Building workflows

Node Template Language

The Node Template Language — NTL — is how data moves between nodes. A small syntax lets one node read another's output, build response bodies, and inject credentials, without hardcoding anything.

The syntax

An expression is a keyword and an operand inside double curly braces:

{{POUT response.articles[0].publisher}}

Reading that left to right:

  • {{ }} — the start and end of the expression.
  • POUT — the keyword. Here, "previous node output".
  • response.articles[0].publisher — the operand: the path to the value you want.

So this node will use the publisher field of the first item in the articles array the previous node returned.

You rarely need to type this. Click a node, open its Input section, and drag a key from the connected node's Response JSON Keys into the field. Flux writes the expression for you — point, drag, drop.
Drag a key from the previous node output into any input field.

The six keywords

1. IN

{{IN operand}}

Injects a value from the current node's own input. Use it when designing a response template — the body of an API Action node's reply, for instance — where you need to echo something that was handed to this node.

2. POUT

{{POUT operand}}

Injects a value from the previous node's output. This is the workhorse — it is how data travels down the chain. An API node's result into a GPT node's prompt, a model's answer into a chat response.

3. RESPONSE

{{RESPONSE operand}}

References the current node's own final response, after it has executed. Used to shape what a node passes on — for example, taking a GPT node's execution result and placing it into that same node's output.

4. RANDOM

{{RANDOM SESSION | STRING | NUMBER}}
OperandProduces
SESSIONA random session identifier
STRINGA random alphanumeric string
NUMBERA random numeric value

Useful for correlation IDs, idempotency keys, and generating a session identifier when one is not supplied by the trigger.

5. DESC

{{DESC 'The topic to search news for'}}

Used in tool nodes to describe a parameter. This is not a value — it is an instruction to the model about what to put here. When a GPT or Gemini node decides to call the tool, it reads these descriptions and fills the fields in itself.

DESC is the single biggest lever on whether an agent uses its tools well. A vague description — 'the input' — produces vague tool calls. Say what the value is, what format it takes, and what it is for.

6. CREDENTIAL

{{CREDENTIAL operand}}
OperandMeans
DEFAULT Flux's own built-in API key for that provider. Applied automatically when you use the built-in keys, and the usage is billed to your project
an id One of your own credentials from the credential store, referenced by its id
Do not write CREDENTIAL by hand. It is filled in for you when you pick a credential from the dropdown on a supported node. Typing it manually causes a compilation exception, which you will see in the workflow logs.

Putting it together

A news agent's API tool might carry a URL built from all three of the common keywords: 

https://api.example.com/news
  ?q={{DESC 'The topic the user asked about'}}
  &key={{CREDENTIAL DEFAULT}}
  &ref={{RANDOM STRING}}

The model supplies q, Flux injects the key, and each call carries a unique reference.

When an expression does not resolve

It becomes empty rather than erroring, and the node runs anyway. That is why an AI node can return a confident answer to nothing at all. Check the compiled log, which shows each expression as it actually resolved on that run — the mismatch is usually a key path that changed shape.