GFM requires space between # and string to create an hN. % curl -d '{"text": "#asdf", "mode": "gfm"}' https://api.github.com/markdown <p>#asdf</p> % curl -d '{"text": "# asdf", "mode": "gfm"}' https://api.github.com/markdown <h1>asdf</h1> markdown doesn't. % curl -d '{"text": "#asdf", "mode": "markdown"}' https://api.github.com/markdown <h1> <a name="asdf" class="anchor" href="#asdf"><span class="mini-icon mini-icon-link"></span></a>asdf</h1> % curl -d '{"text": "# asdf", "mode": "markdown"}' https://api.github.com/markdown <h1> <a name="asdf" class="anchor" href="#asdf"><span class="mini-icon mini-icon-link"></span></a>asdf</h1>