Error message reference
What you might see, what it means, and what to do. Grouped by where the error surfaces.
In a node's output / inspector
[EXPR-ERROR: Node "X" not found. Available: [A, B, C]]
Meaning: An expression references a node name that doesn't exist (or hasn't run in this execution).
Fix: Verify the display name matches exactly (case-sensitive). Use the Data Mapper to insert correctly. See Debugging expressions.
[EXPR-ERROR: unknown reference "x.output.y" — expected $node["Name"], $trigger, $workflow, $input, or $env. Did you mean $node["x"].output.y ?]
Meaning: You used the shorthand form instead of $node["…"].
Fix: Wrap the name: {{ $node["x"].output.y }}.
[EXPR-ERROR: <other reason>]
Meaning: Runtime exception during expression evaluation, usually a type error or null navigation.
Fix: Read the reason. Add a Set node upstream to debug what the actual value looks like, then add a fallback / null-check.
HTTP node
HTTP 401 Unauthorized
Meaning: Credential is rejected, expired, wrong scope, or not active.
Fix: Test the credential under Settings → Credentials. Re-authenticate or rotate.
HTTP 403 Forbidden
Meaning: Credential is valid but lacks permission for this resource.
Fix: Verify the credential's scopes match what the API needs. See the connector's page.
HTTP 404 Not Found
Meaning: The URL is wrong, or the resource doesn't exist.
Fix: Check the URL. Don't forget https://. Test the URL with curl or the browser.
HTTP 429 Too Many Requests
Meaning: You're calling the service faster than its rate limit allows.
Fix: Add Delay between calls, or wrap with the API Pagination & Rate Limit node. Lower workflow concurrency.
HTTP 5xx
Meaning: The upstream service had a problem on its end.
Fix: Add retries (1–3) with exponential backoff. If it's persistent, raise with the service.
TimeoutError
Meaning: The node didn't get a response within its timeout.
Fix: Increase the node's timeout. Check if the URL is reachable. Consider whether you've hit a network egress issue.
NetworkError / ConnectionRefused
Meaning: Couldn't reach the URL at all.
Fix: Check DNS, firewalls, the URL itself. For self-hosted Flero, check the worker's network egress.
Connector nodes
credential_not_found
Meaning: The credential ID referenced by the node has been deleted.
Fix: Select a different credential in the inspector.
credential_revoked
Meaning: The credential exists but has been marked revoked.
Fix: Un-revoke it in Settings → Credentials, or pick a different one.
<service>_auth_error
Meaning: Service-specific, usually a token problem.
Fix: Re-authenticate the credential (OAuth) or rotate (API key).
<service>_rate_limit
Meaning: Per-service rate limit hit.
Fix: Same as HTTP 429, backoff, lower concurrency.
<service>_validation_error: <field>
Meaning: The service rejected your payload because of a specific field.
Fix: Read the field name. Often the value is the wrong shape or missing.
AI nodes
model_not_found
Meaning: The model name you specified doesn't exist (or your credential doesn't have access).
Fix: Check the model name in the provider's docs. Update the AI Agent's Model field.
context_length_exceeded
Meaning: Your prompt + history exceed the model's context window.
Fix: Lower maxMemoryMessages. Truncate the prompt with a Text Processing node. Switch to a larger-context model.
token_budget_exceeded
Meaning: A Token Cost Guard node hit its budget.
Fix: Raise the budget, or fix the workflow's prompt usage so it doesn't burn so many tokens.
tool_call_failed
Meaning: A function-calling tool the AI invoked failed.
Fix: Read the underlying error from the tool's response. Often a downstream HTTP issue.
Workflow-level
workflow_timeout
Meaning: Workflow exceeded the configured execution timeout.
Fix: Identify the slow node and optimise / parallelise. Or raise the timeout in workflow settings.
concurrency_limit
Meaning: Too many simultaneous runs of this workflow; the new run is queued.
Fix: Wait. Or raise the workflow's concurrency limit. Or optimise the slow runs.
worker_unavailable
Meaning: No worker has picked up the run after a long wait.
Fix: Check worker health (admin). For self-hosted, scale workers.
node_disabled
Meaning: A required node has been marked disabled.
Fix: Re-enable it (right-click → Enable) or remove the connection.
Webhook trigger
410 Gone
Workflow is draft / inactive. Set to active.
401 Unauthorized
Webhook secret mismatch. Verify the X-Webhook-Secret header on the caller matches the trigger's secret.
404 Not Found
The trigger has been deleted or the workflow URL changed.
Approval
approval_timeout
The Approval node's timeout fired with no response. Configured action (reject, approve, escalate, keep waiting) takes effect.
approver_not_found
A user listed as an approver has been removed from the workspace.
Fix: Update the approver list and re-trigger the workflow.
API
401 Unauthorized (on the Flero API)
API key invalid, revoked, or expired.
403 Forbidden
API key lacks the required scope for the endpoint, or this workflow is outside the key's allow-list.
429 Too Many Requests
Flero's own rate limit. Slow down or contact support to raise the limit.
How to report something we missed
If an error isn't in this list, file a bug with the exact text of the error (copy-paste, don't paraphrase) and the execution ID. We add new error messages here as they're identified.
Related
Found something out of date? This page lives in the Flero docs content set.