Audit mode="legacy" pins in server tests and top-level stragglers

Removes pins added while making the auto-default suite pass that weren't
actually testing older-protocol-only behavior, and keeps (with a stated
reason) the ones that are. Along the way, fixes two real defects the audit
surfaced in the modern protocol path: PingMiddleware could leak a
_active_sessions entry when a connection's exit_stack closed before its
keepalive task got its first scheduler turn, and FastMCP(experimental_
capabilities=...) was silently dropped from server/discover responses
(it only ever reached the legacy initialize handshake).

fastmcp_slim/fastmcp/server/low_level.py

@@ -461,13 +461,9 @@ class LowLevelServer(_Server[LifespanResultT]):
         # ensure we use the FastMCP notification options
         if notification_options is None:
             notification_options = self.notification_options
-        merged = {
-            **self.fastmcp.experimental_capabilities,
-            **(experimental_capabilities or {}),
-        }
         return super().create_initialization_options(
             notification_options=notification_options,
-            experimental_capabilities=merged or None,
+            experimental_capabilities=experimental_capabilities,
             extensions=extensions,
         )
 
@@ -483,13 +479,23 @@ class LowLevelServer(_Server[LifespanResultT]):
         and advertise the MCP Apps UI extension.
 
         ``ServerCapabilities.tasks`` and ``ServerCapabilities.extensions`` are
-        real declared fields in v2, so we update them directly.
+        real declared fields in v2, so we update them directly. The
+        `FastMCP(experimental_capabilities=...)` merge also lives here rather
+        than in `create_initialization_options`: the modern `server/discover`
+        handler calls this directly, without going through
+        `create_initialization_options` at all, so merging there only reached
+        the handshake-era `initialize` response and silently dropped
+        constructor-configured experimental capabilities from `discover`.
         """
         from fastmcp.server.tasks.capabilities import get_task_capabilities
 
+        merged_experimental = {
+            **self.fastmcp.experimental_capabilities,
+            **(experimental_capabilities or {}),
+        }
         capabilities = super().get_capabilities(
             notification_options,
-            experimental_capabilities,
+            merged_experimental or None,
             extensions,
             protocol_version=protocol_version,
         )

fastmcp_slim/fastmcp/server/middleware/ping.py

@@ -71,6 +71,15 @@ class PingMiddleware(Middleware):
                         ping_task.cancel()
                         with contextlib.suppress(asyncio.CancelledError):
                             await ping_task
+                        # `ping_task` may be cancelled before its first
+                        # scheduler turn (a connection can be built and torn
+                        # down within a single request on the modern,
+                        # per-request `Connection` path), in which case its
+                        # body - and the `finally` in `_ping_loop` that would
+                        # otherwise discard this entry - never runs. Discard
+                        # unconditionally here so a connection that closes
+                        # before the loop starts doesn't leak its entry.
+                        self._active_sessions.discard(connection_id)
 
                     connection.exit_stack.push_async_callback(_cancel_ping)
 

tests/deprecated/test_elicitation.py

@@ -25,6 +25,8 @@ async def test_elicitation_none_response_type_warns_deprecation():
     async def elicitation_handler(message, response_type, params, ctx):
         return ElicitResult(action="accept", content={})
 
+    # `ctx.elicit` sends a server-initiated request down the client's
+    # back-channel, which only the older protocol has, so this pins that era.
     async with Client(
         mcp, mode="legacy", elicitation_handler=elicitation_handler
     ) as client:

tests/server/middleware/test_initialization_middleware.py

@@ -1,4 +1,14 @@
-"""Tests for middleware support during initialization."""
+"""Tests for middleware support during initialization.
+
+`on_initialize` only fires for the `initialize` handshake, which is unique to
+the older protocol version; the modern version connects without it, so a
+default client never triggers this hook. Most tests below pin `mode="legacy"`
+for that reason. `test_session_state_persists_across_tool_calls` pins for a
+different reason: it exercises `ctx.set_state`/`get_state` persisting across
+multiple tool calls in the same client session, which requires the
+handshake-era's persistent session (see `test_session_visibility.py` for the
+same distinction applied to a different feature).
+"""
 
 from collections.abc import Sequence
 from typing import Any
@@ -401,8 +411,7 @@ async def test_state_isolation_between_streamable_http_clients():
         import json
 
         # Client 1 stores its value
-        # Session ids belong to the handshake era, so these pin the legacy era.
+        async with running_server.client() as client1:
-        async with running_server.client(mode="legacy") as client1:
             result1 = await client1.call_tool(
                 "store_and_read", {"value": "client1-value"}
             )
@@ -412,7 +421,7 @@ async def test_state_isolation_between_streamable_http_clients():
             session_id_1 = data1["session_id"]
 
         # Client 2 should have completely isolated state
-        async with running_server.client(mode="legacy") as client2:
+        async with running_server.client() as client2:
             result2 = await client2.call_tool(
                 "store_and_read", {"value": "client2-value"}
             )

tests/server/middleware/test_message_visibility.py

@@ -12,6 +12,7 @@ from typing import Any
 
 import mcp_types
 import pytest
+from mcp.shared.dispatcher import CallOptions
 from mcp.shared.exceptions import MCPError
 from mcp_types import ElicitRequest, ElicitRequestFormParams, InputRequiredResult
 
@@ -65,6 +66,26 @@ def _adder() -> FastMCP:
     return server
 
 
+async def _raw_request(
+    client: Client, method: str, params: dict[str, Any]
+) -> dict[str, Any]:
+    """Send a bare JSON-RPC request through the dispatcher, bypassing the
+    typed `send_request` that normally stamps the outgoing envelope.
+
+    The modern protocol version requires every request's `params._meta` to
+    carry the protocol version, client info, and client capabilities (there is
+    no handshake to establish them once, up front), so a raw request built by
+    hand must stamp them the same way `send_request` would or the server
+    rejects the envelope before dispatch ever sees it.
+    """
+    data: dict[str, Any] = {"method": method, "params": params}
+    opts: CallOptions = {}
+    client.session._stamp(data, opts)
+    return await client.session._dispatcher.send_raw_request(
+        method, data.get("params"), opts
+    )
+
+
 class TestNotificationVisibility:
     async def test_client_cancelled_notification_reaches_on_message(self):
         """A ``notifications/cancelled`` from the client is observed by
@@ -116,11 +137,9 @@ class TestUnroutableAndMalformed:
         recorder = HookRecorder()
         server.add_middleware(recorder)
 
-        async with Client(server, mode="legacy") as client:
+        async with Client(server) as client:
             with pytest.raises(MCPError):
-                await client.session._dispatcher.send_raw_request(
+                await _raw_request(client, "does/not/exist", {})
-                    "does/not/exist", {}, {}
-                )
 
         assert ("on_message", "does/not/exist") in recorder.records
         assert ("on_request", "does/not/exist") in recorder.records
@@ -133,11 +152,9 @@ class TestUnroutableAndMalformed:
         recorder = HookRecorder()
         server.add_middleware(recorder)
 
-        async with Client(server, mode="legacy") as client:
+        async with Client(server) as client:
             with pytest.raises(MCPError):
-                await client.session._dispatcher.send_raw_request(
+                await _raw_request(client, "tools/call", {"not_a_valid": "param"})
-                    "tools/call", {"not_a_valid": "param"}, {}
-                )
 
         assert ("on_message", "tools/call") in recorder.records
         assert ("on_call_tool", "tools/call") not in recorder.records
@@ -216,6 +233,9 @@ class TestMessageModification:
         server = _adder()
         server.add_middleware(RewriteLevel())
 
+        # `logging/setLevel` was dropped from the method registry in the modern
+        # protocol version (logging is opt-in per-request via `_meta` there),
+        # so exercising it needs the older protocol.
         async with Client(server, mode="legacy") as client:
             await client.session._dispatcher.send_raw_request(
                 "logging/setLevel", {"level": "not-a-valid-level"}, {}
@@ -227,6 +247,8 @@ class TestMessageModification:
         recorder = HookRecorder()
         server.add_middleware(recorder)
 
+        # `logging/setLevel` only exists on the older protocol; see the pin
+        # note in `test_modified_message_reaches_sdk_dispatch` above.
         async with Client(server, mode="legacy") as client:
             await client.session._dispatcher.send_raw_request(
                 "logging/setLevel", {"level": "debug"}, {}
@@ -254,6 +276,8 @@ class TestMessageModification:
         server.add_middleware(recorder)
         server.add_middleware(RewriteMethod())
 
+        # `ping` was removed from the modern protocol version, so this pins
+        # the era where it's still a real method to rewrite away from.
         async with Client(server, mode="legacy") as client:
             await client.session._dispatcher.send_raw_request("ping", {}, {})
 
@@ -284,11 +308,9 @@ class TestMessageModification:
         server.add_middleware(RepairAttempt())
         server.add_middleware(recorder)
 
-        async with Client(server, mode="legacy") as client:
+        async with Client(server) as client:
             with pytest.raises(MCPError):
-                await client.session._dispatcher.send_raw_request(
+                await _raw_request(client, "tools/call", {"not_a_valid": "param"})
-                    "tools/call", {"not_a_valid": "param"}, {}
-                )
 
         calls = [r for r in recorder.records if r == ("on_message", "tools/call")]
         assert len(calls) == 1

tests/server/middleware/test_middleware.py

@@ -173,10 +173,13 @@ class TestMiddlewareHooks:
     async def test_call_tool(
         self, mcp_server: FastMCP, recording_middleware: RecordingMiddleware
     ):
-        async with Client(mcp_server, mode="legacy") as client:
+        async with Client(mcp_server) as client:
             await client.call_tool("add", {"a": 1, "b": 2})
 
-        assert recording_middleware.assert_called(at_least=9)
+        # The floor is lower than a legacy connection's 11: the modern
+        # `server/discover` negotiation fires 2 generic hooks, vs. 5 for the
+        # older `initialize` request plus its `notifications/initialized`.
+        assert recording_middleware.assert_called(at_least=8)
         assert recording_middleware.assert_called(method="tools/call", at_least=3)
         assert recording_middleware.assert_called(hook="on_message", at_least=1)
         assert recording_middleware.assert_called(hook="on_request", at_least=1)
@@ -299,6 +302,7 @@ class TestMiddlewareHooks:
     async def test_initialize(
         self, mcp_server: FastMCP, recording_middleware: RecordingMiddleware
     ):
+        # `ping` only exists on the older protocol, so this pins that era.
         async with Client(mcp_server, mode="legacy") as client:
             await client.ping()
 

tests/server/middleware/test_middleware_nested.py

@@ -477,7 +477,7 @@ class TestProxyServer:
         # proxy server will have its tools listed as well as called in order to
         # apply transforms and filters prior to the call.
         proxy_server = create_proxy(mcp_server, name="Proxy Server")
-        async with Client(proxy_server, mode="legacy") as client:
+        async with Client(proxy_server) as client:
             await client.call_tool("add", {"a": 1, "b": 2})
 
         assert recording_middleware.assert_called(at_least=6)

tests/server/middleware/test_ping.py

@@ -193,6 +193,13 @@ class TestPingMiddlewareIntegration:
 
         assert len(middleware._active_sessions) == 0
 
+        # PingMiddleware keys its keepalive loop off the connection, which
+        # persists for the life of a handshake-era session. On the modern
+        # protocol version, a connection lives only for the single request
+        # that built it, so `_active_sessions` never holds a mid-session
+        # entry an outside observer can see — the register-and-clean-up
+        # happens entirely within one call. That per-request lifecycle is
+        # itself the reason this test pins the older era.
         async with Client(mcp, mode="legacy") as client:
             result = await client.call_tool("hello")
             assert result.content[0].text == "Hello!"
@@ -214,6 +221,9 @@ class TestPingMiddlewareIntegration:
         def hello() -> str:
             return "Hello!"
 
+        # See the pin note in `test_ping_middleware_registers_session`: a
+        # mid-session `_active_sessions` entry is only observable when the
+        # connection persists across requests, which is handshake-era only.
         async with Client(mcp, mode="legacy") as client:
             await client.call_tool("hello")
             # Should have one active session

tests/server/telemetry/test_sampling_tracing.py

@@ -4,6 +4,10 @@ Regression focus: the `sampling create_message` span is created with
 `record_exception=False, set_status_on_exception=False` and records the
 exception manually in its `except` block. A failed sampling call must
 therefore produce exactly ONE exception event, not two.
+
+`ctx.sample` requires the server to send a request down to the client, which
+only the older protocol's back-channel supports, so every client below pins
+`mode="legacy"`.
 """
 
 from __future__ import annotations

tests/server/telemetry/test_server_tracing.py

@@ -449,6 +449,8 @@ class TestSeamServerSpan:
     ):
         mcp = FastMCP("test-server")
 
+        # `logging/setLevel` was dropped from the modern protocol version
+        # (SEP-2577), so exercising it needs the older protocol.
         async with Client(mcp, mode="legacy") as client:
             await client.set_logging_level("info")
 
@@ -470,6 +472,8 @@ class TestSeamServerSpan:
         """A seam-spanned method must produce exactly one SERVER span, not two."""
         mcp = FastMCP("test-server")
 
+        # `logging/setLevel` only exists on the older protocol; see the pin
+        # note in `test_set_logging_level_emits_seam_span` above.
         async with Client(mcp, mode="legacy") as client:
             await client.set_logging_level("info")
 

tests/server/test_icons.py

@@ -36,8 +36,8 @@ class TestServerIcons:
         )
 
         # Verify that icons and website_url are passed to the underlying server
-        async with Client(mcp, mode="legacy") as client:
+        async with Client(mcp) as client:
-            server_info = client.initialize_result.server_info
+            server_info = client.session.server_info
             assert server_info.website_url == "https://example.com"
             assert server_info.icons == icons
 
@@ -45,8 +45,8 @@ class TestServerIcons:
         """Test that server works without icons and websiteUrl."""
         mcp = FastMCP(name="TestServer")
 
-        async with Client(mcp, mode="legacy") as client:
+        async with Client(mcp) as client:
-            server_info = client.initialize_result.server_info
+            server_info = client.session.server_info
             assert server_info.website_url is None
             assert server_info.icons is None
 
@@ -290,8 +290,8 @@ class TestIconTypes:
 
         mcp = FastMCP("TestServer", icons=icons)
 
-        async with Client(mcp, mode="legacy") as client:
+        async with Client(mcp) as client:
-            server_info = client.initialize_result.server_info
+            server_info = client.session.server_info
             assert len(server_info.icons) == 3
             assert server_info.icons == icons
 
@@ -319,8 +319,8 @@ class TestIconTypes:
 
         mcp = FastMCP("TestServer", icons=icons)
 
-        async with Client(mcp, mode="legacy") as client:
+        async with Client(mcp) as client:
-            server_info = client.initialize_result.server_info
+            server_info = client.session.server_info
             assert server_info.icons[0].src == "https://example.com/icon.png"
             assert server_info.icons[0].mime_type is None
             assert server_info.icons[0].sizes is None
@@ -336,8 +336,8 @@ class TestIconTheme:
 
         mcp = FastMCP("TestServer", icons=icons)
 
-        async with Client(mcp, mode="legacy") as client:
+        async with Client(mcp) as client:
-            server_info = client.initialize_result.server_info
+            server_info = client.session.server_info
             assert server_info.icons[0].theme == theme
 
     async def test_icon_without_theme_is_none(self):
@@ -346,8 +346,8 @@ class TestIconTheme:
 
         mcp = FastMCP("TestServer", icons=icons)
 
-        async with Client(mcp, mode="legacy") as client:
+        async with Client(mcp) as client:
-            server_info = client.initialize_result.server_info
+            server_info = client.session.server_info
             assert server_info.icons[0].theme is None
 
 

tests/server/test_session_visibility.py

@@ -48,7 +48,16 @@ class RecordingMessageHandler(MessageHandler):
 
 
 class TestSessionVisibility:
-    """Test session-specific visibility control via Context."""
+    """Test session-specific visibility control via Context.
+
+    Session-scoped visibility rules are stored under `ctx.session_id`. The
+    modern protocol version is stateless: each request gets a fresh
+    connection identity, so a rule set in one request is gone by the next.
+    Tests that only check state within a single tool call are era-neutral
+    and stay unpinned; tests that activate a rule in one request and observe
+    its effect in a later request are pinned to the handshake era, where the
+    rule's persistence is the very thing under test.
+    """
 
     async def test_enable_components_stores_rule_dict(self):
         """Test that enable_components stores a rule dict in session state."""
@@ -70,7 +79,7 @@ class TestSessionVisibility:
             assert rules[0]["tags"] == ["finance"]
             return "activated"
 
-        async with Client(mcp, mode="legacy") as client:
+        async with Client(mcp) as client:
             result = await client.call_tool("activate_finance", {})
             assert result.data == "activated"
 
@@ -94,7 +103,7 @@ class TestSessionVisibility:
             assert rules[0]["tags"] == ["internal"]
             return "deactivated"
 
-        async with Client(mcp, mode="legacy") as client:
+        async with Client(mcp) as client:
             result = await client.call_tool("deactivate_internal", {})
             assert result.data == "deactivated"
 
@@ -402,7 +411,7 @@ class TestSessionVisibilityNotifications:
             return "activated"
 
         handler = RecordingMessageHandler()
-        async with Client(mcp, mode="legacy", message_handler=handler) as client:
+        async with Client(mcp, message_handler=handler) as client:
             handler.reset()
             await client.call_tool("activate", {})
 
@@ -432,7 +441,7 @@ class TestSessionVisibilityNotifications:
             return "deactivated"
 
         handler = RecordingMessageHandler()
-        async with Client(mcp, mode="legacy", message_handler=handler) as client:
+        async with Client(mcp, message_handler=handler) as client:
             handler.reset()
             await client.call_tool("deactivate", {})
 
@@ -461,7 +470,7 @@ class TestSessionVisibilityNotifications:
             return "cleared"
 
         handler = RecordingMessageHandler()
-        async with Client(mcp, mode="legacy", message_handler=handler) as client:
+        async with Client(mcp, message_handler=handler) as client:
             handler.reset()
             await client.call_tool("clear", {})
 
@@ -491,7 +500,7 @@ class TestSessionVisibilityNotifications:
             return "activated"
 
         handler = RecordingMessageHandler()
-        async with Client(mcp, mode="legacy", message_handler=handler) as client:
+        async with Client(mcp, message_handler=handler) as client:
             handler.reset()
             await client.call_tool("activate_tools_only", {})
 

tests/server/test_tool_annotations.py

@@ -229,6 +229,8 @@ async def test_task_execution_auto_populated_for_task_enabled_tool():
         """A tool that runs in background."""
         return f"Processed: {data}"
 
+    # `execution.task_support` (SEP-1686) is advertised in the handshake-era
+    # tool listing only; the modern listing omits it.
     async with Client(mcp, mode="legacy") as client:
         tools_result = await client.list_tools()
         assert len(tools_result) == 1

tests/server/transforms/test_search.py

@@ -188,6 +188,9 @@ class TestBaseTransformBehavior:
             await ctx.disable_components(names={"delete_record"})
             return "disabled"
 
+        # Session visibility rules only persist across requests on the
+        # handshake era (see `test_session_visibility.py`); the modern
+        # protocol version has no session for them to persist in.
         async with Client(mcp, mode="legacy") as client:
             # Before disabling, search should find delete_record
             result = await client.call_tool("search_tools", {"pattern": "delete"})

tests/test_apps.py

@@ -417,15 +417,15 @@ class TestExtensionAdvertisement:
             experimental_capabilities={"file_exchange": {"version": "0.3"}},
         )
 
-        async with Client(server, mode="legacy") as client:
+        async with Client(server) as client:
-            experimental = client.initialize_result.capabilities.experimental or {}
+            experimental = client.server_capabilities.experimental or {}
             assert experimental.get("file_exchange") == {"version": "0.3"}
 
     async def test_experimental_capabilities_default_empty(self):
         server = FastMCP("test")
 
-        async with Client(server, mode="legacy") as client:
+        async with Client(server) as client:
-            experimental = client.initialize_result.capabilities.experimental
+            experimental = client.server_capabilities.experimental
             assert not experimental
 
 

tests/test_compat.py

@@ -231,6 +231,7 @@ class TestClientBehaviorCompat:
         assert result.data == "hi"
 
     async def test_ping_returns_bool(self, server):
+        # `ping` only exists on the older protocol, so this pins that era.
         client = Client(transport=FastMCPTransport(server), mode="legacy")
         async with client:
             result = await client.ping()

tests/tools/tool_transform/test_tool_transform.py

@@ -753,7 +753,7 @@ class TestProxy:
         )
         proxy_server.add_tool(new_add_tool)
 
-        async with Client(proxy_server, mode="legacy") as client:
+        async with Client(proxy_server) as client:
             # The tool should be registered with its transformed name
             result = await client.call_tool("add_transformed", {"new_x": 1, "old_y": 2})
             assert isinstance(result.content[0], TextContent)