The A/B has its answer. Across four matched production runs — same recipe, same task, same three offered skills — the MCP-door arm retrieved 1 in 9 and the file arm retrieved 3 of 3, with the judge loop closing on the same run (01a098dd). A signal, not a rate; but 0, 1, 0 → 3 on an otherwise identical task is not noise, and the mechanism is explained rather than guessed: the door is a deferred tool the agents never load, and Read is not. A code default and not CLAWMATES_SKILL_DELIVERY on one server, for the reason always_inject moved into the skill files: a setting that exists only in one deployment is a setting nobody can find. The env var still overrides, and `index` and `inline` stay selectable per mission so the comparison remains runnable against one binary. Garbage in the env var still falls to `inline`, not to the default — an unreadable value must not silently select an arm that needs something installed. A test pins the default so the next change to it is a decision made with the numbers in front of you, not a slip. Co-Authored-By: Claude Opus 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01WZb5A2kfVfjpdwSochkuHz