Vendor lock-in at LLM scale is less about contracts and more about architecture.

In practice, teams that avoid lock-in usually do a few concrete things:

1. Introduce a model abstraction layer
Never call a provider API directly from business logic. Route all model calls through an internal service that standardizes prompts, parameters, and response schemas. This makes swapping providers operationally feasible.

2. Separate prompts from application code
Store prompts in version-controlled configs or a prompt registry. If prompts are tightly embedded in code tuned to one provider’s quirks, migration becomes painful.

3. Maintain cross-model evaluation benchmarks
Build internal eval datasets and scoring pipelines. Regularly test outputs across providers. If you can’t measure equivalence, you can’t switch confidently.

4. Keep retrieval and orchestration provider-agnostic
In RAG or agent systems, ensure your vector store, embedding layer, and orchestration framework are not tightly coupled to one vendor’s ecosystem.

5. Design for fallback from day one
Even if you use one provider initially, structure your system so a second model can be activated with configuration changes, not rewrites.

The key mindset shift is this:
Treat the LLM as a replaceable component, not the foundation of your architecture.

Lock-in happens when convenience overrides design discipline. Flexibility happens when portability is built into the system from the start.