My LESS sourceMap file is missing the "file" when run through grunt - gruntjs

Question

When I run lessc --source-map=styles.map assets/less/00_style.less dest/assets/prod.css in the command line, everything is working. The styles.map file ends in:
...AV2rEF;EAAiB,aAAA","file":"dest/assets/prod.css"}
However, when I run grunt less, the styles.map is missing the "file" part and just ends in:
...AV2rEF;EAAiB,aAAA"}
This stops the SourceMap from working. What could be going wrong? My less config is as follows:
less: {
dist: {
options: {
sourceMap: true,
sourceMapFilename: 'styles.map'
},
files: [{
src : 'assets/less/00_style.less',
dest: 'dest/assets/prod.css'
}]
}
}

Answer 1

Short answer:
Add the following additional option to your less Task in Gruntfile.js:
...
options: {
...
sourceMapURL: '../../styles.map'
},
...
Long answer:
When running the lessc command via the CLI, (as per your example), notice the the following comment is written to the resultant prod.css:
/*# sourceMappingURL=../../styles.map */
However, when running the grunt task, (using your current config), the following comment is written into the resultant prod.css:
/*# sourceMappingURL=styles.map */
Note the missing ../../ - therefore prod.css can't find styles.map
This is why your SourceMap isn't working and not so much to do with the "file:" missing in styles.map when run via grunt. The .css file ultimately points to the .map file - not vice versa.
Even after running the lessc command via the CLI then deleting the "file:" part from styles.map you will find that the SourceMap still works in the browser. Indicating that the "file:" part, whether included in the .map file or not, has no effect on preventing the SourceMap from working.
Besides, as noted in the most recent proposed SourceMap spec (v.3) the "file:" part is optional:
Line 3: An optional name of the generated code that this source map is associated with.
Explicitly defining the sourceMapURL in your grunt Task options will entail having to keep a flat folder structure inside the dest/assets/ directory if you intend on using multiple .less files. (I.e. You'll need to avoid saving any resultant .css files in subfolders)