dagger/dagger · error

failed to connect to session server: %w

Error message

failed to connect to session server: %w

What it means

Returned by mainSession when the TCP dial to 127.0.0.1:<DAGGER_SESSION_PORT> fails. The session server either has not started listening yet, exited, or the port is wrong/firewalled; the subprocess cannot attach its output to the session.

Source

Thrown at cmd/init/main.go:269

func mainSession() error {
	ctx := context.Background()

	// this is closed when the session server is about to run, letting the parent process know that
	pipeW := os.NewFile(3, "session-pipe-w")

	portStr, ok := os.LookupEnv("DAGGER_SESSION_PORT")
	if !ok {
		return fmt.Errorf("DAGGER_SESSION_PORT not set")
	}
	_, err := strconv.Atoi(portStr)
	if err != nil {
		return fmt.Errorf("DAGGER_SESSION_PORT invalid: %w", err)
	}

	conn, err := (&net.Dialer{}).DialContext(ctx, "tcp", "127.0.0.1:"+portStr)
	if err != nil {
		return fmt.Errorf("failed to connect to session server: %w", err)
	}

	attachables := []client.SessionAttachable{
		// secrets
		secretprovider.NewSecretProvider(),
		// sockets
		client.SocketProvider{EnableHostNetworkAccess: true},
		// host=>container networking
		h2c.NewTunnelListenerAttachable(ctx),
		// Git attachable
		git.NewGitAttachable(ctx),
	}
	// filesync
	filesyncer, err := client.NewFilesyncer()
	if err != nil {
		return err
	}
	attachables = append(attachables, filesyncer.AsSource(), filesyncer.AsTarget())

View on GitHub (pinned to 82ba2681db)

Solutions

1. Confirm the session server is running and listening on the given port
2. Check for the parent process crashing before the server started
3. Verify no firewall/namespace rules block localhost TCP on that port

When it happens

Trigger: Thrown at cmd/init/main.go:269 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of dagger/dagger@82ba2681db (2026-09-05). Data as JSON: /api/errors/940ac52bc1f9babc. Report an issue: GitHub.