diff --git a/CHANGELOG.md b/CHANGELOG.md
index f3535d22..cd4d8810 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,7 +20,13 @@ If possible, provide tooling that performs the changes, e.g. a shell-script.
 
 ## API changes
 
-#### I/O
+#### General
+
+* Custom option types must not only model `sharg::istreamable` (`stream >> option`)
+  but must also model `sharg::ostreamable` in order to be used in `parser.add_option()` calls.
+  All standard types as well as types that overload `sharg::named_enumeration` are not affected.
+
+#### Validators
 
 * In order to avoid using the seqan3 I/O module, you now have to give a list of file extensions explicitly to
 `sharg::input_file_validator` and `sharg::output_file_validator`:
diff --git a/include/sharg/auxiliary.hpp b/include/sharg/auxiliary.hpp
index 4ea3f9ce..47bca531 100644
--- a/include/sharg/auxiliary.hpp
+++ b/include/sharg/auxiliary.hpp
@@ -14,8 +14,8 @@
 
 #include <unordered_map>
 #include <vector>
+#include <iostream>
 
-#include <seqan3/core/debug_stream/debug_stream_type.hpp>
 #include <seqan3/core/detail/customisation_point.hpp>
 
 #include <sharg/platform.hpp>
@@ -291,11 +291,11 @@ struct argument_parser_meta_data // holds all meta information
 } // namespace sharg
 
 //!\cond
-namespace seqan3
+namespace std
 {
-template <typename char_t, typename option_type>
+template <typename option_type>
     requires sharg::named_enumeration<std::remove_cvref_t<option_type>>
-inline debug_stream_type<char_t> & operator<<(debug_stream_type<char_t> & s, option_type && op)
+inline ostream & operator<<(ostream & s, option_type && op)
 {
     for (auto & [key, value] : sharg::enumeration_names<option_type>)
     {
@@ -305,5 +305,5 @@ inline debug_stream_type<char_t> & operator<<(debug_stream_type<char_t> & s, opt
 
     return s << "<UNKNOWN_VALUE>";
 }
-}
+} // namespace std
 //!\endcond
diff --git a/include/sharg/concept.hpp b/include/sharg/concept.hpp
index e90501c0..fbf137af 100644
--- a/include/sharg/concept.hpp
+++ b/include/sharg/concept.hpp
@@ -34,6 +34,26 @@ concept istreamable = requires (std::istream & is, value_type & val)
     SHARG_RETURN_TYPE_CONSTRAINT(is >> val, std::same_as, std::istream&);
 };
 
+/*!\concept sharg::ostreamable
+ * \ingroup argument_parser
+ * \brief Concept for types that can be parsed into a std::ostream via the stream operator.
+ * \tparam type The type to check whether it's stremable via std::ostream or it's a container over streamable values.
+ *
+ * ### Requirements
+ *
+ * `std::ostream` must support the (un)formatted output function (`operator<<`) for an l-value of a given `type` or
+ * for an l-value of `type::reference`.
+ */
+template <typename type>
+concept ostreamable = requires (std::ostream & os, type & val)
+{
+    SHARG_RETURN_TYPE_CONSTRAINT(os << val, std::same_as, std::ostream&);
+} ||
+requires (std::ostream & os, type & con)
+{
+    SHARG_RETURN_TYPE_CONSTRAINT(os << con[0], std::same_as, std::ostream&);
+};
+
 /*!\concept sharg::argument_parser_compatible_option
  * \brief Checks whether the the type can be used in an add_(positional_)option call on the argument parser.
  * \ingroup argument_parser
@@ -47,6 +67,7 @@ concept istreamable = requires (std::istream & is, value_type & val)
  * \remark For a complete overview, take a look at \ref argument_parser
  */
 template <typename option_type>
-concept argument_parser_compatible_option = sharg::istreamable<option_type> || named_enumeration<option_type>;
+concept argument_parser_compatible_option = (sharg::istreamable<option_type> && sharg::ostreamable<option_type>) ||
+                                            named_enumeration<option_type>;
 
 } // namespace sharg
diff --git a/include/sharg/detail/format_base.hpp b/include/sharg/detail/format_base.hpp
index a0967497..f9497f24 100644
--- a/include/sharg/detail/format_base.hpp
+++ b/include/sharg/detail/format_base.hpp
@@ -222,7 +222,7 @@ class format_help_base : public format_base
     {
         std::string id = prep_id_for_help(short_id, long_id) + " " + option_type_and_list_info(value);
         std::string info{desc};
-        info += ((spec & option_spec::required) ? std::string{" "} : seqan3::detail::to_string(" Default: ", value, ". "));
+        info += ((spec & option_spec::required) ? std::string{" "} : detail::to_string(" Default: ", value, ". "));
         info += option_validator.get_help_page_message();
         store_help_page_element([this, id, info] () { derived_t().print_list_item(id, info); }, spec);
     }
@@ -253,12 +253,12 @@ class format_help_base : public format_base
         positional_option_calls.push_back([this, &value, desc, msg] ()
         {
             ++positional_option_count;
-            derived_t().print_list_item(seqan3::detail::to_string("\\fBARGUMENT-", positional_option_count, "\\fP ",
-                                                                  option_type_and_list_info(value)),
+            derived_t().print_list_item(detail::to_string("\\fBARGUMENT-", positional_option_count, "\\fP ",
+                                        option_type_and_list_info(value)),
                                         desc +
                                         // a list at the end may be empty and thus have a default value
                                         ((detail::is_container_option<option_type>)
-                                            ? seqan3::detail::to_string(" Default: ", value, ". ")
+                                            ? detail::to_string(" Default: ", value, ". ")
                                             : std::string{" "}) +
                                         msg);
         });
diff --git a/include/sharg/detail/to_string.hpp b/include/sharg/detail/to_string.hpp
new file mode 100644
index 00000000..2cb3138a
--- /dev/null
+++ b/include/sharg/detail/to_string.hpp
@@ -0,0 +1,67 @@
+// -----------------------------------------------------------------------------------------------------------
+// Copyright (c) 2006-2021, Knut Reinert & Freie Universität Berlin
+// Copyright (c) 2016-2021, Knut Reinert & MPI für molekulare Genetik
+// This file may be used, modified and/or redistributed under the terms of the 3-clause BSD-License
+// shipped with this file and also available at: https://github.com/seqan/sharg-parser/blob/master/LICENSE.md
+// -----------------------------------------------------------------------------------------------------------
+
+/*!\file
+ * \author Svenja Mehringer <svenja.mehringer AT fu-berlin.de>
+ * \brief Provides sharg::detail::to_string.
+ */
+
+#pragma once
+
+#include <sstream>
+
+#include <sharg/detail/concept.hpp>
+#include <sharg/concept.hpp>
+
+namespace sharg::detail
+{
+
+/*!\brief Streams all parameters via std::ostringstream and returns a concatenated string.
+ * \tparam    value_types Must be sharg::ostreamable (stream << value).
+ * \param[in] values     Variable number of parameters of any type that implement the stream operator.
+ * \returns A concatenated string of all values (no separator in between is added).
+ */
+template <typename ...value_types>
+    requires (ostreamable<value_types> && ...)
+std::string to_string(value_types && ...values)
+{
+    std::stringstream stream;
+
+    auto print = [&stream] (auto val)
+    {
+        if constexpr (is_container_option<decltype(val)>)
+        {
+            if (val.empty())
+            {
+                stream << "[]";
+            }
+            else
+            {
+                stream << '[';
+                stream << val[0];
+                for (size_t i = 1; i < val.size(); ++i)
+                    stream << ", " << val[i];
+                stream << ']';
+            }
+        }
+        else if constexpr (std::is_same_v<std::remove_cvref_t<decltype(val)>, int8_t> ||
+                           std::is_same_v<std::remove_cvref_t<decltype(val)>, uint8_t>)
+        {
+            stream << static_cast<int16_t>(val);
+        }
+        else
+        {
+            stream << val;
+        }
+    };
+
+    (print(std::forward<value_types>(values)),...);
+
+    return stream.str();
+}
+
+} // namespace sharg::detail
diff --git a/include/sharg/validators.hpp b/include/sharg/validators.hpp
index 1dfa5c3f..72209cf4 100644
--- a/include/sharg/validators.hpp
+++ b/include/sharg/validators.hpp
@@ -15,10 +15,11 @@
 #include <fstream>
 #include <regex>
 
-#include <seqan3/core/debug_stream/detail/to_string.hpp>
-#include <seqan3/core/debug_stream/range.hpp>
+#include <seqan3/std/concepts>
+#include <seqan3/std/ranges>
 
 #include <sharg/detail/safe_filesystem_entry.hpp>
+#include <sharg/detail/to_string.hpp>
 #include <sharg/exceptions.hpp>
 
 namespace sharg
@@ -131,7 +132,7 @@ class arithmetic_range_validator
 /*!\brief A validator that checks whether a value is inside a list of valid values.
  * \ingroup argument_parser
  * \implements sharg::validator
- * \tparam option_value_t \copybrief sharg::value_list_validator::option_value_type
+ * \tparam option_value_t The type the validator is called on. Must model sharg::argument_parser_compatible_option.
  *
  * \details
  *
@@ -147,7 +148,7 @@ class arithmetic_range_validator
  *
  * \remark For a complete overview, take a look at \ref argument_parser
  */
-template <typename option_value_t>
+template <argument_parser_compatible_option option_value_t>
 class value_list_validator
 {
 public:
@@ -201,7 +202,7 @@ class value_list_validator
     void operator()(option_value_type const & cmp) const
     {
         if (!(std::find(values.begin(), values.end(), cmp) != values.end()))
-            throw validation_error{seqan3::detail::to_string("Value ", cmp, " is not one of ", std::views::all(values), ".")};
+            throw validation_error{detail::to_string("Value ", cmp, " is not one of ", values,".")};
     }
 
     /*!\brief Tests whether every element in \p range lies inside values.
@@ -221,11 +222,10 @@ class value_list_validator
     //!\brief Returns a message that can be appended to the (positional) options help page info.
     std::string get_help_page_message() const
     {
-        return seqan3::detail::to_string("Value must be one of ", std::views::all(values), ".");
+        return detail::to_string("Value must be one of ", values, ".");
     }
 
 private:
-
     //!\brief Minimum of the range to test.
     std::vector<option_value_type> values{};
 };
@@ -437,19 +437,6 @@ class file_validator_base
         return true;
     }
 
-    //!\brief Creates a std::string from the extensions list, e.g. "[ext, ext2]".
-    std::string const create_extensions_str() const
-    {
-        if (extensions.empty())
-            return "[]";
-
-        std::string result{'['};
-        for (std::string const & ext : extensions)
-            result += ext + ", ";
-        result.replace(result.size() - 2, 2, "]"); // replace last ", " by "]"
-        return result;
-    }
-
     //!\brief Stores the extensions.
     std::vector<std::string> extensions{};
 
@@ -501,8 +488,8 @@ class input_file_validator : public file_validator_base
      */
     explicit input_file_validator(std::vector<std::string> extensions) : file_validator_base{}
     {
+        file_validator_base::extensions_str = detail::to_string(extensions);
         file_validator_base::extensions = std::move(extensions);
-        file_validator_base::extensions_str = create_extensions_str();
     }
 
     // Import base class constructor.
@@ -613,8 +600,8 @@ class output_file_validator : public file_validator_base
     explicit output_file_validator(output_file_open_options const mode, std::vector<std::string> extensions = {})
         : file_validator_base{}, mode{mode}
     {
+        file_validator_base::extensions_str = detail::to_string(extensions);
         file_validator_base::extensions = std::move(extensions);
-        file_validator_base::extensions_str = create_extensions_str();
     }
 
     // Import base constructor.
diff --git a/test/unit/detail/format_help_test.cpp b/test/unit/detail/format_help_test.cpp
index b522233a..c7f01e6c 100644
--- a/test/unit/detail/format_help_test.cpp
+++ b/test/unit/detail/format_help_test.cpp
@@ -7,6 +7,8 @@
 
 #include <gtest/gtest.h>
 
+#include <seqan3/std/ranges>
+
 #include <sharg/argument_parser.hpp>
 
 // reused global variables
@@ -449,8 +451,8 @@ TEST(help_page_printing, full_information)
                "    -i, --int (signed 32 bit integer)\n"
                "          this is a int option. Default: 5.\n"
                "    -e, --enum (foo)\n"
-               "          this is an enum option. Default: one. Value must be one of\n"
-               "          [three,two,one].\n"
+               "          this is an enum option. Default: one. Value must be one of [three,\n"
+               "          two, one].\n"
                "    -r, --required-int (signed 8 bit integer)\n"
                "          this is another int option.\n"
                "\n"
diff --git a/test/unit/format_parse_test.cpp b/test/unit/format_parse_test.cpp
index 8959ccf2..a5fe3c2a 100644
--- a/test/unit/format_parse_test.cpp
+++ b/test/unit/format_parse_test.cpp
@@ -7,6 +7,8 @@
 
 #include <gtest/gtest.h>
 
+#include <seqan3/std/ranges>
+
 #include <sharg/argument_parser.hpp>
 
 TEST(parse_type_test, add_option_short_id)
diff --git a/test/unit/format_parse_validators_test.cpp b/test/unit/format_parse_validators_test.cpp
index c2c8ab93..2218cb2a 100644
--- a/test/unit/format_parse_validators_test.cpp
+++ b/test/unit/format_parse_validators_test.cpp
@@ -7,6 +7,8 @@
 
 #include <gtest/gtest.h>
 
+#include <seqan3/std/ranges>
+
 #include <sharg/argument_parser.hpp>
 #include <sharg/test/file_access.hpp>
 #include <sharg/test/tmp_filename.hpp>
@@ -830,6 +832,11 @@ enum class foo
     three
 };
 
+auto enumeration_names(foo)
+{
+    return std::unordered_map<std::string_view, foo>{{"one", foo::one}, {"two", foo::two}, {"three", foo::three}};
+}
+
 TEST(validator_test, value_list_validator_success)
 {
     // type deduction
@@ -951,7 +958,7 @@ TEST(validator_test, value_list_validator_success)
                            "\n" +
                            basic_options_str +
                            "    -i, --int-option (List of signed 32 bit integer)\n"
-                           "          desc Default: []. Value must be one of [-10,48,50].\n\n" +
+                           "          desc Default: []. Value must be one of [-10, 48, 50].\n\n" +
                            basic_version_str);
     EXPECT_EQ(my_stdout, expected);
 }