Customer (tenant) ID for the request. This is separate from authentication: you still pass your personal or service account API token in the Authorization header (Bearer <token>). See Get Started.

When to omit (most callers): If your personal or service account token belongs to a single customer, omit this header. The API resolves that customer from the token.

When to send: If your credential can access more than one customer, set X-Tenant-Id to the customer ID you want to act on. Omitting it returns 400 with code tenant_id_required. If the value conflicts with the tenants your credential may access, the request returns 400 with code tenant_id_mismatch. Prefer this header over the legacy customerContext query parameter, which only applies to legacy API keys and is ignored by personal and service account tokens.