diff --git a/lib/mcp/server/transports/streamable_http_transport.rb b/lib/mcp/server/transports/streamable_http_transport.rb
index 7d02841f..1ccadaf4 100644
--- a/lib/mcp/server/transports/streamable_http_transport.rb
+++ b/lib/mcp/server/transports/streamable_http_transport.rb
@@ -2444,22 +2444,24 @@ def create_sse_body(session_id, auth_info = nil)
         # The expiry of the token that authenticated the GET is kept with the stream so the keepalive loop and
         # the delivery paths can close the stream once that token expires.
         def store_stream_for_session(session_id, stream, auth_info = nil)
-          @mutex.synchronize do
+          stored = @mutex.synchronize do
             session = @sessions[session_id]
-            if session && !session[:get_sse_stream]
-              session[:get_sse_stream] = stream
-              session[:get_sse_stream_expires_at] = stream_token_expiry(auth_info)
-              # The expiry is nil for a stream opened without a token, and callers read a falsy return as "not stored",
-              # so the stream itself is the return value.
-              stream
-            else
-              # Either session was removed, or another request already established a stream.
-              stream.close
-              # `stream.close` may return a truthy value depending on the stream class.
-              # Explicitly return nil to guarantee a falsy return for callers.
-              nil
-            end
+            next false unless session && !session[:get_sse_stream]
+
+            session[:get_sse_stream] = stream
+            session[:get_sse_stream_expires_at] = stream_token_expiry(auth_info)
+            true
           end
+
+          # The expiry is nil for a stream opened without a token, and callers read a falsy return as "not stored",
+          # so the stream itself is the return value.
+          return stream if stored
+
+          # Either the session was removed, or another request already established a stream. The refused stream is
+          # closed outside the lock like every other stream this transport closes, so a close that blocks on the peer
+          # cannot hold every other session on `@mutex`.
+          close_stream_safely(stream)
+          nil
         end
 
         # The thread acts on the stream it was started for and on no other: the session may have detached that
diff --git a/test/mcp/server/transports/streamable_http_transport_test.rb b/test/mcp/server/transports/streamable_http_transport_test.rb
index 30be27fe..06fe41ab 100644
--- a/test/mcp/server/transports/streamable_http_transport_test.rb
+++ b/test/mcp/server/transports/streamable_http_transport_test.rb
@@ -743,6 +743,56 @@ def string
           assert stream_b.closed?
         end
 
+        test "store_stream_for_session closes a refused stream outside the mutex" do
+          session_id = initialize_test_session
+          @transport.send(:store_stream_for_session, session_id, StringIO.new)
+
+          mutex = @transport.instance_variable_get(:@mutex)
+          closed_outside_mutex = false
+          refused = Object.new
+          refused.define_singleton_method(:close) do
+            if mutex.try_lock
+              closed_outside_mutex = true
+              mutex.unlock
+            end
+          end
+
+          assert_nil @transport.send(:store_stream_for_session, session_id, refused)
+          assert closed_outside_mutex, "the refused stream was closed while the mutex was held"
+        end
+
+        test "store_stream_for_session swallows an error from closing a refused stream" do
+          session_id = initialize_test_session
+          @transport.send(:store_stream_for_session, session_id, StringIO.new)
+
+          failing = Object.new
+          failing.define_singleton_method(:close) { raise IOError, "already closed by the peer" }
+
+          assert_nil @transport.send(:store_stream_for_session, session_id, failing)
+          assert_equal 200, @transport.handle_request(create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json", "HTTP_MCP_SESSION_ID" => session_id },
+            { jsonrpc: "2.0", method: "ping", id: "ping-1" }.to_json,
+          ))[0]
+        end
+
+        test "store_stream_for_session closes the stream outside the mutex when the session is gone" do
+          # The session was removed between the GET's checks and its body running, so there is nothing to attach to.
+          mutex = @transport.instance_variable_get(:@mutex)
+          closed_outside_mutex = false
+          orphan = Object.new
+          orphan.define_singleton_method(:close) do
+            if mutex.try_lock
+              closed_outside_mutex = true
+              mutex.unlock
+            end
+          end
+
+          assert_nil @transport.send(:store_stream_for_session, "no-such-session", orphan)
+          assert closed_outside_mutex, "the orphaned stream was closed while the mutex was held"
+        end
+
         test "handles GET request with invalid session ID" do
           request = create_rack_request(
             "GET",