diff --git a/backends/native/runtime/graph/Argument.cpp b/backends/native/runtime/graph/Argument.cpp
new file mode 100644
index 00000000000..1eca016a758
--- /dev/null
+++ b/backends/native/runtime/graph/Argument.cpp
@@ -0,0 +1,18 @@
+// Copyright (c) Meta Platforms, Inc. and affiliates.
+// All rights reserved.
+//
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree.
+
+#include <executorch/backends/native/runtime/graph/Argument.h>
+
+#include <stdexcept>
+#include <string>
+
+namespace ptn {
+
+void Argument::throw_bad_kind(const char* what) {
+  throw std::runtime_error(std::string("Argument::") + what);
+}
+
+} // namespace ptn
diff --git a/backends/native/runtime/graph/Argument.h b/backends/native/runtime/graph/Argument.h
new file mode 100644
index 00000000000..28a74942c2c
--- /dev/null
+++ b/backends/native/runtime/graph/Argument.h
@@ -0,0 +1,229 @@
+// Copyright (c) Meta Platforms, Inc. and affiliates.
+// All rights reserved.
+//
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree.
+
+#pragma once
+
+#include <cstdint>
+#include <string>
+#include <utility>
+#include <variant>
+#include <vector>
+
+#include <executorch/backends/native/runtime/graph/Ids.h>
+#include <executorch/backends/native/runtime/graph/ScalarType.h>
+
+namespace ptn {
+
+// One payload struct per kind of value an fx arg/kwarg can hold; mirrors the
+// schema ArgumentValue union, but in-graph references are resolved to ValueRefs
+// (the deserializer turns SSA names into arena indices).
+
+struct NoneArg {};
+
+struct TensorArg {
+  ValueRef ref = kInvalid;
+};
+
+// A scalar int operand: a literal `value`, or (when `ref` is valid) a reference
+// to an in-graph int value — a sym_size / arith node — with `value` ignored.
+struct IntArg {
+  int64_t value = 0;
+  ValueRef ref = kInvalid;
+};
+
+struct FloatArg {
+  double value = 0.0;
+  ValueRef ref = kInvalid;
+};
+
+struct BoolArg {
+  bool value = false;
+  ValueRef ref = kInvalid;
+};
+
+struct StringArg {
+  std::string value;
+};
+
+struct ScalarTypeArg {
+  ScalarType value = ScalarType::Float;
+};
+
+// A list of ints. Element i is a literal (values[i]), or a symbolic reference
+// when `refs` is non-empty and refs[i] is valid (values[i] ignored). Empty
+// `refs` means all literal; otherwise refs.size() == values.size(). E.g. a
+// dynamic view size [s0, -1] is values={0, -1}, refs={<sym>, kInvalid}.
+struct IntListArg {
+  std::vector<int64_t> values;
+  std::vector<ValueRef> refs;
+};
+
+struct FloatListArg {
+  std::vector<double> values;
+};
+
+struct BoolListArg {
+  std::vector<bool> values;
+};
+
+// A list of tensor references (e.g. cat's input list).
+struct TensorListArg {
+  std::vector<ValueRef> refs;
+};
+
+// A list of optional tensor references (Tensor?[]); a kInvalid entry is None.
+struct OptionalTensorListArg {
+  std::vector<ValueRef> refs;
+};
+
+// A subgraph passed to a higher-order op (torch.cond / while_loop / map).
+// `name` is the original submodule attr label (debug only); `subgraph_ref`
+// indexes the subgraph arena (populated once Graph/Model land).
+struct GraphArg {
+  std::string name;
+  GraphRef subgraph_ref = kInvalid;
+};
+
+enum class ArgKind : int8_t {
+  None = 0,
+  Tensor = 1,
+  Int = 2,
+  Float = 3,
+  Bool = 4,
+  String = 5,
+  ScalarType = 6,
+  IntList = 7,
+  FloatList = 8,
+  BoolList = 9,
+  TensorList = 10,
+  OptionalTensorList = 11,
+  Graph = 12,
+};
+
+// A single fx argument value: a std::variant over the payload structs, with the
+// alternatives listed in ArgKind order so kind() is the variant's index and the
+// two can never drift apart.
+//
+// Construct from a payload directly (`Argument a = IntArg{5};`); read the
+// payload via the as_*() accessors after checking kind() (each throws
+// std::runtime_error on a kind mismatch).
+class Argument {
+ private:
+  using Storage = std::variant<
+      NoneArg,
+      TensorArg,
+      IntArg,
+      FloatArg,
+      BoolArg,
+      StringArg,
+      ScalarTypeArg,
+      IntListArg,
+      FloatListArg,
+      BoolListArg,
+      TensorListArg,
+      OptionalTensorListArg,
+      GraphArg>;
+
+  Storage value_;
+
+  // The live payload, or a std::runtime_error reading "Argument::<what>".
+  template <typename T>
+  const T& payload(const char* what) const {
+    const T* p = std::get_if<T>(&value_);
+    if (p == nullptr) {
+      throw_bad_kind(what);
+    }
+    return *p;
+  }
+
+  [[noreturn]] static void throw_bad_kind(const char* what);
+
+ public:
+  Argument() = default; // None
+
+  // Implicit by design: a payload struct is the natural spelling of an
+  // Argument. Spelled out per alternative rather than as one constrained
+  // template, because several payloads are aggregates that would make a
+  // template's overload resolution depend on which of them happens to accept
+  // the argument. The move is omitted on the trivially copyable payloads,
+  // where it buys nothing.
+  //
+  // cppcheck reads every one of these as an unintended converting constructor;
+  // the suppression is scoped to this block so a genuinely accidental implicit
+  // constructor added elsewhere in the class is still reported.
+  // cppcheck-suppress-begin noExplicitConstructor
+  /* implicit */ Argument(TensorArg a) : value_(a) {}
+  /* implicit */ Argument(IntArg a) : value_(a) {}
+  /* implicit */ Argument(FloatArg a) : value_(a) {}
+  /* implicit */ Argument(BoolArg a) : value_(a) {}
+  /* implicit */ Argument(StringArg a) : value_(std::move(a)) {}
+  /* implicit */ Argument(ScalarTypeArg a) : value_(a) {}
+  /* implicit */ Argument(IntListArg a) : value_(std::move(a)) {}
+  /* implicit */ Argument(FloatListArg a) : value_(std::move(a)) {}
+  /* implicit */ Argument(BoolListArg a) : value_(std::move(a)) {}
+  /* implicit */ Argument(TensorListArg a) : value_(std::move(a)) {}
+  /* implicit */ Argument(OptionalTensorListArg a) : value_(std::move(a)) {}
+  /* implicit */ Argument(GraphArg a) : value_(std::move(a)) {}
+  // cppcheck-suppress-end noExplicitConstructor
+
+  ArgKind kind() const {
+    return static_cast<ArgKind>(value_.index());
+  }
+
+  // Typed payload accessors. Each throws std::runtime_error unless kind()
+  // matches; guard with kind().
+  const TensorArg& as_tensor() const {
+    return payload<TensorArg>("as_tensor: argument is not a Tensor");
+  }
+  const IntArg& as_int() const {
+    return payload<IntArg>("as_int: argument is not an Int");
+  }
+  const FloatArg& as_float() const {
+    return payload<FloatArg>("as_float: argument is not a Float");
+  }
+  const BoolArg& as_bool() const {
+    return payload<BoolArg>("as_bool: argument is not a Bool");
+  }
+  const StringArg& as_string() const {
+    return payload<StringArg>("as_string: argument is not a String");
+  }
+  const ScalarTypeArg& as_scalar_type() const {
+    return payload<ScalarTypeArg>(
+        "as_scalar_type: argument is not a ScalarType");
+  }
+  const IntListArg& as_int_list() const {
+    return payload<IntListArg>("as_int_list: argument is not an IntList");
+  }
+  const FloatListArg& as_float_list() const {
+    return payload<FloatListArg>("as_float_list: argument is not a FloatList");
+  }
+  const BoolListArg& as_bool_list() const {
+    return payload<BoolListArg>("as_bool_list: argument is not a BoolList");
+  }
+  const TensorListArg& as_tensor_list() const {
+    return payload<TensorListArg>(
+        "as_tensor_list: argument is not a TensorList");
+  }
+  const OptionalTensorListArg& as_optional_tensor_list() const {
+    return payload<OptionalTensorListArg>(
+        "as_optional_tensor_list: argument is not an OptionalTensorList");
+  }
+  const GraphArg& as_graph() const {
+    return payload<GraphArg>("as_graph: argument is not a Graph");
+  }
+};
+
+// A positional or keyword argument. `name` is the operator-schema parameter
+// name (NOT a value reference; empty for positional-only). `mutated` is true
+// when the op writes this input in place (op-schema Tensor(a!), e.g. counter /
+// kv_cache).
+struct NamedArgument {
+  std::string name;
+  Argument arg;
+  bool mutated = false;
+};
+
+} // namespace ptn
diff --git a/backends/native/runtime/graph/Ids.h b/backends/native/runtime/graph/Ids.h
index 65295791111..90a0f98ce8a 100644
--- a/backends/native/runtime/graph/Ids.h
+++ b/backends/native/runtime/graph/Ids.h
@@ -13,12 +13,14 @@
 namespace ptn {
 
 // Index-arena handles. A NodeRef indexes Graph.nodes; a ValueRef indexes
-// Graph.values. Plain int32_t aliases — they index, compare, and hash
-// directly, at the cost of no NodeRef/ValueRef type distinction. kInvalid
-// marks "no ref" (e.g. a graph input has no producer node; a fresh value has
-// no alias).
+// Graph.values; a GraphRef indexes a subgraph arena (HOP branch bodies; the
+// arena itself lands with Graph/Model). Plain int32_t aliases — they index,
+// compare, and hash directly, at the cost of no type distinction between them.
+// kInvalid marks "no ref" (e.g. a graph input has no producer node; a fresh
+// value has no alias).
 using NodeRef = int32_t;
 using ValueRef = int32_t;
+using GraphRef = int32_t;
 constexpr int32_t kInvalid = -1;
 
 inline bool valid(int32_t ref) {
diff --git a/backends/native/runtime/graph/targets.bzl b/backends/native/runtime/graph/targets.bzl
index 606c9dac305..15196df7d51 100644
--- a/backends/native/runtime/graph/targets.bzl
+++ b/backends/native/runtime/graph/targets.bzl
@@ -67,3 +67,18 @@ def define_common_targets():
         ],
         visibility = ["//executorch/backends/native/..."],
     )
+
+    # One fx argument value: a std::variant over the schema ArgumentValue
+    # payload kinds (in-graph refs resolved to ValueRefs), plus NamedArgument.
+    runtime.cxx_library(
+        name = "argument",
+        srcs = ["Argument.cpp"],
+        exported_headers = [
+            "Argument.h",
+        ],
+        exported_deps = [
+            ":ids",
+            ":scalar_type",
+        ],
+        visibility = ["//executorch/backends/native/..."],
+    )