diff --git a/derive/src/codegen.rs b/derive/src/codegen.rs
index 8118861ae..889a4ca65 100644
--- a/derive/src/codegen.rs
+++ b/derive/src/codegen.rs
@@ -1304,10 +1304,6 @@ fn flag_arm(cli: &Cli, i: usize, field: &Field) -> TokenStream {
     }
 }
 
-/// Whether another occurrence is a command-line mistake rather than another value.
-///
-/// Counts and collections repeat by definition, and `var` explicitly opts a value-taking flag
-/// into repetition. Every other flag matches clap's default of one occurrence.
 /// Whether a repeat of this flag is only a duplicate *within one command*.
 ///
 /// A `global` flag is in scope for every descendant, and clap lets it be given again on a
@@ -1331,6 +1327,26 @@ fn reset_per_level(cli: &Cli) -> TokenStream {
     quote!(#(#resets)*)
 }
 
+/// Whether another occurrence is a command-line mistake rather than another value.
+///
+/// Counts and collections repeat by definition, and `var` explicitly opts a value-taking flag
+/// into repetition. Every other flag matches clap's default of one occurrence.
+///
+/// **This is stricter than the grammar, deliberately.** The corpus specifies the opposite for a
+/// *spec-driven* parse — `long-repeated-keeps-the-last` says "a repeat is a correction, typically
+/// a wrapper appending to a command line it did not write" — and usage-argv's parser and
+/// usage-lib both honour that. The rule lives here, in the derive's post-binding layer, and
+/// nowhere else: `Error::DuplicateFlag` is constructed at one call site, in this file.
+///
+/// Kept, because the two cases are not the same case. A spec parsed at run time may well be
+/// describing someone else's command line, which is the wrapper the corpus has in mind. A
+/// derived CLI is an authored, fixed surface, where `--jobs 1 --jobs 2` is far likelier to be a
+/// mistake than an amendment — and it is what clap does, so an adopter replacing clap sees no
+/// change. Dropping it would make a derived binary quietly accept a line clap rejects today,
+/// which is the direction that costs an adopter rather than the one that helps.
+///
+/// Recorded because it looks like an inconsistency and is not one. `differential.rs` in the gate
+/// carries the same note from the other end.
 fn rejects_duplicate(field: &Field) -> bool {
     matches!(field.kind, Kind::Flag { .. })
         && !matches!(field.shape, Shape::Count | Shape::Many)
diff --git a/tasks/perf-shadow.sh b/tasks/perf-shadow.sh
index 57c0cdff7..e891bbb35 100755
--- a/tasks/perf-shadow.sh
+++ b/tasks/perf-shadow.sh
@@ -66,6 +66,19 @@ verify() {
   fi
 }
 
+# The ratio against clap this file expects to hold, and below which it says so.
+#
+# Not a gate — the workflow runs this with `|| true`, and a shared runner is no place to fail a
+# build on an instruction count. It is an annotation, because the alternative turned out to be
+# nobody noticing: the number went from 117x to 97x across a run of feature work, unremarked,
+# because the only place it appeared was one line of a report nobody diffs. clap is flat, so the
+# movement was ours.
+#
+# 80 rather than 117: the point is to catch a slide, not to pin the current figure. A parse that
+# is only 80x cheaper than clap's has stopped being the thing this project claims, and that is
+# worth a sentence in the log before it is worth an argument about the target.
+CLAP_RATIO_FLOOR=80
+
 # Two significant figures either side of the decimal point, so a framework within a factor
 # of usage reads as 1.5x rather than 1x and one three orders away still reads cleanly.
 ratio() {
@@ -119,3 +132,20 @@ bpaf_cold=$(cold parse-n-bpaf)
 } >"$tmp"
 
 cat "$tmp" >"$out"
+
+# Said after the report is written, so a failure here cannot leave the comment truncated — the
+# same reason the table is built in a temporary file.
+#
+# `::warning::` when running under GitHub Actions, a plain line otherwise, so the same script is
+# useful at a terminal.
+clap_ratio=$(awk -v a="$clap_cold" -v b="$usage_cold" 'BEGIN { printf "%d", a / b }')
+if [ "$clap_ratio" -lt "$CLAP_RATIO_FLOOR" ]; then
+  message="the parse is ${clap_ratio}x cheaper than clap's, under the ${CLAP_RATIO_FLOOR}x this\
+ file expects. clap is flat, so a fall here is usage getting more expensive rather than clap\
+ getting cheaper. See tasks/perf-shadow.sh."
+  if [ -n "${GITHUB_ACTIONS:-}" ]; then
+    echo "::warning title=Shadow ratio below floor::$message"
+  else
+    echo "warning: $message" >&2
+  fi
+fi