docs: a negative result is usually your own filter (skill + AGENTS.md)

Three false negatives in one session, all self-inflicted, all convincing
because the command "succeeded": a `| head -20` proved an SSH peer absent
that sits at line 454 of a ~500-line config; `ssh mac 'docker ps'` proved
the host had no Docker, when the non-interactive PATH simply lacks
/usr/local/bin; and `grep 'ssh '` proved no ControlMaster was running,
when those processes rename themselves to `ssh: <path> [mux]`. Same root
cause each time, so it goes in the skill rather than in a commit message:
a positive result carries its own evidence, absence has to be earned.

The skill (rootfs/, symlinked into ~/.agents/skills) is BAKED, so this is
an image change and is logged in CHANGELOG Unreleased accordingly. Its §3
also now records that a live ControlMaster socket makes later commands
authenticate not at all -- after editing a peer's authorized_keys, "it
still works" proves nothing; prove it with -o ControlPath=none, or the
breakage waits for a future session that has no memory of the edit.

AGENTS.md: corrected a stale CI claim while placing the pointer. It said a
tag push produces two runs including lint; lint.yml has since been scoped
to branches: ['**'], which excludes tag refs, and refs/tags/v1.8.4 duly
produced run 571 (publish) and nothing else. Kept the head_sha + workflow
path filter advice, which is cheap and guards against a future v*-triggered
workflow. Added a short section on verifying this repo from inside a
container, including that docker-compose.yml here is a TEMPLATE pinning
:latest while a real host runs its own per-machine file -- recreating from
the repo copy can silently move a host off :latest-studio.

Placement note: AGENTS.md is only auto-read when the cwd is this repo, so
the durable rule lives in the skill, which loads by description match in
any pi-devbox session.

AGENTS.md

@@ -76,10 +76,15 @@ re-brand of opencode-devbox's `pi-only` variant.
 4. Push tag: `git tag vX.Y.Z && git push origin vX.Y.Z`.
 5. Watch CI: smoke job builds amd64 only and asserts size + extensions +
    pi version + new-base-tooling presence. Variant build is multi-arch
-   (amd64 + arm64) only after smoke passes. **A tag push produces two runs, not
+   (amd64 + arm64) only after smoke passes. A tag push fires **only**
-   one** — `lint.yml` fires on every push (including tag refs) and
+   `docker-publish.yml` — `lint.yml` is scoped to `branches: ['**']`, which
-   `docker-publish.yml` fires on `v*` tags. Watch the **publish** run; see
+   excludes tag refs on purpose (the tagged tree was already linted when the
-   *Gitea API access* below for how to find it without picking lint by mistake.
+   commit hit `main`, and a fast lint run sorting above the slow publish run
+   made releases look finished before anything shipped). Verified on v1.8.4:
+   `refs/tags/v1.8.4` produced run 571 (publish) and nothing else. Still filter
+   discovery on `head_sha` **and** the workflow `path` — see *Gitea API access*
+   below — because that guard costs nothing and a future workflow added on `v*`
+   would silently reintroduce the ambiguity.
 6. Verify the Hub tags appear (latest + vX.Y.Z, the `-studio` pair, plus
    base-latest if the base was rebuilt this run).
 7. **Revoke any short-lived Gitea PAT** used during the release at
@@ -87,6 +92,34 @@ re-brand of opencode-devbox's `pi-only` variant.
    `GITEA_ACCESS_TOKEN` env var instead (see *Gitea API access* below) —
    its lifecycle is managed host-side, nothing to revoke.
 
+## Verifying this repo's reality from inside a container
+
+Most work on this repo happens **inside** a pi-devbox container, inspecting a
+host or a peer over SSH. That setup manufactures convincing false negatives, so
+when you are about to report that something is **absent, unreachable, or not
+running**, suspect your own command first. Recurring instances:
+
+- **`docker` is not on the host's non-interactive SSH `PATH`.** `ssh mac 'docker
+  ps'` says *command not found* on a host that plainly runs Docker; use
+  `/usr/local/bin/docker` (or `command -v docker` first). Every step in the
+  *Release-day checklist* that inspects a running container hits this.
+- **Don't `| head -N` a search whose answer you don't already know.** The host's
+  `~/.ssh/config` is ~500 lines; a `head -20` "proved" a peer absent that was
+  defined at line 454.
+- **The deployment compose file is not this repo's.** `docker-compose.yml` here
+  is a template pinning `:latest`; a real host runs its own per-machine file
+  (find it with `docker inspect <container> --format '{{ index .Config.Labels
+  "com.docker.compose.project.config_files" }}'`). Recreating from the repo copy
+  can silently move a host off `:latest-studio` onto `:latest`.
+- **A live SSH ControlMaster hides remote auth changes** — after editing a
+  peer's `authorized_keys`, prove access with `-o ControlPath=none -o
+  ControlMaster=no`, or the breakage surfaces in a later session instead.
+
+Depth and further mechanisms: the repo-authored `pi-devbox-environment` skill
+(`rootfs/usr/local/share/pi-devbox/skills/pi-devbox-environment/SKILL.md`) §2
+and §3 — that file is the one an agent actually loads mid-session, whereas this
+`AGENTS.md` is only auto-read when the cwd *is* this repo.
+
 ## Gitea API access (env token)
 
 `GITEA_ACCESS_TOKEN` + `GITEA_HOST` are passed into the container from the

CHANGELOG.md

@@ -11,6 +11,42 @@ Pre-v1.0.0 tags followed the pi npm version (`v{pi_version}[letter]`).
 
 ---
 
+## Unreleased
+
+Docs only so far, but one of the two files ships **inside** the image.
+
+### Changed
+
+- **`pi-devbox-environment` skill — new §2 subsection "A negative result is
+  usually your own filter", plus ControlMaster masking in §3.** This is baked
+  (`rootfs/usr/local/share/pi-devbox/skills/`, symlinked to
+  `~/.agents/skills/`), so it is an image-behaviour change even though no
+  package moved. Motivated by three false negatives an agent produced in a
+  single session, each from its own filter rather than from the world: a
+  `| head -20` "proved" an SSH peer absent that was defined at **line 454** of a
+  ~500-line config; `ssh mac 'docker ps'` "proved" the host had no Docker, when
+  the non-interactive SSH `PATH` simply lacks `/usr/local/bin`; and a `grep 'ssh
+  '` "proved" no ControlMaster was running, when master processes **rename
+  themselves** to `ssh: <controlpath> [mux]`. The rule now stated: a positive
+  result carries its own evidence, absence has to be *earned*. §3 additionally
+  documents that a live master socket makes later commands authenticate **not at
+  all**, so "it still works" proves nothing after editing a peer's
+  `authorized_keys` — verify with `-o ControlPath=none -o ControlMaster=no`, or
+  the breakage surfaces in a future session with no memory of the edit.
+- **`AGENTS.md`: a stale CI claim corrected.** It said "a tag push produces two
+  runs, not one — `lint.yml` fires on every push (including tag refs)". That
+  stopped being true when lint was scoped to `branches: ['**']`, which excludes
+  tag refs by design; `refs/tags/v1.8.4` produced run 571 (publish) and nothing
+  else. The `head_sha` + workflow-`path` filter advice stays, because it costs
+  nothing and any future `v*`-triggered workflow would reintroduce the
+  ambiguity. Also adds a short "Verifying this repo's reality from inside a
+  container" section, including the trap that **this repo's `docker-compose.yml`
+  is a template pinning `:latest`** while a real host runs its own per-machine
+  file — so recreating from the repo copy can silently move a host off
+  `:latest-studio`.
+
+---
+
 ## v1.8.4 — 2026-08-22
 
 Patch release, and the one that ends an eight-week bug: **the baked

rootfs/usr/local/share/pi-devbox/skills/pi-devbox-environment/SKILL.md

@@ -130,6 +130,36 @@ are "command not found" there — you must spell out the underlying command.
 If a command "works in my terminal but not when the agent runs it," this alias
 gap is the first thing to suspect.
 
+### A negative result is usually your own filter
+
+**When you are about to report that something is absent, unreachable, or not
+running, the filter you wrote is the prime suspect — not the thing.** This
+environment produces false negatives cheaply, and they are convincing because
+the command "succeeded". Three real instances from one session, all wrong, all
+mine:
+
+| Claim I made | Why it was false |
+|---|---|
+| "`tor-ms22` is not in the SSH config" | `grep … \| head -20` — the entry was at **line 454**. `~/.ssh/config` here is ~500 lines. |
+| "the Docker host has no `docker`" | non-interactive SSH `PATH` lacks `/usr/local/bin` (§2, §3). It was at `/usr/local/bin/docker`. |
+| "no ControlMaster is running" | pattern `ssh ` (trailing space) cannot match a master: those processes **rename themselves** to `ssh: <controlpath> [mux]`. |
+
+Habits that would have caught all three:
+
+```sh
+# don't cap the output of a search whose answer you don't already know
+grep -n -i -A6 'tor-ms22' ~/.ssh/config          # not | head -20
+
+# on the host, resolve the binary instead of trusting PATH
+ssh -F "$HOME/.ssh-local/config" mac 'command -v docker || ls /usr/local/bin/docker'
+
+# match a process's ACTUAL argv, not the name you imagine
+ps -eo pid,etime,args | grep -Ei 'mux|mosh|ssh'
+```
+
+A positive result needs no such scepticism — it carries its own evidence. Only
+absence has to be *earned*, so spend the extra command there.
+
 **`dscp`/`scp` with accented filenames on a macOS host.** macOS stores filenames
 in Unicode **NFD** (decomposed — e.g. `ä` is `a` + combining U+0308), while the
 string you type or paste is usually **NFC** (precomposed `ä`, U+00E4). The bytes
@@ -175,6 +205,23 @@ Two related mechanisms (don't reinvent them):
   CGNAT per-destination flow caps on residential ISPs. If `~/.ssh/config` pins
   a `ControlPath` under the read-only `~/.ssh`, override with
   `-o ControlPath=none` (or use the sidecar, which already redirects it).
+- **A live master socket MASKS auth and config changes on the far end.** Once
+  `~/.ssh-local/cm/<user>@<host>:22` exists, later commands ride it and
+  authenticate **not at all** — so after editing remote `authorized_keys`,
+  `sshd_config`, host keys, or firewall rules, "it still works" proves nothing.
+  A corrupted `authorized_keys` then bites on the next *cold* connect, likely in
+  a future session with no memory of the edit. Prove it immediately instead:
+
+  ```sh
+  ssh -F "$HOME/.ssh-local/config" -O check <host>        # 'Master running (pid=N)'
+  ssh -F "$HOME/.ssh-local/config" -o ControlPath=none -o ControlMaster=no \
+      -o BatchMode=yes <host> 'echo COLD AUTH OK'
+  ```
+
+  To attribute a socket rather than guess whose it is: `ps -p <pid> -o
+  pid,ppid,lstart,etime,args`. A `mosh` the *user* started on the host
+  bootstraps with the **host's** `~/.ssh/cm/` and is invisible from in here;
+  only a mosh started *inside* the container shares `~/.ssh-local/cm/`.
 - **`pi --ssh <host>`** rewires pi's own read/write/edit/bash tools to run on a
   remote host; it has its own writable-socket fallback. See the `pi-extensions`
   skill for that path.
@@ -257,6 +304,10 @@ hardcode. Details are in the `mempalace` skill.
 - [ ] Writing durable output? → `/workspace`, not the ephemeral layer.
 - [ ] Using `dssh`/`dscp`/`ll` in the bash tool? → spell out the real command.
 - [ ] Assuming a hostname / domain / nameserver / host OS? → stop, detect it.
+- [ ] About to report something **absent / unreachable / not running**? → re-run
+      without your own `head`/pattern/`PATH` assumptions first (§2).
+- [ ] Changed remote `authorized_keys` / `sshd_config`? → prove it with a **cold**
+      connect; a live master socket hides breakage (§3).
 - [ ] "Resolves but won't connect"? → check route *and* DNS (§3 + §4).
 - [ ] `apt`/toolchain install? → tell the user it's ephemeral unless imaged.
 - [ ] Editing a skill? → `readlink -f ~/.agents/skills/<name>` first (§1).