dagger/dagger · error

parse runner host: %w

Error message

parse runner host: %w

What it means

Wraps a failure of url.Parse(c.RunnerHost) at the top of startEngine. Before selecting a driver, the client parses the runner host string as a URL to read its scheme; if the string is not a valid absolute URL (bad scheme syntax, control characters, missing scheme), parsing fails and the connect is aborted.

Source

Thrown at engine/client/client.go:455

			c.sessionSrv.Stop()
		}
	}()

	if err := c.subscribeTelemetry(connectCtx); err != nil {
		return nil, fmt.Errorf("subscribe to telemetry: %w", err)
	}

	if err := c.daggerConnect(ctx); err != nil {
		return nil, fmt.Errorf("failed to connect to dagger: %w", err)
	}

	return c, nil
}

func (c *Client) startEngine(ctx context.Context, params Params) (rerr error) {
	remote, err := url.Parse(c.RunnerHost)
	if err != nil {
		return fmt.Errorf("parse runner host: %w", err)
	}

	driver, err := drivers.GetDriver(ctx, remote.Scheme)
	if err != nil {
		return err
	}

	var cloudToken string
	if v, ok := os.LookupEnv(drivers.EnvDaggerCloudToken); ok {
		cloudToken = v
	} else if _, ok := os.LookupEnv(envDaggerCloudCachetoken); ok {
		cloudToken = v
	}

	if c.CloudURLCallback != nil {
		if url, msg, ok := enginetel.URLForTrace(ctx); ok {
			c.CloudURLCallback(ctx, url, msg, ok)
		} else {

View on GitHub (pinned to 82ba2681db)

Solutions

1. Print and fix the RunnerHost value; ensure it is a valid URL like docker://engine or udp://1.2.3.4
2. Check the env var feeding RunnerHost (e.g. DAGGER_RUNNER_HOST) for typos/extra characters
3. Quote the value in shell/scripts to avoid stray spaces

Example fix

// before
os.Setenv("DAGGER_RUNNER_HOST", "docker//my-engine")
client, err := dagger.Connect(ctx)
// after
os.Setenv("DAGGER_RUNNER_HOST", "docker-container://my-engine")
client, err := dagger.Connect(ctx)

runnerHost := os.Getenv("DAGGER_RUNNER_HOST")
if u, err := url.Parse(runnerHost); err != nil || u.Scheme == "" {
    return fmt.Errorf("DAGGER_RUNNER_HOST %q is not a valid URL with a scheme", runnerHost)
}

func isValidRunnerHost(s string) bool {
    u, err := url.Parse(s)
    return err == nil && u.Scheme != ""
}

client, err := dagger.Connect(ctx)
if err != nil && strings.Contains(err.Error(), "parse runner host") {
    return fmt.Errorf("check DAGGER_RUNNER_HOST: %w", err)
}

When it happens

Trigger: Calling Connect or ConnectEngineToEngine with a Params.RunnerHost (or env-provided runner host) that is not a parseable URL — e.g. "docker:/", spaces, or garbage strings.

Common situations: Typo'd DAGGER_RUNNER_HOST value (e.g. missing colon: "docker//container"); shell interpolation injecting whitespace; environment variable set from a mis-templated config.

Understand the failure class

Background: "Invalid URL" / "URL cannot be empty": fix the malformed or missing URL behind request-construction failures — this error's family across 50 libraries.

Related errors

• parse local endpoint URL: %w
• parse port %q from %s: %w
• parse git URL %q: %w
• sshfs endpoint missing user
• sshfs endpoint missing host

AI-assisted analysis of dagger/dagger@82ba2681db (2026-09-05). Data as JSON: /api/errors/b4f06f3128f01585. Report an issue: GitHub.