diff --git a/CHANGELOG.md b/CHANGELOG.md
index c21db01c0..29ea7842c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,13 @@ The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ### Fixed
 
+- **Action Argument Default Type Validation** (#9105): `ActionArgumentSchema` now
+  validates that default values match their declared types at parse time via a
+  `@model_validator`. Supports all argument types (string, integer, float, boolean,
+  list) with correct `bool`/`int` disambiguation and `float` widening from `int`.
+  Invalid configurations that previously passed silently now raise clear, actionable
+  `ValueError` messages indicating the argument name, declared type, and actual type.
+
 - **UKO Runtime Layer 2 (Paradigm) Indexing** (#9351): Added missing `rdf:type
   uko-oo:Class` triple emission in `PythonAnalyzer._extract_class()` so that
   Python class definitions are now correctly classified at layer 2 (paradigm/OO)
diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index 637dd5b31..dc99edcff 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -20,3 +20,4 @@ Below are some of the specific details of various contributions.
 * This project was made possible thanks to considerable donation of time, money, and resources by CleverThis, Inc.
 * HAL 9000 has contributed automated bug fixes, CLI output formatting improvements, and ongoing maintenance as part of the CleverAgents automation system.
 * HAL 9000 has contributed the file edit encoding parameter fix (PR #8258 / issue #7559).
+* HAL 9000 has contributed the action argument default value type validation fix (PR #9178 / issue #9105): added `@model_validator` to `ActionArgumentSchema` that validates default values match their declared types, with correct `bool`/`int` disambiguation and `float` widening from `int`.
diff --git a/features/action_schema_default_type_validation.feature b/features/action_schema_default_type_validation.feature
new file mode 100644
index 000000000..9c71945a4
--- /dev/null
+++ b/features/action_schema_default_type_validation.feature
@@ -0,0 +1,163 @@
+@a2a @domain @action
+Feature: Action Schema Default Value Type Validation
+  Validates that ActionArgumentSchema enforces type matching between
+  the declared 'type' field and the 'default' value.
+
+  Background:
+    Given an action YAML string with only required fields
+
+  # ============================================================
+  # String Type Tests
+  # ============================================================
+
+  Scenario: String argument with string default passes validation
+    Given an action YAML string with an argument of type "string" and default "hello"
+    When I validate the action schema
+    Then the action schema validation should succeed
+
+  Scenario: String argument with integer default fails validation
+    Given an action YAML string with an argument of type "string" and default 42
+    When I validate the action schema expecting failure
+    Then the action schema validation should fail
+    And the action schema error should mention "has type 'string' but default value"
+
+  Scenario: String argument with boolean default fails validation
+    Given an action YAML string with an argument of type "string" and default true
+    When I validate the action schema expecting failure
+    Then the action schema validation should fail
+    And the action schema error should mention "has type 'string' but default value"
+
+  # ============================================================
+  # Integer Type Tests
+  # ============================================================
+
+  Scenario: Integer argument with integer default passes validation
+    Given an action YAML string with an argument of type "integer" and default 42
+    When I validate the action schema
+    Then the action schema validation should succeed
+
+  Scenario: Integer argument with string default fails validation
+    Given an action YAML string with an argument of type "integer" and default "42"
+    When I validate the action schema expecting failure
+    Then the action schema validation should fail
+    And the action schema error should mention "has type 'integer' but default value"
+
+  Scenario: Integer argument with boolean default fails validation
+    Given an action YAML string with an argument of type "integer" and default true
+    When I validate the action schema expecting failure
+    Then the action schema validation should fail
+    And the action schema error should mention "has type 'integer' but default value"
+    And the action schema error should mention "is bool, not int"
+
+  Scenario: Integer argument with float default fails validation
+    Given an action YAML string with an argument of type "integer" and default 3.14
+    When I validate the action schema expecting failure
+    Then the action schema validation should fail
+    And the action schema error should mention "has type 'integer' but default value"
+
+  # ============================================================
+  # Float Type Tests
+  # ============================================================
+
+  Scenario: Float argument with float default passes validation
+    Given an action YAML string with an argument of type "float" and default 3.14
+    When I validate the action schema
+    Then the action schema validation should succeed
+
+  Scenario: Float argument with integer default passes validation
+    Given an action YAML string with an argument of type "float" and default 42
+    When I validate the action schema
+    Then the action schema validation should succeed
+
+  Scenario: Float argument with string default fails validation
+    Given an action YAML string with an argument of type "float" and default "3.14"
+    When I validate the action schema expecting failure
+    Then the action schema validation should fail
+    And the action schema error should mention "has type 'float' but default value"
+
+  Scenario: Float argument with boolean default fails validation
+    Given an action YAML string with an argument of type "float" and default true
+    When I validate the action schema expecting failure
+    Then the action schema validation should fail
+    And the action schema error should mention "has type 'float' but default value"
+
+  # ============================================================
+  # Boolean Type Tests
+  # ============================================================
+
+  Scenario: Boolean argument with boolean true default passes validation
+    Given an action YAML string with an argument of type "boolean" and default true
+    When I validate the action schema
+    Then the action schema validation should succeed
+
+  Scenario: Boolean argument with boolean false default passes validation
+    Given an action YAML string with an argument of type "boolean" and default false
+    When I validate the action schema
+    Then the action schema validation should succeed
+
+  Scenario: Boolean argument with integer default fails validation
+    Given an action YAML string with an argument of type "boolean" and default 1
+    When I validate the action schema expecting failure
+    Then the action schema validation should fail
+    And the action schema error should mention "has type 'boolean' but default value"
+
+  Scenario: Boolean argument with string default fails validation
+    Given an action YAML string with an argument of type "boolean" and default "true"
+    When I validate the action schema expecting failure
+    Then the action schema validation should fail
+    And the action schema error should mention "has type 'boolean' but default value"
+
+  # ============================================================
+  # List Type Tests
+  # ============================================================
+
+  Scenario: List argument with list default passes validation
+    Given an action YAML string with an argument of type "list" and default ["item1", "item2"]
+    When I validate the action schema
+    Then the action schema validation should succeed
+
+  Scenario: List argument with empty list default passes validation
+    Given an action YAML string with an argument of type "list" and default []
+    When I validate the action schema
+    Then the action schema validation should succeed
+
+  Scenario: List argument with string default fails validation
+    Given an action YAML string with an argument of type "list" and default "item1"
+    When I validate the action schema expecting failure
+    Then the action schema validation should fail
+    And the action schema error should mention "has type 'list' but default value"
+
+  Scenario: List argument with integer default fails validation
+    Given an action YAML string with an argument of type "list" and default 42
+    When I validate the action schema expecting failure
+    Then the action schema validation should fail
+    And the action schema error should mention "has type 'list' but default value"
+
+  # ============================================================
+  # None Default Tests (Always Valid)
+  # ============================================================
+
+  Scenario: String argument with None default passes validation
+    Given an action YAML string with an argument of type "string" and no default
+    When I validate the action schema
+    Then the action schema validation should succeed
+
+  Scenario: Integer argument with None default passes validation
+    Given an action YAML string with an argument of type "integer" and no default
+    When I validate the action schema
+    Then the action schema validation should succeed
+
+  Scenario: Float argument with None default passes validation
+    Given an action YAML string with an argument of type "float" and no default
+    When I validate the action schema
+    Then the action schema validation should succeed
+
+  Scenario: Boolean argument with None default passes validation
+    Given an action YAML string with an argument of type "boolean" and no default
+    When I validate the action schema
+    Then the action schema validation should succeed
+
+  Scenario: List argument with None default passes validation
+    Given an action YAML string with an argument of type "list" and no default
+    When I validate the action schema
+    Then the action schema validation should succeed
diff --git a/features/steps/action_schema_steps.py b/features/steps/action_schema_steps.py
index 7d567e76b..335b2fc82 100644
--- a/features/steps/action_schema_steps.py
+++ b/features/steps/action_schema_steps.py
@@ -7,6 +7,7 @@ environment variable interpolation, and error messages.
 
 from __future__ import annotations
 
+import json
 from pathlib import Path
 from typing import Any
 
@@ -131,6 +132,82 @@ def step_given_yaml_with_name(context: Context, name: str) -> None:
     context.action_yaml_string = _MINIMAL_YAML.replace("local/simple-action", name)
 
 
+# NOTE: More specific steps with "and default" / "and no default" MUST be
+# defined BEFORE the shorter pattern 'with an argument of type "{arg_type}"'
+# so that Behave's parse matcher selects the longer match first.
+
+
+@given(
+    'an action YAML string with an argument of type "{arg_type}" and default {default_value}'
+)
+def step_given_yaml_with_arg_and_default(
+    context: Context, arg_type: str, default_value: str
+) -> None:
+    """Provide YAML with an argument that has a specific type and default value."""
+    # Parse the default value from the string representation
+    # Handle special cases: true/false for booleans, numbers, strings, lists
+    if default_value.lower() == "true":
+        parsed_default: str | int | float | bool | list[str] = True
+    elif default_value.lower() == "false":
+        parsed_default = False
+    elif default_value.startswith("[") and default_value.endswith("]"):
+        # Parse list
+        parsed_default = json.loads(default_value)
+    elif default_value.startswith('"') and default_value.endswith('"'):
+        # String value
+        parsed_default = default_value[1:-1]
+    else:
+        # Try to parse as number
+        try:
+            if "." in default_value:
+                parsed_default = float(default_value)
+            else:
+                parsed_default = int(default_value)
+        except ValueError:
+            # Treat as string if not a number
+            parsed_default = default_value
+
+    # Format the default value for YAML
+    if isinstance(parsed_default, bool):
+        yaml_default = "true" if parsed_default else "false"
+    elif isinstance(parsed_default, list):
+        yaml_default = json.dumps(parsed_default)
+    elif isinstance(parsed_default, str):
+        yaml_default = f'"{parsed_default}"'
+    else:
+        yaml_default = str(parsed_default)
+
+    # Build YAML with the argument
+    context.action_yaml_string = f"""\
+name: local/test-default-type
+description: Test argument with default value
+strategy_actor: openai/gpt-4
+execution_actor: openai/gpt-4
+definition_of_done: Done
+arguments:
+  - name: test_arg
+    type: {arg_type}
+    required: false
+    default: {yaml_default}
+"""
+
+
+@given('an action YAML string with an argument of type "{arg_type}" and no default')
+def step_given_yaml_with_arg_no_default(context: Context, arg_type: str) -> None:
+    """Provide YAML with an argument that has no default value."""
+    context.action_yaml_string = f"""\
+name: local/test-no-default
+description: Test argument without default value
+strategy_actor: openai/gpt-4
+execution_actor: openai/gpt-4
+definition_of_done: Done
+arguments:
+  - name: test_arg
+    type: {arg_type}
+    required: false
+"""
+
+
 @given('an action YAML string with an argument of type "{arg_type}"')
 def step_given_yaml_with_bad_arg_type(context: Context, arg_type: str) -> None:
     """Provide YAML with an invalid argument type."""
diff --git a/src/cleveragents/action/schema.py b/src/cleveragents/action/schema.py
index 8ce054207..548687956 100644
--- a/src/cleveragents/action/schema.py
+++ b/src/cleveragents/action/schema.py
@@ -119,6 +119,11 @@ class ActionArgumentSchema(BaseModel):
         description="Maximum value for numeric arguments.",
     )
 
+    model_config = ConfigDict(
+        str_strip_whitespace=True,
+        extra="forbid",
+    )
+
     @field_validator("name")
     @classmethod
     def validate_name(cls, v: str) -> str:
@@ -140,10 +145,62 @@ class ActionArgumentSchema(BaseModel):
             raise ValueError(f"Invalid argument type '{v}'. Allowed types: {valid}.")
         return v_lower
 
-    model_config = ConfigDict(
-        str_strip_whitespace=True,
-        extra="forbid",
-    )
+    @model_validator(mode="after")
+    def validate_default_type(self) -> ActionArgumentSchema:
+        """Validate that the default value's type matches the declared type field.
+
+        Raises:
+            ValueError: If the default value's type does not match the declared type.
+        """
+        # None default is always valid (means no default provided)
+        if self.default is None:
+            return self
+
+        arg_type = self.type.lower()
+        default_value = self.default
+
+        # Check type compatibility
+        if arg_type == "string":
+            if not isinstance(default_value, str):
+                raise ValueError(
+                    f"Argument '{self.name}' has type 'string' but default value "
+                    f"'{default_value}' is {type(default_value).__name__}, not str."
+                )
+        elif arg_type == "integer":
+            # Explicitly check for bool first, since bool is a subclass of int
+            if isinstance(default_value, bool):
+                raise ValueError(
+                    f"Argument '{self.name}' has type 'integer' but default value "
+                    f"'{default_value}' is bool, not int."
+                )
+            if not isinstance(default_value, int):
+                raise ValueError(
+                    f"Argument '{self.name}' has type 'integer' but default value "
+                    f"'{default_value}' is {type(default_value).__name__}, not int."
+                )
+        elif arg_type == "float":
+            # float or int are acceptable for float type
+            is_bool = isinstance(default_value, bool)
+            is_numeric = isinstance(default_value, (float, int))
+            if not is_numeric or is_bool:
+                raise ValueError(
+                    f"Argument '{self.name}' has type 'float' but default value "
+                    f"'{default_value}' is {type(default_value).__name__}, "
+                    "not float or int."
+                )
+        elif arg_type == "boolean":
+            if not isinstance(default_value, bool):
+                raise ValueError(
+                    f"Argument '{self.name}' has type 'boolean' but default value "
+                    f"'{default_value}' is {type(default_value).__name__}, not bool."
+                )
+        elif arg_type == "list" and not isinstance(default_value, list):
+            raise ValueError(
+                f"Argument '{self.name}' has type 'list' but default value "
+                f"'{default_value}' is {type(default_value).__name__}, not list."
+            )
+
+        return self
 
 
 # ────────────────────────────────────────────────────────────