# Expression Evaluator – Variable Inspector

> Expression Evaluator – Variable Inspector is a paid API for AI agents from math.openverbs.com, paid per call via x402, $0.004/call, status unknown (last checked 2026-09-14).

Inspects a mathematical or logical expression and extracts the variables contained within it

## Facts

- Endpoint: POST https://math.openverbs.com/v1/variables
- Price: $0.004/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-14
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/expression-evaluator-variable-inspector-22a2c712
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_VHMHNrcTzKEySYUpCz3pU

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability expression-evaluator-variable-inspector-22a2c712 -d '<json body>'
```

Example prompt: Can you inspect the expression '2*x^2 + 3*y - z' and tell me what variables are in it?

## When to prefer this

Use this endpoint when you need to programmatically determine which variables or identifiers are present in a mathematical or logical expression before evaluating it, building a solver, or validating inputs. It is particularly useful in dynamic formula engines, educational tools, or symbolic math pipelines where expressions are user-supplied and the required variable bindings are unknown ahead of time.

## Known failure modes

- Expression string is empty or missing — returns validation error
- Expression exceeds 1000 character limit — rejected with schema error
- Malformed or unparseable expression — may return parse error or empty variable list
- Network or service unavailability — HTTP 5xx error

## How this service works

Parse an expression and return the free variables it references, without evaluating it — so an agent can discover which inputs a formula needs before supplying values. A syntax error is a clean 400.

## Output

A structured response listing all variable names (identifiers) found within the provided expression string, allowing the caller to know exactly which unknowns or parameters the expression depends on.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method",
    "bodyType",
    "body"
   ],
   "properties": {
    "body": {
     "type": "object",
     "required": [
      "expression"
     ],
     "properties": {
      "expression": {
       "type": "string",
       "maxLength": 1000,
       "minLength": 1,
       "description": "The expression to inspect."
      }
     },
     "additionalProperties": false
    },
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "POST"
     ],
     "type": "string"
    },
    "bodyType": {
     "enum": [
      "json",
      "form-data",
      "text"
     ],
     "type": "string"
    }
   },
   "additionalProperties": false
  }
 }
}
```

## More

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