# PennyRail JSON Key Lister

> PennyRail JSON Key Lister is a paid API for AI agents from pennyrail.vercel.app, paid per call via x402, $0.001/call, status unknown (last checked 2026-09-14).

Extracts and returns a flat list of all top-level keys present in a given JSON object

## Facts

- Endpoint: POST https://pennyrail.vercel.app/api/p/nano/json.keys--list-json-keys
- Price: $0.001/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-14
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/pennyrail-json-key-lister-7a26e481
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_m8S-sqoUFTcZHR2CQmnI6

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 pennyrail-json-key-lister-7a26e481 -d '<json body>'
```

Example prompt: Can you list all the top-level keys in this JSON object for me: {"name": "Alice", "age": 30, "roles": ["admin"], "active": true}?

## When to prefer this

Choose this endpoint when you need a quick, paid, machine-readable enumeration of top-level JSON keys without setting up any local parsing logic. It is useful in agent pipelines where the JSON structure of a dynamic or unknown payload needs to be discovered programmatically before downstream processing. Prefer it over custom code when operating in a tool-calling context where invoking a microservice is more convenient than running code.

## Known failure modes

- Missing or malformed 'input' field returns an error — input must be a JSON object, not a scalar or array
- Empty object input returns an empty key list rather than an error
- Non-object values nested inside the input are not recursively enumerated — only top-level keys are returned
- Payment failure (402) if USDC balance or x402 authorization is insufficient
- Vercel cold-start latency may cause occasional slow responses on first call

## How this service works

Machine-readable settlement service

## Output

A JSON object containing a list of all top-level key names extracted from the provided input object. The response is machine-readable and enumerates each property name present at the root level of the submitted JSON.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object"
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "object",
 "additionalProperties": true
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/pennyrail-json-key-lister-7a26e481/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from pennyrail.vercel.app](https://www.zero.xyz/host/pennyrail.vercel.app/llms.txt)