diff --git a/src/hb-buffer-verify.cc b/src/hb-buffer-verify.cc
index c97729403..dea2c11c3 100644
--- a/src/hb-buffer-verify.cc
+++ b/src/hb-buffer-verify.cc
@@ -397,7 +397,8 @@ hb_buffer_t::verify (hb_buffer_t        *text_buffer,
     ret = false;
   if (!buffer_verify_unsafe_to_break (this, text_buffer, font, features, num_features, shapers))
     ret = false;
-  if (!buffer_verify_unsafe_to_concat (this, text_buffer, font, features, num_features, shapers))
+  if ((flags & HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT) != 0 &&
+      !buffer_verify_unsafe_to_concat (this, text_buffer, font, features, num_features, shapers))
     ret = false;
   if (!ret)
   {
diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index ee72dfafb..cad45979e 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -137,7 +137,11 @@ typedef struct hb_glyph_info_t {
  * 				   clusters.
  * 				   The #HB_GLYPH_FLAG_UNSAFE_TO_BREAK flag will
  * 				   always imply this flag.
- * 				   Since: 3.3.0
+ *				   To use this flag, you must enable the buffer flag
+ *				   @HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT during
+ *				   shaping, otherwise the buffer flag will not be
+ *				   reliably produced.
+ * 				   Since: REPLACEME
  * @HB_GLYPH_FLAG_DEFINED: All the currently defined flags.
  *
  * Flags for #hb_glyph_info_t.
@@ -365,7 +369,10 @@ hb_buffer_guess_segment_properties (hb_buffer_t *buffer);
  *                      handler is installed on the buffer, or a message is written
  *                      to standard error.  In either case, the shaping result might
  *                      be modified to show the failed output. Since: 3.4.0
- *
+ * @HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT:
+ *                      flag indicating that the @HB_GLYPH_FLAG_UNSAFE_TO_CONCAT
+ *                      glyph-flag should be produced by the shaper. By default
+ *                      it will not be produced since it incurs a cost. Since: REPLACEME
  * Flags for #hb_buffer_t.
  *
  * Since: 0.9.20
@@ -377,7 +384,8 @@ typedef enum { /*< flags >*/
   HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES	= 0x00000004u,
   HB_BUFFER_FLAG_REMOVE_DEFAULT_IGNORABLES	= 0x00000008u,
   HB_BUFFER_FLAG_DO_NOT_INSERT_DOTTED_CIRCLE	= 0x00000010u,
-  HB_BUFFER_FLAG_VERIFY				= 0x00000020u
+  HB_BUFFER_FLAG_VERIFY				= 0x00000020u,
+  HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT	= 0x00000040u
 } hb_buffer_flags_t;
 
 HB_EXTERN void
diff --git a/src/hb-buffer.hh b/src/hb-buffer.hh
index cc20f3aef..bc6992905 100644
--- a/src/hb-buffer.hh
+++ b/src/hb-buffer.hh
@@ -460,6 +460,8 @@ struct hb_buffer_t
   }
   void unsafe_to_concat (unsigned int start = 0, unsigned int end = -1)
   {
+    if (likely ((flags & HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT) == 0))
+      return;
     _set_glyph_flags (HB_GLYPH_FLAG_UNSAFE_TO_CONCAT,
 		      start, end,
 		      true);
@@ -472,6 +474,8 @@ struct hb_buffer_t
   }
   void unsafe_to_concat_from_outbuffer (unsigned int start = 0, unsigned int end = -1)
   {
+    if (likely ((flags & HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT) == 0))
+      return;
     _set_glyph_flags (HB_GLYPH_FLAG_UNSAFE_TO_CONCAT,
 		      start, end,
 		      false, true);
diff --git a/test/shape/data/in-house/tests/unsafe-to-concat.tests b/test/shape/data/in-house/tests/unsafe-to-concat.tests
index 24906fa6d..64381a3a5 100644
--- a/test/shape/data/in-house/tests/unsafe-to-concat.tests
+++ b/test/shape/data/in-house/tests/unsafe-to-concat.tests
@@ -1 +1 @@
-../fonts/34da9aab7bee86c4dfc3b85e423435822fdf4b62.ttf;--show-flags;U+0628,U+200C,U+0628;[uni0628=1+993#2|uni0628=0+993#2]
+../fonts/34da9aab7bee86c4dfc3b85e423435822fdf4b62.ttf;--show-flags --unsafe-to-concat;U+0628,U+200C,U+0628;[uni0628=1+993#2|uni0628=0+993#2]
diff --git a/util/shape-options.hh b/util/shape-options.hh
index 806b34b62..5a15c0798 100644
--- a/util/shape-options.hh
+++ b/util/shape-options.hh
@@ -52,6 +52,7 @@ struct shape_options_t
 				  (bot ? HB_BUFFER_FLAG_BOT : 0) |
 				  (eot ? HB_BUFFER_FLAG_EOT : 0) |
 				  (verify ? HB_BUFFER_FLAG_VERIFY : 0) |
+				  (unsafe_to_concat ? HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT : 0) |
 				  (preserve_default_ignorables ? HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES : 0) |
 				  (remove_default_ignorables ? HB_BUFFER_FLAG_REMOVE_DEFAULT_IGNORABLES : 0) |
 				  0));
@@ -137,6 +138,7 @@ struct shape_options_t
   hb_buffer_cluster_level_t cluster_level = HB_BUFFER_CLUSTER_LEVEL_DEFAULT;
   hb_bool_t normalize_glyphs = false;
   hb_bool_t verify = false;
+  hb_bool_t unsafe_to_concat = false;
   unsigned int num_iterations = 1;
 };
 
@@ -260,6 +262,7 @@ shape_options_t::add_options (option_parser_t *parser)
     {"utf8-clusters",	0, 0, G_OPTION_ARG_NONE,	&this->utf8_clusters,		"Use UTF8 byte indices, not char indices",	nullptr},
     {"cluster-level",	0, 0, G_OPTION_ARG_INT,		&this->cluster_level,		"Cluster merging level (default: 0)",	"0/1/2"},
     {"normalize-glyphs",0, 0, G_OPTION_ARG_NONE,	&this->normalize_glyphs,	"Rearrange glyph clusters in nominal order",	nullptr},
+    {"unsafe-to-concat",0, 0, G_OPTION_ARG_NONE,	&this->unsafe_to_concat,	"Produce unsafe-to-concat glyph flag",	nullptr},
     {"verify",		0, 0, G_OPTION_ARG_NONE,	&this->verify,			"Perform sanity checks on shaping results",	nullptr},
     {"num-iterations", 'n', G_OPTION_FLAG_IN_MAIN,
 			      G_OPTION_ARG_INT,		&this->num_iterations,		"Run shaper N times (default: 1)",	"N"},