directory-structure.txt

fez.js (*)
test-src/
  test-bdd.sjs (*)
test/
node_modules/
  fez/
  fez-sweet.js/
    plugin.js (*)
    node_modules/
      sweet.js/
      bluebird/

(*) Included in this gist.

fez.js

#!/usr/bin/env node

var fez = require('fez');
var sweetjs = require('fez-sweet.js');

exports.build = function(rule) {
  rule.each("test-src/*.sjs", fez.mapFile("test/%f.js"), sweetjs({'modules': ['sweet-bdd']}));
};

exports.default = exports.build;

fez(module);

plugin.js

var SweetJS = require('sweet.js');
var Promise = require('bluebird');
var inspect = require('util').inspect;

module.exports = function (options) {
    console.log(inspect(options, {depth: null, showColors: true}));
    var cwd = process.cwd();
    var modules = (options.modules || []).map(function (modulename) {
        return SweetJS.loadNodeModule(cwd, modulename);
    });

    function sjs(promise) {
        return promise
        .then(function (input) {
            var src = input.toString();
            console.log("SRC:", "\n", src, "\n");
            console.log("Modules:", "\n", inspect(modules[0], {depth: 2, showColors: true}), "\n");
            console.log("Compilation Result:");
            var outfile = SweetJS.compile(src, modules).code;
            console.log(outfile);
            return outfile;
        });
    }

    return function (inputs) {
        return Promise.all(inputs.map(function (input) { return input.asBuffer(); }).map(sjs));
    }
}

stdout.txt

$ ./fez.js 
{ modules: [ 'sweet-bdd' ] }
Creating test/test-bdd.js
SRC: 
 it "x" { } 

Modules: 
 { env: 
   { __data: 
      { 'quoteSyntax$3': [Object],
        'syntax$13': [Object],
        '#$15': [Object],
        'syntaxCase$77': [Object],
        'macro$106': [Object],
        'withSyntax_unzip$170': [Object],
        'withSyntax$196': [Object],
        'letstx$246': [Object],
        'quoteSyntax$247': [Object],
        'syntax$248': [Object],
        '#$249': [Object],
        'syntaxCase$250': [Object],
        'macro$251': [Object],
        'withSyntax$252': [Object],
        'letstx$253': [Object],
        'describe$278': [Object],
        'it$306': [Object],
        'beforeEach$334': [Object],
        'afterEach$362': [Object] } },
  defscope: undefined,
  templateMap: 
   { __data: 
      { '275': [Object],
        '300': [Object],
        '303': [Object],
        '328': [Object],
        '331': [Object],
        '356': [Object],
        '359': [Object] } },
  mark: undefined,
  exports: 
   [ { oldExport: [Object], newParam: [Object] },
     { oldExport: [Object], newParam: [Object] },
     { oldExport: [Object], newParam: [Object] },
     { oldExport: [Object], newParam: [Object] } ] } 

Compilation Result:
{ _bitField: 134217728,
  _settledValue: 
   { [Error: Line 1: Unexpected string
   [... it x { } ...]]
     index: 3,
     lineNumber: 1,
     column: 4,
     description: 'Unexpected string' } }
An operation has failed. Aborting.
Nothing to be done.

test-bdd.sjs

it "x" { }

modules isn't the second arg to sweet.compile. It should be an options object with a modules key:

sweet.compile(src, {
    modules: mods
});

I feel so stupid. Thanks.