Migrating from the Assistants API to the Responses API

A migration is safest when it preserves user-visible behavior before it adopts every new capability. Inventory the current assistant instructions, stored files, thread state, function calls, streaming events, and operational dashboards. That inventory becomes the compatibility contract for the new implementation.

Map concepts before touching production

Document how each existing object is used rather than assuming names map one-to-one. Conversation state may move into application-owned storage, built-in tools may replace custom wrappers, and tracing can become part of the platform. The goal is to preserve outcomes and auditability, not old abstractions.

• Record every assistant instruction, tool schema, file dependency, and thread lifecycle
• Build golden conversations with expected tool calls and final outputs
• Create an adapter so old and new implementations share the same product interface
• Run shadow traffic and compare quality, latency, cost, and failure recovery
• Migrate stored state with a reversible cutover and a defined rollback window

Do not migrate blind spots

Use the transition to add trace IDs, structured tool results, retry policies, and evaluations. If the old system could fail without showing why, copying that behavior exactly only preserves technical debt.

Retire the compatibility layer deliberately

Keep the adapter only until traffic, state, and support paths have moved. Then remove it, update runbooks, and freeze the migration test set as a regression suite for future model and tool changes.