diff --git a/scripts/diag-water-pin.mjs b/scripts/diag-water-pin.mjs
new file mode 100644
index 0000000..835317d
--- /dev/null
+++ b/scripts/diag-water-pin.mjs
@@ -0,0 +1,158 @@
+#!/usr/bin/env node
+/**
+ * diag-water-pin.mjs — diagnostic script for the "PIN only works
+ * once" bug in the water log module.
+ *
+ * What it does:
+ *   1. Snapshots every row in `water_irrigators` and `water_sessions`.
+ *   2. For each irrigator, verifies the stored `pin_hash` format is
+ *      a parseable `scrypt$N$r$p$salt$hash` string.
+ *   3. Cross-checks: any session whose `expires_at < now()` would
+ *      fail `requireFieldSession` but should NOT affect `verifyWaterPin`
+ *      (verifyWaterPin re-issues a session on success).
+ *   4. Optionally tests a provided PIN against a provided pin_hash to
+ *      see if the cryptographic verify matches.
+ *
+ * Usage:
+ *   DATABASE_URL="postgresql://..." node scripts/diag-water-pin.mjs
+ *   DATABASE_URL="..." \
+ *     node scripts/diag-water-pin.mjs --pin=4739 --id=<irrigator-uuid>
+ *
+ * Run this BEFORE and AFTER a failing second-login attempt to confirm
+ * whether `pin_hash` is being modified mid-session.
+ */
+import "dotenv/config";
+import { Pool } from "pg";
+import { scryptSync, timingSafeEqual } from "node:crypto";
+
+const args = Object.fromEntries(
+  process.argv.slice(2).map((a) => {
+    const m = a.match(/^--([^=]+)(?:=(.*))?$/);
+    return m ? [m[1], m[2] ?? true] : [a, true];
+  }),
+);
+
+const pin = typeof args.pin === "string" ? args.pin : null;
+const targetId = typeof args.id === "string" ? args.id : null;
+
+if (!process.env.DATABASE_URL) {
+  console.error("DATABASE_URL is not set");
+  process.exit(1);
+}
+
+const pool = new Pool({
+  connectionString: process.env.DATABASE_URL,
+  ssl: process.env.DATABASE_URL.includes("sslmode")
+    ? { rejectUnauthorized: false }
+    : undefined,
+});
+
+function verifyPin(rawPin, stored) {
+  if (!stored || !stored.startsWith("scrypt$")) return false;
+  const parts = stored.split("$");
+  if (parts.length !== 6) return false;
+  const [, nStr, rStr, pStr, saltB64, hashB64] = parts;
+  const n = parseInt(nStr, 10),
+    r = parseInt(rStr, 10),
+    p = parseInt(pStr, 10);
+  if (![n, r, p].every(Number.isFinite)) return false;
+  let salt, expected;
+  try {
+    salt = Buffer.from(saltB64, "base64");
+    expected = Buffer.from(hashB64, "base64");
+  } catch {
+    return false;
+  }
+  if (expected.length !== 32) return false;
+  let candidate;
+  try {
+    candidate = scryptSync(rawPin.normalize("NFKC"), salt, 32, {
+      N: n,
+      r,
+      p,
+    });
+  } catch {
+    return false;
+  }
+  return (
+    candidate.length === expected.length &&
+    timingSafeEqual(candidate, expected)
+  );
+}
+
+function truncate(s, n = 60) {
+  return s && s.length > n ? s.slice(0, n) + "..." : s;
+}
+
+async function main() {
+  const client = await pool.connect();
+  try {
+    console.log("=== SNAPSHOT @ " + new Date().toISOString() + " ===\n");
+
+    // Bypass RLS via withPlatformAdmin equivalent (raw app vars).
+    await client.query("BEGIN");
+    await client.query(
+      "SELECT set_config('app.current_brand_id', '', true)",
+    );
+    await client.query(
+      "SELECT set_config('app.platform_admin', 'true', true)",
+    );
+
+    const irrigators = await client.query(
+      "SELECT id, brand_id, name, role, active, " +
+        "substr(pin_hash, 1, 40) || '...' AS pin_hash_preview, " +
+        "length(pin_hash) AS pin_hash_len, last_used_at, created_at " +
+        "FROM water_irrigators ORDER BY name",
+    );
+
+    console.log("water_irrigators (" + irrigators.rowCount + " rows):");
+    for (const r of irrigators.rows) {
+      console.log(`  • [${r.id}] ${r.name}`);
+      console.log(`      brand=${r.brand_id}`);
+      console.log(`      role=${r.role}  active=${r.active}`);
+      console.log(`      pin_hash_preview="${r.pin_hash_preview}" (len=${r.pin_hash_len})`);
+      console.log(`      last_used_at=${r.last_used_at}`);
+
+      // Format check
+      const fullRow = await client.query(
+        "SELECT pin_hash FROM water_irrigators WHERE id = $1",
+        [r.id],
+      );
+      const ph = fullRow.rows[0]?.pin_hash;
+      if (!ph || !ph.startsWith("scrypt$") || ph.split("$").length !== 6) {
+        console.log(`      ⚠️  pin_hash has UNEXPECTED format`);
+      } else {
+        console.log(`      ✓ pin_hash is well-formed scrypt string`);
+      }
+
+      // Targeted PIN verify
+      if (pin && targetId === r.id) {
+        const ok = verifyPin(pin, ph);
+        console.log(`      ${ok ? "✓" : "✗"} pin '${pin}' matches pin_hash for ${r.name} → ${ok}`);
+      }
+      console.log();
+    }
+
+    const sessions = await client.query(
+      "SELECT id, irrigator_id, expires_at, created_at FROM water_sessions ORDER BY created_at DESC LIMIT 50",
+    );
+    console.log("\nwater_sessions (" + sessions.rowCount + " recent rows):");
+    for (const s of sessions.rows) {
+      const expired = new Date(s.expires_at).getTime() < Date.now();
+      const irrigator = irrigators.rows.find((i) => i.id === s.irrigator_id);
+      console.log(
+        `  • [${s.id}]  irrigator=${s.irrigator_id} (${irrigator?.name ?? "?"})  expires_at=${s.expires_at.toISOString()} ${expired ? "(EXPIRED)" : "(active)"}`,
+      );
+    }
+
+    await client.query("COMMIT");
+  } finally {
+    client.release();
+    await pool.end();
+  }
+}
+
+main().catch((err) => {
+  console.error("error:", err.message);
+  process.exit(1);
+});
diff --git a/src/actions/water-log/field.ts b/src/actions/water-log/field.ts
index c4fed79..9819b3c 100644
--- a/src/actions/water-log/field.ts
+++ b/src/actions/water-log/field.ts
@@ -144,32 +144,40 @@ export async function verifyWaterPin(
   }
 
   // Create a session in the user's brand context. The session insert
-  // and the last_used_at bump are independent writes — fire them in
-  // parallel and wait on both before returning.
-  const [sessionId, cookieStore] = await Promise.all([
-    withBrand(match.brandId, async (db) => {
-      const expiresAt = new Date(
-        Date.now() + FIELD_SESSION_HOURS * 60 * 60 * 1000,
-      );
-      const [insertResult] = await Promise.all([
-        db
-          .insert(waterSessions)
-          .values({
-            irrigatorId: match.id,
-            expiresAt,
-          })
-          .returning({ id: waterSessions.id }),
-        db
-          .update(waterIrrigators)
-          .set({ lastUsedAt: new Date() })
-          .where(eq(waterIrrigators.id, match.id)),
-      ]);
-      const row = insertResult[0];
-      if (!row) throw new Error("Failed to create session");
-      return row.id;
-    }),
-    cookies(),
-  ]);
+  // and the last_used_at bump run *sequentially* on the same pg
+  // connection inside the `withBrand` transaction — never concurrently.
+  //
+  // The previous shape used a nested `Promise.all([insert, update])`
+  // *and* an outer `Promise.all([withBrand, cookies()])`. That was
+  // known-fragile per the codebase's own commit `ca79896`: "cookies()
+  // corrupts Next.js's request-scoped AsyncLocalStorage if called from
+  // inside a pg transaction." The admin portal has been on the
+  // sequential shape (see `src/lib/water-admin-pin-auth.ts`) since
+  // that commit; this brings the field portal in line.
+  const sessionId = await withBrand(match.brandId, async (db) => {
+    const expiresAt = new Date(
+      Date.now() + FIELD_SESSION_HOURS * 60 * 60 * 1000,
+    );
+    const [inserted] = await db
+      .insert(waterSessions)
+      .values({
+        irrigatorId: match.id,
+        expiresAt,
+      })
+      .returning({ id: waterSessions.id });
+    if (!inserted) throw new Error("Failed to create session");
+
+    await db
+      .update(waterIrrigators)
+      .set({ lastUsedAt: new Date() })
+      .where(eq(waterIrrigators.id, match.id));
+
+    return inserted.id;
+  });
+
+  // Cookie set is fully outside the DB transaction. Same rule as
+  // `verifyAdminPin` (see comment in `src/lib/water-admin-pin-auth.ts`).
+  const cookieStore = await cookies();
   cookieStore.set("wl_session", sessionId, {
     httpOnly: true,
     secure: process.env.NODE_ENV === "production",