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+# Argument Parser API Documentation
+
+This document covers the public API exposed by:
+
+- `src/headers/parser/argument_parser.hpp`: the original v1 parser surface.
+- `src/headers/parser/parser_v2.hpp`: the v2 map/initializer-list facade.
+- `src/headers/parser/argument_builder.hpp`: the staged fluent builder for v2.
+
+The lower-level v1 API still powers the platform parsers and v2. New code will
+usually be clearest with `argument_parser::v2` and
+`argument_parser::builder::new_argument()`, but the v1 types remain public and
+useful when you need direct control over actions, stored values, or positional
+registration.
+
+## Common Concepts
+
+Argument names are registered without command-line prefixes. Use `"v"` and
+`"verbose"`, not `"-v"` or `"--verbose"`. The active convention objects decide
+which concrete syntaxes are accepted during parsing and how help text is
+rendered.
+
+Typed values are parsed through
+`argument_parser::parsing_traits::parser_trait<T>::parse(std::string const&)`.
+The built-in traits cover common scalar types such as `std::string`, `bool`,
+`int`, `float`, and `double`; custom value types need a trait specialization.
+Trait `format_hint` and `purpose_hint` members are used in help text and parse
+error messages when available.
+
+Stored values are type-erased internally. Retrieve them with the same type used
+when the argument was registered.
+
+## v1 API: `argument_parser.hpp`
+
+The public v1 types live in the `argument_parser` namespace unless otherwise
+noted.
+
+### `action_base`
+
+```cpp
+class action_base {
+public:
+    virtual ~action_base() = default;
+    [[nodiscard]] virtual bool expects_parameter() const = 0;
+    virtual void invoke() const = 0;
+    virtual void invoke_with_parameter(std::string const& param) const = 0;
+    [[nodiscard]] virtual std::pair<std::string, std::string> get_trait_hints() const = 0;
+    [[nodiscard]] virtual std::unique_ptr<action_base> clone() const = 0;
+};
+```
+
+`action_base` is the polymorphic interface stored by registered arguments.
+Application code normally uses `action_with_param<T>`, `action_no_param`, or
+the overloaded `helpers::make_action(...)` factories instead of implementing
+this interface directly.
+
+- `expects_parameter()` reports whether the action consumes a value token.
+- `invoke()` runs a no-value action. On `action_with_param<T>`, it throws
+  `std::runtime_error`.
+- `invoke_with_parameter(param)` runs a value action. On
+  `action_no_param`, the parameter is ignored.
+- `get_trait_hints()` returns `{format_hint, purpose_hint}` for typed actions
+  when the parser trait exposes those members.
+- `clone()` returns a heap-allocated copy used by `argument`.
+
+### `action_with_param<T>`
+
+```cpp
+template <typename T>
+class action_with_param : public action_base {
+public:
+    using parameter_type = T;
+
+    explicit action_with_param(std::function<void(T const&)> const& handler);
+    void invoke(T const& arg) const;
+    [[nodiscard]] bool expects_parameter() const override;
+    void invoke() const override;
+    void invoke_with_parameter(std::string const& param) const override;
+    [[nodiscard]] std::pair<std::string, std::string> get_trait_hints() const override;
+    [[nodiscard]] std::unique_ptr<action_base> clone() const override;
+};
+```
+
+Adapts a `std::function<void(T const&)>` into a parser action. During
+`invoke_with_parameter`, the raw string is converted with
+`parser_trait<T>::parse(param)` and the parsed value is passed to the handler.
+Parse failures are reported as `std::runtime_error` with trait-derived hints
+when available.
+
+### `action_no_param`
+
+```cpp
+class action_no_param : public action_base {
+public:
+    explicit action_no_param(std::function<void()> const& handler);
+
+    void invoke() const override;
+    [[nodiscard]] bool expects_parameter() const override;
+    void invoke_with_parameter(std::string const& param) const override;
+    [[nodiscard]] std::pair<std::string, std::string> get_trait_hints() const override;
+    [[nodiscard]] std::unique_ptr<action_base> clone() const override;
+};
+```
+
+Adapts a `std::function<void()>` into a flag-style action.
+`expects_parameter()` returns `false`; `invoke_with_parameter(...)` ignores the
+parameter and runs the no-value handler.
+
+### `argument_parser::helpers`
+
+```cpp
+namespace argument_parser::helpers {
+    template <typename T>
+    action_with_param<T>
+    make_action(std::function<void(T const&)> const& function);
+
+    action_no_param
+    make_action(std::function<void()> const& function);
+}
+```
+
+Factory helpers for creating action objects used by `base_parser` and v2.
+
+### `argument`
+
+```cpp
+class argument {
+public:
+    argument();
+
+    template <typename ActionType>
+    argument(int id, std::string name, ActionType const& action);
+
+    argument(argument const& other);
+    argument& operator=(argument const& other);
+    argument(argument&& other) noexcept = default;
+    argument& operator=(argument&& other) noexcept = default;
+
+    [[nodiscard]] bool is_required() const;
+    [[nodiscard]] std::string get_name() const;
+    [[nodiscard]] bool is_invoked() const;
+    [[nodiscard]] bool expects_parameter() const;
+    [[nodiscard]] std::string get_help_text() const;
+    [[nodiscard]] bool is_positional() const;
+    [[nodiscard]] bool is_positional_accumulator() const;
+    [[nodiscard]] std::optional<int> get_position_index() const;
+};
+```
+
+`argument` is the public descriptor returned by parser lookup operations. It is
+copyable; copies clone their stored action.
+
+- `is_required()` reports whether parsing must see this argument.
+- `get_name()` returns the registered display name. Named options are stored as
+  `"<short>|<long>"`; positional arguments use their positional name.
+- `is_invoked()` reports whether the action ran during `handle_arguments(...)`.
+- `expects_parameter()` mirrors the stored action.
+- `get_help_text()` returns the registration help text.
+- `is_positional()` identifies positional arguments.
+- `is_positional_accumulator()` is true for positional accumulators.
+- `get_position_index()` returns the explicit requested position, or
+  `std::nullopt`.
+
+### `base_parser`
+
+```cpp
+class base_parser {
+public:
+    template <typename T>
+    void add_argument(
+        std::string const& short_arg,
+        std::string const& long_arg,
+        std::string const& help_text,
+        action_with_param<T> const& action,
+        bool required);
+
+    template <typename T>
+    void add_argument(
+        std::string const& short_arg,
+        std::string const& long_arg,
+        std::string const& help_text,
+        bool required);
+
+    void add_argument(
+        std::string const& short_arg,
+        std::string const& long_arg,
+        std::string const& help_text,
+        action_no_param const& action,
+        bool required);
+
+    void add_argument(
+        std::string const& short_arg,
+        std::string const& long_arg,
+        std::string const& help_text,
+        bool required);
+
+    template <typename T>
+    void add_positional_argument(
+        std::string const& name,
+        std::string const& help_text,
+        action_with_param<T> const& action,
+        bool required,
+        std::optional<int> position = std::nullopt);
+
+    template <typename T>
+    void add_positional_argument(
+        std::string const& name,
+        std::string const& help_text,
+        bool required,
+        std::optional<int> position = std::nullopt);
+
+    template <typename T>
+    void add_positional_accumulator(
+        std::string const& name,
+        std::string const& help_text,
+        action_with_param<T> const& action,
+        bool required,
+        std::optional<int> position = std::nullopt);
+
+    void on_complete(std::function<void(base_parser const&)> const& action);
+
+    template <typename T>
+    std::optional<T> get_optional(std::string const& arg) const;
+
+    [[nodiscard]] std::string build_help_text(
+        std::initializer_list<conventions::convention const* const> convention_types) const;
+
+    argument& get_argument(conventions::parsed_argument const& arg);
+    [[nodiscard]] std::optional<int> find_argument_id(std::string const& arg) const;
+    void handle_arguments(
+        std::initializer_list<conventions::convention const* const> convention_types);
+    void display_help(
+        std::initializer_list<conventions::convention const* const> convention_types) const;
+
+protected:
+    base_parser() = default;
+};
+```
+
+`base_parser` owns registrations, parsed command-line tokens, stored values,
+required-argument checks, and completion hooks. Its constructor is protected;
+users normally instantiate a concrete parser from `<argparse>` or a platform
+header. Platform parser constructors populate `program_name` and
+`parsed_arguments` from the native process arguments.
+
+`base_parser` is not thread-safe. `handle_arguments(...)` must be called on the
+same thread that created the parser or it throws `std::runtime_error`.
+
+#### Named Arguments
+
+Use `add_argument(...)` for short and/or long options. The v1 API uses `"-"` as
+the sentinel for an omitted short or long name.
+
+```cpp
+argument_parser::parser parser;
+
+parser.add_argument<std::string>(
+    "o",
+    "output",
+    "Output file.",
+    argument_parser::helpers::make_action<std::string>(
+        [](std::string const& value) {
+            // use value
+        }),
+    true);
+
+parser.add_argument(
+    "v",
+    "verbose",
+    "Enable verbose output.",
+    false);
+```
+
+Registration fails with `std::runtime_error("The key already exists!")` if the
+short or long key is already registered. Because duplicate checks also see the
+`"-"` sentinel, avoid registering multiple v1 arguments that pass `"-"` for the
+same side unless you have verified that behavior.
+
+The overloads behave as follows:
+
+- `add_argument<T>(..., action_with_param<T> const& action, bool required)`
+  parses a value and invokes the supplied typed action.
+- `add_argument<T>(..., bool required)` parses a value and stores it internally
+  for `get_optional<T>(...)`.
+- `add_argument(..., action_no_param const& action, bool required)`
+  invokes a no-value action.
+- `add_argument(..., bool required)` stores `true` internally when the option is
+  present.
+
+#### Positional Arguments
+
+```cpp
+parser.add_positional_argument<std::string>(
+    "input",
+    "Input path.",
+    true,
+    0);
+
+parser.add_positional_argument<int>(
+    "count",
+    "Number of iterations.",
+    argument_parser::helpers::make_action<int>(
+        [](int value) {
+            // use value
+        }),
+    false);
+```
+
+Positional arguments are matched against tokens that do not match any supplied
+convention. You can provide an explicit zero-based `position`, or omit it to use
+the next available positional slot.
+
+`add_positional_argument<T>(..., bool required, ...)` stores the parsed value
+internally under the positional name. The action overload invokes the supplied
+typed action.
+
+`add_positional_accumulator<T>(...)` registers a typed positional action that can
+consume repeated positional tokens at its slot. Only one positional accumulator
+is allowed, and it must be the last positional argument. Duplicate names,
+negative explicit positions, or invalid accumulator placement throw
+`std::runtime_error`.
+
+#### Parsing
+
+```cpp
+parser.handle_arguments({
+    &argument_parser::conventions::gnu_argument_convention,
+    &argument_parser::conventions::gnu_equal_argument_convention,
+    &argument_parser::conventions::windows_argument_convention,
+    &argument_parser::conventions::windows_equal_argument_convention,
+});
+```
+
+`handle_arguments(...)` invokes matching actions, records stored values, checks
+required arguments, then runs completion hooks. For each token, conventions are
+tried in order. If no convention matches, the token is consumed as the next
+positional value when one is available. `--` forces all following tokens to be
+treated as positional values.
+
+Unknown options, missing values, parse failures, unexpected positional values,
+and action errors are reported with `std::runtime_error`. Missing required
+arguments print diagnostics and help text, then call `std::exit(1)`.
+
+If a registered argument named `h` or `help` is found, the parser invokes that
+help argument and returns from argument invocation early. Required checks and
+completion hooks still run after invocation returns.
+
+#### Lookup And Help
+
+```cpp
+if (auto output = parser.get_optional<std::string>("output")) {
+    // *output is the stored value
+}
+
+auto id = parser.find_argument_id("output");
+std::string help = parser.build_help_text({
+    &argument_parser::conventions::gnu_argument_convention,
+});
+parser.display_help({
+    &argument_parser::conventions::gnu_argument_convention,
+});
+```
+
+`get_optional<T>(arg)` returns a stored value for a registered short name, long
+name, or positional name. It returns `std::nullopt` when no argument by that name
+exists or when no value was stored. If a value exists but `T` does not match the
+stored type, `std::any_cast<T>` throws `std::bad_any_cast`.
+
+`find_argument_id(arg)` returns the internal integer id for a registered short
+name, long name, or positional name. `get_argument(parsed_argument)` resolves a
+parsed convention result to an `argument&` and throws when no match exists.
+
+`build_help_text(...)` returns formatted usage text. `display_help(...)` writes
+that text to `std::cout`.
+
+#### Completion Hooks
+
+```cpp
+parser.on_complete([](argument_parser::base_parser const& state) {
+    // inspect state.get_optional<T>(...)
+});
+```
+
+`on_complete(...)` appends a callback that runs after successful argument
+invocation and required-argument checks.
+
+## v2 API: `parser_v2.hpp`
+
+The v2 API is declared under `argument_parser::v2`. It provides a
+map/initializer-list facade over the v1 `base_parser`, while still accepting the
+same convention objects and action helpers.
+
+Typical use is through the platform alias from `<argparse>`:
+
+```cpp
+#include <argparse>
+
+using namespace argument_parser::v2::flags;
+
+int main() {
+    argument_parser::v2::parser parser;
+
+    parser.add_argument<int>({
+        {LongArgument, "count"},
+        {HelpText, "Number of items to process."},
+        {Required, true},
+    });
+
+    parser.handle_arguments({
+        &argument_parser::conventions::gnu_argument_convention,
+        &argument_parser::conventions::windows_argument_convention,
+    });
+
+    auto count = parser.get_optional<int>("count");
+}
+```
+
+### `enum class add_argument_flags`
+
+These keys describe an argument passed to `v2::base_parser::add_argument`.
+
+| Flag | Value type | Meaning |
+| --- | --- | --- |
+| `ShortArgument` | `std::string` | Short option name, such as `"v"`. |
+| `LongArgument` | `std::string` | Long option name, such as `"verbose"`. |
+| `Positional` | `std::string` | Registers a positional argument. When present, the positional registration path is used. |
+| `Position` | `int` | Optional zero-based position for a positional argument. |
+| `HelpText` | `std::string` | Help text shown in generated help output. |
+| `Action` | `action_with_param<T>` or `action_no_param` | Callback invoked when the argument is found. |
+| `Required` | `bool` | Marks the argument required when `true`. |
+| `Reference` | `T*` | Stores a parsed typed value into an external object. Cannot be combined with `Action`. |
+| `Accumulate` | `bool` or `T*` | Repeated-value mode. For `std::vector<U>`, each parsed `U` is appended. For `int`, each occurrence increments the count. |
+
+The `argument_parser::v2::flags` namespace exposes inline constants with the
+same names for concise initializer-list calls.
+
+```cpp
+using namespace argument_parser::v2::flags;
+
+parser.add_argument({
+    {ShortArgument, "q"},
+    {HelpText, "Quiet mode."},
+});
+```
+
+### `argument_parser::v2::deducers`
+
+```cpp
+template <typename, typename = void>
+struct has_value_type;
+
+template <typename T>
+struct is_vector;
+
+template <typename T>
+constexpr bool is_vector_v = is_vector<T>::test();
+```
+
+These public type traits are used by v2 accumulator logic. `has_value_type`
+detects `T::value_type`. `is_vector<T>::test()` and `is_vector_v<T>` are true
+only when `T` is exactly a `std::vector` specialization.
+
+### `argument_parser::v2::base_parser`
+
+`v2::base_parser` is the v2 facade. Concrete platform parsers such as
+`argument_parser::v2::linux_parser`, `macos_parser`, `windows_parser`, and the
+portable `<argparse>` alias `argument_parser::v2::parser` derive from it.
+
+The class privately inherits from the v1 `argument_parser::base_parser` and
+re-exposes selected operations.
+
+```cpp
+template <typename T>
+using typed_flag_value =
+    std::variant<std::string, action_with_param<T>, bool, int, T*>;
+
+using non_typed_flag_value =
+    std::variant<std::string, action_no_param, bool, int>;
+
+template <typename T>
+using typed_argument_pair =
+    std::pair<add_argument_flags, typed_flag_value<T>>;
+
+using non_typed_argument_pair =
+    std::pair<add_argument_flags, non_typed_flag_value>;
+```
+
+Use typed aliases for arguments that parse a value of `T`, run an
+`action_with_param<T>`, store into a `T`, or accumulate values. Use non-typed
+aliases for boolean flags and no-parameter actions.
+
+#### Typed `add_argument`
+
+```cpp
+template <typename T>
+void add_argument(
+    std::unordered_map<add_argument_flags, typed_flag_value<T>> const& argument_pairs);
+
+template <typename T>
+void add_argument(std::initializer_list<typed_argument_pair<T>> const& pairs);
+```
+
+Registers a typed option or positional argument. For non-positional options,
+provide at least one of `ShortArgument` or `LongArgument`. If only one name is
+supplied, the other is internally set to `"-"` and not registered.
+
+Without `Action`, `Reference`, or `Accumulate`, the parsed `T` is stored
+internally and can be read with `get_optional<T>()`. For positional arguments,
+include `Positional`; `Position` can place the argument at a specific zero-based
+slot.
+
+```cpp
+int threshold = 0;
+
+parser.add_argument<int>({
+    {LongArgument, "threshold"},
+    {Reference, &threshold},
+    {HelpText, "Store the parsed threshold."},
+});
+```
+
+#### Non-Typed `add_argument`
+
+```cpp
+void add_argument(
+    std::unordered_map<add_argument_flags, non_typed_flag_value> const& argument_pairs);
+
+void add_argument(std::initializer_list<non_typed_argument_pair> const& pairs);
+```
+
+Registers a non-typed option or positional argument. For options, provide at
+least one of `ShortArgument` or `LongArgument`. Without `Action`, the argument
+behaves as a boolean flag: when present, it stores `true` for
+`get_optional<bool>()`.
+
+With `Action`, provide an `action_no_param`. Non-typed arguments cannot use
+`Reference` or `Accumulate`. A non-typed positional argument is stored as
+`std::string`.
+
+```cpp
+parser.add_argument({
+    {ShortArgument, "v"},
+    {LongArgument, "verbose"},
+    {HelpText, "Enable verbose output."},
+});
+```
+
+#### Other Public Operations
+
+```cpp
+argument_parser::base_parser& to_v1();
+
+void handle_arguments(
+    std::initializer_list<conventions::convention const* const> convention_types);
+
+template <typename T>
+std::optional<T> get_optional(std::string const& arg);
+
+using argument_parser::base_parser::display_help;
+using argument_parser::base_parser::on_complete;
+```
+
+`to_v1()` returns a mutable reference to the underlying v1 parser interface.
+`handle_arguments(...)`, `get_optional<T>(...)`, `display_help(...)`, and
+`on_complete(...)` follow the v1 behavior described above.
+
+### Accumulation
+
+`Accumulate` has special typed behavior:
+
+- `T = std::vector<U>` registers an argument that accepts repeated `U` values.
+- `T = int` registers a counter-style option; each occurrence increments the
+  count.
+- `Accumulate` set to `true` stores the accumulated result internally and makes
+  it available through `get_optional<T>()`.
+- `Accumulate` set to `T*`, or `Accumulate` combined with `Reference`, writes
+  directly to an external object.
+
+For vector accumulators, the public argument type is `std::vector<U>`, but each
+individual command-line occurrence is parsed as `U`.
+
+```cpp
+parser.add_argument<std::vector<int>>({
+    {LongArgument, "include-id"},
+    {Accumulate, true},
+    {HelpText, "May be repeated."},
+});
+
+auto ids = parser.get_optional<std::vector<int>>("include-id");
+```
+
+## Builder API: `argument_builder.hpp`
+
+The public builder API lives under `argument_parser::builder`. It provides a
+staged fluent interface for registering arguments with
+`argument_parser::v2::base_parser`.
+
+```cpp
+#include <argument_builder.hpp>
+// or
+#include <argparse>
+
+using argument_parser::builder::new_argument;
+```
+
+### Public Types
+
+`non_type` is a marker type used before a value-producing mode has been
+selected. Application code normally does not need to name it.
+
+`builder_mask` is a public namespace containing the compile-time machinery used
+by `argument<mask, store_type>`. It exposes `mask_type`, `value_mode`,
+`extra_capability`, capability constants such as `short_argument`,
+`long_argument`, `positional`, `help_text`, `required`, `reference`,
+`accumulate`, `count`, `store`, and `flag`, plus constexpr helpers such as
+`bit`, `has`, `remove`, `replace`, `is_buildable`, and
+`is_build_and_gettable`. These are mainly useful for type-level tests or
+advanced wrappers.
+
+### `container<store_type>`
+
+```cpp
+template <typename store_type>
+class container;
+```
+
+Returned by `build_and_get(parser)`. It is populated after
+`parser.handle_arguments(...)` completes.
+
+- `store_type get() const` returns the stored value. Call only after the
+  container is populated.
+- `store_type& operator*()` dereferences the stored value.
+- `store_type* operator->()` accesses members of the stored value.
+- `operator bool()` reports whether a value is available.
+
+For builder-managed storage, `build_and_get()` registers an `on_complete`
+callback that reads the parsed value by lookup key. For
+`accumulate(existing_vector)`, the container references the provided vector
+directly.
+
+### `argument<mask, store_type>`
+
+```cpp
+template <
+    argument_parser::builder::builder_mask::mask_type mask =
+        argument_parser::builder::builder_mask::initial,
+    typename store_type = argument_parser::builder::non_type>
+class argument;
+```
+
+The staged fluent builder. `mask` enables only valid next methods at compile
+time. `store_type` is the type produced by the selected value mode. Most users
+should create builders with `new_argument()`.
+
+### Entry Points
+
+```cpp
+static auto argument<>::start() -> argument<builder_mask::initial>;
+static inline auto new_argument();
+```
+
+`new_argument()` is equivalent to `argument<>::start()`.
+
+```cpp
+new_argument()
+    .long_argument("output")
+    .store<std::string>()
+    .required()
+    .build(parser);
+```
+
+### Identifier Methods
+
+At least one identifier method must be used before `build(parser)` or
+`build_and_get(parser)`.
+
+```cpp
+auto short_argument(std::string short_name) const -> argument<..., store_type>;
+auto long_argument(std::string long_name) const -> argument<..., store_type>;
+auto positional(std::string positional_name) const -> argument<..., store_type>;
+auto position(int index) const -> argument<..., store_type>;
+```
+
+`short_argument(...)` and `long_argument(...)` set named option identifiers and
+can be combined with each other. `positional(...)` registers a positional
+argument and is mutually exclusive with named identifiers. `position(...)` is
+available only after `positional(...)` and only once. If no value mode is
+selected for a positional argument, `build(parser)` registers a `std::string`
+positional value.
+
+### Metadata Methods
+
+```cpp
+auto help_text(std::string help) const -> argument<..., store_type>;
+auto required(bool value = true) const -> argument<..., store_type>;
+```
+
+`help_text(...)` sets generated help text and is single-use. If omitted, v2
+generates default text, often using parser trait hints for typed value modes.
+
+`required(...)` marks the argument required when `value` is `true`. There is no
+public `setRequired(...)` method in `argument_builder.hpp`; the public fluent API
+is `required(...)`.
+
+### Terminal Value Modes
+
+Exactly one terminal value mode can be selected. Once selected, the other modes
+are removed from the builder type.
+
+```cpp
+template <typename T = std::string>
+auto store() const -> argument<..., T>;
+
+template <typename T>
+auto reference(T& value) const -> argument<..., T>;
+
+auto flag() const -> argument<..., bool>;
+
+template <typename T = std::string>
+auto accumulate() const -> argument<..., std::vector<T>>;
+
+template <typename T>
+auto accumulate(T& value) const -> argument<..., T>;
+
+auto count() const -> argument<..., int>;
+auto count(int& value) const -> argument<..., int>;
+
+template <typename Callable>
+auto action(Callable&& handler) const -> argument<..., non_type>;
+
+template <typename T = std::string, typename Callable>
+auto action(Callable&& handler) const -> argument<..., T>;
+```
+
+`store<T>()` parses a value and stores it in the parser for later lookup.
+`store<void>()` is rejected at compile time.
+
+`reference(T&)` parses a value and assigns it directly to an object that must
+outlive parsing.
+
+`flag()` registers a boolean presence flag. It is not available for positional
+arguments. Named arguments also default to flag behavior when no terminal mode
+is selected.
+
+`accumulate<T>()` accepts repeated values and stores them as `std::vector<T>`.
+`accumulate(existing)` appends repeated values into an existing vector. The
+existing object must be a `std::vector` and must outlive parsing.
+
+`count()` counts how many times a named argument appears. `count(int&)` writes
+the count into an existing integer. `count()` is not available for positional
+arguments.
+
+`action(handler)` registers a no-value action when `handler` is invocable as
+`void()`. For positional arguments, the builder adapts it through a
+`std::string` positional action and ignores the parsed positional text.
+
+`action<T>(handler)` registers a typed action when `handler` is invocable as
+`void(T const&)`. `action<void>(...)` is rejected at compile time.
+
+### Build Methods
+
+```cpp
+auto build(argument_parser::v2::base_parser& parser) const -> void;
+
+auto build_and_get(argument_parser::v2::base_parser& parser) const
+    -> container<store_type>;
+```
+
+`build(parser)` registers the configured argument with the parser. It requires
+an identifier, dispatches to the selected terminal mode, and defaults to a named
+boolean flag or `std::string` positional value when no terminal mode was
+selected. It may propagate validation errors from `v2::base_parser`.
+
+`build_and_get(parser)` registers the argument and returns a
+`container<store_type>` for storable modes such as `store<T>()`, `flag()`,
+default named flags, `accumulate<T>()`, `accumulate(vector&)`, `count()`, and
+`count(int&)`. It throws `std::logic_error` for unsupported terminal modes such
+as actions. The returned container is normally empty until
+`parser.handle_arguments(...)` completes.
+
+### Compile-Time Builder Rules
+
+Invalid fluent chains fail to compile rather than failing at runtime.
+
+- `build(...)` requires `short_argument(...)`, `long_argument(...)`, or
+  `positional(...)`.
+- `short_argument(...)` and `long_argument(...)` can be combined.
+- `positional(...)` is mutually exclusive with named identifiers.
+- `position(...)` is available only after `positional(...)` and only once.
+- `help_text(...)` and `required(...)` are single-use.
+- Terminal value modes are mutually exclusive.
+- `flag()` and `count()` are not available for positional arguments.
+- `store<void>()` and `action<void>(...)` are rejected with `static_assert`.
+- `accumulate(existing)` requires `existing` to be a `std::vector`.
+
+The header also exposes `argument_parser::builder::assertions`, a namespace of
+small compile-time detection helpers and `static_assert` checks used to verify
+these staged-builder rules. They are not needed for normal parser
+configuration.
+
+### Complete Builder Example
+
+```cpp
+#include <argparse>
+#include <iostream>
+#include <string>
+#include <vector>
+
+using argument_parser::builder::new_argument;
+
+int main() {
+    argument_parser::v2::parser parser(false);
+
+    int threshold = 0;
+    std::vector<int> ids;
+
+    new_argument()
+        .long_argument("file")
+        .store<std::string>()
+        .required()
+        .help_text("Input file.")
+        .build(parser);
+
+    new_argument()
+        .long_argument("threshold")
+        .reference(threshold)
+        .build(parser);
+
+    new_argument()
+        .long_argument("id")
+        .accumulate(ids)
+        .help_text("Repeatable numeric id.")
+        .build(parser);
+
+    auto verbose = new_argument()
+        .short_argument("v")
+        .count()
+        .build_and_get(parser);
+
+    parser.handle_arguments({
+        &argument_parser::conventions::gnu_argument_convention,
+        &argument_parser::conventions::windows_argument_convention,
+    });
+
+    if (auto file = parser.get_optional<std::string>("file")) {
+        std::cout << "file: " << *file << '\n';
+    }
+
+    if (verbose) {
+        std::cout << "verbosity: " << *verbose << '\n';
+    }
+
+    std::cout << "threshold: " << threshold << '\n';
+    std::cout << "ids: " << ids.size() << '\n';
+}
+```
+
+## Caveats
+
+- The v1 API is retained as public compatibility surface, but the newer v2 and
+  builder APIs are generally more ergonomic.
+- Parser registration and parsing are single-threaded; `handle_arguments(...)`
+  enforces the parser creation thread.
+- `handle_arguments(...)` resets the current convention list after it returns
+  or throws.
+- Namespaces named `argument_parser::internal::*` are implementation details
+  despite being present in public headers.
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # argument-parser

-A lightweight, modern, and highly customizable C++17 argument parser with native platform argument collection, trait-driven typed parsing, pluggable option conventions, and a fluent `v2` builder API.
+A lightweight, modern, and highly customizable C++17 argument parser with native platform argument collection, trait-driven typed parsing, repeatable argument accumulation, pluggable option conventions, and a fluent `v2` builder API.

 > `v1` is deprecated and mainly kept as implementation history. For new projects, use `argument_parser::v2` together with `argument_parser::builder`.

@@ -8,11 +8,13 @@ A lightweight, modern, and highly customizable C++17 argument parser with native

 - Native platform parser alias: `argument_parser::v2::parser` resolves to the current platform parser and reads arguments directly from OS APIs.
 - Fluent builder API with compile-time builder constraints that prevent invalid combinations after a terminal/mutually exclusive mode has been selected.
- Type-safe parsing and extraction. Just extend `parser_trait<T>` for your types and if just want to store use `get_optional<T>()`!
+- Type-safe parsing and extraction. Extend `parser_trait<T>` for your own types and retrieve stored values with `get_optional<T>()`.
+- Repeatable value accumulation with `accumulate<T>()`, `accumulate(vector&)`, `count()`, and `count(int&)`.
+- `build_and_get(parser)` for storable builder modes, returning a small container that is populated after parsing completes.
 - Positional arguments with optional explicit ordering and support for `--` as a positional separator.
 - Trait-driven `format_hint` and `purpose_hint` metadata used in generated help text and parse errors.
 - Automatic help flag on `argument_parser::v2::parser` (`-h`, `--help`) with configurable exit behavior.
- Auto-formatted help output..
+- Auto-formatted help output.
 - Completion hooks via `parser.on_complete(...)`.
 - Pluggable conventions for GNU next-token, GNU equal-style, Windows next-token, and Windows inline `=` / `:` parsing, or bring your own!
 - Testing helper + pseudo command handler `argument_parser::v2::fake_parser`.
@@ -28,15 +30,17 @@ A lightweight, modern, and highly customizable C++17 argument parser with native
 #include <argparse>
 #include <iostream>
 #include <string>
+#include <vector>

-using argument = argument_parser::builder::argument<>;
+using argument_parser::builder::new_argument;

 int main() {
    argument_parser::v2::parser parser(false); // --help prints without exiting immediately

    int threshold = 0;
+    std::vector<int> ids;

-    argument::start()
+    new_argument()
        .short_argument("e")
        .long_argument("echo")
        .action<std::string>([](std::string const& text) {
@@ -44,26 +48,32 @@ int main() {
        })
        .build(parser);

-    argument::start()
+    new_argument()
        .long_argument("file")
        .store<std::string>()
        .required()
        .help_text("Input file to process.")
        .build(parser);

-    argument::start()
+    new_argument()
        .long_argument("threshold")
        .reference(threshold)
+        .help_text("Numeric threshold.")
        .build(parser);

-    argument::start()
+    auto verbose = new_argument()
        .short_argument("v")
-        .long_argument("verbose")
-        .flag()
-        .help_text("Enable verbose output.")
+        .help_text("Increase verbosity. Repeat for a higher level.")
+        .count()
+        .build_and_get(parser);
+
+    new_argument()
+        .long_argument("id")
+        .help_text("Collect an id. May be repeated.")
+        .accumulate(ids)
        .build(parser);

-    argument::start()
+    new_argument()
        .positional("output")
        .position(0)
        .help_text("Output file.")
@@ -89,6 +99,11 @@ int main() {
    }

    std::cout << "threshold: " << threshold << '\n';
+    std::cout << "ids: " << ids.size() << '\n';
+
+    if (verbose) {
+        std::cout << "verbosity: " << *verbose << '\n';
+    }
 }
 ```

@@ -129,7 +144,7 @@ struct argument_parser::parsing_traits::parser_trait<Point> {
 Then use the type directly from the builder:

 ```cpp
-argument::start()
+new_argument()
    .long_argument("point")
    .store<Point>()
    .build(parser);
@@ -161,9 +176,15 @@ parser.display_help(conventions);

 Mix any of them in the same parser by passing the conventions you want to `handle_arguments()`.

-## Builder Modes
+## Builder API

-`argument_parser::builder::argument<>` is a staged builder. `build(parser)` is the terminal call.
+`argument_parser::builder::argument<>` is a staged builder. Prefer `argument_parser::builder::new_argument()` as the entry point:
+
+```cpp
+using argument_parser::builder::new_argument;
+```
+
+`build(parser)` registers the argument and returns `void`. `build_and_get(parser)` is available for storable modes and returns a lightweight `builder::container<T>`. The container is filled by an internal completion hook after `handle_arguments(...)` runs.

 Before `build(...)`, you compose an argument from three kinds of steps:

@@ -173,13 +194,75 @@ Before `build(...)`, you compose an argument from three kinds of steps:
  - `store<T>()` to parse and retain a value for later `get_optional<T>()`
  - `flag()` to store a boolean presence flag
  - `reference(value)` to write the parsed result directly into an existing variable
+  - `accumulate<T>()` to collect repeated values into a stored `std::vector<T>`
+  - `accumulate(vector)` to collect repeated values into an existing `std::vector<T>`
+  - `count()` to store how many times an option appears
+  - `count(value)` to write the occurrence count into an existing `int`
  - `action([] { ... })` for no-value callbacks
  - `action<T>([](T const&) { ... })` for typed value callbacks

-Once you select one value behavior, the other value behavior methods are disabled at compile time, so combinations like `store<T>().action(...)` or `flag().reference(value)` are rejected by the type system. Also you cannot use the same method repeatedly as it is also disabled at compile time by the type system.
+Once you select one value behavior, the other value behavior methods are disabled at compile time, so combinations like `store<T>().action(...)` or `flag().reference(value)` are rejected by the type system. The same staged typing also prevents repeating one-shot methods such as `help_text(...)`, `position(...)`, or `store<T>()` on the same builder chain.

 If you do not select a value behavior explicitly, `build(parser)` uses the default for the argument kind: named arguments become boolean flags, while positional arguments store a `std::string`.

+## Accumulators and Counts
+
+Use `accumulate<T>()` when the parser should accept the same value-bearing argument multiple times and store all parsed values:
+
+```cpp
+auto values = new_argument()
+    .short_argument("n")
+    .long_argument("number")
+    .accumulate<int>()
+    .build_and_get(parser);
+
+parser.handle_arguments(conventions);
+
+if (values) {
+    for (int value : *values) {
+        std::cout << value << '\n';
+    }
+}
+```
+
+Use `accumulate(target)` to append into a vector you own:
+
+```cpp
+std::vector<int> ids;
+
+new_argument()
+    .long_argument("id")
+    .accumulate(ids)
+    .build(parser);
+```
+
+Use `count()` for repeatable flags such as `-v -v -v`:
+
+```cpp
+auto verbosity = new_argument()
+    .short_argument("v")
+    .count()
+    .build_and_get(parser);
+```
+
+The lower-level `v2::base_parser::add_argument` API exposes the same accumulator behavior through the `Accumulate` flag:
+
+```cpp
+using namespace argument_parser::v2::flags;
+
+parser.add_argument<std::vector<int>>({
+    {LongArgument, "id"},
+    {HelpText, "Collect an id. May be repeated."},
+    {Accumulate, true},
+});
+
+std::vector<int> captured_ids;
+parser.add_argument<std::vector<int>>({
+    {LongArgument, "captured-id"},
+    {Accumulate, &captured_ids},
+});
+```
+
 ## Testing

 For unit tests or synthetic argument lists, use `argument_parser::v2::fake_parser` instead of the native platform parser: