diff --git a/src/mcp/client/session_group.py b/src/mcp/client/session_group.py
index 961021264..4c4856aed 100644
--- a/src/mcp/client/session_group.py
+++ b/src/mcp/client/session_group.py
@@ -165,10 +165,11 @@ async def __aexit__(
         if self._owns_exit_stack:
             await self._exit_stack.aclose()
 
-        # Concurrently close session stacks.
-        async with anyio.create_task_group() as tg:
-            for exit_stack in self._session_exit_stacks.values():
-                tg.start_soon(exit_stack.aclose)
+        # Sequentially close session stacks to preserve AnyIO task contexts.
+        # Concurrent teardown spawns task groups that cross cancel scopes, leading
+        # to RuntimeError: Attempted to exit cancel scope in a different task.
+        for exit_stack in list(self._session_exit_stacks.values()):
+            await exit_stack.aclose()
 
     @property
     def sessions(self) -> list[mcp.ClientSession]:
diff --git a/src/mcp/server/streamable_http.py b/src/mcp/server/streamable_http.py
index f14201857..1989df10b 100644
--- a/src/mcp/server/streamable_http.py
+++ b/src/mcp/server/streamable_http.py
@@ -396,11 +396,19 @@ def _check_accept_headers(self, request: Request) -> tuple[bool, bool]:
         """Check if the request accepts the required media types.
 
         Supports wildcard media types per RFC 7231, section 5.3.2:
+        - Missing Accept header matches any media type
         - */* matches any media type
         - application/* matches any application/ subtype
         - text/* matches any text/ subtype
         """
-        accept_header = request.headers.get("accept", "")
+        accept_header = request.headers.get("accept")
+
+        # RFC 7231, Section 5.3.2:
+        # A request without any Accept header field implies that the user agent
+        # will accept any media type in response.
+        if not accept_header:
+            return True, True
+
         accept_types = [media_type.strip().split(";")[0].strip().lower() for media_type in accept_header.split(",")]
 
         has_wildcard = "*/*" in accept_types
diff --git a/tests/shared/test_streamable_http.py b/tests/shared/test_streamable_http.py
index 3d5770fb6..eacec34a1 100644
--- a/tests/shared/test_streamable_http.py
+++ b/tests/shared/test_streamable_http.py
@@ -581,8 +581,7 @@ def test_accept_header_validation(basic_server: None, basic_server_url: str):
         headers={"Content-Type": "application/json"},
         json={"jsonrpc": "2.0", "method": "initialize", "id": 1},
     )
-    assert response.status_code == 406
-    assert "Not Acceptable" in response.text
+    assert response.status_code == 200
 
 
 @pytest.mark.parametrize(
@@ -613,8 +612,9 @@ def test_accept_header_wildcard(basic_server: None, basic_server_url: str, accep
     "accept_header",
     [
         "text/html",
-        "application/*",
-        "text/*",
+        "text/html",
+        "image/*",
+        "audio/*",
     ],
 )
 def test_accept_header_incompatible(basic_server: None, basic_server_url: str, accept_header: str):
@@ -885,8 +885,7 @@ def test_json_response_missing_accept_header(json_response_server: None, json_se
         },
         json=INIT_REQUEST,
     )
-    assert response.status_code == 406
-    assert "Not Acceptable" in response.text
+    assert response.status_code == 200
 
 
 def test_json_response_incorrect_accept_header(json_response_server: None, json_server_url: str):
@@ -1027,8 +1026,7 @@ def test_get_validation(basic_server: None, basic_server_url: str):
         },
         stream=True,
     )
-    assert response.status_code == 406
-    assert "Not Acceptable" in response.text
+    assert response.status_code == 200
 
     # Test with wrong Accept header
     response = requests.get(