diff --git a/go/internal/stack/postgres_image.go b/go/internal/stack/postgres_image.go
new file mode 100644
index 00000000..c2d3a4f9
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+++ b/go/internal/stack/postgres_image.go
@@ -0,0 +1,26 @@
+//go:build unix
+
+package stack
+
+// DefaultPostgresImage is the container image the S4 container-backed postgres
+// component (the T8 adapter) runs by default: the stock upstream postgres:18,
+// pinned BY DIGEST. This is Matt's OQ-5 ruling (DL-260) — the bundled stock
+// service, NOT a nix2container build of nixpkgs postgresql (that mechanism is
+// for the NixOS+devenv agent shell, not a third-party service) and NOT a custom
+// wrapper-entrypoint image. The official postgres entrypoint already does
+// initdb-if-needed + createdb + a clean SIGTERM drain, so no wrapper is built.
+//
+// The pin is by DIGEST, not a mutable tag, for the same reason
+// go/internal/pgtest/pgtest.go pins its CI image: postgres behavior the stack
+// depends on (the store-of-record's on-disk format, and the same
+// generated-tsvector / websearch_to_tsquery behavior the suites assert) is
+// major/minor-version-sensitive, and a mutable tag would ship an unreviewed
+// database under the installed stack.
+//
+// Bump procedure: advance the digest below when the postgres minor/major moves,
+// then re-run the T8 container integration test (up -> probe DSN -> fresh-process
+// down -> container gone) against the new digest before landing. Keep the major
+// aligned with pgtest.go's pin discipline so a dev-box stack and an installed
+// stack never skew on-disk format. This is a Go const Renovate cannot see (like
+// pgtest.go's pgImage), so it moves only via a reviewed manual PR.
+const DefaultPostgresImage = "docker.io/library/postgres:18@sha256:1957b2ff3137e4ef7f3bc813e74fff50b1e1ffddc85c8b9d6f14ade972be8687"