params

A workflow that hardcodes the same env name, image tag, or retry count across several jobs and steps drifts the moment one copy changes. params declares those values once as typed, validated compile-time inputs; reference scalars with {{ params.x }} and objects/lists with {{ toJSON(params.x) }}. The values are resolved by actio build and baked into the generated YAML:

name: Deploy
on: [push]
params:
  env:
    type: enum
    values: [dev, staging, prod]
    default: staging
  retries:
    type: number
    default: 3
jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - run: echo "env={{ params.env }} retries={{ params.retries }}"

jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - run: echo "env=staging retries=3"

name: Deploy
on: [push]
params:
  env:
    type: enum
    values: [dev, staging, prod]
    default: staging
  retries:
    type: number
    default: 3
jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - run: echo "env={{ params.env }} retries={{ params.retries }}"

jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - run: echo "env=staging retries=3"

params vs reusable.inputs

Both feed values into a workflow, at different times. params are compile-time: actio build resolves them and bakes the result into the output, so there is no runtime params (and ${{ params.* }} is invalid). reusable.inputs are runtime inputs a caller or dispatcher sets per run. Use params to deduplicate build-time constants; use reusable.inputs when the value must vary per invocation.

See interpolation tokens for the full {{ }} vs ${{ }} rules.