diff --git a/build_system/clerk_driver.ml b/build_system/clerk_driver.ml
index 468cb638..c8a38dd4 100644
--- a/build_system/clerk_driver.ml
+++ b/build_system/clerk_driver.ml
@@ -347,9 +347,7 @@ module Poll = struct
 
   let ocaml_link_flags : string list Lazy.t =
     lazy
-      (let link_libs =
-         ["yojson"; "ppx_yojson_conv_lib"; "zarith"; "dates_calc"]
-       in
+      (let link_libs = ["zarith"; "dates_calc"] in
        let link_libs_flags =
          List.concat_map
            (fun lib ->
diff --git a/catala.opam b/catala.opam
index 6314cfe8..f3d5c774 100644
--- a/catala.opam
+++ b/catala.opam
@@ -31,8 +31,6 @@ depends: [
   "ocaml" {>= "4.14.0"}
   "ocamlfind" {!= "1.9.5"}
   "ocamlgraph" {>= "1.8.8"}
-  "yojson" {>= "2.0" }
-  "ppx_yojson_conv" {>= "0.14.0"}
   "re" {>= "1.9.0"}
   "sedlex" {>= "2.4"}
   "uutf" {>= "1.0.3"}
diff --git a/compiler/lcalc/to_ocaml.ml b/compiler/lcalc/to_ocaml.ml
index c1e94115..3fdeb7bc 100644
--- a/compiler/lcalc/to_ocaml.ml
+++ b/compiler/lcalc/to_ocaml.ml
@@ -462,7 +462,7 @@ let format_struct_embedding
       format_struct_name struct_name format_to_module_name (`Sname struct_name)
   else
     Format.fprintf fmt
-      "@[<hov 2>let embed_%a (x: %a.t) : runtime_value =@ Struct([\"%a\"],@ \
+      "@[<hov 2>let embed_%a (x: %a.t) : runtime_value =@ Struct(\"%a\",@ \
        @[<hov 2>[%a]@])@]@\n\
        @\n"
       format_struct_name struct_name format_to_module_name (`Sname struct_name)
@@ -485,7 +485,7 @@ let format_enum_embedding
   else
     Format.fprintf fmt
       "@[<hv 2>@[<hov 2>let embed_%a@ @[<hov 2>(x:@ %a.t)@]@ : runtime_value \
-       =@]@ Enum([\"%a\"],@ @[<hov 2>match x with@ %a@])@]@\n\
+       =@]@ Enum(\"%a\",@ @[<hov 2>match x with@ %a@])@]@\n\
        @\n"
       format_enum_name enum_name format_to_module_name (`Ename enum_name)
       EnumName.format enum_name
diff --git a/compiler/shared_ast/interpreter.ml b/compiler/shared_ast/interpreter.ml
index 1e596e40..7824706d 100644
--- a/compiler/shared_ast/interpreter.ml
+++ b/compiler/shared_ast/interpreter.ml
@@ -994,7 +994,7 @@ let interpret_program_dcalc p s : (Uid.MarkedString.info * ('a, 'm) gexpr) list
                built-in interpreter does not have a way to retrieve input \
                values from the command line, so it cannot execute this scope. \
                Please create another scope that provides the input arguments \
-               to this one and execute it instead. ")
+               to this one and execute it instead.")
         taus
     in
     let to_interpret =
diff --git a/runtimes/jsoo/runtime.ml b/runtimes/jsoo/runtime.ml
index 7854ce62..aaa35847 100644
--- a/runtimes/jsoo/runtime.ml
+++ b/runtimes/jsoo/runtime.ml
@@ -84,83 +84,75 @@ let event_manager : event_manager Js.t =
     method resetLog = Js.wrap_meth_callback R_ocaml.reset_log
 
     method retrieveEvents =
-      Js.wrap_meth_callback (fun () ->
-          Js.array
-            (Array.of_list
-               (R_ocaml.retrieve_log ()
-               |> R_ocaml.EventParser.parse_raw_events
-               |> List.map (fun event ->
-                      object%js
-                        val mutable data =
-                          event
-                          |> R_ocaml.yojson_of_event
-                          |> Yojson.Safe.to_string
-                          |> Js.string
-                      end))))
+      Js.wrap_meth_callback
+      @@ fun () ->
+      R_ocaml.retrieve_log ()
+      |> R_ocaml.EventParser.parse_raw_events
+      |> List.map (fun event ->
+             object%js
+               val mutable data = event |> R_ocaml.Json.event |> Js.string
+             end)
+      |> Array.of_list
+      |> Js.array
 
     method retrieveRawEvents =
-      Js.wrap_meth_callback (fun () ->
-          Js.array
-            (Array.of_list
-               (List.map
-                  (fun evt ->
-                    object%js
-                      val mutable eventType =
-                        Js.string
-                          (match evt with
-                          | R_ocaml.BeginCall _ -> "Begin call"
-                          | EndCall _ -> "End call"
-                          | VariableDefinition _ -> "Variable definition"
-                          | DecisionTaken _ -> "Decision taken")
+      let evt_to_js evt =
+        (* FIXME: ideally this could be just a Json.parse (R_ocaml.Json.event
+           foo) ? *)
+        object%js
+          val mutable eventType =
+            (match evt with
+            | R_ocaml.BeginCall _ -> "Begin call"
+            | EndCall _ -> "End call"
+            | VariableDefinition _ -> "Variable definition"
+            | DecisionTaken _ -> "Decision taken")
+            |> Js.string
 
-                      val mutable information =
-                        Js.array
-                          (Array.of_list
-                             (match evt with
-                             | BeginCall info
-                             | EndCall info
-                             | VariableDefinition (info, _, _) ->
-                               List.map Js.string info
-                             | DecisionTaken _ -> []))
+          val mutable information =
+            (match evt with
+            | BeginCall info | EndCall info | VariableDefinition (info, _, _) ->
+              List.map Js.string info
+            | DecisionTaken _ -> [])
+            |> Array.of_list
+            |> Js.array
 
-                      val mutable loggedIOJson =
-                        match evt with
-                        | VariableDefinition (_, io, _) ->
-                          io
-                          |> R_ocaml.yojson_of_io_log
-                          |> Yojson.Safe.to_string
-                          |> Js.string
-                        | EndCall _ | BeginCall _ | DecisionTaken _ ->
-                          "unavailable" |> Js.string
+          val mutable loggedIOJson =
+            match evt with
+            | VariableDefinition (_, io, _) ->
+              io |> R_ocaml.Json.io_log |> Js.string
+            | EndCall _ | BeginCall _ | DecisionTaken _ ->
+              "unavailable" |> Js.string
 
-                      val mutable loggedValueJson =
-                        (match evt with
-                        | VariableDefinition (_, _, v) -> v
-                        | EndCall _ | BeginCall _ | DecisionTaken _ ->
-                          R_ocaml.unembeddable ())
-                        |> R_ocaml.yojson_of_runtime_value
-                        |> Yojson.Safe.to_string
-                        |> Js.string
+          val mutable loggedValueJson =
+            (match evt with
+            | VariableDefinition (_, _, v) -> v
+            | EndCall _ | BeginCall _ | DecisionTaken _ ->
+              R_ocaml.unembeddable ())
+            |> R_ocaml.Json.runtime_value
+            |> Js.string
 
-                      val mutable sourcePosition =
-                        match evt with
-                        | DecisionTaken pos ->
-                          Js.def
-                            (object%js
-                               val mutable fileName = Js.string pos.filename
-                               val mutable startLine = pos.start_line
-                               val mutable endLine = pos.end_line
-                               val mutable startColumn = pos.start_column
-                               val mutable endColumn = pos.end_column
+          val mutable sourcePosition =
+            match evt with
+            | DecisionTaken pos ->
+              Js.def
+                (object%js
+                   val mutable fileName = Js.string pos.filename
+                   val mutable startLine = pos.start_line
+                   val mutable endLine = pos.end_line
+                   val mutable startColumn = pos.start_column
+                   val mutable endColumn = pos.end_column
 
-                               val mutable lawHeadings =
-                                 Js.array
-                                   (Array.of_list
-                                      (List.map Js.string pos.law_headings))
-                            end)
-                        | _ -> Js.undefined
-                    end)
-                  (R_ocaml.retrieve_log ()))))
+                   val mutable lawHeadings =
+                     List.map Js.string pos.law_headings
+                     |> Array.of_list
+                     |> Js.array
+                end)
+            | _ -> Js.undefined
+        end
+      in
+      Js.wrap_meth_callback
+      @@ fun () ->
+      R_ocaml.retrieve_log () |> List.map evt_to_js |> Array.of_list |> Js.array
   end
 
 let execute_or_throw_error f =
diff --git a/runtimes/ocaml/dune b/runtimes/ocaml/dune
index 03c76b5c..ca18edc3 100644
--- a/runtimes/ocaml/dune
+++ b/runtimes/ocaml/dune
@@ -1,7 +1,5 @@
 (library
  (name runtime_ocaml)
  (public_name catala.runtime_ocaml)
- (preprocess
-  (pps ppx_yojson_conv))
  (libraries dates_calc zarith zarith_stubs_js)
  (modules runtime))
diff --git a/runtimes/ocaml/runtime.ml b/runtimes/ocaml/runtime.ml
index 0c642beb..e9592785 100644
--- a/runtimes/ocaml/runtime.ml
+++ b/runtimes/ocaml/runtime.ml
@@ -13,8 +13,6 @@
    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
    License for the specific language governing permissions and limitations under
    the License. *)
-open Ppx_yojson_conv_lib.Yojson_conv.Primitives
-
 type nonrec unit = unit
 type nonrec bool = bool
 
@@ -30,8 +28,8 @@ module Eoption = struct
   type 'a t = ENone of unit | ESome of 'a
 end
 
-type io_input = NoInput | OnlyInput | Reentrant [@@deriving yojson_of]
-type io_log = { io_input : io_input; io_output : bool } [@@deriving yojson_of]
+type io_input = NoInput | OnlyInput | Reentrant
+type io_log = { io_input : io_input; io_output : bool }
 
 type source_position = {
   filename : string;
@@ -41,7 +39,6 @@ type source_position = {
   end_column : int;
   law_headings : string list;
 }
-[@@deriving yojson_of]
 
 exception EmptyError
 exception AssertionFailed of source_position
@@ -190,12 +187,6 @@ let duration_to_string (d : duration) : string =
 let duration_to_years_months_days (d : duration) : int * int * int =
   Dates_calc.Dates.period_to_ymds d
 
-let yojson_of_money (m : money) = `Float (money_to_float m)
-let yojson_of_integer (i : integer) = `Int (integer_to_int i)
-let yojson_of_decimal (d : decimal) = `Float (decimal_to_float d)
-let yojson_of_date (d : date) = `String (date_to_string d)
-let yojson_of_duration (d : duration) = `String (duration_to_string d)
-
 type runtime_value =
   | Unit
   | Bool of bool
@@ -204,11 +195,10 @@ type runtime_value =
   | Decimal of decimal
   | Date of date
   | Duration of duration
-  | Enum of string list * (string * runtime_value)
-  | Struct of string list * (string * runtime_value) list
+  | Enum of string * (string * runtime_value)
+  | Struct of string * (string * runtime_value) list
   | Array of runtime_value array
   | Unembeddable
-[@@deriving yojson_of]
 
 let unembeddable _ = Unembeddable
 let embed_unit () = Unit
@@ -220,7 +210,7 @@ let embed_date x = Date x
 let embed_duration x = Duration x
 let embed_array f x = Array (Array.map f x)
 
-type information = string list [@@deriving yojson_of]
+type information = string list
 
 type raw_event =
   | BeginCall of information
@@ -236,7 +226,6 @@ type event =
       inputs : var_def list;
       body : event list;
     }
-[@@deriving yojson_of]
 
 and var_def = {
   pos : source_position option;
@@ -253,6 +242,108 @@ and fun_call = {
   output : var_def;
 }
 
+module BufferedJson = struct
+  let rec list f buf = function
+    | [] -> ()
+    | [x] -> f buf x
+    | x :: r ->
+      f buf x;
+      Buffer.add_char buf ',';
+      list f buf r
+
+  let quote buf str =
+    Buffer.add_char buf '"';
+    String.iter
+      (function
+        | ('"' | '\\') as c ->
+          Buffer.add_char buf '\\';
+          Buffer.add_char buf c
+        | '\n' -> Buffer.add_string buf "\\n"
+        | '\t' -> Buffer.add_string buf "\\t"
+        | '\r' -> Buffer.add_string buf "\\r"
+        | '\x00' .. '\x1F' as c -> Printf.bprintf buf "\\u%04x" (int_of_char c)
+        | c -> Buffer.add_char buf c)
+      str;
+    Buffer.add_char buf '"'
+
+  (* Note: the output format is made for transition with what Yojson gave us,
+     but we could change it to something nicer (e.g. objects for structures) *)
+  let rec runtime_value buf = function
+    | Unit -> Buffer.add_string buf {|"Unit"|}
+    | Bool b -> Buffer.add_string buf (string_of_bool b)
+    | Money m -> Buffer.add_string buf (money_to_string m)
+    | Integer i -> Buffer.add_string buf (integer_to_string i)
+    | Decimal d ->
+      Buffer.add_string buf (decimal_to_string ~max_prec_digits:10 d)
+    | Date d -> quote buf (date_to_string d)
+    | Duration d -> quote buf (duration_to_string d)
+    | Enum (name, (constr, v)) ->
+      Printf.bprintf buf {|[["%s"],["%s",%a]]|} name constr runtime_value v
+    | Struct (name, elts) ->
+      Printf.bprintf buf {|["%s",[%a]]|} name
+        (list (fun buf (cstr, v) ->
+             Printf.bprintf buf {|"%s":%a|} cstr runtime_value v))
+        elts
+    | Array elts ->
+      Printf.bprintf buf "[%a]" (list runtime_value) (Array.to_list elts)
+    | Unembeddable -> Buffer.add_string buf {|"unembeddable"|}
+
+  let information buf info = Printf.bprintf buf "[%a]" (list quote) info
+
+  let source_position buf pos =
+    Printf.bprintf buf {|{"filename":%a|} quote pos.filename;
+    Printf.bprintf buf {|,"start_line":%d|} pos.start_line;
+    Printf.bprintf buf {|,"start_column":%d|} pos.start_column;
+    Printf.bprintf buf {|,"end_line":%d|} pos.end_line;
+    Printf.bprintf buf {|,"end_column":%d|} pos.end_column;
+    Printf.bprintf buf {|,"law_headings":[%a]}|} (list quote) pos.law_headings
+
+  let io_input buf = function
+    | NoInput -> quote buf "NoInput"
+    | OnlyInput -> quote buf "OnlyInput"
+    | Reentrant -> quote buf "Reentrant"
+
+  let io_log buf iol =
+    Printf.bprintf buf {|{"io_input":%a|} io_input iol.io_input;
+    Printf.bprintf buf {|,"io_output":%b}|} iol.io_output
+
+  let rec event buf = function
+    | VarComputation vd ->
+      Printf.bprintf buf {|"VarComputation",%a]|} var_def vd
+    | FunCall fc -> Printf.bprintf buf {|"FunCall",%a]|} fun_call fc
+    | SubScopeCall { name; inputs; body } ->
+      Printf.bprintf buf {|{"name":%a,"inputs":[%a],"body":[%a]}|} information
+        name (list var_def) inputs (list event) body
+
+  and var_def buf def =
+    Option.iter (Printf.bprintf buf {|{"pos":%a|} source_position) def.pos;
+    Printf.bprintf buf {|,"name":%a|} information def.name;
+    Printf.bprintf buf {|,"io":%a|} io_log def.io;
+    Printf.bprintf buf {|,"value":%a|} runtime_value def.value;
+    Option.iter
+      (Printf.bprintf buf {|,"fun_calls":[%a]}|} (list fun_call))
+      def.fun_calls
+
+  and fun_call buf fc =
+    Printf.bprintf buf {|{"fun_name":%a|} information fc.fun_name;
+    Printf.bprintf buf {|,"fun_inputs":[%a]|} (list var_def) fc.fun_inputs;
+    Printf.bprintf buf {|,"body":[%a]|} (list event) fc.body;
+    Printf.bprintf buf {|,"output":%a}|} var_def fc.output
+end
+
+module Json = struct
+  let str f x =
+    let buf = Buffer.create 800 in
+    f buf x;
+    Buffer.contents buf
+
+  open BufferedJson
+
+  let runtime_value = str runtime_value
+  let io_log = str io_log
+  let event = str event
+end
+
 let log_ref : raw_event list ref = ref []
 let reset_log () = log_ref := []
 let retrieve_log () = List.rev !log_ref
@@ -313,8 +404,7 @@ let rec pp_events ?(is_first_call = true) ppf events =
     | Duration x -> Format.fprintf ppf "%s" (duration_to_string x)
     | Enum (_, (name, _)) -> Format.fprintf ppf "%s" name
     | Struct (name, attrs) ->
-      Format.fprintf ppf "@[<hv 2>%s = {@ %a@;<1 -2>}@]"
-        (String.concat "." name)
+      Format.fprintf ppf "@[<hv 2>%s = {@ %a@;<1 -2>}@]" name
         (Format.pp_print_list
            ~pp_sep:(fun ppf () -> Format.fprintf ppf ",@,")
            (fun fmt (name, value) ->
@@ -414,7 +504,7 @@ module EventParser = struct
     | VariableDefinition (name, io, value) ->
       Printf.sprintf "VariableDefinition([ %s ], %s, %s)"
         (String.concat ", " name) (io_log_to_string io)
-        (yojson_of_runtime_value value |> Yojson.Safe.to_string)
+        (Json.runtime_value value)
     | DecisionTaken pos ->
       Printf.sprintf "DecisionTaken(%s:%d.%d-%d.%d)" pos.filename pos.start_line
         pos.start_column pos.end_line pos.end_column
diff --git a/runtimes/ocaml/runtime.mli b/runtimes/ocaml/runtime.mli
index b5c4fa9f..7bc3a095 100644
--- a/runtimes/ocaml/runtime.mli
+++ b/runtimes/ocaml/runtime.mli
@@ -57,13 +57,11 @@ type io_input =
   | Reentrant
       (** For variables defined in the scope that can also be redefined by the
           caller as they appear in the input. *)
-[@@deriving yojson_of]
 
 type io_log = {
   io_input : io_input;
   io_output : bool;  (** [true] if the variable is an output *)
 }
-[@@deriving yojson_of]
 
 (** {1 Exceptions} *)
 
@@ -85,11 +83,10 @@ type runtime_value =
   | Decimal of decimal
   | Date of date
   | Duration of duration
-  | Enum of string list * (string * runtime_value)
-  | Struct of string list * (string * runtime_value) list
+  | Enum of string * (string * runtime_value)
+  | Struct of string * (string * runtime_value) list
   | Array of runtime_value Array.t
   | Unembeddable
-[@@deriving yojson_of]
 
 val unembeddable : 'a -> runtime_value
 val embed_unit : unit -> runtime_value
@@ -115,7 +112,7 @@ val embed_array : ('a -> runtime_value) -> 'a Array.t -> runtime_value
 
 (** {2 Data structures} *)
 
-type information = string list [@@deriving yojson_of]
+type information = string list
 (** Represents information about a name in the code -- i.e. variable name,
     subscope name, etc...
 
@@ -188,7 +185,6 @@ type event =
       inputs : var_def list;
       body : event list;
     }
-[@@deriving yojson_of]
 
 and var_def = {
   pos : source_position option;
@@ -230,6 +226,16 @@ val log_decision_taken : source_position -> bool -> bool
 
 (** {3 Pretty printers} *)
 
+(** {4 Conversions to JSON} *)
+module Json : sig
+  (* val io_input: io_input -> string *)
+  val io_log : io_log -> string
+  val runtime_value : runtime_value -> string
+
+  (* val information: information -> string *)
+  val event : event -> string
+end
+
 val pp_events : ?is_first_call:bool -> Format.formatter -> event list -> unit
 (** [pp_events ~is_first_call ppf events] pretty prints in [ppf] the string
     representation of [events].