Simplify ContextVar management - trust Context to set them at request time

Remove redundant ContextVar handling:
- _lifespan_manager no longer re-sets ContextVars in early-return branch
- Handler fallback logic removed (no more `if docket is None: docket = server._docket`)

The authoritative place for request-context ContextVars is now Context.__aenter__,
which sets _current_docket and _current_worker from server instance attributes.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/fastmcp/server/server.py

@@ -613,32 +613,13 @@ class FastMCP(Generic[LifespanResultT]):
         logger.info(f"[{instance_id}] _lifespan_manager ENTERING")
 
         if self._lifespan_result_set:
-            # Lifespan already ran in a parent async context. We need to set ContextVars
-            # again in THIS context because ContextVars don't propagate across different
-            # async contexts (e.g., Starlette's ASGI lifespan vs FastMCP's outer lifespan).
-            # Request handlers inherit from Starlette's context, so we must set here.
-            from fastmcp.server.dependencies import (
-                _current_docket,
-                _current_server,
-                _current_worker,
-            )
-
+            # Lifespan already ran - ContextVars will be set by Context.__aenter__
+            # at request time, so we just yield here.
             logger.info(
-                f"[{instance_id}] _lifespan_manager: already set, setting ContextVars "
-                f"for this context (docket={self._docket}, worker={self._worker})"
+                f"[{instance_id}] _lifespan_manager: already set, yielding "
+                f"(ContextVars managed by Context at request time)"
             )
-
-            server_token = _current_server.set(weakref.ref(self))
-            docket_token = _current_docket.set(self._docket) if self._docket else None
-            worker_token = _current_worker.set(self._worker) if self._worker else None
-            try:
-                yield
-            finally:
-                _current_server.reset(server_token)
-                if docket_token is not None:
-                    _current_docket.reset(docket_token)
-                if worker_token is not None:
-                    _current_worker.reset(worker_token)
+            yield
             return
 
         logger.info(

src/fastmcp/server/tasks/handlers.py

@@ -63,16 +63,11 @@ async def handle_tool_as_task(
     ctx = get_context()
     session_id = ctx.session_id
 
-    # Try ContextVar first, fall back to server instance attribute.
-    # ContextVars don't propagate to request handlers in ASGI servers because
-    # request tasks are created from the main event loop, not the lifespan context.
+    # Get Docket from ContextVar (set by Context.__aenter__ at request time)
     docket = _current_docket.get()
-    if docket is None:
-        docket = server._docket
     _logger.info(
         f"[{instance_id}] handle_tool_as_task: tool={tool_name}, "
-        f"_current_docket.get()={_current_docket.get()}, server._docket={server._docket}, "
-        f"using docket={docket}"
+        f"_current_docket.get()={docket}"
     )
     if docket is None:
         _logger.error(
@@ -225,10 +220,8 @@ async def handle_prompt_as_task(
     ctx = get_context()
     session_id = ctx.session_id
 
-    # Try ContextVar first, fall back to server instance attribute
+    # Get Docket from ContextVar (set by Context.__aenter__ at request time)
     docket = _current_docket.get()
-    if docket is None:
-        docket = server._docket
     if docket is None:
         raise McpError(
             ErrorData(
@@ -337,10 +330,8 @@ async def handle_resource_as_task(
     ctx = get_context()
     session_id = ctx.session_id
 
-    # Try ContextVar first, fall back to server instance attribute
+    # Get Docket from ContextVar (set by Context.__aenter__ at request time)
     docket = _current_docket.get()
-    if docket is None:
-        docket = server._docket
     if docket is None:
         raise McpError(
             ErrorData(