diff --git a/src/mcp/types.py b/src/mcp/types.py
index 654c00660..1a3f78857 100644
--- a/src/mcp/types.py
+++ b/src/mcp/types.py
@@ -1290,6 +1290,12 @@ class ToolAnnotations(BaseModel):
     Default: true
     """
 
+    agencyHint: bool | None = None
+    """
+    If true, this tool encapsulates an internal "agent loop" (e.g., plan–act–observe cycles, tool-chaining, or autonomous retries).
+    Default: false
+    """
+
     model_config = ConfigDict(extra="allow")
 
 
diff --git a/tests/server/fastmcp/test_tool_manager.py b/tests/server/fastmcp/test_tool_manager.py
index d83d48474..a766f4bc0 100644
--- a/tests/server/fastmcp/test_tool_manager.py
+++ b/tests/server/fastmcp/test_tool_manager.py
@@ -428,6 +428,7 @@ def read_data(path: str) -> str:  # pragma: no cover
             title="File Reader",
             readOnlyHint=True,
             openWorldHint=False,
+            agencyHint=True,
         )
 
         manager = ToolManager()
@@ -437,6 +438,7 @@ def read_data(path: str) -> str:  # pragma: no cover
         assert tool.annotations.title == "File Reader"
         assert tool.annotations.readOnlyHint is True
         assert tool.annotations.openWorldHint is False
+        assert tool.annotations.agencyHint is True
 
     @pytest.mark.anyio
     async def test_tool_annotations_in_fastmcp(self):
@@ -444,7 +446,7 @@ async def test_tool_annotations_in_fastmcp(self):
 
         app = FastMCP()
 
-        @app.tool(annotations=ToolAnnotations(title="Echo Tool", readOnlyHint=True))
+        @app.tool(annotations=ToolAnnotations(title="Echo Tool", readOnlyHint=True, agencyHint=False))
         def echo(message: str) -> str:  # pragma: no cover
             """Echo a message back."""
             return message
@@ -454,6 +456,7 @@ def echo(message: str) -> str:  # pragma: no cover
         assert tools[0].annotations is not None
         assert tools[0].annotations.title == "Echo Tool"
         assert tools[0].annotations.readOnlyHint is True
+        assert tools[0].annotations.agencyHint is False
 
 
 class TestStructuredOutput:
diff --git a/tests/server/test_lowlevel_tool_annotations.py b/tests/server/test_lowlevel_tool_annotations.py
index f812c4877..31595475c 100644
--- a/tests/server/test_lowlevel_tool_annotations.py
+++ b/tests/server/test_lowlevel_tool_annotations.py
@@ -35,6 +35,7 @@ async def list_tools():  # pragma: no cover
                 annotations=ToolAnnotations(
                     title="Echo Tool",
                     readOnlyHint=True,
+                    agencyHint=True,
                 ),
             )
         ]
@@ -98,3 +99,4 @@ async def handle_messages():
     assert tools_result.tools[0].annotations is not None
     assert tools_result.tools[0].annotations.title == "Echo Tool"
     assert tools_result.tools[0].annotations.readOnlyHint is True
+    assert tools_result.tools[0].annotations.agencyHint is True