From fca339084b2e6b79173bad1a6fe2fd8b3c774e2f Mon Sep 17 00:00:00 2001
From: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Date: Sun, 26 Jul 2026 14:39:31 -0400
Subject: [PATCH] Era-gate client.set_logging_level on modern connections

---
 fastmcp_slim/fastmcp/client/client.py | 17 ++++++++++++++++-
 tests/server/test_protocol_eras.py    | 22 ++++++++--------------
 2 files changed, 24 insertions(+), 15 deletions(-)

diff --git a/fastmcp_slim/fastmcp/client/client.py b/fastmcp_slim/fastmcp/client/client.py
index 0bf9b9b54..42192830a 100644
--- a/fastmcp_slim/fastmcp/client/client.py
+++ b/fastmcp_slim/fastmcp/client/client.py
@@ -1351,7 +1351,22 @@ class Client(
         )
 
     async def set_logging_level(self, level: mcp_types.LoggingLevel) -> None:
-        """Send a logging/setLevel request."""
+        """Send a logging/setLevel request.
+
+        Handshake-era servers only. `logging/setLevel` asks the server to
+        remember a level for the rest of the session, and the 2026-07-28
+        protocol has no session to remember it in — the method is absent from
+        that era's registry. Log *notifications* are unaffected: they ride the
+        request's own stream, so a server's `ctx.info()` still reaches you.
+        Filter by level on the receiving side instead, in your `log_handler`.
+        """
+        if self.protocol_version in MODERN_PROTOCOL_VERSIONS:
+            raise RuntimeError(
+                "logging/setLevel is not available on MCP 2026-07-28 "
+                "connections; the method requires per-session server state that "
+                "the modern protocol does not have. Filter incoming log "
+                "messages by level in your log_handler instead."
+            )
         # Deprecated upstream in SDK v2 but deliberately kept per compat directive;
         # removed with the multi-round-trip follow-up.
         await self._await_with_session_monitoring(
diff --git a/tests/server/test_protocol_eras.py b/tests/server/test_protocol_eras.py
index 47bb0ce75..90cae9bb4 100644
--- a/tests/server/test_protocol_eras.py
+++ b/tests/server/test_protocol_eras.py
@@ -353,23 +353,17 @@ async def test_session_id_access_does_not_crash_on_modern(sessionless_server, mo
 
 
 @pytest.mark.parametrize("mode", MODERN_MODES)
-async def test_set_logging_level_does_not_crash_on_modern(sessionless_server, mode):
-    """logging/setLevel is a session-id-keyed, deprecated-at-2026 operation.
-    On a sessionless modern in-memory connection it must degrade cleanly (either
-    succeed as a no-op or raise a surfaced MCPError) rather than crash the
-    connection. Characterization: capture whichever the current contract is.
+async def test_set_logging_level_is_era_gated_on_modern(sessionless_server, mode):
+    """`logging/setLevel` asks a server to remember a level for the session, and
+    the modern era has no session — the method is absent from its registry. The
+    FastMCP client says so plainly instead of no-opping or surfacing the SDK's
+    opaque "Method not found", and the connection stays usable afterward.
     """
-    async with SDKClient(_server(sessionless_server), mode=mode) as client:
-        outcome: str
-        try:
-            await client.set_logging_level("debug")  # ty: ignore[deprecated]
-            outcome = "ok"
-        except MCPError:
-            outcome = "mcperror"
-        # Either way the connection is still usable afterward.
+    async with FastMCPClient(sessionless_server, mode=mode) as client:
+        with pytest.raises(RuntimeError, match="2026-07-28"):
+            await client.set_logging_level("debug")
         result = await client.call_tool("read_session_id", {})
     assert result.is_error is False
-    assert outcome in {"ok", "mcperror"}
 
 
 # ---------------------------------------------------------------------------