Expose get_session_id callback

src/fastmcp/client/transports.py

@@ -253,6 +253,7 @@ def __init__(
         if isinstance(sse_read_timeout, int | float):
             sse_read_timeout = datetime.timedelta(seconds=float(sse_read_timeout))
         self.sse_read_timeout = sse_read_timeout
+        self.get_session_id_cb = None
 
     def _set_auth(self, auth: httpx.Auth | Literal["oauth"] | str | None):
         if auth == "oauth":
@@ -287,12 +288,18 @@ async def connect_session(
             auth=self.auth,
             **client_kwargs,
         ) as transport:
-            read_stream, write_stream, _ = transport
+            read_stream, write_stream, get_session_id = transport
+            self.get_session_id_cb = get_session_id
             async with ClientSession(
                 read_stream, write_stream, **session_kwargs
             ) as session:
                 yield session
 
+    def get_session_id(self) -> str | None:
+        if self.get_session_id_cb:
+            return self.get_session_id_cb()
+        return None
+
     def __repr__(self) -> str:
         return f"<StreamableHttpTransport(url='{self.url}')>"
 

tests/client/test_streamable_http.py

@@ -175,6 +175,15 @@ async def test_http_headers(streamable_http_server: str):
         assert json_result["x-demo-header"] == "ABC"
 
 
+async def test_session_id_callback(streamable_http_server: str):
+    """Test getting mcp-session-id from the transport."""
+    transport = StreamableHttpTransport(streamable_http_server)
+    assert transport.get_session_id() is None
+    async with Client(transport=transport):
+        session_id = transport.get_session_id()
+        assert session_id is not None
+
+
 @pytest.mark.parametrize("streamable_http_server", [True, False], indirect=True)
 async def test_greet_with_progress_tool(streamable_http_server: str):
     """Test calling the greet tool."""