diff --git a/src/halmos/calldata.py b/src/halmos/calldata.py
index 5dd28dd0..dcfdc361 100644
--- a/src/halmos/calldata.py
+++ b/src/halmos/calldata.py
@@ -103,19 +103,10 @@ class FunctionInfo:
 
 
 class Calldata:
-    args: HalmosConfig
-
-    # `arrlen` holds the parsed value of --array-lengths, specifying the sizes for certain dynamic parameters.
-    # `default_bytes_lengths` holds the parsed value of --default-bytes-lengths.
-    #
     # For dynamic parameters not explicitly listed in --array-lengths, default sizes are used:
     # - For dynamic arrays: the size ranges from 0 to the value of --loop (inclusive).
     # - For bytes or strings: the size candidates are given by --default-bytes-lengths.
-    #
-    # TODO: Extend `args` to include `arrlen` and `default_bytes_lengths`
-    #       to prevent re-parsing --array-lengths and --default-bytes-lengths multiple times.
-    arrlen: dict[str, list[int]]
-    default_bytes_lengths: list[int]
+    args: HalmosConfig
 
     # `dyn_params` will be updated to include the fully resolved size information for all dynamic parameters.
     dyn_params: list[DynamicParam]
@@ -130,10 +121,6 @@ def __init__(
         new_symbol_id: Callable | None,
     ) -> None:
         self.args = args
-        self.arrlen = mk_arrlen(args)
-        self.default_bytes_lengths = [
-            int(x.strip()) for x in self.args.default_bytes_lengths.split(",")
-        ]
         self.dyn_params = []
         self.new_symbol_id = new_symbol_id if new_symbol_id else lambda: ""
 
@@ -144,13 +131,13 @@ def get_dyn_sizes(self, name: str, typ: Type) -> tuple[list[int], BitVecRef]:
         The candidates are derived from --array_lengths if provided; otherwise, default values are used.
         """
 
-        sizes = self.arrlen.get(name)
+        sizes = self.args.array_lengths.get(name)
 
         if sizes is None:
             sizes = (
                 list(range(self.args.loop + 1))
                 if isinstance(typ, DynamicArrayType)
-                else self.default_bytes_lengths  # bytes or string
+                else self.args.default_bytes_lengths  # bytes or string
             )
             if self.args.debug:
                 print(
@@ -345,15 +332,3 @@ def mk_calldata(
     new_symbol_id: Callable = None,
 ) -> tuple[ByteVec, list[DynamicParam]]:
     return Calldata(args, new_symbol_id).create(abi, fun_info)
-
-
-def mk_arrlen(args: HalmosConfig) -> dict[str, list[int]]:
-    if not args.array_lengths:
-        return {}
-
-    # TODO: update syntax: name1=size1,size2; name2=size3,...; ...
-    name_sizes_pairs = args.array_lengths.split(",")
-    return {
-        name.strip(): [int(x.strip()) for x in sizes.split(";")]
-        for name, sizes in [x.split("=") for x in name_sizes_pairs]
-    }
diff --git a/src/halmos/config.py b/src/halmos/config.py
index 738fdba6..fad3b0d3 100644
--- a/src/halmos/config.py
+++ b/src/halmos/config.py
@@ -2,6 +2,7 @@
 import os
 import sys
 from collections import OrderedDict
+from collections.abc import Callable
 from dataclasses import MISSING, dataclass, fields
 from dataclasses import field as dataclass_field
 from typing import Any
@@ -33,6 +34,7 @@ def arg(
     short: str | None = None,
     countable: bool = False,
     global_default_str: str | None = None,
+    action: Callable = None,
 ):
     return dataclass_field(
         default=None,
@@ -45,10 +47,39 @@ def arg(
             "short": short,
             "countable": countable,
             "global_default_str": global_default_str,
+            "action": action,
         },
     )
 
 
+class ParseCSV(argparse.Action):
+    def __call__(self, parser, namespace, values, option_string=None):
+        values = ParseCSV.parse(values)
+        setattr(namespace, self.dest, values)
+
+    @staticmethod
+    def parse(values: str) -> list[int]:
+        return [int(x.strip()) for x in values.split(",")]
+
+
+class ParseArrayLengths(argparse.Action):
+    def __call__(self, parser, namespace, values, option_string=None):
+        values = ParseArrayLengths.parse(values)
+        setattr(namespace, self.dest, values)
+
+    @staticmethod
+    def parse(values: str | None) -> dict[str, list[int]]:
+        if not values:
+            return {}
+
+        # TODO: update syntax: name1=size1,size2; name2=size3,...; ...
+        name_sizes_pairs = values.split(",")
+        return {
+            name.strip(): [int(x.strip()) for x in sizes.split(";")]
+            for name, sizes in [x.split("=") for x in name_sizes_pairs]
+        }
+
+
 # TODO: add kw_only=True when we support Python>=3.10
 @dataclass(frozen=True)
 class Config:
@@ -152,12 +183,14 @@ class Config:
         help="set the length of dynamic-sized arrays including bytes and string (default: loop unrolling bound)",
         global_default=None,
         metavar="NAME1=LENGTH1,NAME2=LENGTH2,...",
+        action=ParseArrayLengths,
     )
 
     default_bytes_lengths: str = arg(
         help="set the default length candidates for bytes and string not specified in --array-lengths",
         global_default="0,32,1024,65",  # 65 is ECDSA signature size
         metavar="LENGTH1,LENGTH2,...",
+        action=ParseCSV,
     )
 
     storage_layout: str = arg(
@@ -528,7 +561,12 @@ def _create_default_config() -> "Config":
         if default == MISSING:
             continue
 
-        values[field.name] = default() if callable(default) else default
+        # retrieve the default value
+        raw_value = default() if callable(default) else default
+
+        # parse the default value, if a custom parser is provided
+        action = field.metadata.get("action", None)
+        values[field.name] = action.parse(raw_value) if action else raw_value
 
     return Config(_parent=None, _source="default", **values)
 
@@ -583,6 +621,8 @@ def _create_arg_parser() -> argparse.ArgumentParser:
             }
             if choices := field_info.metadata.get("choices", None):
                 kwargs["choices"] = choices
+            if action := field_info.metadata.get("action", None):
+                kwargs["action"] = action
             group.add_argument(*names, **kwargs)
 
     return parser