Great question! Short answer: placement strategy is not currently exposed as a first-class field in Prefect 3’s ECS worker template. You can use capacity provider strategy, and you can enforce placement constraints via the task definition. Placement strategy (the RunTask parameter) is only available on EC2, not on Fargate. A few concrete options and examples: 1) Check what the ECS worker supports by default Run this to see the default ECS base job template:

prefect work-pool get-default-base-job-template --type ecs

You’ll see

task_run_request

fields like

cluster

,

overrides

,

taskDefinition

, and

capacityProviderStrategy

, but not

placementStrategy

or

placementConstraints

. Docs: - Prefect ECS guide - Customize job variables - AWS RunTask params: RunTask API - AWS TaskDefinition placement constraints: TaskDefinitionPlacementConstraint 2) If you need EC2-style placement constraints You can bake placement constraints into the task definition (supported by AWS). Add to the work pool’s base job template under

task_definition

or use a pre-created task definition ARN: Example base job template snippet (work pool):

{
  "job_configuration": {
    "task_definition": {
      "family": "my-family",
      "cpu": "512",
      "memory": "1024",
      "containerDefinitions": [
        {
          "name": "{{ container_name }}",
          "image": "{{ image }}"
        }
      ],
      "placementConstraints": [
        {
          "type": "memberOf",
          "expression": "attribute:ecs.instance-type =~ t3.*"
        }
      ]
    },
    "task_run_request": {
      "launchType": "EC2",
      "cluster": "my-cluster"
    }
  }
}

Or reference a pre-created task definition that already has constraints:

"task_run_request": {
  "taskDefinition": "arn:aws:ecs:...:task-definition/my-family:42",
  "launchType": "EC2",
  "cluster": "my-cluster"
}

3) Capacity provider strategy (supported) If your goal is to steer tasks to specific capacity, you can use

capacityProviderStrategy

(works with both EC2 capacity providers and FARGATE/FARGATE_SPOT):

"task_run_request": {
  "cluster": "my-cluster",
  "capacityProviderStrategy": [
    {"capacityProvider": "FARGATE_SPOT", "weight": 1}
  ]
}

4) What about placementStrategy (spread/binpack) on EC2? - Prefect’s default ECS template does not expose

placementStrategy

under

task_run_request

. Even if you add it to job variables, some versions of prefect-aws validate/shape this payload and may reject unknown keys. - If you try to put:

"task_run_request": {
  "launchType": "EC2",
  "cluster": "my-cluster",
  "placementStrategy": [
    {"type": "spread", "field": "attribute:ecs.availability-zone"},
    {"type": "binpack", "field": "cpu"}
  ]
}

and get an “extra fields not permitted” error, that means your version does not pass these through. Workarounds if you need true placementStrategy: - Use capacity providers and/or constraints in the task definition to approximate placement. - If you absolutely must use RunTask placementStrategy, you’d need a small customization (e.g., a custom worker that adds these fields to the boto3 call). See Developing a custom worker. 5) How to apply via deployment job variables You can override the pool’s template at deploy-time using

job_variables

in your `prefect.yaml`:

deployments:
- name: my-ecs-deployment
  work_pool:
    name: my-ecs-pool
  job_variables:
    task_run_request:
      launchType: EC2
      cluster: my-cluster
    task_definition:
      placementConstraints:
        - type: memberOf
          expression: attribute:ecs.instance-type =~ t3.*

Questions to make sure we’re steering you right: - Are you running on Fargate or EC2? (Placement strategies/constraints apply to EC2; Fargate does not support them.) - Do you prefer to reference a pre-existing task definition (with constraints) or let Prefect register one per run? If you share those details, I can provide an exactly-tailored snippet you can paste into your work pool or deployment.

Great question. A few reasons it isn’t in the default ECS worker template: - It’s EC2-only.

placementStrategy

is a RunTask parameter that AWS does not support on Fargate. Since Prefect’s ECS worker supports both Fargate and EC2, the default template sticks to fields that won’t error on Fargate. - Keep the job template small and predictable. Prefect’s ECS worker passes a curated set of RunTask fields (cluster, overrides, taskDefinition, capacityProviderStrategy, etc.). Exposing every RunTask option would bloat the schema and increase validation/compat risk. - Capacity providers are the preferred, broadly applicable control. The template includes

capacityProviderStrategy

(works for EC2 capacity providers and FARGATE/FARGATE_SPOT), which covers many placement needs without EC2-only knobs. - Defaults work for most users. ECS’s scheduler does a reasonable job without explicit placement strategies, so the default avoids adding a field that can fail or confuse in mixed (Fargate/EC2) setups. If you need it on EC2, here are your options today: - Use capacity providers to steer placement:

"task_run_request": {
  "cluster": "my-cluster",
  "capacityProviderStrategy": [
    {"capacityProvider": "EC2ProviderA", "weight": 1},
    {"capacityProvider": "EC2ProviderB", "weight": 2}
  ]
}

- Enforce constraints via the task definition (EC2-only). While

placementStrategy

is RunTask-only, you can still filter with

placementConstraints

inside the task definition:

"task_definition": {
  "family": "my-family",
  "cpu": "512",
  "memory": "1024",
  "containerDefinitions": [
    {"name": "{{ container_name }}", "image": "{{ image }}"}
  ],
  "placementConstraints": [
    {"type": "memberOf", "expression": "attribute:ecs.instance-type =~ t3.*"}
  ]
}

- Try adding

placementStrategy

in your work pool’s base job template (EC2 only). Example:

"task_run_request": {
  "launchType": "EC2",
  "cluster": "my-cluster",
  "placementStrategy": [
    {"type": "spread", "field": "attribute:ecs.availability-zone"},
    {"type": "binpack", "field": "cpu"}
  ]
}

Note: depending on your prefect-aws version, extra keys may be ignored or rejected by the worker’s schema. If it doesn’t pass through, we can open a feature request to add first-class support. Links: - Prefect ECS guide - Customize job variables - Developing a custom worker - AWS RunTask API If you’re on EC2 and specifically need `spread`/`binpack`, want me to check whether your current version will pass

placementStrategy

through if we add it to your base job template? If you share EC2 vs Fargate and your prefect/prefect-aws versions, I can provide an exact template you can drop in.