Use this skill to write, debug, evaluate, and validate FEEL (Friendly Enough Expression Language) expressions for Camunda 8 — the expression language Zeebe uses in BPMN, DMN, and Camunda Forms.
Use for: gateway conditions and conditional sequence flows; service-task input/output mappings; timer durations and cycles (ISO 8601 PT.../R...); DMN input/output entries; Camunda Form validation rules and conditional visibility; connector result and error expressions; list filters, projections, and quantifiers; date and duration arithmetic; type coercion (number-to-string); null-safe access; debugging FEEL_RESOLUTION_ERROR or "Can't add 'N' to ..." warnings.
Do not use for: writing the BPMN XML around expressions (use camunda-bpmn) or designing form structure (use camunda-forms).
**Utility skill** — FEEL is reused inside BPMN, DMN, forms, and connector configuration. Covers c8ctl feel evaluate for validating expressions.
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Use this skill to write, debug, evaluate, and validate FEEL (Friendly Enough Expression Language) expressions for Camunda 8 — the expression language Zeebe uses in BPMN, DMN, and Camunda Forms.
Use for: gateway conditions and conditional sequence flows; service-task input/output mappings; timer durations and cycles (ISO 8601 PT.../R...); DMN input/output entries; Camunda Form validation rules and conditional visibility; connector result and error expressions; list filters, projections, and quantifiers; date and duration arithmetic; type coercion (number-to-string); null-safe access; debugging FEEL_RESOLUTION_ERROR or "Can't add 'N' to ..." warnings.
Do not use for: writing the BPMN XML around expressions (use camunda-bpmn) or designing form structure (use camunda-forms).
**Utility skill** — FEEL is reused inside BPMN, DMN, forms, and connector configuration. Covers c8ctl feel evaluate for validating expressions.
Camunda FEEL Expressions
Write, debug, and evaluate FEEL expressions used in Camunda 8 BPMN processes, DMN decisions, and forms.
To validate and debug FEEL expressions, use c8ctl feel evaluate. By default this runs against the configured cluster's Scala FEEL engine — the same engine that Zeebe uses at runtime, so results match production behavior exactly.
# Simple expression
c8ctl feel evaluate '1 + 2'# Expression with individual variables (leading = optional)
c8ctl feel evaluate '=amount * 1.15' --var amount=100
# Multiple variables
c8ctl feel evaluate 'a + b' --var a=1 --var b=2
# JSON values for complex types
c8ctl feel evaluate 'sum(items)' --var 'items=[1,2,3]'# Bulk variables as a single JSON object
c8ctl feel evaluate 'orderTotal > 1000 and customer.tier = "premium"' \
--vars '{"orderTotal": 1500, "customer": {"tier": "premium"}}'# Dot-path nesting on the CLI
c8ctl feel evaluate 'customer.name' --var customer.name=Alice
Debugging workflow:
Write the expression
Identify the expected variable context
Evaluate via c8ctl feel evaluate to validate against the cluster engine
If evaluation fails, fix based on error message and retry
Offline evaluation (--engine local)
c8ctl feel evaluate --engine local evaluates expressions locally using the feelin JavaScript engine — useful when no cluster is available. Use only when explicitly requested or when no cluster is reachable AND the user has confirmed the fallback. Never silently fall back.
feelin behaves DIFFERENTLY from the Scala FEEL engine that Zeebe runs in production. Subtle differences in type coercion, function support, and date/time handling can cause an expression that passes locally to fail in the cluster (and vice versa). Always re-validate against the cluster before relying on a result obtained with --engine local.
c8ctl feel evaluate '=amount * 1.15' --var amount=100 --engine local
Concrete divergence: today() returns a different type. On the cluster engine, today() returns a date (e.g. 2026-05-12). On --engine local (feelin), it returns a date-time at midnight in the local timezone (e.g. 2026-05-12T00:00:00.000+02:00). This breaks downstream comparisons:
The string() wrapper is required, not stylistic. FEEL does not auto-coerce types in arithmetic — "prefix-" + userId (where userId is a number) silently evaluates to null with a Can't add 'N' to '"prefix-"' warning, not an error. See references/common-patterns.md § Type Coercion Pitfalls for the full rule and debugging tip.
=if response.statusCode >= 400 then bpmnError("HTTP_ERROR", string(response.statusCode)) else null
Timer duration (FEEL required):
="PT" + string(delayHours) + "H"
Null-safe access:
=if customer != null then customer.name else "Unknown"
Troubleshooting
FEEL_RESOLUTION_ERROR — a variable referenced in the expression doesn't exist in the variable context at evaluation time. Run c8ctl feel evaluate '<expr>' --vars '<json>' with the variables you expect at runtime to confirm.
Expression returns null unexpectedly — typical causes: type mismatch in + concatenation (see string() rule above); 0-based indexing (use events[1], not events[0]); calling a non-existent helper (first() doesn't exist); missing commas between context entries. c8ctl feel evaluate prints an actual null result as <null> in text mode to distinguish it from the FEEL string "null" (which still prints as null).
References
For detailed reference material, read from references/:
function-reference.md — complete list of built-in FEEL functions (string, numeric, list, context, date/time, boolean)
common-patterns.md — date arithmetic, list filtering, multi-entry context patterns, fromAi() for agentic AI
