diff --git a/ci/spec22-gems/fixtures/probe.rb b/ci/spec22-gems/fixtures/probe.rb
index 2150cd1..bb2e437 100644
--- a/ci/spec22-gems/fixtures/probe.rb
+++ b/ci/spec22-gems/fixtures/probe.rb
@@ -59,6 +59,18 @@
 gem_loaded = $LOADED_FEATURES.any? { |f| f.end_with?("/tebako-runtime.rb") }
 puts "PROBE gem-loaded #{gem_loaded ? 'yes' : 'no'}"
 
+# Boot-environment tripwire (every leg, never gates): the harness hands
+# the runtime its whole contract through the env (TEBAKO_RUNTIME_IMAGE
+# mounts the stdlib env image; TEBAKO_HOME/HOME scope the store and the
+# user dirs). Incident 13 round 5: a comment line inside run_probe's
+# env -i continuation silently split the command, the runtime booted
+# with NONE of these, and the only symptom was the rubygems prelude
+# warning + the line-66 NoMethodError below — a full CI cycle to
+# diagnose. Report presence up front so a lost handoff names itself in
+# the first three lines of the proof log.
+puts "PROBE-DIAG env TEBAKO_RUNTIME_IMAGE=#{ENV['TEBAKO_RUNTIME_IMAGE'] ? 'set' : 'UNSET'} " \
+     "TEBAKO_HOME=#{ENV['TEBAKO_HOME'] ? 'set' : 'UNSET'} HOME=#{ENV['HOME'] ? 'set' : 'UNSET'}"
+
 # Redirect rubygems at the in-image probe gem home BEFORE requiring any
 # probe gem.
 ENV["GEM_HOME"] = GEM_HOME_IN_IMAGE
diff --git a/ci/spec22-gems/run-msys.sh b/ci/spec22-gems/run-msys.sh
index c5fbe82..3af615e 100755
--- a/ci/spec22-gems/run-msys.sh
+++ b/ci/spec22-gems/run-msys.sh
@@ -470,6 +470,19 @@ fi
 run_probe() {
   local leg="$1" img="${2:-$PAYLOAD_IMG}"
   set +e
+  # TEBAKO_DEBUG_TFS: the closure-walk traces (dlmap2file's answer, the
+  # PE parse verdict, each dep's resolve/materialize verdict) ride
+  # stderr into the per-leg proof log — incident 13's 126 was mute
+  # without them. env -i strips the ambient copy, so it rides inside
+  # the list; overridable for a quiet local run.
+  # WARNING: a comment line INSIDE a backslash continuation terminates
+  # the command at that point (the continuation splices the comment
+  # line in, the comment ends at its own newline) — the trailing lines
+  # then run as a SEPARATE command with the ambient environment. The
+  # round-4 insert below did exactly that: env -i ran with no command
+  # and the runtime booted without TEBAKO_RUNTIME_IMAGE (no env image
+  # mounted, rubygems prelude failed, probe.rb:66 NoMethodError on all
+  # four legs — run 32188296332). Comments stay OUTSIDE the list.
   env -i \
     MSYS2_ARG_CONV_EXCL='*' \
     MSYS2_ENV_CONV_EXCL='*' \
@@ -487,11 +500,6 @@ run_probe() {
     APPDATA="$(w "$SCRATCH/home")/AppData/Roaming" \
     TEBAKO_HOME="$(w "$SCRATCH/tebako-home")" \
     TEBAKO_RUNTIME_IMAGE="$(w "$RUNTIME_IMAGE")" \
-    # TEBAKO_DEBUG_TFS: the closure-walk traces (dlmap2file's answer, the
-    # PE parse verdict, each dep's resolve/materialize verdict) ride
-    # stderr into the per-leg proof log — incident 13's 126 was mute
-    # without them. env -i strips the ambient copy, so it rides inside
-    # the list; overridable for a quiet local run.
     TEBAKO_JAIL="deny;$(w "$SCRATCH"):/host-scratch:rw" \
     TEBAKO_DEBUG_TFS="${TEBAKO_DEBUG_TFS:-1}" \
     "$RUNTIME_EXE" --tebako-image "$(w "$img"):-:/" --tebako-entry /probe/probe.rb "$leg" \