What are we doing? Determining which tenant a request belongs to, right after authentication. Why? Every downstream decision — what data this request can access, what quota applies — depends on knowing the tenant first.

def handle_request(request):
    user = authenticate(request)
    tenant_id = resolve_tenant(user)
    return process(request, tenant_id)

What are we doing? Filtering every retrieval query by the resolved tenant. Why? A model has no inherent concept of tenant boundaries — isolation has to be enforced by the retrieval layer, not left to the model's judgment.

def retrieve(query, tenant_id):
    return vector_store.search(query, filter={"tenant": tenant_id})

Most systems start with logical isolation and move specific tenants to physical isolation only when a contractual or regulatory requirement demands it.

What are we doing? Limiting usage per tenant, not just globally. Why? Without this, one tenant's heavy usage can degrade latency or exhaust shared quota for every other tenant — a noisy-neighbor problem.

What are we doing? Recording token usage and cost attributed to each tenant. Why? Without this, cost can't be allocated, billed, or investigated per tenant — it just shows up as one combined number.

• Relying on the model to keep tenants separate

  A model has no inherent concept of tenant boundaries — isolation must be enforced by the application.

• Forgetting tenant scoping on a shared cache

  A cache key without a tenant identifier can serve one tenant's cached response to another.

• No per-tenant rate limiting

  One tenant can degrade service or dominate cost for everyone else on shared infrastructure.

• Inconsistent metadata tagging at ingestion

  If tenant metadata isn't reliably attached when data is ingested, filtering at query time becomes unreliable.

• Assuming logical isolation is sufficient for every tenant

  Some tenants may require physical isolation for regulatory or contractual reasons.