diff --git a/gemma/gm/tools/_manager.py b/gemma/gm/tools/_manager.py
index 00012cfa..10028a08 100644
--- a/gemma/gm/tools/_manager.py
+++ b/gemma/gm/tools/_manager.py
@@ -129,9 +129,16 @@ def _normalize_response(
   if response.structuredContent is not None:
     return response
 
-  # Assume the response is a single text block.
-  assert len(response.content) == 1
-  assert response.content[0].type == 'text'
+  if len(response.content) != 1:
+    raise ValueError(
+        f'Expected a single content block in the tool response, got'
+        f' {len(response.content)}: {response.content!r}'
+    )
+  if response.content[0].type != 'text':
+    raise ValueError(
+        f'Expected a text content block in the tool response, got'
+        f' type {response.content[0].type!r}: {response.content[0]!r}'
+    )
 
   content = response.content[0].text
   if response.isError:
diff --git a/gemma/gm/tools/_manager_test.py b/gemma/gm/tools/_manager_test.py
new file mode 100644
index 00000000..b414d669
--- /dev/null
+++ b/gemma/gm/tools/_manager_test.py
@@ -0,0 +1,97 @@
+# Copyright 2026 DeepMind Technologies Limited.
+#
+# 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 _manager._normalize_response."""
+
+import pytest
+from gemma.gm.tools import _manager
+from unittest import mock
+
+
+def _make_text_content(text: str):
+  content = mock.MagicMock()
+  content.type = 'text'
+  content.text = text
+  return content
+
+
+def _make_image_content():
+  content = mock.MagicMock()
+  content.type = 'image'
+  return content
+
+
+def _make_tool_result(
+    *,
+    content,
+    is_error: bool = False,
+    structured_content=None,
+):
+  result = mock.MagicMock()
+  result.content = list(content)
+  result.isError = is_error
+  result.structuredContent = structured_content
+  return result
+
+
+def test_normalize_response_passthrough_when_structured_content_present():
+  """If structuredContent is already set, return the response unchanged."""
+  result = _make_tool_result(
+      content=[_make_text_content('hello')],
+      structured_content={'result': 'already set'},
+  )
+  out = _manager._normalize_response(result)
+  assert out is result
+
+
+def test_normalize_response_success_single_text_block():
+  """Single text block with no error should map to {'result': <text>}."""
+  result = _make_tool_result(content=[_make_text_content('hello world')])
+  out = _manager._normalize_response(result)
+  assert out.structuredContent == {'result': 'hello world'}
+
+
+def test_normalize_response_error_single_text_block():
+  """Single text block with isError=True should map to {'error': <text>}."""
+  result = _make_tool_result(
+      content=[_make_text_content('something went wrong')], is_error=True
+  )
+  out = _manager._normalize_response(result)
+  assert out.structuredContent == {'error': 'something went wrong'}
+
+
+def test_normalize_response_raises_for_multiple_content_blocks():
+  """ValueError (not AssertionError) is raised when content has >1 blocks.
+
+  Bare `assert` statements are silently removed under `python -O`, making
+  this guard disappear in optimized production deployments.  The fix
+  replaces them with explicit ValueError so the check is always active and
+  the error message identifies what was returned.
+  """
+  result = _make_tool_result(
+      content=[_make_text_content('block1'), _make_text_content('block2')]
+  )
+  with pytest.raises(ValueError, match='Expected a single content block'):
+    _manager._normalize_response(result)
+
+
+def test_normalize_response_raises_for_non_text_content_type():
+  """ValueError is raised when the single content block is not type 'text'.
+
+  MCP tools may return image or blob content; this path is unsupported and
+  must raise a clear, actionable error rather than an opaque AssertionError.
+  """
+  result = _make_tool_result(content=[_make_image_content()])
+  with pytest.raises(ValueError, match="Expected a text content block"):
+    _manager._normalize_response(result)