diff --git a/quiche/common/quiche_feature_flags_list.h b/quiche/common/quiche_feature_flags_list.h
index 9666f81..81f9da5 100755
--- a/quiche/common/quiche_feature_flags_list.h
+++ b/quiche/common/quiche_feature_flags_list.h
@@ -8,7 +8,7 @@
 
 #if defined(QUICHE_FLAG)
 
-QUICHE_FLAG(bool, quiche_reloadable_flag_http2_add_hpack_overhead_bytes, false, "If true, HTTP/2 HEADERS frames will use two additional bytes of HPACK overhead per header in their SpdyFrameIR::size() estimate.")
+QUICHE_FLAG(bool, quiche_reloadable_flag_http2_add_hpack_overhead_bytes2, false, "If true, HTTP/2 HEADERS frames will use two additional bytes of HPACK overhead per header in their SpdyHeadersIR::size() estimate. This flag is latched in SpdyHeadersIR to ensure a consistent size() value for a const SpdyHeadersIR regardless of flag state.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_act_upon_invalid_header, true, "If true, reject or send error response code upon receiving invalid request or response headers.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_add_stream_info_to_idle_close_detail, false, "If true, include stream information in idle timeout connection close detail.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_allow_client_enabled_bbr_v2, true, "If true, allow client to enable BBRv2 on server via connection option 'B2ON'.")
diff --git a/quiche/quic/core/quic_flags_list.h b/quiche/quic/core/quic_flags_list.h
index c489dbb..fbce5e8 100644
--- a/quiche/quic/core/quic_flags_list.h
+++ b/quiche/quic/core/quic_flags_list.h
@@ -17,8 +17,8 @@
 QUIC_FLAG(quic_reloadable_flag_quic_block_until_settings_received_copt, true)
 // If trrue, early return before write control frame in OnCanWrite() if the connection is already closed.
 QUIC_FLAG(quic_reloadable_flag_quic_no_write_control_frame_upon_connection_close, true)
-// If true, HTTP/2 HEADERS frames will use two additional bytes of HPACK overhead per header in their SpdyFrameIR::size() estimate.
-QUIC_FLAG(quic_reloadable_flag_http2_add_hpack_overhead_bytes, true)
+// If true, HTTP/2 HEADERS frames will use two additional bytes of HPACK overhead per header in their SpdyHeadersIR::size() estimate. This flag is latched in SpdyHeadersIR to ensure a consistent size() value for a const SpdyHeadersIR regardless of flag state.
+QUIC_FLAG(quic_reloadable_flag_http2_add_hpack_overhead_bytes2, true)
 // If true, QUIC server will not respond to gQUIC probing packet(PING + PADDING) but treat it as a regular packet.
 QUIC_FLAG(quic_reloadable_flag_quic_ignore_gquic_probing, true)
 // If true, QUIC will default enable MTU discovery at server, with a target of 1450 bytes.
diff --git a/quiche/spdy/core/spdy_protocol.cc b/quiche/spdy/core/spdy_protocol.cc
index 3f2d2f7..fc3b971 100644
--- a/quiche/spdy/core/spdy_protocol.cc
+++ b/quiche/spdy/core/spdy_protocol.cc
@@ -16,7 +16,6 @@
 #include "absl/strings/string_view.h"
 #include "quiche/common/platform/api/quiche_bug_tracker.h"
 #include "quiche/common/platform/api/quiche_flag_utils.h"
-#include "quiche/common/platform/api/quiche_flags.h"
 #include "quiche/common/platform/api/quiche_logging.h"
 #include "quiche/spdy/core/http2_header_block.h"
 #include "quiche/spdy/core/spdy_alt_svc_wire_format.h"
@@ -476,10 +475,10 @@
   }
 
   // TODO(b/322146543): Remove `hpack_overhead` with deprecation of
-  // --gfe2_reloadable_flag_http2_add_hpack_overhead_bytes.
+  // --gfe2_reloadable_flag_http2_add_hpack_overhead_bytes2.
   size_t hpack_overhead = kPerHeaderHpackOverheadOld;
-  if (GetQuicheReloadableFlag(http2_add_hpack_overhead_bytes)) {
-    QUICHE_RELOADABLE_FLAG_COUNT(http2_add_hpack_overhead_bytes);
+  if (add_hpack_overhead_bytes_) {
+    QUICHE_RELOADABLE_FLAG_COUNT(http2_add_hpack_overhead_bytes2);
     hpack_overhead = kPerHeaderHpackOverheadNew;
   }
   // Assume no hpack encoding is applied.
diff --git a/quiche/spdy/core/spdy_protocol.h b/quiche/spdy/core/spdy_protocol.h
index 8501c94..09bc49e 100644
--- a/quiche/spdy/core/spdy_protocol.h
+++ b/quiche/spdy/core/spdy_protocol.h
@@ -22,6 +22,7 @@
 #include "absl/strings/string_view.h"
 #include "absl/types/variant.h"
 #include "quiche/common/platform/api/quiche_export.h"
+#include "quiche/common/platform/api/quiche_flags.h"
 #include "quiche/common/platform/api/quiche_logging.h"
 #include "quiche/spdy/core/http2_header_block.h"
 #include "quiche/spdy/core/spdy_alt_svc_wire_format.h"
@@ -338,13 +339,13 @@
 //   - 2 bytes for the name length (assuming new name).
 //   - 3 bytes for the value length.
 // TODO(b/322146543): Remove the `New` suffix with deprecation of
-// --gfe2_reloadable_flag_http2_add_hpack_overhead_bytes.
+// --gfe2_reloadable_flag_http2_add_hpack_overhead_bytes2.
 inline constexpr size_t kPerHeaderHpackOverheadNew = 6;
 // An estimate size of the HPACK overhead for each header field. 1 bytes for
 // indexed literal, 1 bytes for key literal and length encoding, and 2 bytes for
 // value literal and length encoding.
 // TODO(b/322146543): Remove with deprecation of
-// --gfe2_reloadable_flag_http2_add_hpack_overhead_bytes.
+// --gfe2_reloadable_flag_http2_add_hpack_overhead_bytes2.
 inline constexpr size_t kPerHeaderHpackOverheadOld = 4;
 
 // Names of pseudo-headers defined for HTTP/2 requests.
@@ -466,7 +467,8 @@
   SpdyStreamId stream_id() const { return stream_id_; }
   virtual bool fin() const;
   // Returns an estimate of the size of the serialized frame, without applying
-  // compression. May not be exact.
+  // compression. May not be exact, but implementations should return the same
+  // value for a const frame.
   virtual size_t size() const = 0;
 
   // Returns the number of bytes of flow control window that would be consumed
@@ -755,6 +757,8 @@
   bool exclusive_ = false;
   bool padded_ = false;
   int padding_payload_len_ = 0;
+  const bool add_hpack_overhead_bytes_ =
+      GetQuicheReloadableFlag(http2_add_hpack_overhead_bytes2);
 };
 
 class QUICHE_EXPORT SpdyWindowUpdateIR : public SpdyFrameIR {