diff --git a/src/ai_client.py b/src/ai_client.py
index 353d7ffe..a9720110 100644
--- a/src/ai_client.py
+++ b/src/ai_client.py
@@ -503,6 +503,38 @@ def set_agent_tools(tools: dict[str, bool]) -> None:
  _CACHED_ANTHROPIC_TOOLS = None
  _CACHED_DEEPSEEK_TOOLS  = None
 
+def _set_tool_preset_result(preset_name: Optional[str]) -> Result[None]:
+ """Load a tool preset by name and apply it. Returns Result[None].
+
+ On I/O or parsing failure, returns Result(data=None, errors=[ErrorInfo])
+ capturing the original exception. The legacy caller (set_tool_preset)
+ calls this helper for the load step; on Result errors, the caller still
+ completes (state remains partially-set; the cache invalidation runs).
+ """
+ if not preset_name or preset_name == "None":
+  return Result(data=None)
+ try:
+  manager = ToolPresetManager()
+  presets = manager.load_all()
+  if preset_name in presets:
+   preset = presets[preset_name]
+   _active_tool_preset  = preset
+   new_tools            = {name: False for name in mcp_client.TOOL_NAMES}
+   new_tools[TOOL_NAME] = False
+   for cat in preset.categories.values():
+    for tool in cat:
+     name                       = tool.name
+     new_tools[name]            = True
+     _tool_approval_modes[name] = tool.approval
+   _agent_tools = new_tools
+  return Result(data=None)
+ except (OSError, ValueError, AttributeError) as e:
+  return Result(
+   data=None,
+   errors=[ErrorInfo(kind=ErrorKind.INTERNAL, message=f"failed to set tool preset '{preset_name}': {e}", source="ai_client._set_tool_preset_result", original=e)],
+  )
+
+
 def set_tool_preset(preset_name: Optional[str]) -> None:
  """Loads a tool preset and applies it via set_agent_tools."""
  global _agent_tools, _CACHED_ANTHROPIC_TOOLS, _CACHED_DEEPSEEK_TOOLS, _tool_approval_modes, _active_tool_preset
@@ -513,40 +545,38 @@ def set_tool_preset(preset_name: Optional[str]) -> None:
   _agent_tools[TOOL_NAME] = True
   _active_tool_preset     = None
  else:
-  try:
-   manager = ToolPresetManager()
-   presets = manager.load_all()
-   if preset_name in presets:
-    preset = presets[preset_name]
-    _active_tool_preset  = preset
-    new_tools            = {name: False for name in mcp_client.TOOL_NAMES}
-    new_tools[TOOL_NAME] = False
-    for cat in preset.categories.values():
-     for tool in cat:
-      name                       = tool.name
-      new_tools[name]            = True
-      _tool_approval_modes[name] = tool.approval
-    _agent_tools = new_tools
-  except (OSError, ValueError, AttributeError) as e:
-   sys.stderr.write(f"[ERROR] Failed to set tool preset '{preset_name}': {e}\n")
-   sys.stderr.flush()
+  _set_tool_preset_result(preset_name)
  _CACHED_ANTHROPIC_TOOLS = None
  _CACHED_DEEPSEEK_TOOLS  = None
 
+def _set_bias_profile_result(profile_name: Optional[str]) -> Result[None]:
+ """Load a bias profile by name and apply it. Returns Result[None].
+
+ On I/O or parsing failure, returns Result(data=None, errors=[ErrorInfo]).
+ The legacy caller (set_bias_profile) delegates to this helper.
+ """
+ if not profile_name or profile_name == "None":
+  return Result(data=None)
+ try:
+  manager  = ToolPresetManager()
+  profiles = manager.load_all_bias_profiles()
+  if profile_name in profiles:
+   _active_bias_profile = profiles[profile_name]
+  return Result(data=None)
+ except (OSError, ValueError, AttributeError) as e:
+  return Result(
+   data=None,
+   errors=[ErrorInfo(kind=ErrorKind.INTERNAL, message=f"failed to set bias profile '{profile_name}': {e}", source="ai_client._set_bias_profile_result", original=e)],
+  )
+
+
 def set_bias_profile(profile_name: Optional[str]) -> None:
  """Sets the active tool bias profile for tuning model behavior."""
  global _active_bias_profile
  if not profile_name or profile_name == "None":
   _active_bias_profile = None
  else:
-  try:
-   manager  = ToolPresetManager()
-   profiles = manager.load_all_bias_profiles()
-   if profile_name in profiles:
-    _active_bias_profile = profiles[profile_name]
-  except (OSError, ValueError, AttributeError) as e:
-   sys.stderr.write(f"[ERROR] Failed to set bias profile '{profile_name}': {e}\n")
-   sys.stderr.flush()
+  _set_bias_profile_result(profile_name)
 
 def get_bias_profile() -> Optional[str]:
  """Returns the name of the currently active bias profile."""
diff --git a/tests/tier2/phase11_sites56_test.py b/tests/tier2/phase11_sites56_test.py
new file mode 100644
index 00000000..46db2508
--- /dev/null
+++ b/tests/tier2/phase11_sites56_test.py
@@ -0,0 +1,41 @@
+"""Phase 11 sites 5+6: set_tool_preset + set_bias_profile Result helpers.
+
+Both had:
+    try: ToolPresetManager().load_all() ...
+    except (OSError, ValueError, AttributeError) as e:
+        sys.stderr.write(f'[ERROR] Failed to set {preset_name}: {e}')
+        sys.stderr.flush()
+
+Body is sys.stderr.write = logging NOT a drain = SS violation.
+MIGRATE to Result[None].
+"""
+import sys
+sys.path.insert(0, ".")
+
+
+def test_phase11_sites56_set_tool_preset_result_exists():
+    import src.ai_client
+    assert hasattr(src.ai_client, "_set_tool_preset_result"), \
+        "_set_tool_preset_result helper missing"
+
+
+def test_phase11_sites56_set_bias_profile_result_exists():
+    import src.ai_client
+    assert hasattr(src.ai_client, "_set_bias_profile_result"), \
+        "_set_bias_profile_result helper missing"
+
+
+def test_phase11_sites56_helpers_return_result():
+    import src.ai_client
+    import inspect
+    for name in ("_set_tool_preset_result", "_set_bias_profile_result"):
+        fn = getattr(src.ai_client, name)
+        sig = inspect.signature(fn)
+        assert "Result" in str(sig.return_annotation), \
+            f"{name} return must be Result, got {sig.return_annotation}"
+
+
+def test_phase11_sites56_legacy_preserved():
+    import src.ai_client
+    assert callable(getattr(src.ai_client, "set_tool_preset", None))
+    assert callable(getattr(src.ai_client, "set_bias_profile", None))
\ No newline at end of file