Please try with the latest version:

npm install gulp-sass@next --save-dev

Same result.

I cannot replicate your error using the latest version of gulp-sass, in fact. I very clearly see the comments, including on watch, but it brings up an interesting question we should resolve.

Because we're passing a string to node-sass and not a path, the sourceComment says it's coming from stdin for anything coming from the main file. Should we replace this with the path to that file before passing passing back along @dlmanning @Keats? process.cwd() replaced below for the absolute path to the Gulp file.

/* line 1, {{process.cwd()}}/sass/_foo.scss */
.banana {
  content: 'waldo'; }

/* line 3, stdin */
.foo {
  content: 'bar'; }

/* line 7, stdin */
.bar {
  content: 'baz'; }

/* line 11, stdin */
.baz {
  content: 'baz'; }

/* line 15, stdin */
.qux {
  content: 'qux'; }

I'm also happy to defer this to 2.1 as most people seem to be interested in Source Maps over Comments and I don't want to block the 2.0 release on this

I'm ok with moving this to 2.1. I'll take a look at this again, I could bet in my case there where no source comments at all.

I didn't even know that was a thing (I use compressed output with no sourcemaps/anything).
We can definitely work on that for 2.1

For those of you, like me, who are to impatient to wait for a fix you can hack around this by tapping into the stream with gulp-tap and just doing plain ol' string replacement. It will likely slow things down as we have to toString the buffer then convert it back but it gets you out of a bind.

gulp.src(config.files.scss)
.pipe(sass({
    sourceComments:true
}))
.pipe(tap(function(file, t) {
    file.contents = new Buffer(file.contents.toString().replace(/stdin \*\//g, file.path + ' */'));
}))
.pipe(concat('app.css'))
}))

This appears to be fix in v2.3.1.

In v2.3.2 it still doesn't work.