Fix execute_custom_tool bypassed when unity_instance is specified (#724)

* Fix execute_custom_tool bypassed when unity_instance is specified

Move execute_custom_tool handling outside the if/else block so it's
executed regardless of whether unity_instance is provided.

Previously, when unity_instance was specified, the code would take the
if branch (match by hash/name) and return 404 if not found,
but the execute_custom_tool block was only in the else branch,
causing custom tools to fall through to PluginHub.send_command.

Fixes #649

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Handle empty sessions gracefully when selecting default session

When no active sessions exist, next(iter(sessions.sessions.keys()))
raises StopIteration. Catch this exception and set session_id
and session_details to None, allowing execute_custom_tool guard to work
as intended.

Suggested by Sourcery review.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Move default session selection into execute_custom_tool block

Per Sourcery review: moving the default session selection
inside execute_custom_tool ensures the empty sessions guard works
correctly. If sessions becomes empty between the initial check
and this code, the StopIteration will now properly trigger the
session validation guard and return a 503 error.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* chore: remove unintended uv.lock changes from PR

* 优化 execute_custom_tool 逻辑以优先使用可用会话

---------

Co-authored-by: whatevertogo <whatevertogo@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: whatevertogo

Server/src/main.py

@@ -394,63 +394,83 @@ async def cli_command_route(request: Request) -> JSONResponse:
                             session_details = details
                             break
 
-                    # If a specific unity_instance was requested but not found, return an error
-                    if not session_id:
+                # If a specific unity_instance was requested but not found, return an error
+                # (Check done here so execute_custom_tool can also validate the instance)
+                if unity_instance and not session_id:
+                    return JSONResponse(
+                        {
+                            "success": False,
+                            "error": f"Unity instance '{unity_instance}' not found",
+                        },
+                        status_code=404,
+                    )
+
+                # If no specific unity_instance requested, use first available session
+                # (Must be done before execute_custom_tool check so all command types benefit)
+                if not session_id:
+                    try:
+                        session_id = next(iter(sessions.sessions.keys()))
+                        session_details = sessions.sessions.get(session_id)
+                    except StopIteration:
+                        # No sessions available - sessions.sessions is empty
+                        # This should not happen since we checked at line 378, but handle gracefully
+                        return JSONResponse({
+                            "success": False,
+                            "error": "No Unity instances connected. Make sure Unity is running with MCP plugin."
+                        }, status_code=503)
+
+                # Custom tool execution - must be checked BEFORE the final PluginHub.send_command call
+                # This applies to both cases: with or without explicit unity_instance
+                if command_type == "execute_custom_tool":
+                    # session_id and session_details are already set above
+                    if not session_id or not session_details:
                         return JSONResponse(
-                            {
-                                "success": False,
-                                "error": f"Unity instance '{unity_instance}' not found",
-                            },
-                            status_code=404,
+                            {"success": False,
+                                "error": "No valid Unity session available for custom tool execution"},
+                            status_code=503,
                         )
-                else:
-                    # No specific unity_instance requested: use first available session
-                    session_id = next(iter(sessions.sessions.keys()))
-                    session_details = sessions.sessions.get(session_id)
-
-                    if command_type == "execute_custom_tool":
-                        tool_name = None
+                    tool_name = None
+                    tool_params = {}
+                    if isinstance(params, dict):
+                        tool_name = params.get(
+                            "tool_name") or params.get("name")
+                        tool_params = params.get(
+                            "parameters") or params.get("params") or {}
+
+                    if not tool_name:
+                        return JSONResponse(
+                            {"success": False,
+                                "error": "Missing 'tool_name' for execute_custom_tool"},
+                            status_code=400,
+                        )
+                    if tool_params is None:
                         tool_params = {}
-                        if isinstance(params, dict):
-                            tool_name = params.get(
-                                "tool_name") or params.get("name")
-                            tool_params = params.get(
-                                "parameters") or params.get("params") or {}
-
-                        if not tool_name:
-                            return JSONResponse(
-                                {"success": False,
-                                    "error": "Missing 'tool_name' for execute_custom_tool"},
-                                status_code=400,
-                            )
-                        if tool_params is None:
-                            tool_params = {}
-                        if not isinstance(tool_params, dict):
-                            return JSONResponse(
-                                {"success": False,
-                                    "error": "Tool parameters must be an object/dict"},
-                                status_code=400,
-                            )
-
-                        # Prefer a concrete hash for project-scoped tools.
-                        unity_instance_hint = unity_instance
-                        if session_details and session_details.hash:
-                            unity_instance_hint = session_details.hash
-
-                        project_id = resolve_project_id_for_unity_instance(
-                            unity_instance_hint)
-                        if not project_id:
-                            return JSONResponse(
-                                {"success": False,
-                                    "error": "Could not resolve project id for custom tool"},
-                                status_code=400,
-                            )
-
-                        service = CustomToolService.get_instance()
-                        result = await service.execute_tool(
-                            project_id, tool_name, unity_instance_hint, tool_params
+                    if not isinstance(tool_params, dict):
+                        return JSONResponse(
+                            {"success": False,
+                                "error": "Tool parameters must be an object/dict"},
+                            status_code=400,
+                        )
+
+                    # Prefer a concrete hash for project-scoped tools.
+                    unity_instance_hint = unity_instance
+                    if session_details and session_details.hash:
+                        unity_instance_hint = session_details.hash
+
+                    project_id = resolve_project_id_for_unity_instance(
+                        unity_instance_hint)
+                    if not project_id:
+                        return JSONResponse(
+                            {"success": False,
+                                "error": "Could not resolve project id for custom tool"},
+                            status_code=400,
                         )
-                        return JSONResponse(result.model_dump())
+
+                    service = CustomToolService.get_instance()
+                    result = await service.execute_tool(
+                        project_id, tool_name, unity_instance_hint, tool_params
+                    )
+                    return JSONResponse(result.model_dump())
 
                 # Send command to Unity
                 result = await PluginHub.send_command(session_id, command_type, params)