diff --git a/src/google/adk/flows/llm_flows/prompt/_instructions_utils.py b/src/google/adk/flows/llm_flows/prompt/_instructions_utils.py
index 4b194df3c9..0cb697ef81 100644
--- a/src/google/adk/flows/llm_flows/prompt/_instructions_utils.py
+++ b/src/google/adk/flows/llm_flows/prompt/_instructions_utils.py
@@ -97,7 +97,8 @@ async def build_instruction(
     use_jinja2: If True, render the template with Jinja2 instead of the
       default regex-based engine.  Defaults to False for backward
       compatibility.  Jinja2 is an optional dependency and must be installed
-      separately to use this.
+      separately to use this.  Templates are rendered in Jinja2's immutable
+      sandbox, so they cannot reach Python internals or modify session state.
 
   Returns:
     The instruction template with values populated.
@@ -192,7 +193,7 @@ async def _render_with_jinja2(
     template: str,
     readonly_context: ReadonlyContext,
 ) -> str:
-  """Renders *template* using a Jinja2 environment.
+  """Renders *template* using a sandboxed Jinja2 environment.
 
   Session state variables are exposed as top-level template variables.
   Artifacts can be loaded with the ``artifact(filename)`` async callable
@@ -213,6 +214,7 @@ async def _render_with_jinja2(
   """
   try:
     import jinja2
+    from jinja2.sandbox import ImmutableSandboxedEnvironment
   except ImportError as e:
     raise ImportError(
         'Rendering an instruction with Jinja2 requires the optional jinja2'
@@ -237,7 +239,10 @@ async def _load_artifact(filename: str) -> str:
       )
     return str(artifact)
 
-  env = jinja2.Environment(
+  # Template text is not always fully trusted, and the context holds the live
+  # session state objects. The immutable sandbox blocks access to Python
+  # internals and in-place mutation of those objects.
+  env = ImmutableSandboxedEnvironment(
       enable_async=True,
       undefined=jinja2.StrictUndefined,
       autoescape=False,
diff --git a/tests/unittests/flows/llm_flows/prompt/test_instructions_utils.py b/tests/unittests/flows/llm_flows/prompt/test_instructions_utils.py
index 942e492e98..4d32e62d10 100644
--- a/tests/unittests/flows/llm_flows/prompt/test_instructions_utils.py
+++ b/tests/unittests/flows/llm_flows/prompt/test_instructions_utils.py
@@ -23,6 +23,7 @@
 from google.adk.flows.llm_flows.prompt._instructions_utils import _is_valid_state_name
 from google.adk.flows.llm_flows.prompt._instructions_utils import InstructionProvider
 from google.adk.sessions.session import Session
+from jinja2.exceptions import SecurityError
 import pytest
 
 from .... import testing_utils
@@ -390,6 +391,44 @@ async def test_inject_session_state_jinja2_artifact_with_filter():
   assert populated_instruction == "Content: ARTIFACT DATA"
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "instruction_template",
+    [
+        "{{ ''.__class__.__mro__[1].__subclasses__() }}",
+        "{{ artifact.__globals__['__builtins__'] }}",
+    ],
+)
+async def test_inject_session_state_jinja2_blocks_python_internals(
+    instruction_template,
+):
+  invocation_context = await _create_test_readonly_context(
+      artifact_service=MockArtifactService({})
+  )
+
+  with pytest.raises(SecurityError):
+    await instructions_utils.inject_session_state(
+        instruction_template, invocation_context, use_jinja2=True
+    )
+
+
+@pytest.mark.asyncio
+async def test_inject_session_state_jinja2_cannot_mutate_state():
+  state = {"items": ["a"], "user": {"name": "Foo"}}
+  invocation_context = await _create_test_readonly_context(state=state)
+
+  for instruction_template in (
+      "{{ items.append('b') }}",
+      "{{ user.update(name='Bar') }}",
+  ):
+    with pytest.raises(SecurityError):
+      await instructions_utils.inject_session_state(
+          instruction_template, invocation_context, use_jinja2=True
+      )
+
+  assert state == {"items": ["a"], "user": {"name": "Foo"}}
+
+
 def test_module_imports_without_jinja2_installed():
   # Jinja2 ships only in the eval and test extras, but this module is on the
   # import path of google.adk.agents, so a module-scope import of it would