Make agent-callable writes idempotent, or lose data

MCP server write tools exposed to agents must assume at-least-once delivery because retries, timeouts, and parallel calls can repeat actions.

Servers should enforce idempotency with client-generated keys, atomic deduplication, and cached results so duplicate requests do not create duplicate side effects.

Without this, repeated agent writes can cause incorrect financial records, phantom invoices, and other data integrity failures.

The core lesson: agent-native write operations need safe retries and recoverable errors by default.