diff --git a/.github/workflows/migrate-staging.yml b/.github/workflows/migrate-staging.yml
index 85644276..d7ac79ed 100644
--- a/.github/workflows/migrate-staging.yml
+++ b/.github/workflows/migrate-staging.yml
@@ -12,6 +12,26 @@
 # separate `production` branch promotion, and auto-applying irreversible DDL to
 # prod on merge is a different risk decision. This runner has no down
 # migrations.
+#
+# THE SECRET MUST BE THE SESSION-MODE POOLER URI, NOT THE DIRECT ONE.
+# `db.<ref>.supabase.co` publishes only an AAAA record, and GitHub-hosted
+# runners have no outbound IPv6 — a direct string fails with "Network is
+# unreachable" / "server closed the connection unexpectedly" before it ever
+# authenticates. (Same wall on a home network without a global IPv6 address;
+# it is why staging migrations had to be applied by hand.)
+#
+# Use the pooler host on port 5432 — SESSION mode. Not 6543 (transaction mode),
+# which drops the session-level behaviour psycopg and DDL rely on. Session mode
+# behaves like a direct connection. Note the pooler also changes the username to
+# `postgres.<ref>`.
+#
+# Take the host from the dashboard's Connect panel rather than assembling it:
+# projects are assigned to NUMBERED pooler clusters (`aws-0-`, `aws-1-`, ...)
+# and the number is NOT derivable from the region — staging and production are
+# both us-west-2 yet sit on different clusters. A wrong prefix fails with
+# "Tenant or user not found", which is at least distinguishable from a bad
+# password. `backend/scripts/pooler_url.py` builds the URI from an env file so
+# the password is never copied by hand.
 name: Migrate (staging)
 
 on:
@@ -56,7 +76,7 @@ jobs:
           SUPABASE_DB_URL: ${{ secrets.STAGING_SUPABASE_DB_URL }}
         run: |
           if [ -z "${SUPABASE_DB_URL}" ]; then
-            echo "::notice::STAGING_SUPABASE_DB_URL is not set — skipping. Add the secret (the DIRECT connection string, port 5432, not the pooler) to enable."
+            echo "::notice::STAGING_SUPABASE_DB_URL is not set — skipping. Add the secret to enable: the SESSION-mode pooler URI on port 5432, user postgres.<ref>. Take the host from the dashboard's Connect panel — projects are assigned to NUMBERED pooler clusters (aws-0-, aws-1-, ...) and the number is not derivable from the region, so do not assume aws-0. 'python scripts/pooler_url.py .env.staging <cluster-prefix> --raw' builds the URI from the env file. Not the direct db.<ref>.supabase.co string — that is IPv6-only and unreachable from GitHub runners — and not port 6543, which is transaction mode."
             echo "skip=true" >> "$GITHUB_OUTPUT"
             exit 0
           fi
diff --git a/CLAUDE.md b/CLAUDE.md
index 833fc93e..1807f71d 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -45,14 +45,18 @@ python -m pytest tests/ -q      # backend test suite
 Database (run from `backend/`; migrations are raw DDL, never dashboard SQL):
 
 ```
-python -m db.migrate              # apply pending migrations (needs SUPABASE_DB_URL = direct conn string)
+python -m db.migrate              # apply pending migrations (SUPABASE_DB_URL = SESSION-mode pooler URI, port 5432)
 python -m db.migrate --baseline   # record migrations as applied without running them
 python -m db.seed_staging         # idempotent fake demo dataset on the new schema
 ```
 
 The `db/` scripts read `.env` by default; for staging/prod ops run them under
 `dotenv -f .env.staging run -- python -m db.<script>` so they hit the right project.
-Migrations are immutable once applied — add a new numbered file, never edit an old one.
+Migrations are immutable once applied — add a new timestamp-prefixed file
+(`date -u +%Y%m%d%H%M%S`), never edit an old one. `SUPABASE_DB_URL` must be the
+SESSION-mode pooler URI (port 5432, user `postgres.<ref>`); the direct
+`db.<ref>.supabase.co` host is IPv6-only and unreachable from most networks, and
+port 6543 is transaction mode and breaks DDL. `scripts/pooler_url.py` builds it.
 
 Docker (full stack from repo root):
 
diff --git a/README.md b/README.md
index 65d6ec6b..16e18313 100644
--- a/README.md
+++ b/README.md
@@ -233,7 +233,7 @@ npm run dev                # → http://localhost:3000
 | `GOOGLE_CLIENT_SECRET` | — | Google OAuth client secret |
 | `SESSION_SECRET` | — | HMAC secret for session tokens (min 32 bytes) |
 | `ALLOWED_EMAIL_DOMAINS` | — | Comma-separated sign-in email-domain allowlist (default `bu.edu`). Empty value disables the check (any domain may sign in). |
-| `SUPABASE_DB_URL` | — | Supabase **direct** connection string (port 5432, not the pooler) — used only by the `db.migrate` migration runner, never at app runtime |
+| `SUPABASE_DB_URL` | — | Supabase **session-mode pooler** URI (port 5432, user `postgres.<ref>`) — used only by the `db.migrate` migration runner, never at app runtime. Not the direct `db.<ref>` host (IPv6-only, unreachable from most networks); not port 6543 (transaction mode, breaks DDL) |
 | `LOGFIRE_TOKEN` | — | If set, traces ship to logfire.pydantic.dev. Without it, Logfire stays local-only. The Sapling scrubber redacts prompt/output content before egress regardless. |
 | `SAPLING_MODEL_CLASSIFIER` | — | Override classifier-agent model (default `gemini-2.5-flash-lite`) |
 | `SAPLING_MODEL_SUMMARY` | — | Override summary-agent model (default `gemini-2.5-flash-lite`) |
@@ -312,7 +312,7 @@ SAPLING_EVAL_UPDATE_BASELINES=1 python tests/evals/run_all.py  # refresh baselin
 
 ## Migrations
 
-Schema lives as ordered SQL files in `backend/db/migrations/` (numeric prefix = apply order, `0001`–`0030`). A minimal runner (`backend/db/migrate.py`) applies pending files in order and records each in a tracking table, so it's idempotent — re-running only applies what's new. The runner connects directly with `psycopg` over the Supabase **direct** connection string (`SUPABASE_DB_URL`, not the pooler); this is the one sanctioned exception to the `db/connection.py::table()`-only convention, since runtime PostgREST can't execute DDL.
+Schema lives as ordered SQL files in `backend/db/migrations/`, applied in filename order. New migrations use a UTC timestamp prefix (`date -u +%Y%m%d%H%M%S`); the legacy `NNNN_` files are frozen and must never be renamed, since the ledger keys on basename and a rename re-runs the migration. See `backend/db/migrations/README.md`. A minimal runner (`backend/db/migrate.py`) applies pending files in order and records each in a tracking table, so it's idempotent — re-running only applies what's new. The runner connects with `psycopg` over the **session-mode pooler** URI (`SUPABASE_DB_URL`, port 5432, user `postgres.<ref>`); this is the one sanctioned exception to the `db/connection.py::table()`-only convention, since runtime PostgREST can't execute DDL.
 
 ```bash
 cd backend
diff --git a/backend/db/migrate.py b/backend/db/migrate.py
index 1aaf943d..5a9a1ad2 100644
--- a/backend/db/migrate.py
+++ b/backend/db/migrate.py
@@ -1,9 +1,26 @@
 """Minimal migration runner for Supabase Postgres (#197).
 
 App runtime uses db/connection.py::table() (PostgREST), which cannot execute DDL.
-Migrations are raw DDL, so this admin tool connects directly with psycopg over the
-Supabase *direct* connection string (SUPABASE_DB_URL, NOT the pooler). This is the
-one sanctioned exception to the table()-only convention.
+Migrations are raw DDL, so this admin tool connects with psycopg over
+SUPABASE_DB_URL. This is the one sanctioned exception to the table()-only
+convention.
+
+WHICH CONNECTION STRING: the SESSION-mode pooler, port 5432.
+
+This file used to say "the direct connection string, NOT the pooler". That
+warning was about TRANSACTION mode (port 6543), which drops the session-level
+behaviour psycopg and DDL depend on — and it is still correct about 6543. But
+it predates Supabase moving the direct host to an IPv6-only endpoint:
+db.<ref>.supabase.co now publishes only an AAAA record, so it is unreachable
+from GitHub-hosted runners and from any network without a global IPv6 address.
+Session mode behaves like a direct connection and its host publishes an A
+record, so it is the reachable substitute — not a compromise.
+
+Two details that are easy to miss: the pooler changes the username to
+`postgres.<ref>`, and projects sit on NUMBERED clusters (`aws-0-`, `aws-1-`,
+...) whose number is not derivable from the region. Take the host from the
+dashboard's Connect panel, or let scripts/pooler_url.py assemble the URI from
+an env file.
 
 Usage:
     SUPABASE_DB_URL=postgresql://... python -m db.migrate            # apply pending
@@ -120,8 +137,10 @@ def main() -> int:
     db_url = os.environ.get("SUPABASE_DB_URL", "").strip()
     if not db_url:
         print(
-            "ERROR: SUPABASE_DB_URL is not set "
-            "(Supabase → Settings → Database → Connection string → Direct).",
+            "ERROR: SUPABASE_DB_URL is not set (Supabase → Connect → "
+            "Session pooler, port 5432, user postgres.<ref>). The direct "
+            "db.<ref>.supabase.co host is IPv6-only and unreachable from most "
+            "networks; port 6543 is transaction mode and breaks DDL.",
             file=sys.stderr,
         )
         return 1
diff --git a/backend/scripts/migration_drift_report.py b/backend/scripts/migration_drift_report.py
new file mode 100644
index 00000000..4c2010a3
--- /dev/null
+++ b/backend/scripts/migration_drift_report.py
@@ -0,0 +1,204 @@
+"""Read-only drift report between backend/db/migrations/ and a live database.
+
+Answers the question you must answer before applying a backlog of migrations to
+an environment that has been touched outside the repo (#317): **is the ledger
+merely behind, or is it lying?**
+
+    SUPABASE_DB_URL=... python scripts/migration_drift_report.py
+
+Writes nothing. Runs no DDL. Safe against production.
+
+Four sections:
+
+  PENDING   — on disk, not in schema_migrations.
+  ORPHANS   — in schema_migrations, not on disk. Means the environment ran SQL
+              that never existed in this repo (dashboard editor, ad-hoc script).
+              A filename collision here is the dangerous shape: staging having
+              recorded `0032_retire_summer_2026.sql` while the repo's own
+              `0032_rooms_missing_columns.sql` is pending means two different
+              migrations share a number.
+  ALREADY   — objects a PENDING migration would create that ALREADY EXIST.
+              Every row here is the ledger lying: the schema moved without
+              being recorded, so "pending" overstates what will actually run.
+              Migrations written with IF NOT EXISTS will no-op safely; ones
+              without it will fail the whole run.
+  BLOCKERS  — live rows that already violate a UNIQUE index a PENDING migration
+              would create. IF NOT EXISTS cannot save this one: it is a DATA
+              conflict, invisible to a schema-only diff, and it is what turns a
+              clean-looking backlog into a half-applied run.
+
+The object list is parsed from the migration SQL itself (CREATE TABLE / ADD
+COLUMN / CREATE INDEX), so it stays correct as migrations are added — nothing
+to keep in sync by hand.
+
+Exit codes: 2 no SUPABASE_DB_URL, 1 drift found (no ledger, orphans, a
+non-idempotent collision, or a data blocker), 0 clean. Nonzero means "do not
+apply on top of this" — so the report can gate CI directly rather than being
+re-implemented inline, which is how the workflow preflight and this script
+drifted apart in the first place.
+"""
+from __future__ import annotations
+
+import os
+import pathlib
+import re
+import sys
+
+import psycopg
+
+MIGRATIONS = pathlib.Path(__file__).resolve().parent.parent / "db" / "migrations"
+
+RE_TABLE = re.compile(r"create\s+table\s+(?:if\s+not\s+exists\s+)?([a-z0-9_.]+)", re.I)
+RE_COLUMN = re.compile(
+    r"alter\s+table\s+([a-z0-9_.]+)\s+add\s+column\s+(?:if\s+not\s+exists\s+)?([a-z0-9_]+)",
+    re.I,
+)
+RE_INDEX = re.compile(r"create\s+(?:unique\s+)?index\s+(?:if\s+not\s+exists\s+)?([a-z0-9_]+)", re.I)
+
+# A UNIQUE index is the one thing in a pending migration that IF NOT EXISTS
+# cannot make safe: it still fails if the live data already violates it. That
+# is a DATA problem, invisible to a schema-only diff, and it is what turns a
+# clean-looking backlog into a half-applied run. Parsed so the check follows
+# whatever migrations are actually pending.
+RE_UNIQUE_INDEX = re.compile(
+    r"create\s+unique\s+index\s+(?:if\s+not\s+exists\s+)?([a-z0-9_]+)\s+"
+    r"on\s+([a-z0-9_.]+)\s*\(([^)]*)\)(?:\s*where\s+([^;]+))?",
+    re.I | re.S,
+)
+
+
+def bare(name: str) -> str:
+    return name.split(".")[-1]
+
+
+def main() -> int:
+    url = os.getenv("SUPABASE_DB_URL")
+    if not url:
+        print("SUPABASE_DB_URL is not set", file=sys.stderr)
+        return 2
+
+    files = sorted(p.name for p in MIGRATIONS.glob("*.sql"))
+    conn = psycopg.connect(url)
+
+    has_ledger = conn.execute(
+        "SELECT count(*) FROM information_schema.tables WHERE table_name = %s",
+        ("schema_migrations",),
+    ).fetchone()[0]
+    if not has_ledger:
+        print("NO LEDGER — this database has never been migrated by db/migrate.py.")
+        print("Reconcile with `python -m db.migrate --baseline` against a schema you")
+        print("have verified is current, rather than applying.")
+        return 1
+
+    recorded = {
+        r[0] for r in conn.execute("SELECT filename FROM schema_migrations").fetchall()
+    }
+    pending = [f for f in files if f not in recorded]
+    orphans = sorted(recorded - set(files))
+
+    print(f"on disk {len(files)} | recorded {len(recorded)} | pending {len(pending)}\n")
+
+    print("PENDING")
+    for p in pending:
+        print(f"  {p}")
+    if not pending:
+        print("  none")
+
+    print("\nORPHANS (recorded here, absent from the repo)")
+    for o in orphans:
+        clash = [f for f in files if f.split("_")[0] == o.split("_")[0]]
+        note = f"  <-- NUMBER COLLIDES WITH {clash[0]}" if clash else ""
+        print(f"  {o}{note}")
+    if not orphans:
+        print("  none")
+
+    # Which objects the pending migrations would create, that already exist.
+    print("\nALREADY EXISTS (pending migration, object already present)")
+    found_any = False
+    unsafe_already = False
+    for name in pending:
+        sql = (MIGRATIONS / name).read_text()
+        hits: list[str] = []
+
+        for tbl in {bare(t) for t in RE_TABLE.findall(sql)}:
+            n = conn.execute(
+                "SELECT count(*) FROM information_schema.tables WHERE table_name = %s",
+                (tbl,),
+            ).fetchone()[0]
+            if n:
+                hits.append(f"table {tbl}")
+
+        for tbl, col in {(bare(t), c) for t, c in RE_COLUMN.findall(sql)}:
+            n = conn.execute(
+                "SELECT count(*) FROM information_schema.columns "
+                "WHERE table_name = %s AND column_name = %s",
+                (tbl, col),
+            ).fetchone()[0]
+            if n:
+                hits.append(f"column {tbl}.{col}")
+
+        for idx in set(RE_INDEX.findall(sql)):
+            n = conn.execute(
+                "SELECT count(*) FROM pg_indexes WHERE indexname = %s", (idx,)
+            ).fetchone()[0]
+            if n:
+                hits.append(f"index {idx}")
+
+        if hits:
+            found_any = True
+            idempotent = "if not exists" in sql.lower()
+            if not idempotent:
+                unsafe_already = True
+            flag = "safe: uses IF NOT EXISTS" if idempotent else "!! NO IF NOT EXISTS — would fail"
+            print(f"  {name}  ({flag})")
+            for h in sorted(hits):
+                print(f"      {h}")
+    if not found_any:
+        print("  none — the ledger is behind, not lying")
+
+    # Data blockers: a pending UNIQUE index that live rows already violate.
+    print("\nDATA BLOCKERS (pending UNIQUE index vs rows already present)")
+    blocked = False
+    for name in pending:
+        sql = (MIGRATIONS / name).read_text()
+        for idx, table, cols, pred in RE_UNIQUE_INDEX.findall(sql):
+            cols_sql = ", ".join(c.strip() for c in cols.split(","))
+            where = f" WHERE {pred.strip()}" if pred else ""
+            try:
+                dupes = conn.execute(
+                    f"SELECT {cols_sql}, count(*) FROM {bare(table)}{where} "
+                    f"GROUP BY {cols_sql} HAVING count(*) > 1 LIMIT 5"
+                ).fetchall()
+            except Exception as exc:  # table may not exist yet — that's fine
+                conn.rollback()
+                print(f"  {name}: {idx} — could not check ({type(exc).__name__})")
+                continue
+            if dupes:
+                blocked = True
+                print(f"  {name}: {idx} WOULD FAIL — duplicate rows exist:")
+                for d in dupes:
+                    print(f"      {d}")
+    if not blocked:
+        print("  none")
+
+    # Exit nonzero on anything that makes "apply the backlog" unsafe, so this
+    # can gate CI as-is. PENDING alone is not drift — that is the normal state
+    # of an environment that is merely behind.
+    problems = []
+    if orphans:
+        problems.append(f"{len(orphans)} orphan(s) recorded but absent from the repo")
+    if unsafe_already:
+        problems.append("an object already exists for a migration without IF NOT EXISTS")
+    if blocked:
+        problems.append("live rows already violate a pending UNIQUE index")
+    if problems:
+        print("\nDRIFT — do not apply on top of this:")
+        for p in problems:
+            print(f"  {p}")
+        return 1
+
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/backend/scripts/pooler_url.py b/backend/scripts/pooler_url.py
new file mode 100644
index 00000000..edc80c22
--- /dev/null
+++ b/backend/scripts/pooler_url.py
@@ -0,0 +1,91 @@
+"""Build the SESSION-mode pooler connection string for an environment.
+
+Why this exists: `db.<ref>.supabase.co` publishes only an AAAA record, so a
+direct connection is unreachable from any host without global IPv6 — which
+includes this laptop and GitHub-hosted runners. The Supavisor pooler hosts
+publish A records, so they are the reachable path.
+
+    python scripts/pooler_url.py .env.staging aws-1-us-west-2         # print (masked)
+    python scripts/pooler_url.py .env.staging aws-1-us-west-2 --raw   # print usable URI
+
+Pass the pooler host PREFIX, not a bare region: Supabase assigns projects to
+numbered pooler clusters (`aws-0-…`, `aws-1-…`) and the number is not derivable
+from the region — staging is `aws-1-us-west-2`. A bare region is still accepted
+and assumes `aws-0-`, which is a guess; take the prefix from the dashboard's
+Connect panel instead. A wrong prefix fails fast and harmlessly with
+"Tenant or user not found", which is distinguishable from a bad password
+("password authentication failed").
+
+The URI is derived from the password already in the env file, so the secret
+never has to be copied by hand. `--raw` is what you feed to db.migrate; without
+it the password is masked so the value is safe to look at (and to paste into a
+chat or an issue).
+
+SESSION mode is port 5432 — NOT 6543. Transaction mode drops the session-level
+behaviour psycopg and DDL rely on. db/migrate.py's "NOT the pooler" warning is
+about transaction mode and predates the IPv6-only direct endpoint.
+
+The pooler also requires the username to carry the project ref
+(`postgres.<ref>`), which is the detail most easily missed when assembling this
+by hand.
+"""
+from __future__ import annotations
+
+import pathlib
+import re
+import sys
+from urllib.parse import quote, unquote, urlparse
+
+SESSION_PORT = 5432
+
+
+def pooler_host(prefix: str) -> str:
+    """`aws-1-us-west-2` -> full host. A bare region gets the `aws-0-` guess."""
+    if prefix.endswith(".pooler.supabase.com"):
+        return prefix
+    if not prefix.startswith("aws-"):
+        prefix = f"aws-0-{prefix}"
+    return f"{prefix}.pooler.supabase.com"
+
+
+def build(env_file: str, region: str) -> str:
+    lines = pathlib.Path(env_file).read_text().splitlines()
+    matches = [ln for ln in lines if ln.startswith("SUPABASE_DB_URL=")]
+    if not matches:
+        raise SystemExit(f"{env_file}: no SUPABASE_DB_URL")
+    parsed = urlparse(matches[0].split("=", 1)[1].strip())
+    if not parsed.password:
+        raise SystemExit(f"{env_file}: SUPABASE_DB_URL carries no password")
+    host = parsed.hostname or ""
+    # db.<ref>.supabase.co -> <ref>
+    ref = host.split(".")[1] if host.startswith("db.") else host.split(".")[0]
+    # urlparse() hands back the password STILL percent-encoded, so quoting it
+    # again double-escapes: a stored `p%40ss` would go out as `p%2540ss` and
+    # authenticate as the literal text `p%40ss` rather than `p@ss`. Supabase
+    # generates passwords containing reserved characters, so this is reached in
+    # practice — and it fails as "password authentication failed", which reads
+    # like a wrong secret rather than a broken builder. Decode then re-encode;
+    # the round trip is a no-op on an already-correct value.
+    pw = quote(unquote(parsed.password), safe="")
+    return (
+        f"postgresql://postgres.{ref}:{pw}"
+        f"@{pooler_host(region)}:{SESSION_PORT}/postgres"
+    )
+
+
+def mask(uri: str) -> str:
+    return re.sub(r"://([^:]+):[^@]+@", r"://\1:********@", uri)
+
+
+def main() -> int:
+    args = [a for a in sys.argv[1:] if a != "--raw"]
+    if len(args) != 2:
+        print(__doc__)
+        return 2
+    uri = build(args[0], args[1])
+    print(uri if "--raw" in sys.argv[1:] else mask(uri))
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/backend/tests/test_pooler_url.py b/backend/tests/test_pooler_url.py
new file mode 100644
index 00000000..77113d34
--- /dev/null
+++ b/backend/tests/test_pooler_url.py
@@ -0,0 +1,96 @@
+"""Tests for scripts.pooler_url — the pure URI-assembly logic (#508).
+
+The password round trip is the one that matters. `urlparse()` returns the
+password STILL percent-encoded, so re-quoting it double-escapes, and the
+resulting URI authenticates as the literal escape text rather than the real
+password. Supabase generates passwords containing reserved characters, so this
+is reached in practice, and it surfaces as "password authentication failed" —
+indistinguishable from simply having the wrong secret, which is what makes it
+expensive to diagnose.
+"""
+from urllib.parse import urlparse
+
+
+def _env(tmp_path, db_url: str):
+    p = tmp_path / ".env.test"
+    p.write_text(f"SUPABASE_URL=https://ref.supabase.co\nSUPABASE_DB_URL={db_url}\n")
+    return str(p)
+
+
+class TestPasswordEncoding:
+    def test_a_password_with_reserved_characters_survives_the_round_trip(self, tmp_path):
+        """`p@ss/word` is stored percent-encoded and must come back out the same.
+
+        Before the fix this emitted `p%2540ss%252Fword` — the escapes escaped.
+        """
+        from scripts.pooler_url import build
+
+        env = _env(tmp_path, "postgresql://postgres:p%40ss%2Fword@db.abcd.supabase.co:5432/postgres")
+        uri = build(env, "aws-1-us-west-2")
+
+        assert "p%40ss%2Fword" in uri
+        assert "%25" not in uri, "password was double-encoded"
+        assert urlparse(uri).password == "p%40ss%2Fword"
+
+    def test_a_plain_password_is_unchanged(self, tmp_path):
+        from scripts.pooler_url import build
+
+        env = _env(tmp_path, "postgresql://postgres:simplepw@db.abcd.supabase.co:5432/postgres")
+
+        assert "simplepw" in build(env, "aws-1-us-west-2")
+
+
+class TestUriShape:
+    def test_the_username_carries_the_project_ref(self, tmp_path):
+        """The pooler rejects a bare `postgres` user — this is the detail most
+        easily missed when assembling the URI by hand."""
+        from scripts.pooler_url import build
+
+        env = _env(tmp_path, "postgresql://postgres:pw@db.abcd.supabase.co:5432/postgres")
+
+        assert build(env, "aws-1-us-west-2").startswith("postgresql://postgres.abcd:")
+
+    def test_session_mode_port(self, tmp_path):
+        """5432, never 6543 — transaction mode drops the session-level
+        behaviour psycopg and DDL rely on."""
+        from scripts.pooler_url import build
+
+        env = _env(tmp_path, "postgresql://postgres:pw@db.abcd.supabase.co:5432/postgres")
+        uri = build(env, "aws-1-us-west-2")
+
+        assert ":5432/postgres" in uri
+        assert "6543" not in uri
+
+
+class TestPoolerHost:
+    def test_an_explicit_cluster_prefix_is_preserved(self):
+        """The cluster number is not derivable from the region, so an explicit
+        prefix must survive untouched."""
+        from scripts.pooler_url import pooler_host
+
+        assert pooler_host("aws-1-us-west-2") == "aws-1-us-west-2.pooler.supabase.com"
+
+    def test_a_bare_region_assumes_cluster_zero(self):
+        """Documented as a guess, not a derivation — kept so a bare region is
+        still usable, but it is why the dashboard is the real source."""
+        from scripts.pooler_url import pooler_host
+
+        assert pooler_host("us-west-2") == "aws-0-us-west-2.pooler.supabase.com"
+
+    def test_a_full_host_passes_through(self):
+        from scripts.pooler_url import pooler_host
+
+        host = "aws-1-us-west-2.pooler.supabase.com"
+        assert pooler_host(host) == host
+
+
+class TestMask:
+    def test_mask_hides_the_password(self):
+        """The default output is meant to be safe to paste into a chat or an
+        issue, so the masking has to actually hold."""
+        from scripts.pooler_url import mask
+
+        masked = mask("postgresql://postgres.abcd:sup3rs3cret@aws-1-us-west-2.pooler.supabase.com:5432/postgres")
+
+        assert "sup3rs3cret" not in masked
+        assert "postgres.abcd" in masked
diff --git a/docs/staging/setup-checklist.md b/docs/staging/setup-checklist.md
index 4134426d..be605314 100644
--- a/docs/staging/setup-checklist.md
+++ b/docs/staging/setup-checklist.md
@@ -16,11 +16,14 @@ variables, Cloudflare worker secrets, or a local gitignored `backend/.env.stagin
 - [ ] Create a new project `sapling-staging` (same region as prod).
 - [ ] Copy **Settings → API → Project URL** → `SUPABASE_URL`.
 - [ ] Copy **Settings → API → service_role key** → `SUPABASE_SERVICE_KEY` (backend only — never the frontend).
-- [ ] Copy **Settings → Database → Connection string → "Direct connection" (port 5432, not the pooler)** → `SUPABASE_DB_URL`.
+- [ ] Copy **Connect → "Session pooler" (port 5432)** → `SUPABASE_DB_URL`. Not the
+      "Direct connection" — that host is IPv6-only and unreachable from most networks
+      and from GitHub runners. Not port 6543 either (transaction mode breaks DDL).
+      Note the pooler username carries the project ref: `postgres.<ref>`.
 - [ ] Generate a staging encryption key → `ENCRYPTION_KEY`:
       `python -c "import secrets; print(secrets.token_hex(32))"`
 - [ ] Create storage buckets `avatars` and `cosmetic-assets` (match prod).
-- [ ] Once you have the keys locally (Step 6), apply schema: `SUPABASE_DB_URL=<direct> python -m db.migrate`.
+- [ ] Once you have the keys locally (Step 6), apply schema: `SUPABASE_DB_URL=<session-pooler> python -m db.migrate`.
 
 ### Step 2 — Google Cloud (OAuth) — you can do this NOW
 - [ ] Create OAuth client "Sapling Staging" (Web application).