Short answer: wrap the parameter value as a Jinja placeholder object:

parameters:
  output_uri:
    __prefect_kind: jinja
    template: "{{ event.payload.output_uri }}"

Details and examples (Prefect 3.x): - In a deployment’s event trigger inside prefect.yaml

deployments:
  - name: my-deployment
    entrypoint: flows.py:my_flow
    work_pool:
      name: default
    triggers:
      - type: event
        enabled: true
        match:
          prefect.resource.id: my.app.resource
        expect:
          - my.event.type
        parameters:
          output_uri:
            __prefect_kind: jinja
            template: "{{ event.payload.output_uri }}"
          # static parameter (no templating)
          mode: "fast"

- If you are defining a top-level automation with a RunDeployment action, the same parameter wrapper goes under the action’s parameters:

automations:
  - name: run-on-event
    trigger:
      type: event
      match:
        prefect.resource.id: my.app.resource
      expect:
        - my.event.type
    actions:
      - type: run-deployment
        deployment: "my-flow/my-deployment"
        parameters:
          output_uri:
            __prefect_kind: jinja
            template: "{{ event.payload.output_uri }}"

Notes: - Always include

__prefect_kind: jinja

with a

template

key; without it, Prefect treats the value as a literal. - If you want the result to be a structured object (e.g., pass the entire payload as a dict), use a JSON wrapper so the rendered template is parsed back into a Python object:

parameters:
  full_payload:
    __prefect_kind: json
    value:
      __prefect_kind: jinja
      template: "{{ event.payload | tojson }}"

If you share a snippet of your prefect.yaml, I can sanity-check the placement and syntax.

Your YAML shape for templated parameters is correct, but there are a few common gotchas that can make it look like “it didn’t work.” What you wrote (explicit Jinja object) is valid:

parameters:
  grib_file:
    __prefect_kind: jinja
    template: "{{ event.payload.output_uri }}"

You can also use the simpler string form (also supported):

parameters:
  grib_file: "{{ event.payload.output_uri }}"

If it’s still not working, check these points: - Verify the event actually has payload.output_uri - Add a temporary debug parameter to inspect what the automation sees:

parameters:
      grib_file: "{{ event.payload.output_uri }}"
      _debug_event:
        __prefect_kind: jinja
        template: "{{ event | tojson }}"

Then look at the automation firing in the UI and confirm the payload shape. - Ensure your match selector is correct - Flow run resource IDs look like

prefect.flow-run.<UUID>

. If you want to match flow-run events broadly, use either:

match:
      prefect.resource.role: "flow-run"

or

match:
      prefect.resource.id: "prefect.flow-run.*"

Using

flow-run.*

without the

prefect.

prefix typically won’t match. - Confirm the event type and payload source -

maps.decompress.completed

must be emitted with

payload={"output_uri": ...}

. If the event was not emitted with that payload key, the template will render empty/undefined. - Make sure your flow parameter name matches exactly - Your flow must accept a

grib_file

parameter; otherwise the parameter may be ignored or cause an error. - Re-deploy after changing prefect.yaml - Edit →

prefect deploy

again so the updated trigger and parameters are applied. - You can also try the “JSON wrapper” if you need a dict object (not needed for a single string, but useful for debugging):

parameters:
    full_payload:
      __prefect_kind: json
      value:
        __prefect_kind: jinja
        template: "{{ event.payload | tojson }}"

If you can share: - a sample event (or the

_debug_event

output), - your exact

match

selector, - and the flow signature, I can pinpoint why the value isn’t rendering and suggest the minimal change.