diff --git a/infra/run-observability-check.sh b/infra/run-observability-check.sh
index a8d01fa..b340f64 100755
--- a/infra/run-observability-check.sh
+++ b/infra/run-observability-check.sh
@@ -1,41 +1,45 @@
 #!/usr/bin/env bash
 #
-# S-16a (#122): assert the observability backplane is live against the running stack.
-# Not just "the containers started" — it asks Grafana to health-check its provisioned
-# Tempo and Prometheus datasources, which proves Grafana can actually reach both over
-# the `cg` network and that provisioning landed. Polls, so it tolerates a cold Grafana.
+# S-16a (#122): assert the observability backplane is live against an ALREADY-RUNNING
+# stack. Runs curl INSIDE the compose network (like the other verify checks) because
+# the stack's published ports aren't on the CI runner's localhost — the stack is a set
+# of sibling containers on the host daemon. It asks Grafana to reach its provisioned
+# datasources — Prometheus via its health method, Tempo via the datasource proxy (Tempo's
+# Grafana plugin implements no health method) — so it proves the datasources are wired,
+# not merely that the containers started. Polls, so it tolerates a cold Grafana.
 #
-# Usage: run-observability-check.sh   (override GRAFANA_URL / GRAFANA_AUTH / OBS_TIMEOUT)
+# Does NOT manage the stack lifecycle (the caller owns bring-up + teardown).
 set -euo pipefail
 
-GRAFANA="${GRAFANA_URL:-http://localhost:3000}"
-AUTH="${GRAFANA_AUTH:-admin:admin}"
 TIMEOUT="${OBS_TIMEOUT:-60}"
+AUTH="${GRAFANA_AUTH:-admin:admin}"
 
-# poll <description> <url> <python-bool-expr over `d` (parsed JSON)>
-# Passes when the expression prints True within TIMEOUT. Tempo's Grafana plugin
-# doesn't implement the datasource /health method, so instead of the Prometheus-
-# style health probe we prove reachability through Grafana's datasource proxy.
+gf="$(docker ps -q --filter 'name=[-_]grafana[-_]' | head -1)"
+[ -n "$gf" ] || { echo "ERROR: no running grafana container — bring the stack up first" >&2; exit 1; }
+net="$(docker inspect -f '{{range $k,$_ := .NetworkSettings.Networks}}{{$k}}{{"\n"}}{{end}}' "$gf" | head -1)"
+gf_ip="$(docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' "$gf")"
+base="http://$gf_ip:3000"
+echo ">> grafana=$gf_ip network=$net"
+
+# Run curl inside a throwaway container on the stack network (reaches services by IP).
+net_curl() { docker run --rm --network "$net" curlimages/curl:latest "$@"; }
+
+# poll <description> <grep -E pattern> <curl args...>
 poll() {
-  local desc="$1" url="$2" expr="$3" got deadline
-  deadline=$(( $(date +%s) + TIMEOUT ))
+  local desc="$1" pat="$2"; shift 2
+  local deadline=$(( $(date +%s) + TIMEOUT ))
   while :; do
-    got="$(curl -fsS -u "$AUTH" "$url" 2>/dev/null \
-      | python3 -c "import sys,json; d=json.load(sys.stdin); print($expr)" 2>/dev/null || true)"
-    [ "$got" = "True" ] && { echo "  ✓ $desc"; return 0; }
-    if [ "$(date +%s)" -ge "$deadline" ]; then
-      echo "  ✗ $desc — check failed ($url)" >&2
-      return 1
-    fi
-    sleep 2
+    if net_curl -fsS "$@" 2>/dev/null | grep -Eq "$pat"; then echo "  ✓ $desc"; return 0; fi
+    if [ "$(date +%s)" -ge "$deadline" ]; then echo "  ✗ $desc ($*)" >&2; return 1; fi
+    sleep 3
   done
 }
 
-echo "Checking observability backplane at $GRAFANA ..."
+echo "Checking observability backplane at $base ..."
 poll "Grafana is healthy" \
-  "$GRAFANA/api/health" "d['database'] == 'ok'"
+  '"database":[[:space:]]*"ok"' "$base/api/health"
 poll "Prometheus datasource reachable" \
-  "$GRAFANA/api/datasources/uid/prometheus/health" "d['status'] == 'OK'"
+  '"status":[[:space:]]*"OK"' -u "$AUTH" "$base/api/datasources/uid/prometheus/health"
 poll "Tempo datasource reachable (via Grafana proxy)" \
-  "$GRAFANA/api/datasources/proxy/uid/tempo/api/status/buildinfo" "bool(d.get('version'))"
+  '"version"' -u "$AUTH" "$base/api/datasources/proxy/uid/tempo/api/status/buildinfo"
 echo "Observability backplane OK."