Start with one data flow.

// Local learning example — Node.js 18+; no packages or API calls.
// Fixed fictional mappings illustrate the boundary, not cryptography.
const source = {
  customer: "Alex Morgan",
  email: "alex@example.com",
  plan: "Business",
  request: "Help me change my plan"
};
const mappings = {
  customer: "person_7f2a",
  email: "email_9b4c"
};
const modelContext = {
  customer: mappings.customer,
  email: mappings.email,
  plan: source.plan,
  request: source.request
};
if (JSON.stringify(modelContext).includes(source.email)) {
  throw new Error("Original email crossed the example boundary");
}
console.log(JSON.stringify(modelContext, null, 2));
// Production: replace fixture mappings with a provisioned vault,
// authenticate callers and enforce retrieval permissions server-side.

Run with node boundary-example.mjs. The fixed fixture is fully local; it does not tokenize real data or call Provyn.