Build send_discover params via DiscoverRequest; gate preconnect cancel-suppression by method

- send_discover: construct params through types.DiscoverRequest/RequestParams
  instead of a hand-rolled dict (same wire shape; routing stays raw because
  the probe's _meta version is per-call, not session state).
- _preconnect_stamp: only suppress cancel_on_abandon for initialize and
  server/discover, so lowlevel ClientSession callers that skip the handshake
  keep the courtesy cancel on timed-out/abandoned requests.
- send_discover docstring: drop the nonexistent ProbeNotRecognized entry;
  fold the transport-4xx case into the MCPError Raises line.
- migration.md: note the MCP_PROTOCOL_VERSION header constant move to
  mcp.shared.inbound.MCP_PROTOCOL_VERSION_HEADER.

Author: maxisbey

docs/migration.md

@@ -163,6 +163,8 @@ Note: `sse_client` retains its `headers`, `timeout`, `sse_read_timeout`, and `au
 
 The transport no longer holds per-connection protocol state; era-dependent headers (e.g. `MCP-Protocol-Version`) are now supplied per-message by the session. If you were reading `transport.protocol_version` to learn the negotiated version, read `session.protocol_version` (or `client.protocol_version` on the high-level `Client`) instead.
 
+The `MCP_PROTOCOL_VERSION` header-name constant has moved: import `MCP_PROTOCOL_VERSION_HEADER` from `mcp.shared.inbound` instead of `MCP_PROTOCOL_VERSION` from `mcp.client.streamable_http`.
+
 ### `terminate_windows_process` removed
 
 The deprecated `mcp.os.win32.utilities.terminate_windows_process` function has been

src/mcp/client/session.py

@@ -52,8 +52,10 @@
 
 
 def _preconnect_stamp(data: dict[str, Any], opts: CallOptions) -> None:
-    # Only initialize/discover go out before connect; both forbid cancellation.
-    opts["cancel_on_abandon"] = False
+    # initialize/discover forbid cancellation; other pre-handshake requests (lowlevel
+    # ClientSession callers may skip the handshake entirely) keep the courtesy cancel.
+    if data["method"] in ("initialize", "server/discover"):
+        opts["cancel_on_abandon"] = False
 
 
 def _make_handshake_stamp(protocol_version: str) -> Callable[[dict[str, Any], CallOptions], None]:
@@ -402,25 +404,28 @@ async def send_discover(self, version: str) -> dict[str, Any]:
         the connect-time auto-negotiation policy.
 
         Raises:
-            MCPError: The server returned a JSON-RPC error.
-            ProbeNotRecognized: The transport bounced the request at its own
-                layer (HTTP 4xx without a JSON-RPC error body).
+            MCPError: The server returned a JSON-RPC error, or the transport
+                bounced the request at its own layer (a bare HTTP 4xx is
+                synthesized into a JSON-RPC error by the transport).
         """
         client_info = self._client_info.model_dump(by_alias=True, mode="json", exclude_none=True)
         capabilities = self._build_capabilities().model_dump(by_alias=True, mode="json", exclude_none=True)
-        params = {
-            "_meta": {
-                PROTOCOL_VERSION_META_KEY: version,
-                CLIENT_INFO_META_KEY: client_info,
-                CLIENT_CAPABILITIES_META_KEY: capabilities,
-            }
-        }
+        request = types.DiscoverRequest(
+            params=types.RequestParams(
+                _meta={
+                    PROTOCOL_VERSION_META_KEY: version,
+                    CLIENT_INFO_META_KEY: client_info,
+                    CLIENT_CAPABILITIES_META_KEY: capabilities,
+                }
+            )
+        )
+        data = request.model_dump(by_alias=True, mode="json", exclude_none=True)
         opts: CallOptions = {
             "timeout": DISCOVER_TIMEOUT_SECONDS,
             "cancel_on_abandon": False,
             "headers": {MCP_PROTOCOL_VERSION_HEADER: version},
         }
-        return await self._dispatcher.send_raw_request("server/discover", params, opts)
+        return await self._dispatcher.send_raw_request(data["method"], data.get("params"), opts)
 
     async def discover(self) -> types.DiscoverResult:
         """Probe `server/discover` and adopt the result.