diff --git a/apps/api/src/plugins/mcp/interfaces.py b/apps/api/src/plugins/mcp/interfaces.py
index f2ee317e..6c2d75c2 100644
--- a/apps/api/src/plugins/mcp/interfaces.py
+++ b/apps/api/src/plugins/mcp/interfaces.py
@@ -30,7 +30,9 @@ class MCPTool:
     name: str
     description: str
     input_schema: dict[str, Any]
-    server_name: str
+    # 2026-05-06 Codex: 部分舊 provider 的 list_tools() 尚未傳 server_name。
+    # 先給 DTO 預設值，registry 會以 provider.name 補正，避免啟動登記直接 crash。
+    server_name: str = ""
 
 
 @dataclass
diff --git a/apps/api/src/services/mcp_tool_registry.py b/apps/api/src/services/mcp_tool_registry.py
index 5d8e0b34..4a22e6ad 100644
--- a/apps/api/src/services/mcp_tool_registry.py
+++ b/apps/api/src/services/mcp_tool_registry.py
@@ -21,8 +21,8 @@ MASTER §3.1.3 (B) AI 自主工具選擇
 
 from __future__ import annotations
 
-from dataclasses import dataclass, field
-from enum import Enum
+from dataclasses import dataclass, field, replace
+from enum import StrEnum
 from typing import Any
 
 import structlog
@@ -33,7 +33,7 @@ from src.plugins.mcp.registry import AuditedMCPToolProvider
 logger = structlog.get_logger(__name__)
 
 
-class SensorDimension(str, Enum):
+class SensorDimension(StrEnum):
     """8D 感官維度分類"""
     D1_K8S_STATE = "d1_k8s_state"
     D2_LOGS = "d2_logs"
@@ -111,6 +111,8 @@ class MCPToolRegistry:
 
         count = 0
         for tool in tools:
+            if not tool.server_name:
+                tool = replace(tool, server_name=provider.name)
             audited_provider = AuditedMCPToolProvider(provider)
             reg = _classify_tool(tool, audited_provider)
             self._tools.append(reg)
diff --git a/apps/api/tests/test_mcp_tool_result_compat.py b/apps/api/tests/test_mcp_tool_result_compat.py
index 686ce92f..baefeb06 100644
--- a/apps/api/tests/test_mcp_tool_result_compat.py
+++ b/apps/api/tests/test_mcp_tool_result_compat.py
@@ -1,4 +1,7 @@
-from src.plugins.mcp.interfaces import MCPToolResult
+import pytest
+
+from src.plugins.mcp.interfaces import MCPTool, MCPToolProvider, MCPToolResult
+from src.services.mcp_tool_registry import MCPToolRegistry
 
 
 def test_mcp_tool_result_accepts_legacy_data_alias() -> None:
@@ -26,3 +29,31 @@ def test_mcp_tool_result_allows_failure_without_execution_id() -> None:
 
     assert result.execution_id.startswith("mcp-")
     assert result.error == "blocked"
+
+
+def test_mcp_tool_allows_legacy_missing_server_name() -> None:
+    tool = MCPTool(name="legacy_tool", description="legacy", input_schema={})
+
+    assert tool.server_name == ""
+
+
+@pytest.mark.asyncio
+async def test_mcp_registry_stamps_missing_server_name_from_provider() -> None:
+    class LegacyProvider(MCPToolProvider):
+        @property
+        def name(self) -> str:
+            return "legacy_provider"
+
+        async def list_tools(self) -> list[MCPTool]:
+            return [MCPTool(name="legacy_tool", description="legacy", input_schema={})]
+
+        async def execute(self, tool_name: str, parameters: dict) -> MCPToolResult:
+            return MCPToolResult(success=True, output={"tool": tool_name, "parameters": parameters})
+
+    registry = MCPToolRegistry()
+
+    count = await registry.register_provider(LegacyProvider())
+
+    assert count == 1
+    registered = registry.get_all_tools()[0]
+    assert registered.tool.server_name == "legacy_provider"
diff --git a/docs/LOGBOOK.md b/docs/LOGBOOK.md
index 4b8b894d..469195f9 100644
--- a/docs/LOGBOOK.md
+++ b/docs/LOGBOOK.md
@@ -5,7 +5,8 @@
 **本次修補**：
 - `MCPToolResult` 對舊 provider 介面做向後相容：`data` 自動映射到 `output`。
 - 缺少 `execution_id` 時自動產生 `mcp-<uuid>`，避免失敗/blocked 回傳因建構 DTO 就爆掉。
-- 補回歸測試，鎖住 `data` alias、明確 `output` 優先，以及 failure without execution_id。
+- `MCPTool` 對舊 provider 介面做向後相容：允許 `server_name` 暫缺，並由 `MCPToolRegistry.register_provider()` 以 `provider.name` 補正。
+- 補回歸測試，鎖住 `data` alias、明確 `output` 優先、failure without execution_id，以及舊 provider 缺 `server_name` 時仍可登記工具。
 
 **後續**：
 - 這是相容層止血；後續仍應逐步把 provider call-sites 改成明確 `output=` 與穩定 `execution_id`。