diff --git a/modules/billing/migrations/20260511224500-grant-signup-credits-to-existing-free-orgs.js b/modules/billing/migrations/20260511224500-grant-signup-credits-to-existing-free-orgs.js
new file mode 100644
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--- /dev/null
+++ b/modules/billing/migrations/20260511224500-grant-signup-credits-to-existing-free-orgs.js
@@ -0,0 +1,118 @@
+/**
+ * Migration: Backfill 500 signup-grant credits to existing Free-plan orgs.
+ *
+ * Credits every organization currently on the Free plan (plan === 'free')
+ * that has not been canceled (status !== 'canceled') and does not already hold
+ * a 'signup_grant' ledger entry, mirroring what new Free signups receive via
+ * BillingSignupGrantService (N2 — one-shot 500-compute grant).
+ *
+ * Idempotent: the synthetic idempotency key `signup_grant-<orgId>` stored as
+ * `ledger[].refId` is the same key `creditGrant` uses at signup time.
+ * Running this migration a second time skips every org (all are already credited).
+ *
+ * Safe to run while the app is live: each updateOne is atomic (single-document
+ * write) and the migration runner serialises execution via a DB-level claim so
+ * only one pod runs this at startup even in multi-replica deploys.
+ *
+ * down() removes all signup_grant ledger entries.  Note: cachedBalance is NOT
+ * adjusted on rollback — run a reconcile sweep if down() is invoked in prod.
+ */
+import mongoose from 'mongoose';
+
+const SIGNUP_GRANT_AMOUNT = 500;
+const GRANT_SOURCE = 'signup_grant';
+
+/**
+ * @returns {Promise<void>}
+ */
+export async function up() {
+  const db = mongoose.connection.db;
+  const subscriptions = db.collection('subscriptions');
+  const extraBalances = db.collection('billingextrabalances');
+
+  const cursor = subscriptions.find(
+    { plan: 'free', status: { $ne: 'canceled' } },
+    { projection: { organization: 1 } },
+  );
+
+  let granted = 0;
+  let skipped = 0;
+
+  for await (const sub of cursor) {
+    const orgId = sub.organization;
+    if (!orgId) { skipped += 1; continue; }
+
+    const idempotencyKey = `${GRANT_SOURCE}-${orgId.toString()}`;
+
+    // Skip if this org already has a signup_grant entry (idempotent re-run).
+    const existing = await extraBalances.findOne(
+      { organization: orgId, 'ledger.refId': idempotencyKey },
+      { projection: { _id: 1 } },
+    );
+    if (existing) { skipped += 1; continue; }
+
+    // Step 1: ensure the ExtraBalance document exists (no-op if already present).
+    await extraBalances.updateOne(
+      { organization: orgId },
+      {
+        $setOnInsert: {
+          organization: orgId,
+          ledger: [],
+          cachedBalance: 0,
+          cachedBalanceAt: new Date(),
+          createdAt: new Date(),
+          updatedAt: new Date(),
+        },
+      },
+      { upsert: true },
+    );
+
+    // Step 2: push the grant entry (idempotency-guarded, no upsert).
+    const result = await extraBalances.updateOne(
+      { organization: orgId, 'ledger.refId': { $ne: idempotencyKey } },
+      {
+        $push: {
+          ledger: {
+            kind: 'topup',
+            amount: SIGNUP_GRANT_AMOUNT,
+            source: GRANT_SOURCE,
+            refId: idempotencyKey,
+            at: new Date(),
+          },
+        },
+        $inc: { cachedBalance: SIGNUP_GRANT_AMOUNT },
+        $set: { cachedBalanceAt: new Date(), updatedAt: new Date() },
+      },
+    );
+
+    if (result.modifiedCount > 0) {
+      granted += 1;
+    } else {
+      // Another concurrent writer beat us to this org — harmless race, already credited.
+      skipped += 1;
+    }
+  }
+
+  console.info(`[migration] grant-backfill: complete — granted=${granted} skipped=${skipped}`);
+}
+
+/**
+ * Reverse: remove all signup_grant ledger entries.
+ *
+ * WARNING: cachedBalance is NOT adjusted. If down() is applied to a live
+ * database, run a cachedBalance reconcile sweep afterwards.
+ *
+ * @returns {Promise<void>}
+ */
+export async function down() {
+  const db = mongoose.connection.db;
+
+  const result = await db.collection('billingextrabalances').updateMany(
+    { 'ledger.source': GRANT_SOURCE },
+    { $pull: { ledger: { source: GRANT_SOURCE } } },
+  );
+
+  console.warn(
+    `[migration] grant-backfill DOWN: removed signup_grant entries from ${result.modifiedCount} docs. cachedBalance NOT adjusted — run reconcile if applied in prod.`,
+  );
+}