one is adding a rule. Set the Switch's options.fallbackOutput to extra, or an unmatched error is dropped and the caller hangs. A single Respond with a computed code is fine for exactly two outcomes, as one short ternary in its Response Code field. The Switch JSON is in API_WORKFLOWS.md.