Run a Custom Harness Provider in a Container

Use a custom harness provider when the runner image exposes a CLI that is not one of Dagu's built-in harness providers, or when the image exposes a wrapper script around a provider CLI.

The image must contain the configured binary in PATH.

Shell Provider Smoke Test

This DAG uses Alpine and a custom provider named shell. It verifies that harness.run executes inside the configured container.

type: graph

container:
  image: alpine:3.20
  pull_policy: missing
  env:
    - SMOKE_TOKEN=container-env-ok

harnesses:
  shell:
    binary: sh
    prefix_args:
      - -c
    prompt_mode: arg

steps:
  - id: smoke
    action: harness.run
    with:
      provider: shell
      prompt: |
        set -eu
        echo "HARNESS_CONTAINER_SMOKE=ok"
        echo "SMOKE_TOKEN=$SMOKE_TOKEN"
        echo "ALPINE_RELEASE=$(cat /etc/alpine-release)"

The output must include:

HARNESS_CONTAINER_SMOKE=ok
SMOKE_TOKEN=container-env-ok
ALPINE_RELEASE=

The Alpine release value depends on the image tag.

Custom CLI Provider

This example assumes the runner image contains /usr/local/bin/review-agent. The CLI receives its prompt as the final command-line argument.

type: graph

harnesses:
  review_agent:
    binary: review-agent
    prefix_args:
      - run
    prompt_mode: arg

steps:
  - id: review
    action: harness.run
    container:
      image: dagu-review-agent:local
      pull_policy: never
      working_dir: /workspace
      volumes:
        - .:/workspace:ro
      env:
        - REVIEW_AGENT_TOKEN=${REVIEW_AGENT_TOKEN}
    with:
      provider: review_agent
      prompt: |
        Review this repository.

Dagu builds this command inside the container:

review-agent run "Review this repository."

Prompt as a Flag

If the CLI expects a prompt flag, set prompt_mode: flag and prompt_flag.

harnesses:
  review_agent:
    binary: review-agent
    prefix_args:
      - run
    prompt_mode: flag
    prompt_flag: --prompt

With this provider definition, the command shape is:

review-agent run --prompt "Review this repository."

Container Requirements

For the examples above, the image must contain:

• the configured provider binary
• any runtime required by that binary
• any tools the provider is expected to run
• a writable home or cache directory if the provider writes files there

If the provider needs credentials, pass them with container.env or mount the credential files the provider reads.

Unsupported Prompt Input

Containerized harness.run does not support stdin prompt mode. Do not combine container: with:

• with.stdin
• script:
• a custom provider that sets prompt_mode: stdin

Use prompt_mode: arg or prompt_mode: flag for containerized providers.

Links

• Harness overview
• Harness runner images
• Harness container credentials