diff --git a/.github/workflows/gc-native-roots.yml b/.github/workflows/gc-native-roots.yml
index a0c8bf679b..e187683f7a 100644
--- a/.github/workflows/gc-native-roots.yml
+++ b/.github/workflows/gc-native-roots.yml
@@ -535,15 +535,26 @@ jobs:
           # Same answer as the shadow stack, and a collection that actually
           # moved something. Without the movement assert this passes with the
           # conservative scan doing all the rooting (#7336, #7338).
+          #
+          # PERRY_GC_DIAG=1 is what makes the evacuation assert able to see
+          # anything: `[gc-copy-minor] ran copied_objects=...` is printed only
+          # under that variable (`gc/copying.rs`). Without it the trace holds
+          # nothing but the probe's own `#gcmetric` lines, the assert reads
+          # 0 copying minors / 0 objects copied off an empty file, and the step
+          # fails no matter how the collector behaved — which is how this arm
+          # read on its first-ever execution (three of four arms in this matrix
+          # were permanently queued until #7393). Diagnostics go to stderr only,
+          # so the control diff below is unaffected.
           ./target/perry-dev/perry "$probe" -o /tmp/inproc-09-control
           PERRY_GC_HEAP_LIMIT=8 PERRY_GC_INCREMENTAL=0 PERRY_CONSERVATIVE_STACK_SCAN=off \
             /tmp/inproc-09-control > /tmp/inproc-09.control.out 2>/dev/null
-          PERRY_GC_FORCE_EVACUATE=1 PERRY_GC_VERIFY_EVACUATION=1 \
+          PERRY_GC_DIAG=1 PERRY_GC_FORCE_EVACUATE=1 PERRY_GC_VERIFY_EVACUATION=1 \
           PERRY_GC_HEAP_LIMIT=8 PERRY_GC_INCREMENTAL=0 PERRY_CONSERVATIVE_STACK_SCAN=off \
             /tmp/inproc-09 > /tmp/inproc-09.out 2> /tmp/inproc-09.err
           diff /tmp/inproc-09.control.out /tmp/inproc-09.out \
             || { echo "::error::in-process RS4GC diverged from the shadow-stack control"; exit 1; }
-          python3 scripts/gc_evacuation_liveness_assert.py /tmp/inproc-09.err
+          python3 scripts/gc_evacuation_liveness_assert.py /tmp/inproc-09.err \
+            --probe "09_try_catch_roots (in-process)"
 
           # And it must be RS4GC doing the lowering, not a per-function bail to
           # the bridge -- which would make this arm green while testing the
diff --git a/changelog.d/7414-macos-rs4gc-inprocess-gc-diag.md b/changelog.d/7414-macos-rs4gc-inprocess-gc-diag.md
new file mode 100644
index 0000000000..3290451409
--- /dev/null
+++ b/changelog.d/7414-macos-rs4gc-inprocess-gc-diag.md
@@ -0,0 +1,12 @@
+**Fixed** the macOS `native-roots-rs4gc` arm's in-process step could never pass.
+
+It asserts that a copying minor moved objects by counting
+`[gc-copy-minor] ran copied_objects=` lines, but both prints are gated on
+`PERRY_GC_DIAG` and the step never set it. The trace held only the probe's own
+`#gcmetric` lines, so the assert read 0/0 off an effectively empty file and
+failed regardless of collector behaviour — the inverse of a gate that cannot
+fail. It shipped with the step in #7339 and had never run, because three of four
+arms in this matrix were permanently queued until #7393.
+
+The liveness assert now distinguishes "no collector diagnostics at all" from
+"the collector moved nothing", which the old message conflated.
diff --git a/scripts/gc_evacuation_liveness_assert.py b/scripts/gc_evacuation_liveness_assert.py
index 5501bf25ef..0525534027 100755
--- a/scripts/gc_evacuation_liveness_assert.py
+++ b/scripts/gc_evacuation_liveness_assert.py
@@ -26,6 +26,13 @@
 COPIED = re.compile(r"copied_objects=(\d+)")
 ELIGIBLE = re.compile(r"\[gc-copy-minor\] eligible=(\w+)(?: fallback=(\S+))?")
 MANUAL = re.compile(r"\[gc-scan-fallback\] site=manual_collect")
+# Any collector diagnostic at all. Every one of them is printed behind
+# `PERRY_GC_DIAG`, so a trace with none of them was produced by a run that did
+# not set it — which is indistinguishable, by counts alone, from a collector
+# that moved nothing. Told apart below, because guessing wrong costs a build:
+# the `gc-native-roots` in-process arm read as "evacuated NOTHING" on its
+# first-ever execution purely for want of the variable.
+ANY_DIAG = re.compile(r"^\[gc-[a-z-]+\]", re.MULTILINE)
 
 
 def main() -> int:
@@ -44,6 +51,14 @@ def main() -> int:
         print(f"{args.probe}: evacuation live — {ran} copying minor(s), {copied} objects copied")
         return 0
 
+    if not ANY_DIAG.search(text):
+        print(f"::error::{args.probe}: {args.trace} carries no collector diagnostics at all, "
+              "so this assert measured nothing about the collector. Every line it reads is "
+              "printed behind PERRY_GC_DIAG; re-run the binary with PERRY_GC_DIAG=1 set "
+              "alongside PERRY_GC_FORCE_EVACUATE=1. (Diagnostics go to stderr, so this does "
+              "not disturb an stdout oracle diff.)")
+        return 1
+
     print(f"::error::{args.probe}: the forced-evacuation arm evacuated NOTHING "
           f"({ran} copying minors, {copied} objects copied). The arm is vacuous: "
           f"it proves the program ran, not that a moving collector did (#7336).")