CEL Expressions

Data flows between steps through CEL (Common Expression Language) expressions. CEL is a small, fast, non-Turing-complete expression language designed by Google for security and policy evaluation.

Three Namespaces

Where CEL Appears

Conditional execution — the if field on a step:

if: "steps.fetch-data.output.status_code == 200"

The step runs only when this expression evaluates to true. If the expression evaluates to false, the step is skipped.

Template interpolation — double-brace syntax in params:

params:
  url: "{{ inputs.url }}"
  prompt: "Summarize: {{ steps.fetch-data.output.body }}"

Template expressions are evaluated and their results are substituted into the string.

Data Flow Example

Consider this workflow:

inputs:
  url:
    type: string

steps:
  - name: fetch-data
    action: http/request
    params:
      method: GET
      url: "{{ inputs.url }}"

  - name: summarize
    action: ai/completion
    params:
      provider: openai
      model: gpt-4o
      prompt: "Summarize: {{ steps.fetch-data.output.body }}"

The data flows like this:

1. The caller provides url as an input when triggering the workflow.
2. Step fetch-data reads inputs.url and makes an HTTP GET request.
3. The HTTP connector returns output with fields like status_code, headers, and body.
4. Step summarize reads steps.fetch-data.output.body to build its prompt.
5. The AI connector returns the completion result.

Each step can only reference outputs from steps that have completed before it runs. The engine detects these references automatically and treats them as implicit dependencies. When combined with explicit depends_on declarations, this enables parallel execution — see Execution Model.

CEL Syntax Quick Reference

Access step output:

url: "{{ steps['step-name'].output.json.field }}"

Access inputs:

url: "{{ inputs.field_name }}"

Bracket notation is required when step names contain hyphens:

# Correct
if: "steps['get-user'].output.status == 200"

# Incorrect in if expressions (hyphen interpreted as subtraction)
if: "steps.get-user.output.status == 200"

Dot notation works for step names without hyphens:

prompt: "{{ steps.summarize.output.json.summary }}"

Template strings use {{ }} delimiters inside params values.

Type Safety

CEL is a strongly typed language. If you compare values of different types, the expression will fail at evaluation time. For example, inputs.count > "5" fails because you are comparing a number to a string.

Expression Examples

Boolean logic:

if: "inputs.verbose == true && steps.fetch-data.output.status == 200"

String operations:

if: "steps.fetch-data.output.body.contains('error') == false"

Size checks:

if: "steps.summarize.output.key_points.size() > 0"

See the Workflow Reference for the full expression reference.