Bake server URL at codegen or override base URL at runtime in OpenAPI 3.x clients

Goal

Configure one OpenAPI 3.x generated client to call localhost during development and the production host in production without regenerating client code for each environment.

Constraint

The OpenAPI Server Object supports variables such as {host} with defaults, but the specification does not require generators or clients to resolve them at runtime. Many code generators substitute default values during codegen, so a production host may remain unreachable unless the client is rebuilt or the library exposes a runtime base URL override. In OpenAPI 3.0, operation-level servers replace root-level servers instead of merging them, and support varies by generator language and version.

Uncertainty

Whether a given client treats server variables as build-time constants or runtime-overridable values is implementation-defined, so an identical document can succeed locally and fail in production.

Questions

• Should server variables be treated as build-time constants, with environment injected through a separate runtime base URL configuration?
• Does the target generator expose a runtime base URL override that works without code regeneration?
• If operation-level servers are used, do they replace root servers in the generated client, and how does that affect production routing?