Add migration note

Author: Kludex

docs/migration.md

@@ -44,10 +44,58 @@ async with http_client:
     async with streamable_http_client(
         url="http://localhost:8000/mcp",
         http_client=http_client,
-    ) as (read_stream, write_stream, get_session_id):
+    ) as (read_stream, write_stream):
         ...
 ```
 
+### `get_session_id` callback removed from `streamable_http_client`
+
+The `get_session_id` callback (third element of the returned tuple) has been removed from `streamable_http_client`. The function now returns a 2-tuple `(read_stream, write_stream)` instead of a 3-tuple.
+
+If you need to capture the session ID (e.g., for session resumption testing), you can use httpx event hooks to capture it from the response headers:
+
+**Before (v1):**
+
+```python
+from mcp.client.streamable_http import streamable_http_client
+
+async with streamable_http_client(url) as (read_stream, write_stream, get_session_id):
+    async with ClientSession(read_stream, write_stream) as session:
+        await session.initialize()
+        session_id = get_session_id()  # Get session ID via callback
+```
+
+**After (v2):**
+
+```python
+import httpx
+from mcp.client.streamable_http import streamable_http_client
+
+# Option 1: Simply ignore if you don't need the session ID
+async with streamable_http_client(url) as (read_stream, write_stream):
+    async with ClientSession(read_stream, write_stream) as session:
+        await session.initialize()
+
+# Option 2: Capture session ID via httpx event hooks if needed
+captured_session_ids: list[str] = []
+
+async def capture_session_id(response: httpx.Response) -> None:
+    session_id = response.headers.get("mcp-session-id")
+    if session_id:
+        captured_session_ids.append(session_id)
+
+http_client = httpx.AsyncClient(
+    event_hooks={"response": [capture_session_id]},
+    follow_redirects=True,
+)
+
+async with http_client:
+    async with streamable_http_client(url, http_client=http_client) as (read_stream, write_stream):
+        async with ClientSession(read_stream, write_stream) as session:
+            await session.initialize()
+            session_id = captured_session_ids[0] if captured_session_ids else None
+```
+
 ### `StreamableHTTPTransport` parameters removed
 
 The `headers`, `timeout`, `sse_read_timeout`, and `auth` parameters have been removed from `StreamableHTTPTransport`. Configure these on the `httpx.AsyncClient` instead (see example above).

tests/client/test_session_group.py

@@ -278,6 +278,7 @@ async def test_client_session_group_disconnect_non_existent_server():
         await group.disconnect_from_server(session)
 
 
+# TODO(Marcelo): This is horrible. We should drop this test.
 @pytest.mark.anyio
 @pytest.mark.parametrize(
     "server_params_instance, client_type_name, patch_target_for_client_func",
@@ -310,19 +311,8 @@ async def test_client_session_group_establish_session_parameterized(
             mock_read_stream = mock.AsyncMock(name=f"{client_type_name}Read")
             mock_write_stream = mock.AsyncMock(name=f"{client_type_name}Write")
 
-            # streamable_http_client's __aenter__ returns three values
-            if client_type_name == "streamablehttp":
-                mock_extra_stream_val = mock.AsyncMock(name="StreamableExtra")
-                mock_client_cm_instance.__aenter__.return_value = (
-                    mock_read_stream,
-                    mock_write_stream,
-                    mock_extra_stream_val,
-                )
-            else:
-                mock_client_cm_instance.__aenter__.return_value = (
-                    mock_read_stream,
-                    mock_write_stream,
-                )
+            # All client context managers return (read_stream, write_stream)
+            mock_client_cm_instance.__aenter__.return_value = (mock_read_stream, mock_write_stream)
 
             mock_client_cm_instance.__aexit__ = mock.AsyncMock(return_value=None)
             mock_specific_client_func.return_value = mock_client_cm_instance