diff --git a/swh/model/identifiers.py b/swh/model/identifiers.py
--- a/swh/model/identifiers.py
+++ b/swh/model/identifiers.py
@@ -728,22 +728,19 @@
     def check_namespace(self, attribute, value):
         if value != SWHID_NAMESPACE:
             raise ValidationError(
-                "Wrong format: only supported namespace is '%s'" % SWHID_NAMESPACE
+                f"Wrong format: Supported namespace is '{SWHID_NAMESPACE}'"
             )
 
     @scheme_version.validator
     def check_scheme_version(self, attribute, value):
         if value != SWHID_VERSION:
-            raise ValidationError(
-                "Wrong format: only supported version is %d" % SWHID_VERSION
-            )
+            raise ValidationError(f"Wrong format: Supported version is {SWHID_VERSION}")
 
     @object_type.validator
     def check_object_type(self, attribute, value):
         if value not in _object_type_map:
-            raise ValidationError(
-                "Wrong input: Supported types are %s" % (list(_object_type_map.keys()))
-            )
+            supported_types = ", ".join(list(_object_type_map.keys()))
+            raise ValidationError(f"Wrong input: Supported types are {supported_types}")
 
     @object_id.validator
     def check_object_id(self, attribute, value):
@@ -823,7 +820,22 @@
     swhid_data = swhid_parts.pop(0).split(":")
 
     if len(swhid_data) != 4:
-        raise ValidationError("Wrong format: There should be 4 mandatory values")
+        raise ValidationError(
+            """Wrong format: The supported format should be:
+  <identifier_core> ::= "swh" ":" <scheme_version> ":" <object_type> ":" <object_id> ;
+  <scheme_version> ::= "1" ;
+  <object_type> ::=
+      "snp"  (* snapshot *)
+    | "rel"  (* release *)
+    | "rev"  (* revision *)
+    | "dir"  (* directory *)
+    | "cnt"  (* content *)
+    ;
+  <object_id> ::= 40 * <hex_digit> ;  (* intrinsic object id, as hex-encoded SHA1 *)
+  <dec_digit> ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
+  <hex_digit> ::= <dec_digit> | "a" | "b" | "c" | "d" | "e" | "f" ;
+"""
+        )
 
     # Checking for parsing errors
     _ns, _version, _type, _id = swhid_data
@@ -834,7 +846,13 @@
             break
 
     if not _id:
-        raise ValidationError("Wrong format: Identifier should be present")
+        raise ValidationError(
+            """Wrong format: <object_id> should be provided:
+  <object_id> ::= 40 * <hex_digit> ;  (* intrinsic object id, as hex-encoded SHA1 *)
+  <dec_digit> ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
+  <hex_digit> ::= <dec_digit> | "a" | "b" | "c" | "d" | "e" | "f" ;
+"""
+        )
 
     _metadata = {}
     for part in swhid_parts:
@@ -842,7 +860,23 @@
             key, val = part.split("=")
             _metadata[key] = val
         except Exception:
-            msg = "Contextual data is badly formatted, form key=val expected"
+            msg = """Wrong format: contextual data should be of form:
+  <qualifiers> := ";" <qualifier> [ <qualifiers> ] ;
+  <qualifier> ::=
+      <context_qualifier>
+    | <fragment_qualifier>
+    ;
+  <context_qualifier> ::=
+      <origin_ctxt>
+    | <visit_ctxt>
+    | <anchor_ctxt>
+    | <path_ctxt>
+    ;
+  <origin_ctxt> ::= "origin" "=" <url_escaped> ;
+  <visit_ctxt> ::= "visit" "=" <identifier_core> ;
+  <anchor_ctxt> ::= "anchor" "=" <identifier_core> ;
+  <path_ctxt> ::= "path" "=" <path_absolute_escaped> ;
+"""
             raise ValidationError(msg)
     return SWHID(
         _ns,