diff --git a/src/lib_base/block_locator.ml b/src/lib_base/block_locator.ml
index 04cca9e96..99b7ddea9 100644
--- a/src/lib_base/block_locator.ml
+++ b/src/lib_base/block_locator.ml
@@ -16,6 +16,8 @@ and raw = Block_header.t * Block_hash.t list
 
 let raw x = x
 
+let pp ppf loc = Block_header.pp ppf (fst loc)
+
 let encoding =
   let open Data_encoding in
   (* TODO add a [description] *)
diff --git a/src/lib_base/block_locator.mli b/src/lib_base/block_locator.mli
index 9a3a15ad2..4c332fc77 100644
--- a/src/lib_base/block_locator.mli
+++ b/src/lib_base/block_locator.mli
@@ -15,6 +15,8 @@ and raw = Block_header.t * Block_hash.t list
 
 val raw: t -> raw
 
+val pp : Format.formatter -> t -> unit
+
 val encoding: t Data_encoding.t
 
 val compute: predecessor: (Block_hash.t -> int -> Block_hash.t option Lwt.t) ->
diff --git a/src/lib_shell/bootstrap_pipeline.ml b/src/lib_shell/bootstrap_pipeline.ml
index f935ad87a..107087c39 100644
--- a/src/lib_shell/bootstrap_pipeline.ml
+++ b/src/lib_shell/bootstrap_pipeline.ml
@@ -9,7 +9,7 @@
 
 include Logging.Make(struct let name = "node.validator.bootstrap_pipeline" end)
 
-type error += Invalid_locator of P2p_peer.Id.t * Block_locator.t
+open Validation_errors
 
 type t = {
   canceler: Lwt_canceler.t ;
diff --git a/src/lib_shell/bootstrap_pipeline.mli b/src/lib_shell/bootstrap_pipeline.mli
index 59f0a5b87..cc0c1cda5 100644
--- a/src/lib_shell/bootstrap_pipeline.mli
+++ b/src/lib_shell/bootstrap_pipeline.mli
@@ -9,8 +9,6 @@
 
 type t
 
-type error += Invalid_locator of P2p_peer.Id.t * Block_locator.t
-
 val create:
   ?notify_new_block: (State.Block.t -> unit) ->
   block_header_timeout:float ->
diff --git a/src/lib_shell/distributed_db_functors.ml b/src/lib_shell/distributed_db_functors.ml
index b75230fbb..bb5ab3d01 100644
--- a/src/lib_shell/distributed_db_functors.ml
+++ b/src/lib_shell/distributed_db_functors.ml
@@ -149,7 +149,9 @@ end = struct
   type error += Timeout of key
 
   let () =
-    Error_monad.register_error_kind `Permanent
+    (* Missing data key *)
+    register_error_kind
+      `Permanent
       ~id: ("distributed_db." ^ Hash.name ^ ".missing")
       ~title: ("Missing " ^ Hash.name)
       ~description: ("Some " ^ Hash.name ^ " is missing from the distributed db")
@@ -158,6 +160,7 @@ end = struct
       (Data_encoding.obj1 (Data_encoding.req "key" Hash.encoding))
       (function Missing_data key -> Some key | _ -> None)
       (fun key -> Missing_data key) ;
+    (* Canceled key *)
     register_error_kind
       `Permanent
       ~title: ("Canceled fetch of a " ^ Hash.name)
@@ -168,6 +171,7 @@ end = struct
       Data_encoding.(obj1 (req "key" Hash.encoding))
       (function (Canceled key) -> Some key | _ -> None)
       (fun key -> Canceled key) ;
+    (* Timeout key *)
     register_error_kind
       `Permanent
       ~title: ("Timed out fetch of a " ^ Hash.name)
diff --git a/src/lib_shell/peer_validator.ml b/src/lib_shell/peer_validator.ml
index d8e007d72..142048090 100644
--- a/src/lib_shell/peer_validator.ml
+++ b/src/lib_shell/peer_validator.ml
@@ -81,10 +81,6 @@ type t = Worker.dropbox Worker.t
 let debug w =
   Format.kasprintf (fun msg -> Worker.record_event w (Debug msg))
 
-type error +=
-  | Unknown_ancestor
-  | Known_invalid
-
 let set_bootstrapped pv =
   if not pv.bootstrapped then begin
     pv.bootstrapped <- true ;
@@ -194,7 +190,7 @@ let may_validate_new_head w hash header =
             "ignoring known invalid block %a from peer %a"
             Block_hash.pp_short hash
             P2p_peer.Id.pp_short pv.peer_id ;
-          fail Known_invalid
+          fail Validation_errors.Known_invalid
     end
   | false ->
       only_if_fitness_increases w header @@ fun () ->
@@ -211,7 +207,7 @@ let may_validate_new_branch w distant_hash locator =
         "ignoring branch %a without common ancestor from peer: %a."
         Block_hash.pp_short distant_hash
         P2p_peer.Id.pp_short pv.peer_id ;
-      fail Unknown_ancestor
+      fail Validation_errors.Unknown_ancestor
   | Some (ancestor, unknown_prefix) ->
       bootstrap_new_branch w ancestor distant_header unknown_prefix
 
@@ -246,8 +242,8 @@ let on_completion w r _ st =
 let on_error w r st errs =
   let pv = Worker.state w in
   match errs with
-    ((( Unknown_ancestor
-      | Bootstrap_pipeline.Invalid_locator _
+    ((( Validation_errors.Unknown_ancestor
+      | Validation_errors.Invalid_locator _
       | Block_validator_errors.Invalid_block _ ) :: _) as errors ) ->
       (* TODO ban the peer_id... *)
       debug w
diff --git a/src/lib_shell/prevalidation.ml b/src/lib_shell/prevalidation.ml
index 122cb9173..adbff649d 100644
--- a/src/lib_shell/prevalidation.ml
+++ b/src/lib_shell/prevalidation.ml
@@ -8,6 +8,7 @@
 (**************************************************************************)
 
 open Preapply_result
+open Validation_errors
 
 let rec apply_operations apply_operation state r max_ops ~sort ops =
   Lwt_list.fold_left_s
@@ -96,32 +97,6 @@ let start_prevalidation
   return (State { proto = (module Proto) ; state ;
                   max_number_of_operations ; max_operation_data_length })
 
-type error += Parse_error
-type error += Too_many_operations
-type error += Oversized_operation of { size: int ; max: int }
-
-let () =
-  register_error_kind `Temporary
-    ~id:"prevalidation.too_many_operations"
-    ~title:"Too many pending operations in prevalidation"
-    ~description:"The prevalidation context is full."
-    ~pp:(fun ppf () ->
-        Format.fprintf ppf "Too many operation in prevalidation context.")
-    Data_encoding.empty
-    (function Too_many_operations -> Some () | _ -> None)
-    (fun () -> Too_many_operations) ;
-  register_error_kind `Permanent
-    ~id:"prevalidation.oversized_operation"
-    ~title:"Oversized operation"
-    ~description:"The operation size is bigger than allowed."
-    ~pp:(fun ppf (size, max) ->
-        Format.fprintf ppf "Oversized operation (size: %d, max: %d)"
-          size max)
-    Data_encoding.(obj2
-                     (req "size" int31)
-                     (req "max_size" int31))
-    (function Oversized_operation { size ; max } -> Some (size, max) | _ -> None)
-    (fun (size, max) -> Oversized_operation { size ; max })
 
 let prevalidate
     (State { proto = (module Proto) ; state ;
diff --git a/src/lib_shell/protocol_validator.ml b/src/lib_shell/protocol_validator.ml
index 5f26f5d53..4253d8ba2 100644
--- a/src/lib_shell/protocol_validator.ml
+++ b/src/lib_shell/protocol_validator.ml
@@ -7,6 +7,8 @@
 (*                                                                        *)
 (**************************************************************************)
 
+open Validation_errors
+
 include Logging.Make(struct let name = "node.validator.block" end)
 
 type 'a request =
@@ -26,56 +28,6 @@ type t = {
 
 (** Block validation *)
 
-type protocol_error =
-  | Compilation_failed
-  | Dynlinking_failed
-
-let protocol_error_encoding =
-  let open Data_encoding in
-  union
-    [
-      case (Tag 0)
-        (obj1
-           (req "error" (constant "compilation_failed")))
-        (function Compilation_failed -> Some ()
-                | _ -> None)
-        (fun () -> Compilation_failed) ;
-      case (Tag 1)
-        (obj1
-           (req "error" (constant "dynlinking_failed")))
-        (function Dynlinking_failed -> Some ()
-                | _ -> None)
-        (fun () -> Dynlinking_failed) ;
-    ]
-
-let pp_protocol_error ppf = function
-  | Compilation_failed ->
-      Format.fprintf ppf "compilation error"
-  | Dynlinking_failed ->
-      Format.fprintf ppf "dynlinking error"
-
-type error +=
-  | Invalid_protocol of { hash: Protocol_hash.t ; error: protocol_error }
-
-let () =
-  Error_monad.register_error_kind
-    `Permanent
-    ~id:"validator.invalid_protocol"
-    ~title:"Invalid protocol"
-    ~description:"Invalid protocol."
-    ~pp:begin fun ppf (protocol, error) ->
-      Format.fprintf ppf
-        "@[<v 2>Invalid protocol %a@ %a@]"
-        Protocol_hash.pp_short protocol pp_protocol_error error
-    end
-    Data_encoding.(merge_objs
-                     (obj1 (req "invalid_protocol" Protocol_hash.encoding))
-                     protocol_error_encoding)
-    (function Invalid_protocol { hash ; error } ->
-       Some (hash, error) | _ -> None)
-    (fun (hash, error) ->
-       Invalid_protocol { hash ; error })
-
 let rec worker_loop bv =
   begin
     protect ~canceler:bv.canceler begin fun () ->
diff --git a/src/lib_shell/protocol_validator.mli b/src/lib_shell/protocol_validator.mli
index df48e848e..2abcab9da 100644
--- a/src/lib_shell/protocol_validator.mli
+++ b/src/lib_shell/protocol_validator.mli
@@ -9,14 +9,6 @@
 
 type t
 
-type protocol_error =
-  | Compilation_failed
-  | Dynlinking_failed
-
-type error +=
-  | Invalid_protocol of
-      { hash: Protocol_hash.t ; error: protocol_error }
-
 val create: Distributed_db.t -> t
 
 val validate:
diff --git a/src/lib_shell/state.ml b/src/lib_shell/state.ml
index e2db5bcdb..5ee47dafb 100644
--- a/src/lib_shell/state.ml
+++ b/src/lib_shell/state.ml
@@ -8,51 +8,7 @@
 (**************************************************************************)
 
 open Logging.Node.State
-
-type error +=
-  | Unknown_chain of Chain_id.t
-
-type error += Bad_data_dir
-
-type error += Block_not_invalid of Block_hash.t
-
-let () =
-  let open Error_monad in
-  register_error_kind
-    `Temporary
-    ~id:"state.unknown_chain"
-    ~title:"Unknown chain"
-    ~description:"TODO"
-    ~pp:(fun ppf id ->
-        Format.fprintf ppf "Unknown chain %a" Chain_id.pp id)
-    Data_encoding.(obj1 (req "chain" Chain_id.encoding))
-    (function Unknown_chain x -> Some x | _ -> None)
-    (fun x -> Unknown_chain x) ;
-  register_error_kind
-    `Permanent
-    ~id:"badDataDir"
-    ~title:"Bad data directory"
-    ~description:"The data directory could not be read. \
-                  This could be because it was generated with an \
-                  old version of the tezos-node program. \
-                  Deleting and regenerating this directory \
-                  may fix the problem."
-    Data_encoding.empty
-    (function Bad_data_dir -> Some () | _ -> None)
-    (fun () -> Bad_data_dir) ;
-  register_error_kind
-    `Permanent
-    ~id:"blockNotInvalid"
-    ~title:"Block not invalid"
-    ~description:"The invalid block to be unmarked was not actually invalid."
-    ~pp:(fun ppf block ->
-        Format.fprintf ppf "Block %a was expected to be invalid, but was not actually invalid."
-          Block_hash.pp block)
-    Data_encoding.(obj1 (req "block" Block_hash.encoding))
-    (function Block_not_invalid block -> Some block | _ -> None)
-    (fun block -> Block_not_invalid block) ;
-
-  (** *)
+open Validation_errors
 
 module Shared = struct
   type 'a t = {
@@ -545,28 +501,6 @@ module Block = struct
     Shared.use chain.block_store (fun store ->
         predecessor_n store b distance)
 
-
-  type error += Inconsistent_hash of Context_hash.t * Context_hash.t
-
-  let () =
-    Error_monad.register_error_kind
-      `Permanent
-      ~id:"inconsistentContextHash"
-      ~title:"Inconsistent commit hash"
-      ~description:
-        "When commiting the context of a block, the announced context \
-         hash was not the one computed at commit time."
-      ~pp: (fun ppf (got, exp) ->
-          Format.fprintf ppf
-            "@[<v 2>Inconsistant hash:@ got: %a@ expected: %a"
-            Context_hash.pp got
-            Context_hash.pp exp)
-      Data_encoding.(obj2
-                       (req "wrong_context_hash" Context_hash.encoding)
-                       (req "expected_context_hash" Context_hash.encoding))
-      (function Inconsistent_hash (got, exp) -> Some (got, exp) | _ -> None)
-      (fun (got, exp) -> Inconsistent_hash (got, exp))
-
   let store
       ?(dont_enforce_context_hash = false)
       chain_state block_header operations
diff --git a/src/lib_shell/state.mli b/src/lib_shell/state.mli
index 8a50e7685..568937728 100644
--- a/src/lib_shell/state.mli
+++ b/src/lib_shell/state.mli
@@ -31,12 +31,6 @@ val read:
 val close:
   global_state -> unit Lwt.t
 
-(** {2 Errors} **************************************************************)
-
-type error +=
-  | Unknown_chain of Chain_id.t
-
-
 (** {2 Network} ************************************************************)
 
 (** Data specific to a given chain (e.g the main chain or the current
@@ -105,8 +99,6 @@ module Block : sig
   val read_opt: Chain.t -> Block_hash.t -> block option Lwt.t
   val read_exn: Chain.t -> Block_hash.t -> block Lwt.t
 
-  type error += Inconsistent_hash of Context_hash.t * Context_hash.t
-
   val store:
     ?dont_enforce_context_hash:bool ->
     Chain.t ->
diff --git a/src/lib_shell/test/jbuild b/src/lib_shell/test/jbuild
index dbcc8bd07..973b737a4 100644
--- a/src/lib_shell/test/jbuild
+++ b/src/lib_shell/test/jbuild
@@ -7,6 +7,7 @@
               tezos-storage
               tezos-protocol-updater
               tezos-shell
+              tezos-shell-services
               tezos-embedded-protocol-demo
               tezos-stdlib-unix
               alcotest-lwt))
@@ -16,6 +17,7 @@
                     -open Tezos_storage
                     -open Tezos_protocol_updater
                     -open Tezos_shell
+                    -open Tezos_shell_services
                     -open Tezos_stdlib_unix))))
 
 (alias
diff --git a/src/lib_shell/test/test_state.ml b/src/lib_shell/test/test_state.ml
index d40109fdf..0d9099109 100644
--- a/src/lib_shell/test/test_state.ml
+++ b/src/lib_shell/test/test_state.ml
@@ -101,7 +101,7 @@ let build_valid_chain state vtbl pred names =
            return vblock
          end >>= function
          | Ok v -> Lwt.return v
-         | Error [ State.Block.Inconsistent_hash (got, _) ] ->
+         | Error [ Validation_errors.Inconsistent_hash (got, _) ] ->
              (* Kind of a hack, but at least it tests idempotence to some extent. *)
              attempt (Some got)
          | Error err ->
diff --git a/src/lib_shell/validator.ml b/src/lib_shell/validator.ml
index 627a32c30..0310da7b9 100644
--- a/src/lib_shell/validator.ml
+++ b/src/lib_shell/validator.ml
@@ -55,26 +55,9 @@ let activate v ?max_child_ttl chain_state =
 let get_exn { active_chains } chain_id =
   Chain_id.Table.find active_chains chain_id
 
-type error +=
-  | Inactive_chain of Chain_id.t
-
-let () =
-  register_error_kind `Branch
-    ~id: "node.validator.inactive_chain"
-    ~title: "Inactive chain"
-    ~description: "Attempted validation of a block from an inactive chain."
-    ~pp: (fun ppf chain ->
-        Format.fprintf ppf
-          "Tried to validate a block from chain %a, \
-           that is not currently considered active."
-          Chain_id.pp chain)
-    Data_encoding.(obj1 (req "inactive_chain" Chain_id.encoding))
-    (function Inactive_chain chain -> Some chain | _ -> None)
-    (fun chain -> Inactive_chain chain)
-
 let get v chain_id =
   try get_exn v chain_id >>= fun nv -> return nv
-  with Not_found -> fail (Inactive_chain chain_id)
+  with Not_found -> fail (Validation_errors.Inactive_chain chain_id)
 
 let validate_block v ?(force = false) ?chain_id bytes operations =
   let hash = Block_hash.hash_bytes [bytes] in
diff --git a/src/lib_shell/validator.mli b/src/lib_shell/validator.mli
index 289575adf..2fd9d4aaa 100644
--- a/src/lib_shell/validator.mli
+++ b/src/lib_shell/validator.mli
@@ -27,8 +27,6 @@ val activate:
   ?max_child_ttl:int ->
   State.Chain.t -> Chain_validator.t Lwt.t
 
-type error +=
-  | Inactive_chain of Chain_id.t
 val get: t -> Chain_id.t -> Chain_validator.t tzresult Lwt.t
 val get_exn: t -> Chain_id.t -> Chain_validator.t Lwt.t
 
diff --git a/src/lib_shell_services/validation_errors.ml b/src/lib_shell_services/validation_errors.ml
new file mode 100644
index 000000000..859f6592c
--- /dev/null
+++ b/src/lib_shell_services/validation_errors.ml
@@ -0,0 +1,246 @@
+(**************************************************************************)
+(*                                                                        *)
+(*    Copyright (c) 2014 - 2018.                                          *)
+(*    Dynamic Ledger Solutions, Inc. <contact@tezos.com>                  *)
+(*                                                                        *)
+(*    All rights reserved. No warranty, explicit or implicit, provided.   *)
+(*                                                                        *)
+(**************************************************************************)
+
+(***************** Prevalidation errors ***********************************)
+
+type error += Parse_error
+type error += Too_many_operations
+type error += Oversized_operation of { size: int ; max: int }
+
+let () =
+  (* Parse error *)
+  register_error_kind
+    `Permanent
+    ~id:"prevalidation.parse_error"
+    ~title:"Parsing error in prevalidation"
+    ~description:"Raised when an operation has not been parsed correctly during prevalidation."
+    ~pp:(fun ppf () ->
+        Format.fprintf ppf "Operation parsing error in prevalidation.")
+    Data_encoding.empty
+    (function Parse_error -> Some () | _ -> None)
+    (fun () -> Parse_error) ;
+  (* Too many operations *)
+  register_error_kind
+    `Temporary
+    ~id:"prevalidation.too_many_operations"
+    ~title:"Too many pending operations in prevalidation"
+    ~description:"The prevalidation context is full."
+    ~pp:(fun ppf () ->
+        Format.fprintf ppf "Too many operations in prevalidation context.")
+    Data_encoding.empty
+    (function Too_many_operations -> Some () | _ -> None)
+    (fun () -> Too_many_operations) ;
+  (* Oversized operation *)
+  register_error_kind
+    `Permanent
+    ~id:"prevalidation.oversized_operation"
+    ~title:"Oversized operation"
+    ~description:"The operation size is bigger than allowed."
+    ~pp:(fun ppf (size, max) ->
+        Format.fprintf ppf "Oversized operation (size: %d, max: %d)"
+          size max)
+    Data_encoding.(obj2
+                     (req "size" int31)
+                     (req "max_size" int31))
+    (function Oversized_operation { size ; max } -> Some (size, max) | _ -> None)
+    (fun (size, max) -> Oversized_operation { size ; max })
+
+
+(************************* State errors ***********************************)
+
+type error +=  Unknown_chain of Chain_id.t
+
+type error += Bad_data_dir
+
+type error += Block_not_invalid of Block_hash.t
+
+let () =
+  (* Unknown network *)
+  register_error_kind
+    `Permanent
+    ~id:"state.unknown_chain"
+    ~title:"Unknown chain"
+    ~description:"The chain identifier could not be found in \
+                  the chain identifiers table."
+    ~pp:(fun ppf id ->
+        Format.fprintf ppf "Unknown chain %a" Chain_id.pp id)
+    Data_encoding.(obj1 (req "chain" Chain_id.encoding))
+    (function Unknown_chain x -> Some x | _ -> None)
+    (fun x -> Unknown_chain x) ;
+  register_error_kind
+    `Permanent
+    ~id:"badDataDir"
+    ~title:"Bad data directory"
+    ~description:"The data directory could not be read. \
+                  This could be because it was generated with an \
+                  old version of the tezos-node program. \
+                  Deleting and regenerating this directory \
+                  may fix the problem."
+    ~pp:(fun ppf () -> Format.fprintf ppf "Bad data directory.")
+    Data_encoding.empty
+    (function Bad_data_dir -> Some () | _ -> None)
+    (fun () -> Bad_data_dir) ;
+  (* Block not invalid *)
+  register_error_kind
+    `Permanent
+    ~id:"blockNotInvalid"
+    ~title:"Block not invalid"
+    ~description:"The invalid block to be unmarked was not actually invalid."
+    ~pp:(fun ppf block ->
+        Format.fprintf ppf "Block %a was expected to be invalid, but was not actually invalid."
+          Block_hash.pp block)
+    Data_encoding.(obj1 (req "block" Block_hash.encoding))
+    (function Block_not_invalid block -> Some block | _ -> None)
+    (fun block -> Block_not_invalid block)
+
+(* Block database error *)
+
+type error += Inconsistent_hash of Context_hash.t * Context_hash.t
+
+let () =
+  (* Inconsistent hash *)
+  register_error_kind
+    `Permanent
+    ~id:"inconsistentContextHash"
+    ~title:"Inconsistent commit hash"
+    ~description:
+      "When commiting the context of a block, the announced context \
+       hash was not the one computed at commit time."
+    ~pp: (fun ppf (got, exp) ->
+        Format.fprintf ppf
+          "@[<v 2>Inconsistant hash:@ got: %a@ expected: %a"
+          Context_hash.pp got
+          Context_hash.pp exp)
+    Data_encoding.(obj2
+                     (req "wrong_context_hash" Context_hash.encoding)
+                     (req "expected_context_hash" Context_hash.encoding))
+    (function Inconsistent_hash (got, exp) -> Some (got, exp) | _ -> None)
+    (fun (got, exp) -> Inconsistent_hash (got, exp))
+
+(******************* Bootstrap pipeline errors ****************************)
+
+type error += Invalid_locator of P2p_peer.Id.t * Block_locator.t
+
+let () =
+  (* Invalid locator *)
+  register_error_kind
+    `Permanent
+    ~id:"invalidLocator"
+    ~title:"Invalid block locator"
+    ~description:"Block locator is invalid."
+    ~pp: (fun ppf (id, locator) ->
+        Format.fprintf ppf
+          "Invalid block locator %a on peer %a"
+          Block_locator.pp locator
+          P2p_peer.Id.pp id)
+    Data_encoding.(obj2
+                     (req "id" P2p_peer.Id.encoding)
+                     (req "locator" Block_locator.encoding))
+    (function | Invalid_locator (id, loc) -> Some (id, loc) | _ -> None)
+    (fun (id, loc) -> Invalid_locator (id, loc))
+
+(******************* Protocol validator errors ****************************)
+
+type protocol_error =
+  | Compilation_failed
+  | Dynlinking_failed
+
+type error += Invalid_protocol of { hash: Protocol_hash.t ; error: protocol_error }
+
+let protocol_error_encoding =
+  let open Data_encoding in
+  union
+    [
+      case (Tag 0)
+        (obj1
+           (req "error" (constant "compilation_failed")))
+        (function Compilation_failed -> Some ()
+                | _ -> None)
+        (fun () -> Compilation_failed) ;
+      case (Tag 1)
+        (obj1
+           (req "error" (constant "dynlinking_failed")))
+        (function Dynlinking_failed -> Some ()
+                | _ -> None)
+        (fun () -> Dynlinking_failed) ;
+    ]
+
+let pp_protocol_error ppf = function
+  | Compilation_failed ->
+      Format.fprintf ppf "compilation error"
+  | Dynlinking_failed ->
+      Format.fprintf ppf "dynlinking error"
+
+let () =
+  (* Invalid protocol *)
+  register_error_kind
+    `Permanent
+    ~id:"validator.invalid_protocol"
+    ~title:"Invalid protocol"
+    ~description:"Invalid protocol."
+    ~pp:begin fun ppf (protocol, error) ->
+      Format.fprintf ppf
+        "@[<v 2>Invalid protocol %a@ %a@]"
+        Protocol_hash.pp_short protocol pp_protocol_error error
+    end
+    Data_encoding.(merge_objs
+                     (obj1 (req "invalid_protocol" Protocol_hash.encoding))
+                     protocol_error_encoding)
+    (function Invalid_protocol { hash ; error } ->
+       Some (hash, error) | _ -> None)
+    (fun (hash, error) ->
+       Invalid_protocol { hash ; error })
+
+(********************* Peer validator errors ******************************)
+
+type error +=
+  | Unknown_ancestor
+  | Known_invalid
+
+let () =
+  (* Unknown ancestor *)
+  register_error_kind
+    `Permanent
+    ~id: "node.peer_validator.unknown_ancestor"
+    ~title: "Unknown ancestor"
+    ~description: "Unknown ancestor block found in the peer's chain"
+    ~pp: (fun ppf () -> Format.fprintf ppf "Unknown ancestor")
+    Data_encoding.empty
+    (function Unknown_ancestor -> Some () | _ -> None)
+    (fun () -> Unknown_ancestor) ;
+  (* Known invalid *)
+  register_error_kind
+    `Permanent
+    ~id: "node.peer_validator.known_invalid"
+    ~title: "Known invalid"
+    ~description: "Known invalid block found in the peer's chain"
+    ~pp: (fun ppf () -> Format.fprintf ppf "Known invalid")
+    Data_encoding.empty
+    (function Known_invalid -> Some () | _ -> None)
+    (fun () -> Known_invalid)
+
+(************************ Validator errors ********************************)
+
+type error +=  Inactive_chain of Chain_id.t
+
+let () =
+  (* Inactive network *)
+  register_error_kind
+    `Branch
+    ~id: "node.validator.inactive_chain"
+    ~title: "Inactive chain"
+    ~description: "Attempted validation of a block from an inactive chain."
+    ~pp: (fun ppf chain ->
+        Format.fprintf ppf
+          "Tried to validate a block from chain %a, \
+           that is not currently considered active."
+          Chain_id.pp chain)
+    Data_encoding.(obj1 (req "inactive_chain" Chain_id.encoding))
+    (function Inactive_chain chain -> Some chain | _ -> None)
+    (fun chain -> Inactive_chain chain)