Python: Filter MCP tool kwargs to declared params via allowlist (#6399)

* Filter MCP tool kwargs to declared params via allowlist

Previously MCPTool combined framework runtime kwargs (from
FunctionInvocationContext.kwargs) with the LLM-supplied arguments and
stripped only a hardcoded denylist of known framework keys before
forwarding to the MCP server. Any new framework-injected kwarg leaked to
the server unless the denylist was updated.

Switch to an allowlist built from each tool's declared parameters
(inputSchema.properties). Only declared params are forwarded; everything
else is stripped. Add an `additional_tool_argument_names` constructor
argument so users can opt extra names back in, globally (Sequence[str])
and/or per remote tool name (Mapping with reserved "*" global key). The
existing denylist is kept as a safety net for framework-named params a
server declares in its schema; explicitly opted-in extras always win. The
reserved _meta handling is unchanged.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address MCP allowlist review comments and fix reload arg loss

- Fix pyright reportUnknownArgumentType in _load_tools (cast schema properties).
- Register declared param names before the existing-tool skip guard so that
  tool-list reloads preserve the allowlist for already-loaded tools (previously
  unchanged tools silently dropped all declared args after a background reload).
- Handle bare-string values in an additional_tool_argument_names mapping instead
  of iterating their characters.
- Clarify the framework denylist comment: explicit extras override the denylist.
- Make the extras-override-denylist test unambiguous (opt in a denylisted name).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/packages/core/AGENTS.md

@@ -80,6 +80,8 @@ agent_framework/
 
 - **`MCPTool`** - Base wrapper that owns the MCP `ClientSession` and exposes the remote server's tools as `FunctionTool`s.
 - **`MCPStdioTool`** / **`MCPStreamableHTTPTool`** / **`MCPWebsocketTool`** - Transport-specific subclasses.
+- **Argument allowlist (`_prepare_call_kwargs`)** - Before each `tools/call`, kwargs are filtered to an **allowlist** built from the tool's declared parameters (`inputSchema.properties`) plus any user-configured extras. Framework runtime kwargs injected through the function-invocation pipeline (e.g. `thread`, `conversation_id`, `chat_options`, `options`, `response_format`) are stripped by default rather than forwarded. A tool that declares no usable `properties` (including schemas with `additionalProperties: true`) forwards only the configured extras. The `_MCP_FRAMEWORK_DENYLIST` is a safety net for framework-named params a server *declares* in its schema (those are dropped); names explicitly opted in via `additional_tool_argument_names` always win. The reserved `_meta` key is extracted as MCP request metadata, never forwarded as an argument.
+- **`additional_tool_argument_names`** (constructor arg on all `MCPTool` subclasses) - Opt extra argument names back into the allowlist. Accepts a `Sequence[str]` (applied to every tool) or a `Mapping[str, Sequence[str]]` keyed by **remote tool name**, where the reserved key `"*"` denotes global extras. It is configured only in user code at construction; there is **no per-call/runtime override**, so a model-issued tool call cannot change which names pass through. To use a server that accepts `additionalProperties: true`, list the extra names here and then either (1) manually extend that tool's `inputSchema` (via the `.functions` list after connecting) so the model is prompted to supply them, or (2) supply the values yourself via `function_invocation_kwargs`. If a name is supplied by both the model and `function_invocation_kwargs`, the model-supplied value wins.
 - **`MCPTaskOptions`** (experimental, `MCP_LONG_RUNNING_TASKS` feature, **frozen**) - Per-tool-instance options controlling the SEP-2663 long-running task lifecycle. When the server advertises a tool with `execution.taskSupport == "required"`, `MCPTool.call_tool` transparently routes through `call_tool_as_task`, which sends an augmented `tools/call`, polls `tasks/get` until terminal, and reinterprets `tasks/result` as a normal `CallToolResult`. Instances are immutable; replace via `MCPTool.task_options = MCPTaskOptions(...)`. Fields:
   - `default_ttl: timedelta | None` — forwarded to the server as `params.task.ttl` (milliseconds). When `None`, the server's default applies.
   - `cancel_remote_task_on_local_cancellation: bool = True` — only gates the `CancelledError` path. Abandonment paths (see below) always cancel.