diff --git a/src/io/alsa_io.ml b/src/io/alsa_io.ml
index 5c3e25d1ca..080805b3b6 100644
--- a/src/io/alsa_io.ml
+++ b/src/io/alsa_io.ml
@@ -308,7 +308,8 @@ let () =
         ("", Lang.source_t k, None, None);
       ] )
     ~return_t:k ~category:Lang.Output ~meth:Output.meth
-    ~descr:"Output the source's stream to an ALSA output device."
+    ~descr:"Output the source's stream to an ALSA output device." ~active:true
+    ~self_sync:true
     (fun p ->
       let e f v = f (List.assoc v p) in
       let bufferize = e Lang.to_bool "bufferize" in
@@ -349,7 +350,7 @@ let () =
           Some "Alsa device to use" );
       ] )
     ~meth:(Start_stop.meth ()) ~return_t:k ~category:Lang.Input
-    ~descr:"Stream from an ALSA input device."
+    ~descr:"Stream from an ALSA input device." ~active:true ~self_sync:true
     (fun p ->
       let e f v = f (List.assoc v p) in
       let bufferize = e Lang.to_bool "bufferize" in
diff --git a/src/io/ffmpeg_io.ml b/src/io/ffmpeg_io.ml
index d32c3a2a15..413a6f8143 100644
--- a/src/io/ffmpeg_io.ml
+++ b/src/io/ffmpeg_io.ml
@@ -290,7 +290,8 @@ let register_input is_http =
     if is_http then ("input.http", "Create a http stream using ffmpeg")
     else ("input.ffmpeg", "Create a stream using ffmpeg")
   in
-  Lang.add_operator name ~descr ~category:Lang.Input
+  Lang.add_operator name ~descr ~category:Lang.Input ~active:true
+    ~self_sync:false ~clock_safe:false
     ( List.filter
         (fun (lbl, _, _, _) -> lbl <> "clock_safe")
         (Start_stop.active_source_proto ~fallible_opt:`Nope)
diff --git a/src/io/gstreamer_io.ml b/src/io/gstreamer_io.ml
index 52f690b2b2..916782c00b 100644
--- a/src/io/gstreamer_io.ml
+++ b/src/io/gstreamer_io.ml
@@ -342,7 +342,7 @@ let () =
   Lang.add_operator "output.gstreamer.audio"
     (output_proto ~return_t ~pipeline:"autoaudiosink")
     ~category:Lang.Output ~descr:"Output stream to a GStreamer pipeline."
-    ~return_t (fun p ->
+    ~self_sync:true ~return_t (fun p ->
       let clock_safe = Lang.to_bool (List.assoc "clock_safe" p) in
       let pipeline = Lang.to_string (List.assoc "pipeline" p) in
       let infallible = not (Lang.to_bool (List.assoc "fallible" p)) in
@@ -373,7 +373,7 @@ let () =
   Lang.add_operator "output.gstreamer.video"
     (output_proto ~return_t ~pipeline:"videoconvert ! autovideosink")
     ~category:Lang.Output ~descr:"Output stream to a GStreamer pipeline."
-    ~return_t (fun p ->
+    ~self_sync:true ~return_t (fun p ->
       let clock_safe = Lang.to_bool (List.assoc "clock_safe" p) in
       let pipeline = Lang.to_string (List.assoc "pipeline" p) in
       let infallible = not (Lang.to_bool (List.assoc "fallible" p)) in
@@ -420,7 +420,7 @@ let () =
           Some "Pushing buffers is blocking." );
       ] )
     ~category:Lang.Output ~descr:"Output stream to a GStreamer pipeline."
-    ~return_t
+    ~self_sync:true ~return_t
     (fun p ->
       let clock_safe = Lang.to_bool (List.assoc "clock_safe" p) in
       let pipeline = Lang.to_string (List.assoc "pipeline" p) in
@@ -700,7 +700,7 @@ let () =
       ]
   in
   Lang.add_operator "input.gstreamer.audio_video" proto ~return_t
-    ~category:Lang.Input ~flags:[]
+    ~category:Lang.Input ~flags:[] ~self_sync:true
     ~meth:
       [
         ( "pause",
diff --git a/src/io/oss_io.ml b/src/io/oss_io.ml
index bda6febdbd..75f9965fa7 100644
--- a/src/io/oss_io.ml
+++ b/src/io/oss_io.ml
@@ -146,7 +146,7 @@ let () =
         ("", Lang.source_t k, None, None);
       ] )
     ~return_t:k ~category:Lang.Output
-    ~descr:"Output the source's stream to an OSS output device."
+    ~descr:"Output the source's stream to an OSS output device." ~self_sync:true
     (fun p ->
       let e f v = f (List.assoc v p) in
       let infallible = not (Lang.to_bool (List.assoc "fallible" p)) in
@@ -174,8 +174,8 @@ let () =
           Some (Lang.string "/dev/dsp"),
           Some "OSS device to use." );
       ] )
-    ~meth:(Start_stop.meth ()) ~return_t:k ~category:Lang.Input
-    ~descr:"Stream from an OSS input device."
+    ~meth:(Start_stop.meth ()) ~return_t:k ~category:Lang.Input ~active:true
+    ~self_sync:true ~descr:"Stream from an OSS input device."
     (fun p ->
       let e f v = f (List.assoc v p) in
       let clock_safe = e Lang.to_bool "clock_safe" in
diff --git a/src/io/portaudio_io.ml b/src/io/portaudio_io.ml
index 76d0fd3c78..8266bfd12c 100644
--- a/src/io/portaudio_io.ml
+++ b/src/io/portaudio_io.ml
@@ -190,7 +190,8 @@ let () =
           Some "Length of a buffer in samples." );
         ("", Lang.source_t k, None, None);
       ] )
-    ~return_t:k ~category:Lang.Output ~meth:Output.meth
+    ~return_t:k ~category:Lang.Output ~meth:Output.meth ~active:true
+    ~self_sync:true
     ~descr:"Output the source's stream to a portaudio output device."
     (fun p ->
       let e f v = f (List.assoc v p) in
@@ -219,8 +220,8 @@ let () =
           Some (Lang.int 256),
           Some "Length of a buffer in samples." );
       ] )
-    ~return_t:k ~category:Lang.Input ~meth:(Start_stop.meth ())
-    ~descr:"Stream from a portaudio input device."
+    ~return_t:k ~category:Lang.Input ~meth:(Start_stop.meth ()) ~active:true
+    ~self_sync:true ~descr:"Stream from a portaudio input device."
     (fun p ->
       let e f v = f (List.assoc v p) in
       let buflen = e Lang.to_int "buflen" in
diff --git a/src/io/pulseaudio_io.ml b/src/io/pulseaudio_io.ml
index c647db305e..35091ad418 100644
--- a/src/io/pulseaudio_io.ml
+++ b/src/io/pulseaudio_io.ml
@@ -206,8 +206,9 @@ let () =
   in
   Lang.add_operator "output.pulseaudio"
     (Output.proto @ proto @ [("", Lang.source_t k, None, None)])
-    ~return_t:k ~category:Lang.Output ~meth:Output.meth
-    ~descr:"Output the source's stream to a portaudio output device."
+    ~return_t:k ~category:Lang.Output ~meth:Output.meth ~active:true
+    ~self_sync:true
+    ~descr:"Output the source's stream to a Pulseaudio output device."
     (fun p ->
       let infallible = not (Lang.to_bool (List.assoc "fallible" p)) in
       let start = Lang.to_bool (List.assoc "start" p) in
@@ -223,8 +224,8 @@ let () =
       (new output ~infallible ~on_start ~on_stop ~start ~kind p :> Output.output));
   Lang.add_operator "input.pulseaudio"
     (Start_stop.active_source_proto ~fallible_opt:(`Yep false) @ proto)
-    ~return_t:k ~category:Lang.Input ~meth:(Start_stop.meth ())
-    ~descr:"Stream from a portaudio input device."
+    ~return_t:k ~category:Lang.Input ~meth:(Start_stop.meth ()) ~active:true
+    ~self_sync:true ~descr:"Stream from a Pulseaudio input device."
     (fun p ->
       let kind = Source.Kind.of_kind kind in
       new input ~kind p)
diff --git a/src/io/srt_io.ml b/src/io/srt_io.ml
index 178eff0c80..cef90a9dcf 100644
--- a/src/io/srt_io.ml
+++ b/src/io/srt_io.ml
@@ -901,7 +901,8 @@ class input_caller ~hostname ~port ~kind ~max ~log_overfull ~payload_size
 let () =
   let kind = Lang.any in
   let return_t = Lang.kind_type_of_kind_format kind in
-  Lang.add_operator "input.srt" ~return_t ~category:Lang.Input
+  Lang.add_operator "input.srt" ~return_t ~category:Lang.Input ~active:true
+    ~self_sync:true
     ~meth:(meth () @ Start_stop.meth ())
     ~descr:"Receive a SRT stream from a distant agent."
     ( common_options ~mode:`Listener
diff --git a/src/io/udp_io.ml b/src/io/udp_io.ml
index c55425aff6..936840630b 100644
--- a/src/io/udp_io.ml
+++ b/src/io/udp_io.ml
@@ -249,7 +249,7 @@ let () =
 let () =
   let kind = Lang.any in
   let k = Lang.kind_type_of_kind_format kind in
-  Lang.add_operator "input.udp"
+  Lang.add_operator "input.udp" ~active:true
     ~descr:"Input encoded data from UDP, without any control whatsoever."
     ~category:Lang.Input
     ~flags:[Lang.Hidden; Lang.Deprecated; Lang.Experimental]
diff --git a/src/lang/lang.ml b/src/lang/lang.ml
index 7a9fe21549..e48257f3ad 100644
--- a/src/lang/lang.ml
+++ b/src/lang/lang.ml
@@ -207,13 +207,29 @@ let doc_of_prototype_item ~generalized t d doc =
       | Some d -> Doc.trivial (print_value d) );
   item
 
-type doc_flag = Hidden | Deprecated | Experimental | Extra
+type doc_flag =
+  | Hidden
+  | Deprecated
+  | Experimental
+  | Extra
+  | Active of bool
+  | Fallible of bool
+  | Clock_safe of bool
+  | Self_sync of bool
 
 let string_of_flag = function
   | Hidden -> "hidden"
   | Deprecated -> "deprecated"
   | Experimental -> "experimental"
   | Extra -> "extra"
+  | Active true -> "active"
+  | Active false -> "passive"
+  | Fallible true -> "fallible"
+  | Fallible false -> "infallible"
+  | Clock_safe true -> "clock-safe"
+  | Clock_safe false -> "clock-unsafe"
+  | Self_sync true -> "self-syncing"
+  | Self_sync false -> "not self-syncing"
 
 let builtin_type p t =
   T.make
@@ -866,7 +882,17 @@ type 'a operator_method = string * scheme * string * ('a -> value)
 let add_operator =
   let _meth = meth in
   fun ~category ~descr ?(flags = []) ?(meth = ([] : 'a operator_method list))
-      name proto ~return_t f ->
+      ?(active = false) ?(fallible = true) ?(self_sync = false)
+      ?(clock_safe = true) name proto ~return_t f ->
+    let flags =
+      [
+        Active active;
+        Fallible fallible;
+        Self_sync self_sync;
+        Clock_safe clock_safe;
+      ]
+      @ flags
+    in
     let compare (x, _, _, _) (y, _, _, _) =
       match (x, y) with
         | "", "" -> 0
diff --git a/src/lang/lang.mli b/src/lang/lang.mli
index 76e60e7811..5a630e960a 100644
--- a/src/lang/lang.mli
+++ b/src/lang/lang.mli
@@ -110,6 +110,10 @@ type doc_flag =
   | Deprecated  (** The plugin should not be used. *)
   | Experimental  (** The plugin should not considered as stable. *)
   | Extra  (** Anything that is not part of the essential set of plugins. *)
+  | Active of bool
+  | Fallible of bool
+  | Clock_safe of bool
+  | Self_sync of bool
 
 (** Add an builtin to the language, high-level version for functions. *)
 val add_builtin :
@@ -186,6 +190,10 @@ val add_operator :
   descr:string ->
   ?flags:doc_flag list ->
   ?meth:(< Source.source ; .. > as 'a) operator_method list ->
+  ?active:bool ->
+  ?fallible:bool ->
+  ?self_sync:bool ->
+  ?clock_safe:bool ->
   string ->
   proto ->
   return_t:t ->
diff --git a/src/outputs/ao_out.ml b/src/outputs/ao_out.ml
index 42c522780d..b630ca27b6 100644
--- a/src/outputs/ao_out.ml
+++ b/src/outputs/ao_out.ml
@@ -132,7 +132,7 @@ let () =
           Some "List of parameters, depends on the driver." );
         ("", Lang.source_t return_t, None, None);
       ] )
-    ~category:Lang.Output ~meth:Output.meth
+    ~category:Lang.Output ~meth:Output.meth ~active:true ~self_sync:true
     ~descr:"Output stream to local sound card using libao." ~return_t
     (fun p ->
       let clock_safe = Lang.to_bool (List.assoc "clock_safe" p) in
diff --git a/src/outputs/bjack_out.ml b/src/outputs/bjack_out.ml
index 0f1f61ab27..f1966a3228 100644
--- a/src/outputs/bjack_out.ml
+++ b/src/outputs/bjack_out.ml
@@ -128,6 +128,7 @@ let () =
         ("", Lang.source_t k, None, None);
       ] )
     ~return_t:k ~category:Lang.Output ~descr:"Output stream to jack."
+    ~active:true ~clock_safe:true
     (fun p ->
       let source = List.assoc "" p in
       let clock_safe = Lang.to_bool (List.assoc "clock_safe" p) in
diff --git a/src/outputs/pipe_output.ml b/src/outputs/pipe_output.ml
index fc394d712f..e9de52c46f 100644
--- a/src/outputs/pipe_output.ml
+++ b/src/outputs/pipe_output.ml
@@ -118,7 +118,7 @@ class url_output p =
 let () =
   let return_t = Lang.univ_t () in
   Lang.add_operator "output.url" (url_proto return_t) ~return_t
-    ~category:Lang.Output
+    ~category:Lang.Output ~active:true
     ~descr:
       "Encode and let encoder handle data output. Useful with encoder with no \
        expected output or to encode to files that need full control from the \
@@ -430,7 +430,7 @@ let () =
   let return_t = Lang.univ_t () in
   Lang.add_operator "output.external"
     (pipe_proto return_t "Process to pipe data to.")
-    ~return_t ~category:Lang.Output
+    ~return_t ~category:Lang.Output ~active:true
     ~meth:
       [
         ( "reopen",
diff --git a/src/sources/audio_gen.ml b/src/sources/audio_gen.ml
index 49626955d4..2e8b80565b 100644
--- a/src/sources/audio_gen.ml
+++ b/src/sources/audio_gen.ml
@@ -43,7 +43,7 @@ class gen ~kind ~seek name g freq duration ampl =
 let add name g =
   let kind = Lang.internal in
   let return_t = Lang.kind_type_of_kind_format kind in
-  Lang.add_operator name ~category:Lang.Input
+  Lang.add_operator name ~category:Lang.Input ~fallible:false
     ~descr:("Generate a " ^ name ^ " wave.")
     ~return_t
     [
diff --git a/src/sources/bjack_in.ml b/src/sources/bjack_in.ml
index 0879581aff..d5f70f9f3e 100644
--- a/src/sources/bjack_in.ml
+++ b/src/sources/bjack_in.ml
@@ -134,7 +134,7 @@ let () =
           Some "Jack server to connect to." );
       ] )
     ~meth:(Start_stop.meth ()) ~return_t ~category:Lang.Input
-    ~descr:"Get stream from jack."
+    ~descr:"Get stream from jack." ~active:true ~self_sync:true
     (fun p ->
       let clock_safe = Lang.to_bool (List.assoc "clock_safe" p) in
       let fallible = Lang.to_bool (List.assoc "fallible" p) in
diff --git a/src/sources/blank.ml b/src/sources/blank.ml
index 24046809e1..fc4e9310fb 100644
--- a/src/sources/blank.ml
+++ b/src/sources/blank.ml
@@ -72,7 +72,7 @@ class blank ~kind duration =
 let () =
   let kind = Lang.internal in
   let return_t = Lang.kind_type_of_kind_format kind in
-  Lang.add_operator "blank" ~category:Lang.Input
+  Lang.add_operator "blank" ~category:Lang.Input ~fallible:false
     ~descr:"Produce silence and blank images." ~return_t
     [
       ( "duration",
diff --git a/src/sources/noise.ml b/src/sources/noise.ml
index 4a77a4a990..2d6443bfbf 100644
--- a/src/sources/noise.ml
+++ b/src/sources/noise.ml
@@ -46,7 +46,7 @@ class noise ~kind duration =
 let () =
   let kind = Lang.internal in
   let return_t = Lang.kind_type_of_kind_format kind in
-  Lang.add_operator "noise" ~category:Lang.Input
+  Lang.add_operator "noise" ~category:Lang.Input ~fallible:false
     ~descr:"Generate audio white noise."
     [
       ( "duration",
diff --git a/src/sources/request_simple.ml b/src/sources/request_simple.ml
index 73b55a5b19..6d277b2c66 100644
--- a/src/sources/request_simple.ml
+++ b/src/sources/request_simple.ml
@@ -92,7 +92,7 @@ let log = Log.make ["single"]
 let () =
   let kind = Lang.any in
   let return_t = Lang.kind_type_of_kind_format kind in
-  Lang.add_operator "single" ~category:Lang.Input
+  Lang.add_operator "single" ~category:Lang.Input ~fallible:false
     ~descr:
       "Loop on a request. It never fails if the request is static, meaning \
        that it can be fetched once. Typically, http, ftp, say requests are \
@@ -118,7 +118,7 @@ let () =
 let () =
   let kind = Lang.any in
   let t = Lang.kind_type_of_kind_format kind in
-  Lang.add_operator "single.infallible" ~category:Lang.Input
+  Lang.add_operator "single.infallible" ~category:Lang.Input ~fallible:false
     ~flags:[Lang.Hidden]
     ~descr:
       "Loops on a request, which has to be ready and should be persistent. \
diff --git a/src/synth/keyboard.ml b/src/synth/keyboard.ml
index 0afa47b0a0..48e422b1f7 100644
--- a/src/synth/keyboard.ml
+++ b/src/synth/keyboard.ml
@@ -54,7 +54,7 @@ let note_of_char c = array_index c + 72
 
 class keyboard ~kind =
   object (self)
-    inherit Source.active_source ~name:"input.keyboard" kind
+    inherit Source.source ~name:"input.keyboard" kind
 
     method stype = Source.Infallible
 
@@ -140,7 +140,7 @@ let () =
   let kind = Lang.midi_n 1 in
   let return_t = Lang.kind_type_of_kind_format kind in
   Lang.add_operator "input.keyboard" [] ~return_t ~category:Lang.Input
-    ~flags:[Lang.Hidden; Lang.Experimental]
+    ~fallible:false ~flags:[Lang.Hidden; Lang.Experimental]
     ~descr:"Play notes from the keyboard." (fun _ ->
       let kind = Source.Kind.of_kind kind in
       (new keyboard ~kind :> Source.source))
diff --git a/src/synth/keyboard_sdl.ml b/src/synth/keyboard_sdl.ml
index f6a7f5f735..2ea1e86cc9 100644
--- a/src/synth/keyboard_sdl.ml
+++ b/src/synth/keyboard_sdl.ml
@@ -85,8 +85,7 @@ let note_of_char c =
 class keyboard ~kind velocity =
   let () = Sdl_utils.init [Sdl.Init.events; Sdl.Init.video] in
   object (self)
-    inherit
-      Source.active_source ~name:"input.keyboard.sdl" (Source.Kind.of_kind kind)
+    inherit Source.source ~name:"input.keyboard.sdl" (Source.Kind.of_kind kind)
 
     method stype = Source.Infallible
 
diff --git a/src/tools/doc.ml b/src/tools/doc.ml
index d34079985f..88bf30b620 100644
--- a/src/tools/doc.ml
+++ b/src/tools/doc.ml
@@ -202,8 +202,15 @@ let print_functions_md ~extra (doc : item) print_string =
                   let s = if s = "" then "" else ": " ^ s in
                   Printf.ksprintf print_string "- `%s` (of type `%s`)%s\n" l t s)
                 methods );
-            if List.mem "experimental" flags then
-              print_string "\nThis function is experimental.\n";
+            (let rec concat = function
+               | [] -> ""
+               | [x] -> x
+               | [x; y] -> x ^ " and " ^ y
+               | x :: l -> x ^ ", " ^ concat l
+             in
+             let flags = concat flags in
+             if flags <> "" then
+               Printf.ksprintf print_string "\nThis function is %s.\n" flags);
             print_string "\n" ))
         ff)
     by_cat