diff --git a/bin/check-session-log b/bin/check-session-log
index cb8770a..4a33e58 100755
--- a/bin/check-session-log
+++ b/bin/check-session-log
@@ -25,15 +25,39 @@ for dir in "$ROOT"/skills/*/; do
 
   grep -q 'session_log' "$dir/SKILL.md" || continue
 
-  if ! grep -q 'session_id' "$dir/SKILL.md"; then
-    echo "ERROR: skills/$name documents session_log without session_id — the tool's only required field" >&2
-    fail=1
+  # Read the PAYLOAD, not the file. The first version of this gate grepped the
+  # whole SKILL.md for "session_id", so a payload missing the field passed as
+  # long as the word appeared anywhere — including in prose explaining that the
+  # field is required. A traceability probe demonstrated it in one sed.
+  payloads=$(grep -n 'session_log' "$dir/SKILL.md" | grep '{' || true)
+
+  if [ -z "$payloads" ]; then
+    # Mentions the tool but documents no call. Nothing to check, and nothing to
+    # get wrong.
+    continue
   fi
 
-  if grep -q 'duration_ms:[[:space:]]*<' "$dir/SKILL.md"; then
-    echo "ERROR: skills/$name documents duration_ms as a placeholder an agent cannot measure — omit it or pass a real measurement" >&2
+  printf '%s\n' "$payloads" | while IFS= read -r line; do
+    [ -n "$line" ] || continue
+    body=$(printf '%s' "$line" | sed 's/^[0-9]*://')
+    lineno=$(printf '%s' "$line" | cut -d: -f1)
+
+    printf '%s' "$body" | grep -q 'session_id' \
+      || echo "ERROR: skills/$name:$lineno documents a session_log payload with no session_id — the tool's only required field" >&2
+
+    # Any spelling of the key, quoted or not, followed by anything that is not a
+    # bare number. A placeholder nobody can fill gets fabricated or dropped; a
+    # real measurement is fine.
+    if printf '%s' "$body" | grep -Eq '"?duration_ms"?[[:space:]]*:[[:space:]]*[^0-9,}[:space:]]'; then
+      echo "ERROR: skills/$name:$lineno documents duration_ms as something other than a real measurement — an agent does not observe a clock; omit it" >&2
+    fi
+  done > "$ROOT/.session_log_errors.$$" 2>&1
+
+  if [ -s "$ROOT/.session_log_errors.$$" ]; then
+    cat "$ROOT/.session_log_errors.$$" >&2
     fail=1
   fi
+  rm -f "$ROOT/.session_log_errors.$$"
 done
 
 exit $fail
diff --git a/tests/check_session_log_test.sh b/tests/check_session_log_test.sh
index c29a214..5f15878 100644
--- a/tests/check_session_log_test.sh
+++ b/tests/check_session_log_test.sh
@@ -63,5 +63,38 @@ expect 1 'documenting duration_ms as wall-clock fails' \
   "$(fixture clock 'Call `session_log` with `{session_id: "<id>", skill: "alpha", duration_ms: <wall-clock>}`')" \
   'duration_ms'
 
+# --- the gate must read the PAYLOAD, not the file ---------------------------
+# A traceability probe falsified the original: it grepped the whole file for the
+# string "session_id", so stripping the field from every documented payload and
+# leaving the word once in prose passed cleanly. The exact regression the gate
+# exists to prevent was back in the file and the gate said nothing.
+
+expect 1 'a payload without session_id fails even when prose mentions the word' \
+  "$(fixture prose 'The tool has a required session_id field.
+
+Call `session_log` with `{skill: "alpha", phase: "red", final_status: "pass"}`')" \
+  'session_id'
+
+expect 1 'one bad payload among several fails' \
+  "$(fixture mixed 'Call `session_log` with `{session_id: "<id>", skill: "alpha", phase: "red"}`
+
+Then `session_log` with `{skill: "alpha", phase: "green", final_status: "pass"}`')" \
+  'session_id'
+
+# duration_ms was banned by matching exactly one spelling of it — the historical
+# one. Both of these passed the original gate.
+expect 1 'duration_ms with a JSON-quoted key fails' \
+  "$(fixture jsonkey 'Call `session_log` with `{session_id: "<id>", "duration_ms": <wall-clock>}`')" \
+  'duration_ms'
+
+expect 1 'duration_ms described in prose rather than angle brackets fails' \
+  "$(fixture prosems 'Call `session_log` with `{session_id: "<id>", duration_ms: elapsed wall-clock ms}`')" \
+  'duration_ms'
+
+# A real measurement is allowed: the objection is to a placeholder no agent can
+# fill, not to the field.
+expect 0 'a duration_ms carrying a real value is allowed' \
+  "$(fixture realms 'Call `session_log` with `{session_id: "<id>", duration_ms: 1430}`')"
+
 printf '\ncheck_session_log: %d passed, %d failed\n' "$pass" "$fail"
 [ "$fail" -eq 0 ]