diff --git a/src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py b/src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py
index b9f91f8e21..7bddd00528 100644
--- a/src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py
+++ b/src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py
@@ -75,6 +75,24 @@ def snake_to_lower_camel(snake_case_string: str):
   ])
 
 
+def _to_param_string(value: Any) -> Any:
+  """Serializes a header or cookie parameter value for httpx.
+
+  httpx serializes query values itself but rejects anything other than a string
+  for a header or cookie, so an integer or boolean parameter declared by the
+  spec would fail the whole tool call. Numbers are stringified, booleans use
+  their JSON spelling and arrays are comma-joined, matching OpenAPI's default
+  serialization. Other values, such as strings, are returned unchanged.
+  """
+  if isinstance(value, bool):
+    return "true" if value else "false"
+  if isinstance(value, (int, float)):
+    return str(value)
+  if isinstance(value, list):
+    return ",".join(str(_to_param_string(item)) for item in value)
+  return value
+
+
 HttpxClientFactory = Callable[[], httpx.AsyncClient]
 """Type alias for a zero-argument factory returning an ``httpx.AsyncClient``.
 
@@ -417,9 +435,9 @@ def _prepare_request_params(
         if v is not None:
           query_params[original_k] = v
       elif param_location == "header":
-        header_params[original_k] = v
+        header_params[original_k] = _to_param_string(v)
       elif param_location == "cookie":
-        cookie_params[original_k] = v
+        cookie_params[original_k] = _to_param_string(v)
 
     # Construct URL
     base_url = self.endpoint.base_url or ""
diff --git a/tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py b/tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py
index 5cd2d340e8..8d62b48ea5 100644
--- a/tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py
+++ b/tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py
@@ -1919,6 +1919,59 @@ def test_prepare_request_params_cookie_param(
 
     assert request_params["cookies"]["session_id"] == "cookie_value"
 
+  @pytest.mark.parametrize(
+      "value, expected",
+      [
+          (50, "50"),
+          (1.5, "1.5"),
+          (True, "true"),
+          (False, "false"),
+          (["a", "b", 3], "a,b,3"),
+          ("already-a-string", "already-a-string"),
+      ],
+  )
+  def test_prepare_request_params_serializes_header_and_cookie_values(
+      self,
+      sample_endpoint,
+      sample_operation,
+      value,
+      expected,
+  ):
+    """httpx rejects non-string header and cookie values."""
+    tool = RestApiTool(
+        name="test_tool",
+        description="Test Tool",
+        endpoint=sample_endpoint,
+        operation=sample_operation,
+    )
+    params = [
+        ApiParameter(
+            original_name="X-Page-Size",
+            py_name="x_page_size",
+            param_location="header",
+            param_schema=OpenAPISchema(type="integer"),
+        ),
+        ApiParameter(
+            original_name="page_size",
+            py_name="page_size",
+            param_location="cookie",
+            param_schema=OpenAPISchema(type="integer"),
+        ),
+    ]
+    kwargs = {"x_page_size": value, "page_size": value}
+
+    request_params = tool._prepare_request_params(params, kwargs)
+
+    assert request_params["headers"]["X-Page-Size"] == expected
+    assert request_params["cookies"]["page_size"] == expected
+    # The values must be accepted by httpx when the request is built.
+    httpx.Request(
+        request_params["method"],
+        request_params["url"],
+        headers=request_params["headers"],
+        cookies=request_params["cookies"],
+    )
+
   def test_prepare_request_params_quota_project_id(
       self,
       sample_endpoint,