When --cap-drop ALL Broke the Gate Socket

The dogfood run went green. The gate had governed zero calls. That is the agent-governance-plane's entire job: run an AI coding agent inside a sandbox, route every tool call through a Unix-domain-socket gateway, and write a signed, hash-chained journal of every allow/deny. A green run that gated nothing isn't a pass. It's a governance plane governing air. The gate that catches its own hollowness AGP's CI dogfood doesn't just check that the harness exits 0. evidence-bundle.sh fails on a 0-gated run — if the journal shows no decisions, the build is red regardless of process exit status. That guard is what surfaced this at all: the agent process came up, the harness reported success, but the bundle had no verdicts to verify. Red. That's the last I'll say about hollow-green detection here. It's the door, not the room. The room is why zero calls reached the gate, and the answer turned out to be a collision between two things that look unrelated until you trace the syscall: Linux capabilities and a Unix socket's permission bits. The wrong theory The first hypothesis blamed the execution path. AGP has a dev-sandbox mode where the agent and the gate share a process, and a docker mode where the agent runs in a container talking to a host daemon over a bind-mounted socket. The theory was that the same-process path was short-circuiting the gate — agent and gate in one address space, the socket round-trip optimized away, decisions never journaled. Plausible. Wrong. The dev-sandbox path journaled fine in isolation. The failure only appeared in docker mode, and the moment that became clear the investigation moved from "which code path" to "what's different about the container." What's different about the container is the security posture. The real root cause: caps meet a missing write bit The short version: connecting to a Unix domain socket needs write permission on the socket file. --cap-drop ALL strips CAP_DAC_OVERRIDE — the capability that lets root ignore permission bits — s