googleapis/mcp-toolbox · error

failed to marshal response to JSON: %w

Error message

failed to marshal response to JSON: %w

What it means

Error "failed to marshal response to JSON: %w" thrown in googleapis/mcp-toolbox.

Source

Thrown at internal/server/mcp.go:349

	select {
	// if context is cancelled, return an empty string
	case <-ctx.Done():
		return "", ctx.Err()
	// return error if error is found
	case err := <-errChan:
		return "", err
	// return line if successful
	case line := <-readChan:
		return line, nil
	}
}

// write writes to stdout with response to client
func (s *stdioSession) write(_ context.Context, response any) error {
	res, err := json.Marshal(response)
	if err != nil {
		return fmt.Errorf("failed to marshal response to JSON: %w", err)
	}

	_, err = fmt.Fprintf(s.writer, "%s\n", res)
	return err
}

// mcpRouter creates a router that represents the routes under /mcp
func mcpRouter(s *Server) (chi.Router, error) {
	r := chi.NewRouter()

	r.Use(middleware.AllowContentType("application/json", "application/json-rpc", "application/jsonrequest"))
	r.Use(middleware.StripSlashes)
	r.Use(render.SetContentType(render.ContentTypeJSON))
	// Inject logger into ctx
	r.Use(func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			ctx := util.WithLogger(r.Context(), s.logger)
			next.ServeHTTP(w, r.WithContext(ctx))

View on GitHub (pinned to 8cc6e09de2)

When it happens

Trigger: Thrown at internal/server/mcp.go:349 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/cdbf7e3e47dd5540. Report an issue: GitHub.