diff --git a/src/google/adk/tools/function_tool.py b/src/google/adk/tools/function_tool.py
index 9514c9dd21..a99bc3be01 100644
--- a/src/google/adk/tools/function_tool.py
+++ b/src/google/adk/tools/function_tool.py
@@ -14,6 +14,7 @@
 
 from __future__ import annotations
 
+import copy
 import inspect
 import logging
 from typing import Any
@@ -47,6 +48,13 @@ class FunctionTool(BaseTool):
     func: The function to wrap.
   """
 
+  # Declaration built for the last seen (func, ignore_params, api variant).
+  # Declared on the class so instances created without __init__ (for example
+  # via copy.copy) still resolve it.
+  _declaration_cache: Optional[
+      tuple[Callable[..., Any], tuple[str, ...], Any, types.FunctionDeclaration]
+  ] = None
+
   def __init__(
       self,
       func: Callable[..., Any],
@@ -92,17 +100,33 @@ def __init__(
 
   @override
   def _get_declaration(self) -> Optional[types.FunctionDeclaration]:
-    function_decl = types.FunctionDeclaration.model_validate(
-        build_function_declaration(
-            func=self.func,
-            # The model doesn't understand the function context.
-            # input_stream is for streaming tool
-            ignore_params=self._ignore_params,
-            variant=self._api_variant,
-        )
-    )
-
-    return function_decl
+    variant = self._api_variant
+    ignore_params = tuple(self._ignore_params)
+    cache = self._declaration_cache
+    if (
+        cache is None
+        or cache[0] is not self.func
+        or cache[1] != ignore_params
+        or cache[2] != variant
+    ):
+      function_decl = types.FunctionDeclaration.model_validate(
+          build_function_declaration(
+              func=self.func,
+              # The model doesn't understand the function context.
+              # input_stream is for streaming tool
+              ignore_params=self._ignore_params,
+              variant=variant,
+          )
+      )
+      cache = (self.func, ignore_params, variant, function_decl)
+      self._declaration_cache = cache
+
+    # Callers are allowed to mutate the declaration they receive:
+    # LongRunningFunctionTool appends to its description, TransferToAgentTool
+    # writes an enum into its parameter schema, and BaseToolset rewrites its
+    # name when prefixing. Hand back a private copy so the cached value stays
+    # pristine.
+    return copy.deepcopy(cache[3])
 
   def _preprocess_args(self, args: dict[str, Any]) -> dict[str, Any]:
     """Preprocess and convert function arguments before invocation.
diff --git a/tests/unittests/tools/test_function_tool_declaration_cache.py b/tests/unittests/tools/test_function_tool_declaration_cache.py
new file mode 100644
index 0000000000..b7823a30c5
--- /dev/null
+++ b/tests/unittests/tools/test_function_tool_declaration_cache.py
@@ -0,0 +1,200 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for the FunctionTool declaration cache.
+
+`_get_declaration` is called once per tool on every LLM request, so its result
+is cached. Callers are allowed to mutate the declaration they receive, so each
+call must still get a private copy, and the cache must follow every input the
+declaration depends on.
+"""
+
+import copy
+
+from google.adk.tools import _function_tool_declarations
+from google.adk.tools.function_tool import FunctionTool
+from google.adk.tools.long_running_tool import LongRunningFunctionTool
+from google.adk.tools.transfer_to_agent_tool import TransferToAgentTool
+from google.adk.utils.variant_utils import GoogleLLMVariant
+import pytest
+
+
+def sample_tool(device_id: str, status: str = 'ON') -> str:
+  """Sets a device status.
+
+  Args:
+    device_id: The device.
+    status: The desired status.
+
+  Returns:
+    A confirmation message.
+  """
+  return 'ok'
+
+
+def poll_job(job_id: str) -> str:
+  """Polls a job.
+
+  Args:
+    job_id: The job.
+
+  Returns:
+    The job status.
+  """
+  return 'PENDING'
+
+
+def test_repeated_calls_return_equal_declarations():
+  tool = FunctionTool(sample_tool)
+
+  first = tool._get_declaration()
+  second = tool._get_declaration()
+
+  assert first.model_dump() == second.model_dump()
+
+
+def test_repeated_calls_return_distinct_objects():
+  tool = FunctionTool(sample_tool)
+
+  first = tool._get_declaration()
+  second = tool._get_declaration()
+
+  assert first is not second
+  assert first.parameters_json_schema is not second.parameters_json_schema
+
+
+def test_declaration_is_built_once_for_repeated_calls():
+  tool = FunctionTool(sample_tool)
+  calls = []
+  original = (
+      _function_tool_declarations.build_function_declaration_with_json_schema
+  )
+
+  def counting(*args, **kwargs):
+    calls.append(1)
+    return original(*args, **kwargs)
+
+  _function_tool_declarations.build_function_declaration_with_json_schema = (
+      counting
+  )
+  try:
+    for _ in range(5):
+      tool._get_declaration()
+  finally:
+    _function_tool_declarations.build_function_declaration_with_json_schema = (
+        original
+    )
+
+  assert len(calls) == 1
+
+
+def test_mutating_a_returned_declaration_does_not_affect_later_calls():
+  tool = FunctionTool(sample_tool)
+
+  first = tool._get_declaration()
+  first.name = 'renamed'
+  first.description = 'mutated'
+  first.parameters_json_schema['properties']['device_id']['enum'] = ['x']
+
+  second = tool._get_declaration()
+
+  assert second.name == 'sample_tool'
+  assert second.description != 'mutated'
+  assert 'enum' not in second.parameters_json_schema['properties']['device_id']
+
+
+def test_long_running_tool_description_is_stable_across_calls():
+  """LongRunningFunctionTool appends a note to the declaration it is given."""
+  tool = LongRunningFunctionTool(poll_job)
+
+  descriptions = [tool._get_declaration().description for _ in range(3)]
+
+  assert len(set(descriptions)) == 1
+  assert descriptions[0].count('long-running operation') == 1
+
+
+def test_transfer_to_agent_tool_sets_enum_on_every_call():
+  """TransferToAgentTool writes an enum into the parameter schema it is given."""
+  tool = TransferToAgentTool(agent_names=['alpha', 'beta'])
+
+  for _ in range(3):
+    declaration = tool._get_declaration()
+    assert declaration.parameters_json_schema['properties']['agent_name'][
+        'enum'
+    ] == ['alpha', 'beta']
+
+
+def test_toolset_name_prefixing_does_not_rename_the_original_tool():
+  """BaseToolset prefixing writes .name onto the declaration it is given."""
+  tool = FunctionTool(sample_tool)
+  prefixed = copy.copy(tool)
+  original_get_declaration = tool._get_declaration
+
+  def prefixed_declaration():
+    declaration = original_get_declaration()
+    declaration.name = 'prefix_sample_tool'
+    return declaration
+
+  prefixed._get_declaration = prefixed_declaration
+
+  assert prefixed._get_declaration().name == 'prefix_sample_tool'
+  assert tool._get_declaration().name == 'sample_tool'
+
+
+def test_changing_ignore_params_rebuilds_the_declaration():
+  tool = FunctionTool(sample_tool)
+
+  before = tool._get_declaration().parameters_json_schema['properties']
+  assert 'status' in before
+
+  tool._ignore_params = list(tool._ignore_params) + ['status']
+  after = tool._get_declaration().parameters_json_schema['properties']
+
+  assert 'status' not in after
+
+
+def test_changing_func_rebuilds_the_declaration():
+  tool = FunctionTool(sample_tool)
+  assert tool._get_declaration().name == 'sample_tool'
+
+  tool.func = poll_job
+
+  assert tool._get_declaration().name == 'poll_job'
+
+
+@pytest.mark.parametrize(
+    'variant',
+    [GoogleLLMVariant.GEMINI_API, GoogleLLMVariant.VERTEX_AI],
+)
+def test_changing_api_variant_rebuilds_the_declaration(monkeypatch, variant):
+  """The api variant is resolved from the environment on every call."""
+  tool = FunctionTool(sample_tool)
+
+  monkeypatch.setattr(
+      type(tool),
+      '_api_variant',
+      property(lambda self: GoogleLLMVariant.GEMINI_API),
+  )
+  studio = tool._get_declaration()
+
+  monkeypatch.setattr(
+      type(tool), '_api_variant', property(lambda self: variant)
+  )
+  switched = tool._get_declaration()
+
+  if variant is GoogleLLMVariant.GEMINI_API:
+    assert switched.model_dump() == studio.model_dump()
+  else:
+    assert switched.response_json_schema is not None
+    assert studio.response_json_schema is None