Creating RESTful sensor, but keep getting 'value_json' is undefined

Configuration
1

Trying to create a sensor of the current water level in a local river. I got the following from ChatGPT:

sensor:
  - platform: rest
    name: Mohawk River Stage (NOAA)
    resource: https://api.water.noaa.gov/nwps/v1/gauges/SPRO3/stageflow
    method: GET
    headers:
      Accept: application/json
    value_template: >
      {% set obs = value_json.data.observed %}
      {{ (obs[-1].stage) if obs }}
    unit_of_measurement: 'ft'
    device_class: water
    icon: mdi:river
    scan_interval: 900

I"ve tried several variations, but keep getting ‘value_json’ is undefined in the template editor. Any idea what I’m doing wrong?

2

Two things with your value_template.

  1. Should be value_json.observed.data.
  2. .stage does not exist on the data objects. There is .primary and .secondary. I note that the observation header has information in primaryName and secondaryName. Do you exoect these to ever change. If not just use .primary

Note, you won’t get value_json in template editor. It is special to internals of Home Assistant template engine when an integration passes json, like REST. I test by mocking up an extra line or two. e.g.

{# mock data #}
{% set json = '{
  "observed":  {
    "pedts":  "HGIRG",
    "issuedTime":  "2025-06-29T20:45:00Z",
    "wfo":  "PQR",
    "timeZone":  "PST8PDT",
    "primaryName":  "Stage",
    "primaryUnits":  "ft",
    "secondaryName":  "Flow",
    "secondaryUnits":  "kcfs",
    "data":  [
      {
        "validTime":  "2025-05-30T21:45:00Z",
        "generatedTime":  "2025-05-30T22:10:59Z",
        "primary":  1.43,
        "secondary":  0.11
      }]
    }}' %}
{% set value_json = json | from_json %}
{# rest value_template testing #}
{% set obs = value_json.observed.data %}
{{ (obs[-1].primary) if obs }}
1 Like
3

The code you shared goes into your configuration.yaml file. The template editor is used to test out Jinja templates. Templates are the code that is surrounded by curly brackets.

5

Thanks a lot. I’ve got a lot to learn.

1 Like
6

Thanks. I’ve got a lot to learn.