The rule was correct and written down twice (global AGENTS.md, this skill)
and was still violated by agents that had just read it: on 2026-09-17 all
five fork briefs in one session carried "do not", one returned confident
verbatim quotes that did not exist, and four had disjoint write boundaries
fork cannot enforce (re-run as pi-task, they passed). Three mechanisms,
none of them wording:
1. fork is a TOOL — its self-recommending description ("implementation,
testing, review…") is in the model's face every turn; pi-task was a CLI
to be remembered and reached through bash with a hand-written JSON.
2. The skill is gone after the first compaction; the tool list never is.
The asymmetry widens in exactly the long sessions where fork is worst.
3. Friction: one string vs a spec file + bash + reading result.json.
extensions/task.ts registers pi-task as the `task` tool. Flat parameters
build the spec; the decision rule sits in the description and in
promptGuidelines (appended to the system prompt, so compaction cannot remove
it). Before spending a model run it rejects the two spec errors that make a
boundary violation certain — write_allowed not an exact subset of roots, and
a writable root nested in a watched-only root (the parent's porcelain would
change every time; pi-task keys deltas by root string) — and it serialises
sibling tasks whose roots overlap (parallel siblings saw each other's writes
as violations, 2026-09-17). Returns the CLI's own parent-facing report; a
FAIL verdict is a result, only a CLI refusal is an error.
extensions/fork-gate.ts is a tool_call hook that BLOCKS a fork whose brief
contains a prohibition, a write boundary, or a clause-initial file-changing
imperative, and returns as the reason the exact task(...) to make instead.
Wording, not intent — the message says so and how to rephrase a genuinely
read-only brief. PI_FORK_GATE=off logs instead; /ext disables.
Evidence: test/fork-gate.test.mjs is two-sided (15 must-block incl. the real
shapes, 10 must-pass incl. "Write a summary…", "Report which files were
modified…", "Give me an update…"); the classifier redirects 5/5 of the real
briefs from the motivating session. test/task.test.mjs pins root overlap and
the pre-launch validation. Live in `pi -p`: the fork was intercepted before
any child spawned (no /tmp/pi-fork-* dir) and the model received the
redirect; task returned PASS with an evidence pointer and audit dir, a
budget-overrun returned a FAIL result (isError=false), and a nested-root spec
was rejected with no audit dir created.
skill/SKILL.md: Part 1 now opens with "decide the rung before the brief"
(the table, the three-question pre-flight, the roots contract, overlap,
what isolation does not fix); the ladder section and quick reference no
longer say pi-task "will never appear in your tool list". package.json gains
"type": "module" and a test script.
joakimp
25c1265681