feat: Set instructions in code (#1838)

Author: attiks

src/fastmcp/server/server.py

@@ -329,6 +329,10 @@ def name(self) -> str:
     def instructions(self) -> str | None:
         return self._mcp_server.instructions
 
+    @instructions.setter
+    def instructions(self, value: str | None) -> None:
+        self._mcp_server.instructions = value
+
     @property
     def version(self) -> str | None:
         return self._mcp_server.version

tests/server/test_server.py

@@ -35,6 +35,12 @@ async def test_create_server(self):
         assert mcp.name.startswith("FastMCP-")
         assert mcp.instructions == "Server instructions"
 
+    async def test_change_instruction(self):
+        mcp = FastMCP(instructions="Server instructions")
+        assert mcp.instructions == "Server instructions"
+        mcp.instructions = "New instructions"
+        assert mcp.instructions == "New instructions"
+
     async def test_non_ascii_description(self):
         """Test that FastMCP handles non-ASCII characters in descriptions correctly"""
         mcp = FastMCP()