Agent-friendly tests: implementation-bound white-box suites become cement under coding agents

Why it matters

Chinese-circle claim — @intentplex (https://x.com/i/status/2102297780244877687) says a hire from a big-tech testing background wrote 4000+ unit tests with an all-green merge gate; almost all were implementation-bound white-box tests with heavy mocks. In the AI era, they argue, such suites are mostly liabilities: coding agents such as Codex, when asked to refactor, prioritize keeping stale tests green, refuse to delete old interfaces, and pile compatibility layers into a Frankenstein; after the feature is done they may spend another hour “fixing tests,” pouring cement over an implementation that should stay malleable. What should be protected, they say, is external behavior, API contracts, business invariants, integration and E2E—not historical call paths; large implementation-coupled white-box suites should be removed from agent-native engineering. Verifiable engineering commons — assertions tied to private paths, function names, and mock interaction graphs freeze today’s shape; contract tests, property/invariant tests, golden fixtures, and end-to-end acceptance constrain observable behavior and public contracts. An all-green merge gate without failure semantics or a behavior-coverage bar incentivizes agents (and humans) to buy green bars with compatibility shims instead of converging design. This tension is not tied to one vendor or one agent product. Independent conclusion — when an agent refactors, implementation-bound green tests become cement. Constraints on the path to reliable coding agents are not only stronger models, but whether the test asset is agent-friendly: contracts, properties, and golden examples over mocks of implementation detail. Practice: shift CI weight from mock-path greens to public contracts + business invariants + golden regressions; delete implementation unit tests when old interfaces go; treat agent-generated implementation-bound tests as temporary scaffolding, not long-lived merge gates.