Serialize to JSON: to_json

The to_json filter converts a Python object (like a dictionary, list, string, or number) into a JSON-formatted string. This is the opposite of from_json, which parses a JSON string back into an object.

This is essential when you need to send structured data to external services. Many integrations like MQTT, REST, and webhooks expect JSON-formatted payloads. For example, you might build a dictionary with sensor data and convert it to JSON for an MQTT publish, or format a payload for a REST API call. The optional parameters let you control the output format: pretty_print adds indentation for readability, sort_keys orders the keys alphabetically, and ensure_ascii escapes non-ASCII characters.

Usage

Here’s how to use this template function. Copy any example and adjust it to your setup.

{{ {"temperature": 21.5, "humidity": 45} | to_json }}

{"temperature":21.5,"humidity":45}

Function signature

The signature is a technical summary of this template function. It shows the name of the function, the values (called parameters) it accepts, and what type of data each parameter expects (for example, a piece of text or a number).

Function parameters that have a = with a value after them are optional. If you leave them out, the default value shown is used automatically.

Function parameters without a = are usually required. A few optional parameters have no default value, so the signature shows them without a = as well. To check whether a parameter is required, look at the parameter list below the signature. Each parameter is marked as Required or Optional.

The most common optional parameter without a default value is default. The default parameter sets the value a function returns when the function can’t produce a result, for example, when float gets text that isn’t a number. If you leave out default, there is no fallback value, so most functions raise an error and the template fails. The parameter list of each function says what happens. If you set default, the function returns your value instead.

round(
    value: Any,
    precision: int = 0,
    method: str = "common",
    default: Any,
) -> float | int | Any

value | to_json(
    ensure_ascii: bool = False,
    pretty_print: bool = False,
    sort_keys: bool = False,
) -> str

Function parameters

The following parameters can be provided to this filter.

Pretty-printed output

Use pretty_print to format the JSON with indentation, making it easier to read in logs or on dashboards.

{{
  {"name": "Living Room", "temperature": 21.5}
  | to_json(pretty_print=true)
}}

{
  "name": "Living Room",
  "temperature": 21.5
}

Good to know

• The output has no spaces between keys and values by default, which is ideal for MQTT or API payloads but less readable. Turn on pretty_print for debugging.
• Values that are not JSON-serializable (like datetime objects) raise an error. Convert them to strings or numbers first.
• Dictionary keys are preserved in their original order unless you set sort_keys=true.

Try it yourself

Ready to test this? Open Settings > Tools > Template, paste the example into the Template editor, and watch the result update on the right. Edit the values to see how the function adapts to your own entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more].

More examples

Real scenarios where this function comes up in automations and templates. Copy any example and adapt it to your setup.

Publish sensor data via MQTT

Build a JSON payload from sensor values and publish it to an MQTT topic.

action:
  - action: mqtt.publish
    data:
      topic: "home/sensors/living_room"
      payload: >
        {{
          {
            "temperature": states("sensor.living_room_temperature") | float,
            "humidity": states("sensor.living_room_humidity") | float,
            "timestamp": now() | as_timestamp | int
          } | to_json
        }}

Send data to a REST API

Format a dictionary as JSON for a RESTful notification or webhook.

action:
  - action: rest_command.send_data
    data:
      payload: >
        {{
          {
            "event": "door_opened",
            "entity": "binary_sensor.front_door",
            "state": states("binary_sensor.front_door")
          } | to_json
        }}

Sort keys for consistent output

Use sort_keys when you need deterministic output, for example when comparing JSON strings.

{{ {"z": 1, "a": 2, "m": 3} | to_json(sort_keys=true) }}

{"a":2,"m":3,"z":1}

Still stuck?

The Home Assistant community is quick to help: join Discord for real-time chat, post on the community forum with your template and expected result, or share on our subreddit /r/homeassistant.

Related template functions

These functions work well alongside this one:

• Parse JSON string: from_json - Parses a JSON string into a Python object.

• Merge dictionaries: combine - Merges multiple dictionaries into one.