nektos/act · critical

Unsupported Operation

Error message

Unsupported Operation

What it means

ImageExistsLocally in docker_stub.go returns 'Unsupported Operation' because the binary was compiled without Docker support. The file's build tag is 'WITHOUT_DOCKER || !(linux || darwin || windows || netbsd)', so this code is active when act is built with the WITHOUT_DOCKER tag or for an OS outside the supported set. Every Docker-backed function becomes a stub, and this one reports that image inspection is unavailable in such builds.

Source

Thrown at pkg/container/docker_stub.go:17

//go:build WITHOUT_DOCKER || !(linux || darwin || windows || netbsd)

package container

import (
	"context"
	"runtime"

	"github.com/moby/moby/api/types/system"
	"github.com/nektos/act/pkg/common"
	"github.com/pkg/errors"
)

// ImageExistsLocally returns a boolean indicating if an image with the
// requested name, tag and architecture exists in the local docker image store
func ImageExistsLocally(ctx context.Context, imageName string, platform string) (bool, error) {
	return false, errors.New("Unsupported Operation")
}

// RemoveImage removes image from local store, the function is used to run different
// container image architectures
func RemoveImage(ctx context.Context, imageName string, force bool, pruneChildren bool) (bool, error) {
	return false, errors.New("Unsupported Operation")
}

// NewDockerBuildExecutor function to create a run executor for the container
func NewDockerBuildExecutor(input NewDockerBuildExecutorInput) common.Executor {
	return func(ctx context.Context) error {
		return errors.New("Unsupported Operation")
	}
}

// NewDockerPullExecutor function to create a run executor for the container
func NewDockerPullExecutor(input NewDockerPullExecutorInput) common.Executor {
	return func(ctx context.Context) error {

View on GitHub (pinned to 4f41128141)

Solutions

1. Install an official act release built for a supported OS (linux, darwin, windows, netbsd) without the WITHOUT_DOCKER tag.
2. If building from source, drop -tags WITHOUT_DOCKER from your build command.
3. If you intentionally need a Docker-less build, use act's host execution mode (-P ...=-self-hosted) which avoids the Docker code paths, or accept that image operations are unsupported.
4. When importing act as a library, ensure your build tags match the execution environment you target.

Example fix

# before
go build -tags WITHOUT_DOCKER -o act ./cmd/act
./act -j build   # ImageExistsLocally -> Unsupported Operation

# after
go build -o act ./cmd/act
./act -j build

// Detect a Docker-less build before calling image APIs
package main

import (
	"runtime"
)

//go:build WITHOUT_DOCKER || !(linux || darwin || windows || netbsd)
const dockerSupportCompiled = false

// companion file with the inverse tag sets it to true; then:
func checkImageSupport() error {
	if !dockerSupportCompiled {
		return fmt.Errorf("act built without docker support; image lookups unavailable on %s", runtime.GOOS)
	}
	return nil
}

if ok, err := container.ImageExistsLocally(ctx, image, platform); err != nil {
    if err.Error() == "Unsupported Operation" {
        // Docker-less build: skip local-image optimization and let pull fail or use host mode
        ok = false
    } else {
        return err
    }
}

When it happens

Trigger: Calling container.ImageExistsLocally from code that imports a build of act compiled with -tags WITHOUT_DOCKER, or running an act binary produced for an unsupported GOOS. The function is invoked during job setup to decide whether an image pull is needed.

Common situations: Installing a distro-packaged act built with WITHOUT_DOCKER (common in limited environments); building act from source on an exotic platform; importing act as a library into a project whose CI compiles with the same tags; running act --list works but actual job execution fails because every container operation is stubbed.

Related errors

• Failed to create job container
• --userns: invalid USER mode
• no name set for network
• --pid: invalid PID mode
• --uts: invalid UTS mode

AI-assisted analysis of nektos/act@4f41128141 (2026-08-15). Data as JSON: /api/errors/0b5604b58303e7ca. Report an issue: GitHub.