diff --git a/packages/server/src/utils/inviteService.test.ts b/packages/server/src/utils/inviteService.test.ts
index 569a98b6..b121bf16 100644
--- a/packages/server/src/utils/inviteService.test.ts
+++ b/packages/server/src/utils/inviteService.test.ts
@@ -531,13 +531,13 @@ describe('redeemInvite', () => {
     expect(redemptions).toHaveLength(0);
   });
 
-  it('aborts transaction (no usedCount increment, no redemption row) if insertUser throws', () => {
+  it('does not bump usedCount or write a redemption when insertUser throws synchronously', () => {
     const adminId = seedAdmin();
     const inv = createInvite({ name: 'a', maxUses: 5, expiresAt: null }, adminId);
 
     expect(() => redeemInvite(inv.token, () => {
-      throw new Error('username collision');
-    })).toThrow('username collision');
+      throw new Error('username taken');
+    })).toThrow('username taken');
 
     // usedCount unchanged
     const row = testDb.select().from(schema.inviteLinks).where(eq(schema.inviteLinks.id, inv.id)).get();
@@ -548,6 +548,34 @@ describe('redeemInvite', () => {
     expect(redemptions).toHaveLength(0);
   });
 
+  it('rolls back insertUser writes when a later step throws (true SQLite ROLLBACK)', () => {
+    const adminId = seedAdmin();
+    const inv = createInvite({ name: 'a', maxUses: 5, expiresAt: null }, adminId);
+    const newUserId = 'rollback-user-1';
+
+    // The callback writes a real users row, then throws AFTER the write.
+    // If the txn truly rolls back, the users row must not exist after the call.
+    expect(() => redeemInvite(inv.token, () => {
+      testDb.insert(schema.users).values({
+        id: newUserId,
+        username: 'will-be-rolled-back',
+        passwordHash: 'x',
+        createdAt: Date.now(),
+      }).run();
+      throw new Error('post-insert failure');
+    })).toThrow('post-insert failure');
+
+    // Proves SQLite ROLLBACK reverted the user insert AND the would-be usedCount bump
+    const u = testDb.select().from(schema.users).where(eq(schema.users.id, newUserId)).get();
+    expect(u).toBeUndefined();
+
+    const after = testDb.select().from(schema.inviteLinks).where(eq(schema.inviteLinks.id, inv.id)).get();
+    expect(after?.usedCount).toBe(0);
+
+    const redemptions = testDb.select().from(schema.inviteRedemptions).where(eq(schema.inviteRedemptions.inviteId, inv.id)).all();
+    expect(redemptions).toHaveLength(0);
+  });
+
   it('throws InviteUnavailableError when token not found', () => {
     expect(() => redeemInvite('aaaaaaaaaaaaaaaaaaaaaa', () => ({ id: 'x', username: 'x' }))).toThrow(InviteUnavailableError);
   });
diff --git a/packages/server/src/utils/inviteService.ts b/packages/server/src/utils/inviteService.ts
index 3ef0541c..6130a800 100644
--- a/packages/server/src/utils/inviteService.ts
+++ b/packages/server/src/utils/inviteService.ts
@@ -468,14 +468,23 @@ export function reinstateInvite(id: string, req: ReinstateInviteRequest): Reinst
   });
 }
 
+/**
+ * Discriminant union of reasons an invite cannot be redeemed. Surfaced as a
+ * typed public field on `InviteUnavailableError` so the HTTP register route
+ * can switch on it to produce user-facing copy without parsing the message
+ * string. Mirrors the non-active subset of `InviteStatus` plus `'not found'`
+ * for the missing-token case.
+ */
+export type InviteUnavailableReason = 'not found' | 'revoked' | 'expired' | 'exhausted';
+
 /**
  * Thrown when an invite cannot be redeemed because its current state forbids
  * it (token not found, revoked, expired, exhausted). Caller (HTTP register
- * route) maps this to 403 Forbidden. The reason string is preserved in the
- * message for debugging — the user-facing copy is constructed by the route.
+ * route) maps this to 403 Forbidden. The `reason` field is the structured
+ * discriminant; the message string is preserved for debugging/logging.
  */
 export class InviteUnavailableError extends Error {
-  constructor(reason: string) {
+  constructor(public readonly reason: InviteUnavailableReason) {
     super(`Invite unavailable: ${reason}`);
     this.name = 'InviteUnavailableError';
   }
@@ -513,6 +522,9 @@ export function redeemInvite(
     if (!row) throw new InviteUnavailableError('not found');
     const status = inviteStatus(row);
     if (status !== 'active') {
+      // 'active' is excluded by the guard above, so `status` is necessarily
+      // one of 'revoked' | 'expired' | 'exhausted' — all valid
+      // InviteUnavailableReason values. TS narrows the union here.
       throw new InviteUnavailableError(status);
     }