diff --git a/nanobot/agent/tools/registry.py b/nanobot/agent/tools/registry.py
index 137038c0..3d185d57 100644
--- a/nanobot/agent/tools/registry.py
+++ b/nanobot/agent/tools/registry.py
@@ -14,14 +14,17 @@ class ToolRegistry:
 
     def __init__(self):
         self._tools: dict[str, Tool] = {}
+        self._cached_definitions: list[dict[str, Any]] | None = None
 
     def register(self, tool: Tool) -> None:
         """Register a tool."""
         self._tools[tool.name] = tool
+        self._cached_definitions = None
 
     def unregister(self, name: str) -> None:
         """Unregister a tool by name."""
         self._tools.pop(name, None)
+        self._cached_definitions = None
 
     def get(self, name: str) -> Tool | None:
         """Get a tool by name."""
@@ -46,8 +49,12 @@ class ToolRegistry:
         """Get tool definitions with stable ordering for cache-friendly prompts.
 
         Built-in tools are sorted first as a stable prefix, then MCP tools are
-        sorted and appended.
+        sorted and appended.  The result is cached until the next
+        register/unregister call.
         """
+        if self._cached_definitions is not None:
+            return self._cached_definitions
+
         definitions = [tool.to_schema() for tool in self._tools.values()]
         builtins: list[dict[str, Any]] = []
         mcp_tools: list[dict[str, Any]] = []
@@ -60,7 +67,8 @@ class ToolRegistry:
 
         builtins.sort(key=self._schema_name)
         mcp_tools.sort(key=self._schema_name)
-        return builtins + mcp_tools
+        self._cached_definitions = builtins + mcp_tools
+        return self._cached_definitions
 
     def prepare_call(
         self,
diff --git a/tests/tools/test_tool_registry.py b/tests/tools/test_tool_registry.py
index f9e8ce5e..ca60f30e 100644
--- a/tests/tools/test_tool_registry.py
+++ b/tests/tools/test_tool_registry.py
@@ -71,3 +71,33 @@ def test_prepare_call_other_tools_keep_generic_object_validation() -> None:
     assert tool is not None
     assert params == ["TODO"]
     assert error == "Error: Invalid parameters for tool 'grep': parameters must be an object, got list"
+
+
+def test_get_definitions_returns_cached_result() -> None:
+    registry = ToolRegistry()
+    registry.register(_FakeTool("read_file"))
+    first = registry.get_definitions()
+    assert registry._cached_definitions is not None
+    second = registry.get_definitions()
+    assert first == second
+
+
+def test_register_invalidates_cache() -> None:
+    registry = ToolRegistry()
+    registry.register(_FakeTool("read_file"))
+    first = registry.get_definitions()
+    registry.register(_FakeTool("write_file"))
+    second = registry.get_definitions()
+    assert first is not second
+    assert len(second) == 2
+
+
+def test_unregister_invalidates_cache() -> None:
+    registry = ToolRegistry()
+    registry.register(_FakeTool("read_file"))
+    registry.register(_FakeTool("write_file"))
+    first = registry.get_definitions()
+    registry.unregister("write_file")
+    second = registry.get_definitions()
+    assert first is not second
+    assert len(second) == 1