HI all, I've come up with the following problem I got something like  "filesrc location="<a href="http://file.in">file.in</a>" ! audio/XXX ! plugin ! fakesink" Plugin can take audio/XXX,var=x ... So i'm partially setting the caps with the caps filter The problems seems to be that they caps are never fixated , my fixate function in my plugin isn't called nor is my setcaps... and filesrc pushes me a buffer with caps = NULL :( I"ve solved it by patching with : --- gstreamer-cvs/plugins/elements/gstcapsfilter.c      2005-11-29 13:00:15.000000000 -0500 +++ gstreamer/plugins/elements/gstcapsfilter.c  2005-12-02 15:21:51.000000000 -0500 @@ -256,7 +256,11 @@        out_caps = gst_pad_get_allowed_caps (trans->srcpad);        g_return_val_if_fail (out_caps != NULL, GST_FLOW_ERROR);      } - + +    if(!gst_caps_is_fixed(out_caps) || gst_caps_is_empty (out_caps)) { +       out_caps = gst_capsfilter_transform_caps(trans, GST_PAD_UNKNOWN, out_caps); +       gst_pad_fixate_caps(trans->srcpad, out_caps); +    }      if (gst_caps_is_fixed (out_caps) && !gst_caps_is_empty (out_caps)) {        GST_DEBUG_OBJECT (trans, "Have fixed output caps %"            GST_PTR_FORMAT " to apply to buffer with no caps", out_caps); But i'm really not sure it's a good way to do it :( I made the patch with the rationale that capsfilter wounldn't be able to set the caps if they were not fixed .. so I fixed them before but maybe I should do the fixate on the sinkpad, or do something completly different... I'm also unsure about what happends when you send a buffer with no caps (the 1st) shouldn't some default fixate function be called then ? Thanks a lot Antoine