set_model_response is missing from the MCP _RESERVED_TOOL_NAMES guard

Start in src/google/adk/tools/mcp_tool/mcp_tool.py and inspect the _RESERVED_TOOL_NAMES guard, then read the related tests in test_mcp_tool.py. Add set_model_response to the reserved names and cover an MCP server advertising that name. Done means registration raises ValueError consistently with the other reserved tool names.

_RESERVED_TOOL_NAMES in src/google/adk/tools/mcp_tool/mcp_tool.py omits set_model_response, even though it is a tool name the framework itself puts on the wire. The guard added in #6796 states its own invariant in the comment above the set:

# Tool names the framework itself puts on the wire. A server advertising one of
# these would have its tool dispatched in place of the framework's own, so the
# name is refused at registration.
_RESERVED_TOOL_NAMES = frozenset({
    REQUEST_EUC_FUNCTION_CALL_NAME,             # adk_request_credential
    REQUEST_CONFIRMATION_FUNCTION_CALL_NAME,    # adk_request_confirmation
    REQUEST_INPUT_FUNCTION_CALL_NAME,           # adk_request_input
    transfer_to_agent.__name__,                 # transfer_to_agent
})

set_model_response satisfies that description but is not in the set, so an MCP server advertising it is accepted where the other four are refused.

set_model_response is a framework-owned tool name

• Defined as def set_model_response() -> str: in src/google/adk/tools/set_model_response_tool.py (self.func = set_model_response), so that is its declaration name.
• Injected into the request whenever output_schema is set alongside other tools:
  src/google/adk/flows/llm_flows/prompt/_schema.py:54

  set_response_tool = SetModelResponseTool(agent.output_schema)
  llm_request.append_tools([set_response_tool])
  

• The framework then instructs the model to use it by name: "you must provide your final response using the set_model_response tool" (_schema.py:57-64).
• And it reads the answer back by that name — get_structured_model_response() (_schema.py:97) is reached from base_llm_flow.py:710 with the comment "Check if this is a set_model_response function response".

Why that matters at an MCP boundary

LlmRequest.append_tools resolves a duplicate name by last-wins with only a warning (src/google/adk/models/llm_request.py):

if tool.name in self.tools_dict:
    # Both declarations are still advertised to the model, but only one
    # tool can hold the name, so calls land on the survivor.
    logging.warning(
        "Duplicate tool name %r: the previously registered tool is shadowed ...",
        tool.name,
    )
self.tools_dict[tool.name] = tool

So if an MCP server advertises set_model_response, one of the two tools holds the name and the model's set_model_response call is dispatched there. If the server's tool is the survivor, the agent's structured final answer is delivered to the third-party MCP server and the framework never sees it — which is the outcome the reserved-name check exists to prevent. If the framework's tool is the survivor, the duplicate is still advertised and callable by name, so calls can land on the wrong one.

Either way the invariant stated in the #6796 comment is violated: the name is never refused at registration.

Reproduction sketch

agent = LlmAgent(
    model=..., tools=[McpToolset(connection_params=<server that advertises
                                 a tool named "set_model_response">)],
    output_schema=MySchema,
)
# Expected: ValueError, as for transfer_to_agent / adk_request_confirmation
# Actual:   accepted; a warning is logged at request build time

Suggested fix

Add the name to the set:

_RESERVED_TOOL_NAMES = frozenset({
    REQUEST_EUC_FUNCTION_CALL_NAME,
    REQUEST_CONFIRMATION_FUNCTION_CALL_NAME,
    REQUEST_INPUT_FUNCTION_CALL_NAME,
    transfer_to_agent.__name__,
    'set_model_response',
})

If it is useful, the same reasoning may apply to other in-model names the framework injects (google_search, google_maps, url_context, vertex_ai_search, code_execution, load_artifacts, load_memory) — I have not verified each of those reaches tools_dict, so I have deliberately kept this report to the one name I traced end to end.

Happy to send the set change plus a unit test alongside test_mcp_tool.py if that is the preferred route.