Version Pinning and Upgrade Strategy

Routing decides which pinned model bundle a request reaches. It says nothing about what happens when the model behind that bundle changes on its own schedule, not the organization’s. A model version is a behavioral dependency whose outputs can shift even when application code stays constant. An application version is the controlled bundle of code and configuration released through the normal software pipeline. Pinning exists because a non-deterministic system can drift under the same inputs, and because an upstream change outside the organization can alter behavior without a code diff. Treat a model change as a governed release, with explicit evidence and a rollback path.

Version pinning as an architectural commitment

Use version pinning to define the exact set of runtime dependencies whose change should be visible, reviewed, and attributable. For a copilot under quarterly refresh pressure and occasional provider-driven deprecations, pin model_id or model_version, decoding parameters like temperature, top_p, and max_output_tokens, a system_prompt_version or prompt-template hash, retrieval settings such as retriever_version, index_snapshot_id, embedding_model_id, and tool contracts like a tool_schema_version for structured outputs. ...