diff --git a/.github/scripts/check-pg-healthchecks.sh b/.github/scripts/check-pg-healthchecks.sh
new file mode 100755
index 00000000..8ec08ebc
--- /dev/null
+++ b/.github/scripts/check-pg-healthchecks.sh
@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+# Every pg_isready must name a host, so the healthcheck probes TCP.
+#
+# Without -h, pg_isready uses the Unix socket, and the official postgres image
+# runs initdb against a temporary server started with `listen_addresses=''` --
+# socket up, TCP refused. The healthcheck therefore passes *during* init, and a
+# dependent with `condition: service_healthy` starts into a window where the
+# port it actually connects to does not exist yet.
+#
+# This is not theoretical. From e2e run 35247949580, plc-postgres's own log
+# against plc's crash:
+#
+#   16:55:27.593  temp server: listening on Unix socket ONLY
+#   16:55:27.633  temp server: ready to accept connections   <- healthcheck goes green
+#   16:55:29.441  temp server: shut down
+#   16:55:29.887  real server: listening on IPv4 0.0.0.0:5432
+#   16:55:30.531  plc exits: ECONNREFUSED 172.18.0.6:5432
+#
+# The healthcheck was green 2.25 seconds before TCP existed. That is why
+# TestUploadAndRetrieve/filesystem failed 2 of 40 e2e runs naming a *different*
+# container each time -- whichever dependent lost the race that run.
+#
+# The rule has no list in it. Every pg_isready is either given a host or it is
+# a bug, so nothing here needs updating when a service is added. It covers Go
+# as well as YAML because smelt/pkg/generate builds one of these strings.
+#
+# It matches an *invocation*, not a mention: the name must be preceded by a
+# quote, so it is the start of a quoted command string. The first version of
+# this check did not, and its own step name in ci.yml -- "every pg_isready
+# healthcheck probes TCP" -- failed it. A guard that cannot tell running a
+# thing from naming it is not guarding the thing.
+#
+# The gap that leaves: an unquoted invocation, e.g. `--health-cmd pg_isready`
+# in an Actions services: block. Nothing in this repository writes one, and
+# the alternative -- matching every mention -- is what just misfired.
+set -euo pipefail
+
+cd "$(dirname "$0")/../.."
+
+status=0
+checked=0
+while IFS= read -r hit; do
+  file="${hit%%:*}"
+  rest="${hit#*:}"
+  line="${rest%%:*}"
+  checked=$((checked + 1))
+  case "$rest" in
+    *pg_isready*-h[[:space:]]*|*pg_isready*--host*) echo "ok   $file:$line" ;;
+    *) echo "FAIL $file:$line pg_isready with no -h probes the Unix socket"; status=1 ;;
+  esac
+done < <(grep -rn '["'"'"']pg_isready' --include='*.yml' --include='*.yaml' --include='*.go' . | grep -v '^\./\.git/')
+
+if [ "$checked" -eq 0 ]; then
+  echo "No pg_isready healthchecks found -- this check has nothing to guard."
+  echo "That is suspicious rather than fine: it used to find several."
+  exit 1
+fi
+
+if [ "$status" -eq 0 ]; then
+  echo "All $checked pg_isready probes name a host."
+else
+  echo
+  echo "Add -h 127.0.0.1. Without it the probe uses the Unix socket, which is"
+  echo "up during initdb while TCP is not, so dependents start too early."
+fi
+exit "$status"
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 10d2bff4..b1b9c573 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -56,6 +56,8 @@ jobs:
         run: .github/scripts/check-base-images.sh
       - name: every pulled compose image is pinned by digest
         run: .github/scripts/check-stack-images.sh
+      - name: every pg_isready healthcheck probes TCP
+        run: .github/scripts/check-pg-healthchecks.sh
       - name: gofmt
         run: |
           unformatted=$(gofmt -s -l .)
diff --git a/smelt/pkg/generate/compose.go b/smelt/pkg/generate/compose.go
index ceed4140..e34771fa 100644
--- a/smelt/pkg/generate/compose.go
+++ b/smelt/pkg/generate/compose.go
@@ -150,7 +150,7 @@ func buildPostgresService() ComposeService {
 			"piri-postgres-data:/var/lib/postgresql/data",
 		},
 		Healthcheck: &Healthcheck{
-			Test:          []string{"CMD-SHELL", "pg_isready -U piri -d postgres"},
+			Test:          []string{"CMD-SHELL", "pg_isready -U piri -d postgres -h 127.0.0.1"},
 			StartInterval: "1s",
 			Interval:      "5s",
 			Timeout:       "3s",
diff --git a/smelt/systems/hilt/compose.yml b/smelt/systems/hilt/compose.yml
index cfdadecf..9ce179e8 100644
--- a/smelt/systems/hilt/compose.yml
+++ b/smelt/systems/hilt/compose.yml
@@ -136,7 +136,7 @@ services:
     volumes:
       - hilt-postgres-data:/var/lib/postgresql/data
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U hilt -d hilt"]
+      test: ["CMD-SHELL", "pg_isready -U hilt -d hilt -h 127.0.0.1"]
       start_interval: 1s
       interval: 5s
       timeout: 3s
diff --git a/smelt/systems/ingot/compose.yml b/smelt/systems/ingot/compose.yml
index ebbad34a..c8f476ee 100644
--- a/smelt/systems/ingot/compose.yml
+++ b/smelt/systems/ingot/compose.yml
@@ -135,7 +135,7 @@ services:
     volumes:
       - ingot-postgres-data:/var/lib/postgresql/data
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U ingot -d ingot"]
+      test: ["CMD-SHELL", "pg_isready -U ingot -d ingot -h 127.0.0.1"]
       start_interval: 1s
       interval: 5s
       timeout: 3s
diff --git a/smelt/systems/plc/compose.yml b/smelt/systems/plc/compose.yml
index e91ab0b4..b6dea7b5 100644
--- a/smelt/systems/plc/compose.yml
+++ b/smelt/systems/plc/compose.yml
@@ -48,7 +48,7 @@ services:
     volumes:
       - plc-postgres-data:/var/lib/postgresql/data
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U plc -d plc"]
+      test: ["CMD-SHELL", "pg_isready -U plc -d plc -h 127.0.0.1"]
       start_interval: 1s
       interval: 5s
       timeout: 3s
diff --git a/smelt/systems/swarf/compose.yml b/smelt/systems/swarf/compose.yml
index 1a1e99a1..80ffe422 100644
--- a/smelt/systems/swarf/compose.yml
+++ b/smelt/systems/swarf/compose.yml
@@ -57,7 +57,7 @@ services:
     volumes:
       - swarf-postgres-data:/var/lib/postgresql/data
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U swarf -d swarf"]
+      test: ["CMD-SHELL", "pg_isready -U swarf -d swarf -h 127.0.0.1"]
       start_interval: 1s
       interval: 5s
       timeout: 3s
diff --git a/smelt/systems/upload/compose.yml b/smelt/systems/upload/compose.yml
index 2df53600..2707d1f7 100644
--- a/smelt/systems/upload/compose.yml
+++ b/smelt/systems/upload/compose.yml
@@ -81,7 +81,7 @@ services:
     volumes:
       - upload-postgres-data:/var/lib/postgresql/data
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U sprue -d sprue"]
+      test: ["CMD-SHELL", "pg_isready -U sprue -d sprue -h 127.0.0.1"]
       start_interval: 1s
       interval: 5s
       timeout: 3s