Expression Syntax

Several JSON Transform operations take an expression (or condition) string. There are two flavours, and they are not interchangeable.

Filter Expressions

filter uses a compact comparison syntax of the form:

<field> <operator> `<value>`

• Supported operators: == (equals), != (not equals).
• The right-hand value must be wrapped in backticks.
• <field> is read directly from each array element, which must be an object.

{ "type": "filter", "options": { "expression": "role != `system`" } }

{ "type": "filter", "options": { "expression": "status == `active`" } }

Numbers, booleans, and null are stringified before comparison, so count == \5`andenabled == `true`` both work.

Heads up: other JMESPath features (&&, ||, function calls, nested paths) are not supported by filter. Use replaceMatching (and accept the cost of a JMESPath evaluation per element) when you need them.

JMESPath Context for map, find, findIndex, replaceMatching

These operations evaluate a JMESPath selector against each array element. Inside the expression, the current element is exposed as both item and @, so you can write either:

{ "type": "find", "options": { "expression": "item.role == `assistant`" } }
{ "type": "find", "options": { "expression": "@.role == `assistant`" } }

map projects each element through the expression:

{ "type": "map", "options": { "expression": "content" } }

For an input like:

[
  { "role": "user", "content": "hi" },
  { "role": "assistant", "content": "hello" }
]

…the result is ["hi", "hello"].

replaceMatching couples a condition (JMESPath) with a with value (selector-evaluated) and a mode ("all" or "first"):

{
  "type": "replaceMatching",
  "options": {
    "condition": "item.role == `system`",
    "with": [{ "role": "system", "content": "${{ prompts.system_prompt }}" }],
    "mode": "first"
  }
}

Common Pitfalls

• Backticks are mandatory for literal values (role == `system`). Quoting with "system" will not parse.
• filter cannot combine conditions. To express AND/OR, chain multiple filter operations or switch to replaceMatching with a JMESPath condition.
• The current element's bindings are item and @, not this, $, or the field name.
• For find/findIndex, an expression that resolves to anything other than Bool(true) is treated as a non-match — so the expression must return a boolean.