dagger/dagger · error

host service proxy client %q not found for client %q

Error message

host service proxy client %q not found for client %q

What it means

getHostServiceCaller walks the client's parent chain looking for a parent whose clientID equals the client's hostServiceProxyClientID, to route host-service calls through the proxy client. If no such parent exists in the chain, this error is returned.

Source

Thrown at engine/server/session.go:1077

	ctx context.Context,
	id string,
) (engineutil.SessionCaller, error) {
	if id == client.clientID && client.hostServiceProxyClientID != "" {
		// Synthetic nested clients (e.g. builtin dang evaluation) do not
		// establish their own session attachables. When host-backed services
		// such as git config are requested through the current client ID, fall
		// back to the explicit proxy client chain.
		if caller, ok := client.daggerSession.attachables.Lookup(id); ok {
			return caller, nil
		}

		for i := len(client.parents) - 1; i >= 0; i-- {
			parent := client.parents[i]
			if parent.clientID == client.hostServiceProxyClientID {
				return parent.getHostServiceCaller(ctx, parent.clientID)
			}
		}
		return nil, fmt.Errorf("host service proxy client %q not found for client %q", client.hostServiceProxyClientID, client.clientID)
	}

	return client.getClientCaller(ctx, id)
}

func (srv *Server) clientFromContext(ctx context.Context) (*daggerClient, error) {
	clientMetadata, err := engine.ClientMetadataFromContext(ctx)
	if err != nil {
		return nil, fmt.Errorf("failed to get client metadata for session call: %w", err)
	}
	client, err := srv.clientFromIDs(clientMetadata.SessionID, clientMetadata.ClientID)
	if err != nil {
		return nil, err
	}
	return client, nil
}

func (srv *Server) clientFromIDs(sessID, clientID string) (*daggerClient, error) {

View on GitHub (pinned to 82ba2681db)

Solutions

1. Re-run the pipeline so a fresh parent chain including the host-service proxy client is established
2. Ensure parent sessions stay alive for the duration of child host-service calls
3. Check for early cancellation of the parent session in your workflow
4. If reproducible in a supported setup, report as an engine bug with session/parent IDs from the message

// Go: ensure parent sessions outlive child operations
// structure code so parent queries aren't cancelled before children finish
errGroup, ctx := errgroup.WithContext(baseCtx) // propagate cancellation carefully

// Go
if err != nil && strings.Contains(err.Error(), "host service proxy client") {
    // re-establish the session and retry the operation
}

When it happens

Trigger: A client performs a host-service call, client.hostServiceProxyClientID is set, but no parent in client.parents has that clientID — the proxy parent disconnected, was never registered, or the chain was pruned.

Common situations: Nested function calls where an intermediate parent session exited/cancelled before the child made a host call, custom SDK harnesses that build client chains incorrectly, or race between parent teardown and child host-service usage.

Related errors

• failed to get engine client: %w
• buildkit: %w
• failed to get engine client: %w
• module source engine client: %w
• buildkit: %w

AI-assisted analysis of dagger/dagger@82ba2681db (2026-09-05). Data as JSON: /api/errors/9702aab0dd585d0e. Report an issue: GitHub.