M2 decision: DynamicProxyGenAssembly2 InternalsVisibleTo — keep as recorded mechanism or replace the seam (N1) #25

Source: multi-agent review of PR #16, UNVERIFIED-FINDER. 2026-08-13. M-anchor: M2 (decide once, cheap either way). Maintainer decision — recommendation below is to close as-is.

src/OpenCode.Sdk/Properties/AssemblyInfo.cs:5-6 declares [assembly: InternalsVisibleTo("DynamicProxyGenAssembly2")] so PipelineTests can Substitute.For<IEnvironmentProvider>() (internal seam). The review notes AGENTS.md's "fake only published contracts" posture and that the friend grant exposes all internals to any DynamicProxy user in-process.

Recommendation: keep, close. This is a solved-once, deliberately recorded mechanism in this repo (NSubstitute over internal seams requires the DynamicProxy friend; paired with PolyUseEmbeddedAttribute=true to avoid CS0121 in friend assemblies). Reopening a recorded decision needs new evidence, and "a hostile in-process DynamicProxy user" is not a threat model for an SDK consumer process. If the maintainer prefers encapsulation purity anyway, the alternative is a hand-written internal fake for IEnvironmentProvider (named test collaborator), dropping the friend attribute — small, but trades away NSubstitute ergonomics for every future internal seam. Do not remove the attribute before an approved replacement seam exists (verifier's own caveat).