The agent builds the cart, but checkout re-prices every line from the catalog server-side and re-validates the voucher — a compromised client can't change a single price.
Build notes
Computed upsell, not prompted
A pure module (14 tests) works out the exact gap to a voucher and the cheapest add-on that closes it; the LLM only phrases the sentence. Added after a prompt-only nudge silently failed.
Idempotent checkout
Racing double-submits collapse into one order via a partial unique index on the idempotency key — no double charge, no double loyalty redemption.
The code-switching fix
Mid-sentence Vietnamese/English switching was fixed with two fully monolingual system prompts behind a pinned language toggle — a documented bug-and-fix, not a guess.
Spend guard
Every model call is priced in micro-USD and cut off at $20/day, reset on Vietnam time.
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